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Occurrence of gypsy moth (Lymantria dispar L.) in the Slovak Republic and its outbreaks during 1945–2020

Occurrence of gypsy moth (Lymantria dispar L.) in the Slovak Republic and its outbreaks during... The gypsy moth is one of the most serious pests in forests and fruit tree plantations over prevailing parts of the North- ern Hemisphere. This work is based on a literature review, and presents history of gypsy moth Lymantria dispar L., observed in Slovak forests within the period 1945–2020. The life cycle, hosts, natural enemies, population dynamics of pests, impact of outbreaks on forests and different management methods used in the past are discussed. Since 1945, there were nine gypsy moth outbreaks in Slovakia. Between 1945 and 2020, a total of 155,034 ha of deciduous forests were touched with varying intensity, representing an average annual damage of 2,040 ha. The strongest outbreak culminated in 2004. Totally 51,479 ha were attacked in the period of 2000–2008. We have found outbreak periods that repeat with frequency of 7.8 ±2.2 years and the average outbreak phase lasts 3.1 ±1.1 years. The period between two subsequent outbreaks seems to be more or less constant and duration of the outbreak phase seems to be gradually shortened during the study period. Several factors influencing the gypsy moth population dynamics in Slovakia are discussed. The role of biological control by using entomopathogenic fungus Entomophaga maimaiga is described. Key words: population dynamics leaf-eating insect; periodic outbreaks; natural enemies; Entomophaga maimaiga Editor: Jiří Foit insects in Slovakia (Zúbrik et al. 2017a). The gypsy moth 1. Introduction ranked among key insect pests that feed on leaves in for- The gypsy moth Lymantria dispar L. (Lepidoptera: Ere- ests of Slovakia during the study period, along with Oper- bidae) is one of the most serious forest insect pests, but ophtera brumata L., Erannis defoliaria Clerck, Agriopis also of fruit trees across much of the Northern Hemi- leucophaearia Denis & Schiffermüller, Tortrix viridana sphere. Large areas damaged by the gypsy moth are L., Orthosia spp., Choristoneura murinana Hübner, Epi- reported from the Northeastern United States and Asia notia nigricana Herrich-Schäffer, Diprion pini L., Diprion (Schedl 1936; Doane & McManus 1981; McManus & spp., Melolontha spp., and some other species (Turček Csóka 2007; Zúbrik et al. 2013). The cyclic gypsy moth 1956; Charvát & Patočka 1960; Čapek 1961; Patočka outbreaks (Hlásny et al. 2015) resulted in loss of radial 1955, 1963a, b, 1967a, 1973; Leontovyč et al. 1980; growth (Muzika & Liebhold 1999), changes in fruiting Surovec et al. 1989; Zúbrik 2006; Zúbrik et al. 2015, (Gottschalk 1990), and if repeated, in tree mortality in 2017a, b; Vakula et al. 2015; Sarvašová et al. 2020). Some subsequent years (Patočka & Novotný 1985; Davidson species, such as Pristiphora laricis Hartig, Rhyacionia et al. 1999). In Southeast Europe, outbreaks are more fre- buoliana Denis & Schiffermüller or Coleophora laricella quent (Pernek et al. 2008) and more intense (McManus & Hübner, have caused damage to trees only occasionally Csóka 2007) than in Central Europe (Hlásny et al. 2015). and only in relatively restricted areas (Leontovyč et al. Over the period of 1945–2016, more than 0.5 million hectares was damaged in different ways by leaf-eating 1980; Surovec et al. 1989; Zúbrik et al. 2017b). *Corresponding author. Milan Zúbrik, e-mail: milan.zubrik@nlcsk.org M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Despite direct effect of defoliation on the tree mor- Institute (SHMÚ) in Bratislava. Average monthly tem- peratures and precipitation data recorded at 26 weather tality is questionable, often defoliated drought-stressed stations (Gabčíkovo, Bratislava Airport, Bratislava- trees increase their secondary mortality caused by other Koliba, Dolné Plachtince, Dubník, Dudince, Holíč, Hur- pests such as Scolytus intricatus Ratzeburg (Coleop- banovo, Komárno, Kráľová pri Senci, Kuchyňa, Leles, tera: Scolytinae), Agrilus spp. as well as other species of Lučenec, Malacky, Malé Ripňany, Moldava nad Bodovu, jewel beetles (Coleoptera: Buprestidae) and long-horned Nitra, Nový Tekov, Podhájska, Rimavská Sobota, Somo- beetles (Cerambycidae), etc., developing frequently and tor, Štúrovo, Tesárske Mlyňany, Trnava, Žihárec) over abundantly in weakened trees (Novotný 1986; Zach the period of 1931–2015 were used. The presence of the 1994; Patočka et al. 1999). In the 70s and 80s of the last gypsy moth in the area was the criterion for including century, oak stands in Slovakia, were heavily affected by a particular weather station in the analysis. Data were tracheomycosis disease (Leontovyč 1980; Surovec et al. smooth using Local Polynomial Regression Fitting. 1989). Tracheomycosis disease and also armillaria root disease (Armillaria spp.) significantly reduce the ability of oak stands to resist defoliation caused by gypsy moth. The aim of this study was to summarise the more 2.2. Terms detailed information on the gypsy moth in Slovakia, in In order to be as precise as possible and to avoid confu- particular: to reconstruct its population dynamics, ana- sion due to unclear terms, we provide a short explana- lyse data about life cycle, hosts, natural enemies, impacts tion of some of the most commonly used terms (Fig. 1). on forests and evaluate other signic fi ant aspects related to We defined ‘outbreak’, ‘outbreak length’ or ‘outbreak its biology and ecology over the period of 1945–2020. All period length’ respectively, as a period, during which these aspects are discussed in the context of theoretical gypsy moth caused certain damages in the forest (higher knowledge, and practical expertise obtained from sources than zero). During building and declining phase of out- in Slovakia and others being external. Data from Slova- break, gypsy moth population density often remains on kia are discussed with those, known from other areas of a very low level for a long time. For that reason we also gypsy moth occurrence. introduce a term “outbreak phase“. We defined it as a period, during which more than 1,000 ha of forest stands was damaged annually. This rule was not applied to the 2. Methods period 2013–2014 and 2017–2020. During this period, only minor areas were damaged, but we nevertheless, 2.1. Data sources for certain reasons mentioned in the article, decided To obtain the data we used three main sources for this to label the events as outbreaks of the gypsy moth. The work. ‘outbreak peak’ is considered the year, with the highest A) The primary source was the data present in different registered damage during the outbreak period (Fig. 1). scientific and expert publications. Accurate information If we speak about “outbreak frequency“, we mean the about the gypsy moth presence from the period 1945– period between two outbreak peaks. 1960 is lacking. Most of the data were available on the country level. We also used information about local gra- dations, determined by a place, or certain locality. These 2.3. Statistics and data presentations data are based on published estimations. Recent data, For common statistics and data interpretation, we used in papers published over the last 50 years are of higher Microsoft Excel 2016. We used mean, standard devia- accuracy. tion (mean ± standard deviation) and coefc fi ient of deter - B) For the period since 1961, we used official statisti - mination calculated in this program. Trend lines were cal records reported by forest managers. Damages were calculated using the method of least squares in the same recorded on the accuracy level of the forest districts or program. For picture processing, and figure elaborating country district (Zúbrik et al. 1999). we used Adobe Photoshop® (2016) and R Studio, version C) The most accurate data were obtained through moni- 1.3.1093, package ggplot2 Wickham (2016). toring conducted by forest managers and supervised by the Forest Protection Service in the frame of relevant projects, as our observations and experience. 3. Gypsy moth life cycle We analyzed data from all the above-mentioned sources to reconstruct the long-term trend of the gypsy In Slovakia, the gypsy moth prefers older forest stands moth population occurrence in Slovakia. After analys- that are under warmer conditions in the southwestern ing the data from all these three sources (A, B, C), we and southern regions of the country, as well as in the constructed a table with the estimated area, damaged by eastern lowlands. It can also be found in dry localities, gypsy moth per years. on steep slopes, in sparse and in wet-mesic floodplain To define temperature and precipitation trends, we forests along rivers (Turček 1956; Stolina 1985; Novotný analysed the data from the Slovak Hydrometeorological & Turčáni 1992; Patočka et al. 1999). 56 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Fig. 1. Gypsy moth outbreak phases. The gypsy moth has one generation per year. Adults windy (Patočka 1961). Airborne spread of larvae can be (Fig. 2) are on the wings from July to August (from mid- up to 15 km by wind (Novotný 1986). The larva (Fig. 2) dle June to early September in recent years). Females has 5–6 instars, depending on sexes, that are relatively lay eggs (Fig. 2) preferably at the base of the tree trunk easy to determine by external morphological characters although, during an outbreak, they do it also high in the (Patočka 1954; Gogola 1969). Larvae start to feed in the crowns and even on thin branches (Turček 1956; Patočka crown at the beginning of May, defoliation culminates in 1961, 1973; Novotný 1986; Zúbrik 2006; Vakula et al. mid-June (Fig. 3). They co-occur with other abundant 2015). For assessing pest density on a plot, the Turček’s species of leaf feeding caterpillars, such as Archips xylo- method (based on egg masses count) is commonly used steana L. and Orthosia spp. (“dispar-xylosteana com- (Turček 1956), later this being changed slightly in for- plex”) (Kulfan et al. 2018). Larvae pupate (Fig. 2) in est documents that are officials (STN 43 2715). Dur- late June and early July. The pupal stage lasts two weeks ing an outbreak period, average number of egg masses (Novotný 1986; Vakula et al. 2015; Zúbrik et al. 2020a). per tree can reach 20 to 30 clusters (Hoch et al. 2001; Swarming starts in the earlier part of July and culminates Zúbrik, 2006), but it can be in some exceptional cases in its second half and eggs are laid in early August. At even more, 30 to 70 in heavily infested stands (Patočka the beginning of September, no living adults are seen in 1973; Novotný 1986; Novotný & Turčáni 1992; Zúbrik forests (Turček 1949a; Novotný 1986). & Novotný 1997). Patočka (1973) counted as many as 250 egg masses on one tree. If the number of masses per tree average two or more, the pest outbreak can be 4. Hosts to the gypsy moth expected the following year (Turček 1956; Vakula et al. Gypsy moth hosts in Europe vary somewhat depending 2015), but critical numbers should be considered based on how its distribution corresponds to the predominant on the forest age and health status (Patočka 1961). vegetation in various geographical regions. There are The number of eggs per egg mass varied from 343– slight differences in the gypsy moth food preferences 491 (Novotný, 1986) to 500–1000 (Turček 1949a; 1956). between North, Central and southern Europe (Schopf Turček (1949a; 1956) assumed 500–600 eggs are an et al. 1999; Švestka 1993, 1994, 2004; Hirka 2006; Csóka average value. Hoch et al. (2001) reported similar num- & Hirka 2009; McManus & Csóka 2007). bers (averaging 534 eggs per egg mass) from Klingen- bach (Austria, 60 km southwest of the Slovakia’s border). The gypsy moth is a highly polyphagous species, The gypsy moth overwinters as eggs. Larvae hatch not only in Slovakia, but also in the whole territory of its from the middle of April (Novotný 1986; Vakula et al. occurrence (Kurir 1953; Janković 1958; Jahn & Sinreich 1957; Fuester et al. 1983; Novotný 1986; Zúbrik et al. 2015; Zúbrik et al. 2020a). After hatching, they rest 2013). In Eurasia, gypsy moth larvae are able to consume several days on the surface of egg mass and then crawl to the crown on sunny days or balloon if the weather is about 90 different tree species, while in the United States 57 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Fig. 2. Gypsy moth stages A) male, B) female, C) egg masses, D) pupae, E) larva in a final instar. Table 1. Area defoliated by the gypsy moth in years 2002, and it is about 85 species (Schedl 1936; Doane & McManus 2004–2006, by tree species, according to the official statistics 1981). Wellenstein & Schwenke (1978) state that up to (Varínsky et al. 2003; Kunca et al. 2005, 2006, 2007, 2008). 300 host plants have been recorded worldwide. It seems Tree species Area damaged (in hectares) % that younger larvae prefer mainly oak, possibly other Quercus spp. 38,099 91.09 trees with soft leaves, and only older instars are more Carpinus betulus 2,492 5.96 Robinia pseudoacacia 743 1.78 polyphagous (Patočka 1970; Novotný 1986). Fagus sylvatica 417 1.00 In Slovakia, larvae cause defoliation mostly in oaks Populus spp. 72 0.17 Tilia spp. 2 0.00 (Quercus cerris L., Q. robur L., Q. petraea (Matt.) Liebl., Acer spp. 1 0.00 Q. pubescens Willd.) (Tab. 1), but they can also feed on Alnus spp. 1 0.00 other trees and shrubs such as Carpinus betulus L., Acer Total 41,827 100.00 spp., Robinia pseudoacacia L., Prunus spp., Crataegus 1985; Novotný 1986). Laboratory experiments showed spp., Malus sylvestris Mill., Pyrus communis L., Tilia spp., that gypsy moth populations developed faster, and its lar- Populus spp., Betula spp. as well as others (Turček 1956; vae and pupae gained more weight on Q. cerris than on Patočka 1973; Stolina 1985; Novotný 1986) (Table 1). In Slovakia, Q. cerris (Fig. 3) seems to be the most pre- Q. petraea. Q. cerris females also laid significantly more ferred food source (Turček 1953; Patočka 1961; Stolina eggs than those from Q. petraea. Field studies demon- 58 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Fig. 3. A) Defoliation of Q. cerris stands by the gypsy moth are often severe; Šenkvice, June 2019. B) Defoliation of the spruce trees in 2018 (Vraná nad Vltavou, Czechia) documents gypsy moth is a highly polyphagous species. C) Unusual phenomenon in Slovakia: defoliation of Vaccinium myrtillus by the gypsy moth in a meadow habitat in 2011–2012 and 2020. strated as well, a much faster development with less mor- defoliated too (Fig. 3). Novotný (1986) reported defo- tality of the caged gypsy moth larvae feeding on Q. cerris liation of Picea abies L., Abies alba Mill., Larix decidua compared to those on Q. petraea (Schopf et al. 1999). In Mill., Pinus sylvestris L., Pseudotsuga menziesii Mirb. and Slovakia, there is about 174,500 hectares of forests with Pinus strobus L. Some conifers, as P. abies or P. silvestris, prevalence of oak (Green report 2019) (Fig. 5). occurred in mixed forests were heavily defoliated by the gypsy moth in 2018 and 2020 in Slovakia and Czechia Some tree species remain completely or partially untouched during gypsy moth outbreaks, like Loranthus (Zúbrik, Liška, personal observation). In Slovakia, beech (Fagus sylvatica L.) was defoliated only on a relatively europaeus Jacq., Fraxinus excelsior L., Fraxinus ornus L., restricted area (Patočka 1967b; Novotný 1986), unlike Ligustrum vulgare L. and Morus alba L. (Turček 1956; neighbouring Hungary, where forests of it had heavy Novotný 1986). This is not the same everywhere, as some local insect pest populations may have different feeding defoliation on large areas in the past (Csóka et al. 2015). preferences (Patočka 1973). In 2011–2012, and again in 2020, approximately Coniferous trees are also usually untouched; how- 0.25 ha of meadows (600 m a.s.l.) near Banská Štiavnica ever, taken individually in oak forests, they can be heavily in Slovakia covered by Vaccinium myrtillus L. (Fig. 3) was 59 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 completely defoliated, along with Sorbus spp., Salix spp., 28 insect species from the orders Hymenoptera and Dip- tera and one nematode were recorded during the investi- Malus sylvestris as well as P. abies and P. sylvestris grow- gation in 1991–1996. Species like tachinids Parasetigena ing solitary in that area (Zúbrik, personal observation). silvestris R.–D. and Blepharipa pratensis Meig., braco- Damages to fruit tree plantations, vineyards and even nids Cotesia melanoscelus Ratz., Phobocampe spp., and agricultural crops (Zea mays L.) were also reported in the Glyptapanteles liparidis Bouché were the most impor- past from Slovakia (Turček 1949a, 1956; Patočka 1973; tant (Hoch et al. 2001). A broad range of insect parasi- Leontovyč et al. 1980; Novotný 1986; Alford 2010). toids was also confirmed by other authors (Čapek et al. 1969; Čapek 1988; Zúbrik 1997; Turčáni et al. 2001). At latency sites, C. melanoscelus was the dominant spe- 5. Natural enemy complex cies, followed by Phobocampe spp. and P. sylvestris. The of the gypsy moth oligophagous tachinids P. silvestris, B. pratensis and braconid G. liparidis were the dominant parasitoids In spring, some egg masses eaten by unknown predators at sites of outbreaks and pro-outbreaks (Zúbrik 1997; are often seen in forests (personal experiences). It seems Hoch et al. 2001). About 20% of larvae investigated that predation may play a meaningful role, especially were killed by parasitoids. Mortality by pathogens was during the latency period. Inversely, density-dependent higher more than 30% (Hoch et al. 2001). That of larvae relationship (between the gypsy moth egg mass density by pathogens during the culmination stage can reach and the predation) was found, but this was not signifi - even 60% (Novotný 1989). The most frequently occur- cant (Turčáni et al. 2003). About 30% mortality in eggs ring pathogen, which has been present in Slovakia, is the is caused by birds as predators, such as Certhia familiaris Lymantria dispar multicapsid nuclear polyhedrosis virus L., Sitta europaea L., Parus major L., Cyanistes caeruleus (LdMNPV). LdMNPV is considered the main reason for L. and Aegithalos caudatus L. (Turček 1949a). The gypsy the collapse of the gypsy moth outbreak in 1949 (Charvát moth larvae may fall into the diet of several bird species in 1967). Other pathogens, such as Bacillus thuringiensis forests. However, they appear to be barely able to reduce Berliner, Beauveria bassiana (Bals.-Criv.) Vuill., Nosema the population density of this pest (Krištín 1999). serbica Weiser and Nosema lymantriae Weiser were also There are two egg parasitoids present on the terri- recorded (Novotný 1989; Zúbrik 1997; Hoch et al. 2001). tory of Slovakia, native Anastatus disparis Ruschka and The exotic entomopathogenic fungus Entomophaga mai- Ooencyrtus kuwanae How (Hymenoptera: Eupelmidae), maiga Humber, Shimazu & Soper was found in Slovakia originating in Asia. Both species were released in that for the first time in 2013 (Zúbrik et al. 2014). Further country at certain sites between 1960 and 1965 (Čapek study revealed that the fungus was relatively widely 1966, 1971). They were successfully recorded again spread here (Zúbrik et al. 2018b). It was suggested as well between 1985 and 1988 (Novotný & Čapek 1989) and that it is spreading from the Balkan Peninsula (Zúbrik et also later, from 1992 until 1995 (Zúbrik & Novotný al. 2016). Field study conducted in Slovakia during the 1997), although, parasitisation rates were low, varying years of 2014–2017 documented a narrow host range of from 1 to 3%. E. maimaiga. Therefore, significant negative effects on The natural enemy complex is much broader in the the native lepidopteran fauna are not expected to occur larval and pupal stages than in the egg stage. In total, (Zúbrik et al. 2018a). Fig. 4. Area damaged by the gypsy moth during the period from 1945 to 2020. Period from 1945 to 1960 is coloured in different shade. In this way we wanted to point out, that information we had available about gypsy moth population density in the period up to 1960 is less accurate and not very detailed. While information after 1960 is more accurate and quite comprehensive. 60 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Predation on pupae was also investigated. It shows al. 2020a). Only 4 outbreaks were included into the data that small mammals may play an important role in pupal analyse in Hlásny et al. (2015), and also outbreak period, mortality (Turčáni et al. 2001). Invertebrate preda- not outbreak phase was used in analyse. In our study, 9 outbreaks and outbreak phase was used. This difference tion on gypsy moth caterpillars and pupae can also be caused, our results slightly differ from those published a major mortality factor, mainly Calosoma sycophanta L. and Calosoma inquisitor L. (Coleoptera: Carabidae) by Hlásny et al. (2015). We tried to construct a trend for the length of the out- frequently seen in forests during an outbreak period break phase and the length of the period between two (Zúbrik, personal observations), but they have not yet outbreak peaks (outbreak frequency) (Fig. 6). The trend been systematically studied as predators of larvae in indicates the period between two subsequent outbreaks Slovakia. seems to be more or less constant and duration of the out- Outbreak situation inu fl ences in a different way forest break phase seems to be gradually shortened during the ecosystems. Structure of natural enemies has changed study period. However, the coefficient of determination (Hoch et al. 2001). Impact of defoliation on the presence is very low in both cases. The shortening of the length of of plant diseases was also conr fi med (Patočka & Novotný the outbreak phase may be related to the appearance of 1985). The number of birds increased 1.5 to 2 times dur- the fungus E. maimaiga. Several circumstances suggest ing the outbreak period (Turček 1956). that E. maimaiga has affected the length of gradation in 2013–2014 and 2017–2020 significantly (Zúbrik, per - sonal observation). 6. Dynamics of the gypsy moth population There are some traditional areas for gypsy moth According to our analyses, there were nine gypsy moth outbreaks on Slovakia’s territory that do not change too outbreaks on the territory of Slovakia since 1945 (Table much over time (Fig. 5). These regions are, such as the 2, Fig. 4). Over the period of 1945–2020, 155,034 ha of following: 1) the western part of the country, around deciduous forests were touched with varying intensity, Pezinok, Modra, Šenkvice, Bratislava; 2) around Nitra, representing an average annual damage of 2,040 ha. The Čifáre, Levice and Nové Zámky; 3) around Veľký Krtíš, strongest outbreak was recorded in 2000–2008. Totally Rimavská Sobota and Lučenec; and 4) eastern Slovakia, 51,479 ha were attacked during that period (Tab. 2). The around Michalovce, Ortov and Sobrance (Pfeffer 1961; year with the greatest damage intensity was in 2004, Patočka 1961, 1973; Charvát 1967; Kunca et al. 2005, when 21,304 ha were attacked. On average, 17,226 ha 2006, 2007; Zúbrik et al. 2019). Gypsy moth outbreaks were affected per one outbreak. We have found outbreak usually start in the western and central parts of the coun- periods in Slovakia that repeat with frequency of 7.8 ± try, and then slowly shift to the East (Turček 1956; Char- 2.2 years and the average outbreak phase lasts 3.1 ± vát 1967; Zúbrik et al. 2019). We tried to confirm this 1.1 years overall. This value agrees with previous stud- statement on an example of the outbreak in 2002–2007 ies of Patočka (1954), Novotný (1986). Novotný (1986) (Fig. 7). Defoliation in 2002 was reported only from was speaking about an outbreak period of three years. western part of Slovakia and defoliation in eastern part Sometimes local outbreaks collapse faster from different clearly occurs r fi st only after 3 year in 2005 and continued reasons, after one or two years (Patočka 1973; Zúbrik et to 2006. Fig. 5. Map showing the distribution of all oak forest types in Slovakia (only those with more than 10% of oak in composition were considered as oak forest). Very schematic indication of traditional areas for gypsy moth outbreaks on Slovakia’s territory (see description in text). 61 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Pezinok and Palárikovo. Also, Patočka (1961) mentioned Outbreak No. 1: this outbreak and dated it as 1954–1957. Years with very About this r fi st, it was a very strong gypsy moth outbreak mild weather in 1952, 1954, 1955 and 1956 could have after World War II, reported Hendrych (1959). Patočka initiated this, which started in stands of Fageto-Querce- (1961) mentioned that there was damage to 30,000– tum and Carpineto-Quercetum under warmer climate. 50,000 hectares during two outbreaks in 1946–1949 and Outbreak culminated in 1956 and then the gypsy moth then in 1954–1957. Charvát (1967) conr fi med that about density was reduced, probably due to the unsuitable 30,000 ha were defoliated and it culminated in 1949 dur- weather in 1957 and 1958. Charvát (1967) dated this ing the outbreak period of 1946–1949. He was speak- oubreak approximately to the years 1953–1958. ing about a “large outbreak” in 1946–1949, comparing it with the one in 1953–1958, which he commented as a Fig. 6. The length of the outbreak phase (blue dots, 1-1948–1950, 2-1955–1957, 3-1963–1965, 4-1972–1975, 5-1984–1987, 6-1992–1995, 7-2003–2006, 8-2013–2014, 9-2019) and the length of the period between two outbreak peaks (red dots, 1-1949–1956, 2-1956–1964, 3-1964–1973, 4-1973–1986, 5-1986–1993, 6-1993–2004, 7-2004–2013, 8-2013–2019) with linear trends (duration of the outbreak phase – blue dotted line, duration of the period between two outbreak peaks – red dotted line). Trend lines were calculated using the method of least squares. “smaller outbreak of local importance”. Patočka (1973) Outbreak No. 3 described outbreaks in 1946–1948 as “massive” and Leontovyč et al. (1980) deeply discussed this period of the subsequent two in 1954–1956 and 1964–1966 as 1963–1967. They reported complete defoliation near “smaller ones”. Turček (1950) and Konôpka (1978) Pezinok, Palárikovo, Levice, Šahy, Lučenec and other also reported about a large, area-wide defoliation dur- areas. It is estimated that there was damage to about ing this outbreak period. It was preceded by extremely 2,000 ha annually during this one. Patočka (1967b) and dry and hot weather in 1947. The defoliation started Čapek et al. (1969) also noted this outbreak and dated it in western Slovakia and moved to eastern Slovak low- as 1963–1965, with an abundance reaching culmination lands in subsequent years (Charvát 1967). Forests were especially damaged near Levice, Nitra, Lučenec, Šahy, in 1964. Čapek et al. (1969) studied larval parasitoids in Rimavská Sobota and Sobrance (Patočka 1953; Charvát the gypsy moth population and he therefore independ- 1967). Turček (1949a) described outbreaks at the local ently monitored the density of the latter on seven study level, reporting total defoliation in 1946–1948. In 1949, plots. The results show that the number of gypsy moth after all primary food sources were consumed; hungry egg masses has already increased in 1963 and their abun- caterpillars caused damages even to agricultural crops dance culminated in 1964. Population was locally high (Patočka 1961). even in 1965, but in 1966, it falls down to a low level, apparently also affected by cold spring weather (Patočka Outbreak No. 2 1967b). In 1968, egg masses only occurred sporadically Shortly about this outbreak, reported Pfeffer (1961), (Charvát 1969). largest-scale defoliation was observed in 1955, near Šaštín-Stráže and Smolenice, as well as in 1956, near 62 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 local outbreak in the area of about 200 ha was recognised Outbreak No. 4 (Zúbrik & Turčáni 1997). Leontovyč et al. (1980) r fi st dated this to the years 1971– 1974. They mentioned a situation where 300 hectares, Outbreak No. 7 even of coniferous trees (Pinus spp., P. abies, Pseudot- The pest population density rose slightly in 2002 followed suga spp.), were damaged. Patočka et al. (1999) recorded by a very fast increase in 2003. The outbreak started in an increase in the gypsy moth density, also in that period, the western part of the country and progress to the East and confirmed the outbreak culminating in 1973–1975, (Fig. 7). In 2004, already 21,304 ha were defoliated by with local occurrences in 1976 and 1977.) A very high the gypsy moth, mainly around Nitra and Levice (Kunca pest density in the whole zone was observed in 1975, but et al. 2005). As for 2005, 13,498 ha were damaged, pri- in 1976 and 1977, only small outbreaks were reported marily in the vicinity of Veľký Krtíš, Krupina, Lučenec and the damaged area was increasing from 145 ha in as well as Bratislava and Nové Zámky (Kunca et al. 1976 to 247 ha in 1977 (Surovec et al. 1989). 2006, 2007, 2008; Zúbrik 2006). Aerial applications of insecticides were realised on a territory of 29,831 ha in Outbreak No. 5 2004–2006. Approximately 7,000 hectares were defoli- The exact data about this outbreak are provided by ated (officially 6,025 ha) in 2006 (Kunca et al. 2006). Surovec et al. (1989). It speaks about an outbreak where Kunca et al. (2007) were speaking about the end of the approximately 2,400–4,500 hectares were damaged on outbreak in 2007, when only a very restricted area was an annual basis, especially in Bratislava, and slightly damaged (45 ha) and no application was made. They less in the Banská Bystrica administrative district. First concluded that this was the largest gypsy moth outbreak defoliations were already reported in 1983 (near Tesárske during the last 50 years. It was exceptionally severe not Mlyňany). In 1984, local occurrences were more frequent only in Slovakia, but also in many other EU countries. in every primary affected zone, nearby specic fi ally Čifáre, Hungary, for example, reported record-breaking dam- Podhájska, Nitra and Žitavany. In 1985 and 1986, there ages to 212,000 hectares starting from 2005 (Csóka & was area-wide defoliation reaching impressive intensity. Hirka 2009), while in Croatia the gypsy moth affected Significant reduction in abundance was found in 1986 33,000 ha in this particular year (Hrašovec et al. 2008). and local outbreaks became scarce in 1987. Outbreak Švestka (2004) observed a large gypsy moth outbreak in period had a low-range peak in 1986. These data were the neighbouring Czechia in 2003 and 2004. He expected also confirmed by analysing the official statistical evi - that defoliation will continue in 2005. dence from the state forest (Zúbrik et al. 2013). Patočka et al. (1999) dated this outbreak to the years 1984–1987 Outbreak No. 8 too. This is very unusual outbreak and after consideration, we decided to keep it here, despite it does not full fi all criteria Outbreak No. 6 for a typical outbreak period. In 2011, in certain areas, a The period that was warm and dry at the beginning of the high number of gypsy moth larvae and adults was found nineties of the last century (especially the year 1992) con- in the oak forests. During autumn 2011, high amount of tributed to the rapid progress of the gypsy moth outbreak gypsy moth egg masses in oak stands was confirmed. A in 1993 and 1994 (Zúbrik 1998). Novotný & Turčáni further signic fi ant rise in population density was expected (1993) reported previously in 1992 some changes in the in 2012. However, this did not occur (Zúbrik et al. 2013). population density. The infested area already reached In 2013, defoliation was recognised on about 200 ha in more than 2,000 hectares in 1992, with a tendency to some isolated “spots” (for example, near Ortov in the further increase. Outbreak culminated in 1993, and eastern part of Slovakia) (Kunca et al. 2014). In 2014, very high level of infestation remains in 1994. The most about 150 ha was defoliated (Kunca et al. 2015, 2016). damaged areas were close to Nitra, Levice, Malé Karpaty, A detailed monitoring of the gypsy moth population den- Košice and Prešov. In 1995, damages were not so exten- sity conr fi med these observations; it slightly increased in sive. In 1996, the gypsy moth population was in latency; a 2011–2013. As for 2014, there was a fast decline into a Table 2. Outbreaks overview in the period 1945–2020. Hectares defoliated Hectares defoliated No. of outbreak Outbreak peak Outbreak Outbreak phase Control methods used during outbreak during outbreak phase 1 1949 1946–1951 29,822 1948–1950 28,000 Dynocid (DDT) 2 1956 1953–1958 10,600 1955–1957 10,000 Dynocid (DDT) 3 1964 1963–1967 6,100 1963–1965 5,500 no data available 4 1973 1971–1978 7,268 1972–1975 6,278 no data available 5 1986 1979–1990 13,588 1984–1987 11,799 B. thuringiensis + growth regulators 6 1993 1991–1999 33,639 1992–1995 32,210 B. thuringiensis + growth regulators 7 2004 2000–2008 51,579 2003–2006 49,758 B. thuringiensis + growth regulators 8 2013 2013–2014 350 2013–2014 350 B. thuringiensis 9 2019 2017–2020 2,053 2019 2,000 no control realised Total (ha) 155,034 145,925 63 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Fig 7. Area damaged by the gypsy moth from 2002 to 2007 in different geographical parts of Slovakia, demonstrating, that the defoliation started in the western part of the country and progress to the East. latency stage (Zúbrik, unpublished data). Despite these (food source, natural enemies, etc.) and some are pest´s changes were only occurring at low levels and were not density-independent (temperature, wind, humidity, rain- recognised so significantly at a “macro” one, we do con- fall, etc.) (Clark et al. 1967; Barbosa & Schultz 1987). sider them as a gypsy moth outbreak. In the same years, Gypsy moth outbreaks come, according to some quite strong-evidence outbreaks were also reported in authors, mostly after years of warm weather, as well some other European countries (Tabaković–Tošić 2015). as balanced climate in May and without late frost dur- ing spring months in Slovakia (Patočka 1973; Novotný Outbreak No. 9 1986; Kunca et al. 2013). We can also discuss how much was the decline in pest population density, affected with In 2018, the gypsy moth population density again has a heavy, late frost occurring in southern Slovakia – on increased significantly in many places and a new out - April 18 2012, was measured −9.4 °C (Kunca et al. break started (Zúbrik et al. 2019). Data from a detailed 2013). Patočka (1967b) stated that spring weather in monitoring conr fi med, that gypsy moth population den - sity was very high in 2018 in an area of 2,418 ha, although April and May could have a very significant impact on no significant damage was reported (Kunca et al. 2019a, pest abundance. However, it seems that most of these 2019b). Zúbrik et al. (2019) expected that about 2,000 statements were just expert estimations and were not to 4,000 ha were going to be damaged in 2019, the out- seriously supported by e fi ld experiments. Not any deeper break will continue to reach a peak in 2020, and gradu- research was done on the territory of Slovakia to assess ally decline in 2021. Despite high defoliation occurring the impact intensity of individual climate factors on gypsy in 2019 in some areas, most of the gypsy moth larvae died moth population dynamics. at the beginning of June 2019 infected by E. maimaiga. We addressed temperature and precipitation trends, Very cold and wet conditions in May can be a predispos- analysing data from 26 meteorological stations in the ing factor for strong activity of fungi (Zúbrik et al. 2020a, area of gypsy moth outbreaks (Fig. 8). So far, we have 2020b). This situation resulted in only 91 ha which were not made a statistical comparison between the trend in ofc fi ially reported as damaged in 2019, but the area defo - population dynamics and that of precipitation and tem- liated by the gypsy moth was in reality definitely larger, perature. It is likely that some deeper analysis is needed estimated at a level of 2000 ha (Zúbrik et al. 2020b). Only to estimate more precisely this influence. It seems that very few local areas of the country are staying with high another factor or more than one could deteriorate the gypsy moth population density in 2020. One of these population dynamics besides these two. Definitely more was a forest near Párovské Háje (Nitra region). Field research and investigation is required in this area. observations confirmed almost no defoliation in Slova - Natural enemy complex plays an important role in kia caused by the gypsy moth in 2020 (Zúbrik, personal population dynamics of pests (Novotný 1989; Hoch et al. observation). In neighbouring Czechia about gypsy moth 2001). In 2013, E. maimaiga was found for the first time outbreak in this period reports Liška (2018). in Slovakia (Zúbrik et al. 2014). The impact of this fungus on the L. dispar population should not be underestimated since that time, especially when certain indications are 7. Causes of the origin and the collapse suggesting it was introduced a few years earlier, as was of gypsy moth outbreaks finally recognised (Zúbrik et al. 2014, 2016, 2018b). There are generally several factors determining outbreaks Zúbrik et al. (2016) stated that interactions between of leaf-eating insects. Some are pest´s density-dependent E. maimaiga and gypsy moth population dynamics can 64 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 be very strong. This entomopathogenic fungus can even Novotný 1986; Novotný & Surovec 1992; Csóka & Hirka lead to the collapse of its outbreak. Further observation 2009). documented that local outbreaks in the country were Novotný (1986) divided the tree species into several very heavily attacked by E. maimaiga (near Šenkvice, categories regarding their responses to the defoliation Párovské Háje, Čifáre, etc.) in 2019 as in 2020, and it caused by the gypsy moth. The most resistant ones, such was clearly responsible for fast collapse of outbreaks as Q. cerris, Tilia spp. or Prunus spp., can completely occurring locally (Zúbrik et al. 2020a; Zúbrik personal replace their foliage within about 60 days. Impacts of observation). It can be assumed that due to E. maimaiga, defoliation on the health of trees used to be more severe gypsy moth outbreaks should reach a lesser intensity in if repeated for two or three consecutive years and if it is the future and be more local compared to the past. combined with dry and hot weather conditions. Genetic predisposition and food quality are also Defoliated trees have reduced annual stem growth; important (Patočka 1973). Patočka (1973) concluded they are less tolerant to water stress and easily attacked that outbreaks in Slovakia will occur only during particu- by secondary pests (Patočka et al. 1999; Nakajima 2015; larly favourable years and in the most suitable areas. In Camarero et al. 2018, 2019). The Turkey oak evidently experiments with manipulated water availability, gypsy had the best recovery potential (Csóka et al. 2015). It moth larvae consumed much more leaves (birch Betula almost totally replaced its lost foliage in four months after pendula) and the food conversion efficiency was lower, severe defoliation by gypsy moth caterpillars in western if food comes from plots with no watering (Castagney- Hungary. The pedunculate oak and beech needed two rol et al. 2018). However, the growth rate of these lar- years to reach the same level of recovery. This r fi st species vae was the same for both types of experimental plots, used to suffer from a heavy infection of powdery mildew watered and non-watered. These results suggest that Erysiphe alphitoides, following defoliation, which may larvae compensated the low quality of leaves from areas slow down tree recovery (Csóka et al. 2015; Zúbrik et with no watering by consuming larger amounts of them. al. 2020a). Defoliation can be fatal in case of seedlings Adaptation to lower food quality in drought conditions (Patočka 1973). generally leads to greater consumption, which is accom- Impact of defoliation on plant production can also be panied with more tree damage (Jactel et al. 2012). very important, mainly in areas where natural regenera- tion is expected. There was almost no crop output occur- ring in heavily defoliated stands (Turček 1956; Novotný 8. Impacts of defoliation on forests 1986). Similar impact of it on acorn production is known Generally, direct defoliation-induced tree mortality is in North America (McConnell 1988). ever lower, even in sensitive forests, compared to that Patočka (1973) was also speaking about reducing caused by wind, snow or bark beetles (Kunca et al. 2019b). the recreational value of forests during gypsy moth Some authors pointed out its impacts on radial growth, outbreaks. The invasion of large, hairy caterpillars in presence of secondary pests (Agrilus spp., S. intricatus, the recreation area of Pata, during the pest outbreak of Armillaria spp., etc.) and the one on coniferous trees, 1972–1973, caused problems to forest visitors and resi- which is typically negative (Turček 1950; Patočka 1974; dents in the area. Fig. 8. Development of mean annual temperatures (°C) and annual precipitation (mm) in the period of 1931–2015 calculated as an average values from 26 weather stations of Slovak Hydrometeorological Institute (SHMI). Mean annual temperatures and precipitations were smoothed using smoothed conditional means – local polynomial regression fitting. Grey background behind smooth represents standard error with 95% confidence intervals. 65 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 about 3,000 ha were planned, while B. thuringiensis Bio- 9. p revious and current methods of control bit XL was proposed exclusively (Zúbrik et al. 2019). Intense applications of Dynocid (DDT) against gypsy They were not realised yet, partially due to a restriction moth larvae were realised in the past in Slovakia. Several from the State Nature Conservancy officials and also to authors reported about large aerial applications during high mortality of the larvae by E. maimaiga (Zúbrik et al. the outbreak of 1946–1949 (Turček et al. 1950; Patočka 2020a). The applications were usually taken place in the 1973; Konôpka 1978). Later on, forests were treated by spring, between May 5 and 20 (Zúbrik 2004). insecticide (DDT), about 5,000 ha were in 1955, 4,000 in 1956, and 6,000 in 1957 (Kudler et al. 1958). Some authors observed negative effects of DDT on natural ene- 10. Conclusions mies and environment (Turček 1949b; Patočka 1973). Patočka (1961) discussed a possible control method This is the most complete and most detailed reconstruc- based on collecting and destroying egg masses. This is tion of the gypsy moth population dynamics on the ter- more ecologically friendly but less effective, compared to ritory of Slovakia, although some partial reconstruction chemical applications against larvae. As a disadvantage, was already compiled and published (Zúbrik et al. 2013; this procedure is considering that part of these masses Kunca et al. 2013, 2014; Hlásny et al. 2015; Zúbrik et al. can be overlooked and many are in inaccessible areas in 2017a). We have included these works in our assessment, the crown. and they have been l fi led with new, until now unanalysed Since the 60’s and the 70’s of the last century, intense data. experimentation with biological agents against gypsy – There were nine gypsy moth outbreaks since 1945. In moth larvae was realised in Slovakia. As the most prom- the period of 1945–2020, 155,036 ha were damaged ising one, B. thuringiensis was tested in several trials with varying intensity. The strongest outbreak was in (Hešková 1978; Novotný 1985, 1988a, 1988b; Novotný 2000–2008 when 51,579 ha were attacked. On aver- & Švestka 1986, Turčáni 2001). Larval mortality was age, 17,226 ha were affected per one outbreak period. high and full efficiency was reached after 2–8 days of We have found outbreak periods that repeat with fre- treatment, depending on the dose (Novotný 1988b, quency of 7.8 ± 2,2 years and the average outbreak 1989). Besides B. thuringiensis, also LdMNPV (Novotný phase lasts 3.1 ± 1,1 years. The outbreak started in 1985, 1989), B. bassiana (Novotný 1989), viruses the western part of the country and progress to the (Švestka & Pultar 1997) and microsporidia (Weiser & East. The period between two subsequent outbreaks Novotný 1987; Hoch et al. 2008; Solter et al. 2010) have seems to be more or less constant and the duration of been tested against the gypsy moth on the territory of the outbreak seems to be gradually shortened during Slovakia. the study period. As a result of this intense research in the field of – There are several factors influencing the gypsy moth biomonitoring, gypsy moth outbreaks were almost population density in Slovakia. For example, weather exclusively managed with biological and biotechnical in May, late frosts, genetic predisposition, natural preparation since 1984–1987. Insecticide formula- enemy complex and food quality are considered by tions based on B. thuringiensis were used on 1,126 hec- many authors as key ones. Unfortunately, most pub- tares (24% of total 5,878 ha sprayed) during those lished statements were just an expert estimation and years (Novotný 1988a). In order to prevent damage were not sufficiently supported by field experiments. in 1992–1995, extensive aerial applications were also – Gypsy moth is extremely polyphagous species in the realised. B. thuringiensis-based bioinsecticides and country; however, Q. cerris seems to be the most pre- insect growth inhibitors (e.g. Dimilin) were applied ferred host. on a territory over 15,000 hectares (Turčáni & Zúbrik – Defoliated trees have reduced annual stem growth; 1997; Zúbrik 1998). In the period of 2004–2006 (three they are less tolerant to water stress and easily years) applications from the air were even more intense. attacked by secondary pests. From the total treated area (29,831 hectares; 8,298 ha – Intense applications of Dynocid (DDT) against gypsy in 2004, 15,955 in 2005, 5,538 in 2006), the biological moth larvae were realised in the past. Gypsy moth preparation based on B. thuringiensis (Biobit XL) was outbreaks were almost exclusively managed with used on 6,637 ha (22.2%). Growth regulators were for biological and biotechnical preparation since 1984. the remaining 23,194 ha (77.8%). Aerial applications – The exotic entomopathogenic fungus E. maimaiga were financially covered by the Ministry of Agriculture was found in Slovakia for the first time in 2013. Cer- and co-financed by State Forest Enterprises. Their cost tain circumstances suggest that, due to E. maimaiga, reached c. 365,000 EUR in 2004, c. 740,000 EUR in gypsy moth outbreaks should reach a lesser intensity 2005, and c. 170,000 EUR in 2006 (Zúbrik & Kovalčík 2005; Zúbrik 2006; Zúbrik et al. 2006). In 2019, applica- in the future and will be more local compared to the tions from the air against the gypsy moth on a territory of past. 66 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Csóka, G., Pödör, Z., Nagy, G., Hirka, A., 2015: Canopy Acknowledgements recovery of pedunculate oak, Turkey oak and beech This research was supported by the Slovak Research and Devel- trees after severe defoliation by gypsy moth (Lyman- opment Agency (APVV) via grants No. APVV-0707-12, APVV- tria dispar): Case study from Western Hungary. Cen- 14-0567, APVV-15-0531, APVV-15-0348, APVV-16-0325, tral European Forestry Journal, 61:143–148. APVV-19-0116 and APVV-19-0119, the Scientic fi Grant Agency Čapek, M. et al., 1969: Výskum entomopatogénnych VEGA via grants No. VEGA 2/0012/17, VEGA 2/0032/19 and also by the European Regional Development Fund (ERDF) via článkonožcov kalamitných škodcov lesných drevín. Project No. 313011X531 “Development of biologically and bio- Zvolen, VÚLH, R-VII-3/1, 21 p. technically oriented systems of forest protection against domestic Čapek, M., 1961: Kalamita obaľovača smrekovcov- and non-native (invasive) organisms”. This work also received ého Zeiraphera diniana Guen. na smreku v oblasti support from the Ministry of Defence of the Slovak Republic prašivej. Lesnícky časopis, 7:260–271. via project VLMSR0120 and from the Ministry of Agriculture Čapek, M., 1966: Doterajšie skúsenosti s introdukciou and Rural Development of the Slovak Republic via the project vaječných parazitoidov mníšky veľkohlavej Lyman- SLOVLES. Also supported by Ministry of Agriculture of the tria dispar L. z južnej Európy. In: Možnosti využití Czech Republic via institutional support MZE-RO 0118. The biologického boje v ochraně zemědělských plodin a authors thank Dominique Fournier (Canada) for linguistic and lesních kultur. Souhrn referátů z vědeckého semináře, editorial improvements. 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de Gruyter
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© 2021 Milan Zúbrik et al., published by Sciendo
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DOI
10.2478/forj-2021-0007
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Abstract

The gypsy moth is one of the most serious pests in forests and fruit tree plantations over prevailing parts of the North- ern Hemisphere. This work is based on a literature review, and presents history of gypsy moth Lymantria dispar L., observed in Slovak forests within the period 1945–2020. The life cycle, hosts, natural enemies, population dynamics of pests, impact of outbreaks on forests and different management methods used in the past are discussed. Since 1945, there were nine gypsy moth outbreaks in Slovakia. Between 1945 and 2020, a total of 155,034 ha of deciduous forests were touched with varying intensity, representing an average annual damage of 2,040 ha. The strongest outbreak culminated in 2004. Totally 51,479 ha were attacked in the period of 2000–2008. We have found outbreak periods that repeat with frequency of 7.8 ±2.2 years and the average outbreak phase lasts 3.1 ±1.1 years. The period between two subsequent outbreaks seems to be more or less constant and duration of the outbreak phase seems to be gradually shortened during the study period. Several factors influencing the gypsy moth population dynamics in Slovakia are discussed. The role of biological control by using entomopathogenic fungus Entomophaga maimaiga is described. Key words: population dynamics leaf-eating insect; periodic outbreaks; natural enemies; Entomophaga maimaiga Editor: Jiří Foit insects in Slovakia (Zúbrik et al. 2017a). The gypsy moth 1. Introduction ranked among key insect pests that feed on leaves in for- The gypsy moth Lymantria dispar L. (Lepidoptera: Ere- ests of Slovakia during the study period, along with Oper- bidae) is one of the most serious forest insect pests, but ophtera brumata L., Erannis defoliaria Clerck, Agriopis also of fruit trees across much of the Northern Hemi- leucophaearia Denis & Schiffermüller, Tortrix viridana sphere. Large areas damaged by the gypsy moth are L., Orthosia spp., Choristoneura murinana Hübner, Epi- reported from the Northeastern United States and Asia notia nigricana Herrich-Schäffer, Diprion pini L., Diprion (Schedl 1936; Doane & McManus 1981; McManus & spp., Melolontha spp., and some other species (Turček Csóka 2007; Zúbrik et al. 2013). The cyclic gypsy moth 1956; Charvát & Patočka 1960; Čapek 1961; Patočka outbreaks (Hlásny et al. 2015) resulted in loss of radial 1955, 1963a, b, 1967a, 1973; Leontovyč et al. 1980; growth (Muzika & Liebhold 1999), changes in fruiting Surovec et al. 1989; Zúbrik 2006; Zúbrik et al. 2015, (Gottschalk 1990), and if repeated, in tree mortality in 2017a, b; Vakula et al. 2015; Sarvašová et al. 2020). Some subsequent years (Patočka & Novotný 1985; Davidson species, such as Pristiphora laricis Hartig, Rhyacionia et al. 1999). In Southeast Europe, outbreaks are more fre- buoliana Denis & Schiffermüller or Coleophora laricella quent (Pernek et al. 2008) and more intense (McManus & Hübner, have caused damage to trees only occasionally Csóka 2007) than in Central Europe (Hlásny et al. 2015). and only in relatively restricted areas (Leontovyč et al. Over the period of 1945–2016, more than 0.5 million hectares was damaged in different ways by leaf-eating 1980; Surovec et al. 1989; Zúbrik et al. 2017b). *Corresponding author. Milan Zúbrik, e-mail: milan.zubrik@nlcsk.org M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Despite direct effect of defoliation on the tree mor- Institute (SHMÚ) in Bratislava. Average monthly tem- peratures and precipitation data recorded at 26 weather tality is questionable, often defoliated drought-stressed stations (Gabčíkovo, Bratislava Airport, Bratislava- trees increase their secondary mortality caused by other Koliba, Dolné Plachtince, Dubník, Dudince, Holíč, Hur- pests such as Scolytus intricatus Ratzeburg (Coleop- banovo, Komárno, Kráľová pri Senci, Kuchyňa, Leles, tera: Scolytinae), Agrilus spp. as well as other species of Lučenec, Malacky, Malé Ripňany, Moldava nad Bodovu, jewel beetles (Coleoptera: Buprestidae) and long-horned Nitra, Nový Tekov, Podhájska, Rimavská Sobota, Somo- beetles (Cerambycidae), etc., developing frequently and tor, Štúrovo, Tesárske Mlyňany, Trnava, Žihárec) over abundantly in weakened trees (Novotný 1986; Zach the period of 1931–2015 were used. The presence of the 1994; Patočka et al. 1999). In the 70s and 80s of the last gypsy moth in the area was the criterion for including century, oak stands in Slovakia, were heavily affected by a particular weather station in the analysis. Data were tracheomycosis disease (Leontovyč 1980; Surovec et al. smooth using Local Polynomial Regression Fitting. 1989). Tracheomycosis disease and also armillaria root disease (Armillaria spp.) significantly reduce the ability of oak stands to resist defoliation caused by gypsy moth. The aim of this study was to summarise the more 2.2. Terms detailed information on the gypsy moth in Slovakia, in In order to be as precise as possible and to avoid confu- particular: to reconstruct its population dynamics, ana- sion due to unclear terms, we provide a short explana- lyse data about life cycle, hosts, natural enemies, impacts tion of some of the most commonly used terms (Fig. 1). on forests and evaluate other signic fi ant aspects related to We defined ‘outbreak’, ‘outbreak length’ or ‘outbreak its biology and ecology over the period of 1945–2020. All period length’ respectively, as a period, during which these aspects are discussed in the context of theoretical gypsy moth caused certain damages in the forest (higher knowledge, and practical expertise obtained from sources than zero). During building and declining phase of out- in Slovakia and others being external. Data from Slova- break, gypsy moth population density often remains on kia are discussed with those, known from other areas of a very low level for a long time. For that reason we also gypsy moth occurrence. introduce a term “outbreak phase“. We defined it as a period, during which more than 1,000 ha of forest stands was damaged annually. This rule was not applied to the 2. Methods period 2013–2014 and 2017–2020. During this period, only minor areas were damaged, but we nevertheless, 2.1. Data sources for certain reasons mentioned in the article, decided To obtain the data we used three main sources for this to label the events as outbreaks of the gypsy moth. The work. ‘outbreak peak’ is considered the year, with the highest A) The primary source was the data present in different registered damage during the outbreak period (Fig. 1). scientific and expert publications. Accurate information If we speak about “outbreak frequency“, we mean the about the gypsy moth presence from the period 1945– period between two outbreak peaks. 1960 is lacking. Most of the data were available on the country level. We also used information about local gra- dations, determined by a place, or certain locality. These 2.3. Statistics and data presentations data are based on published estimations. Recent data, For common statistics and data interpretation, we used in papers published over the last 50 years are of higher Microsoft Excel 2016. We used mean, standard devia- accuracy. tion (mean ± standard deviation) and coefc fi ient of deter - B) For the period since 1961, we used official statisti - mination calculated in this program. Trend lines were cal records reported by forest managers. Damages were calculated using the method of least squares in the same recorded on the accuracy level of the forest districts or program. For picture processing, and figure elaborating country district (Zúbrik et al. 1999). we used Adobe Photoshop® (2016) and R Studio, version C) The most accurate data were obtained through moni- 1.3.1093, package ggplot2 Wickham (2016). toring conducted by forest managers and supervised by the Forest Protection Service in the frame of relevant projects, as our observations and experience. 3. Gypsy moth life cycle We analyzed data from all the above-mentioned sources to reconstruct the long-term trend of the gypsy In Slovakia, the gypsy moth prefers older forest stands moth population occurrence in Slovakia. After analys- that are under warmer conditions in the southwestern ing the data from all these three sources (A, B, C), we and southern regions of the country, as well as in the constructed a table with the estimated area, damaged by eastern lowlands. It can also be found in dry localities, gypsy moth per years. on steep slopes, in sparse and in wet-mesic floodplain To define temperature and precipitation trends, we forests along rivers (Turček 1956; Stolina 1985; Novotný analysed the data from the Slovak Hydrometeorological & Turčáni 1992; Patočka et al. 1999). 56 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Fig. 1. Gypsy moth outbreak phases. The gypsy moth has one generation per year. Adults windy (Patočka 1961). Airborne spread of larvae can be (Fig. 2) are on the wings from July to August (from mid- up to 15 km by wind (Novotný 1986). The larva (Fig. 2) dle June to early September in recent years). Females has 5–6 instars, depending on sexes, that are relatively lay eggs (Fig. 2) preferably at the base of the tree trunk easy to determine by external morphological characters although, during an outbreak, they do it also high in the (Patočka 1954; Gogola 1969). Larvae start to feed in the crowns and even on thin branches (Turček 1956; Patočka crown at the beginning of May, defoliation culminates in 1961, 1973; Novotný 1986; Zúbrik 2006; Vakula et al. mid-June (Fig. 3). They co-occur with other abundant 2015). For assessing pest density on a plot, the Turček’s species of leaf feeding caterpillars, such as Archips xylo- method (based on egg masses count) is commonly used steana L. and Orthosia spp. (“dispar-xylosteana com- (Turček 1956), later this being changed slightly in for- plex”) (Kulfan et al. 2018). Larvae pupate (Fig. 2) in est documents that are officials (STN 43 2715). Dur- late June and early July. The pupal stage lasts two weeks ing an outbreak period, average number of egg masses (Novotný 1986; Vakula et al. 2015; Zúbrik et al. 2020a). per tree can reach 20 to 30 clusters (Hoch et al. 2001; Swarming starts in the earlier part of July and culminates Zúbrik, 2006), but it can be in some exceptional cases in its second half and eggs are laid in early August. At even more, 30 to 70 in heavily infested stands (Patočka the beginning of September, no living adults are seen in 1973; Novotný 1986; Novotný & Turčáni 1992; Zúbrik forests (Turček 1949a; Novotný 1986). & Novotný 1997). Patočka (1973) counted as many as 250 egg masses on one tree. If the number of masses per tree average two or more, the pest outbreak can be 4. Hosts to the gypsy moth expected the following year (Turček 1956; Vakula et al. Gypsy moth hosts in Europe vary somewhat depending 2015), but critical numbers should be considered based on how its distribution corresponds to the predominant on the forest age and health status (Patočka 1961). vegetation in various geographical regions. There are The number of eggs per egg mass varied from 343– slight differences in the gypsy moth food preferences 491 (Novotný, 1986) to 500–1000 (Turček 1949a; 1956). between North, Central and southern Europe (Schopf Turček (1949a; 1956) assumed 500–600 eggs are an et al. 1999; Švestka 1993, 1994, 2004; Hirka 2006; Csóka average value. Hoch et al. (2001) reported similar num- & Hirka 2009; McManus & Csóka 2007). bers (averaging 534 eggs per egg mass) from Klingen- bach (Austria, 60 km southwest of the Slovakia’s border). The gypsy moth is a highly polyphagous species, The gypsy moth overwinters as eggs. Larvae hatch not only in Slovakia, but also in the whole territory of its from the middle of April (Novotný 1986; Vakula et al. occurrence (Kurir 1953; Janković 1958; Jahn & Sinreich 1957; Fuester et al. 1983; Novotný 1986; Zúbrik et al. 2015; Zúbrik et al. 2020a). After hatching, they rest 2013). In Eurasia, gypsy moth larvae are able to consume several days on the surface of egg mass and then crawl to the crown on sunny days or balloon if the weather is about 90 different tree species, while in the United States 57 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Fig. 2. Gypsy moth stages A) male, B) female, C) egg masses, D) pupae, E) larva in a final instar. Table 1. Area defoliated by the gypsy moth in years 2002, and it is about 85 species (Schedl 1936; Doane & McManus 2004–2006, by tree species, according to the official statistics 1981). Wellenstein & Schwenke (1978) state that up to (Varínsky et al. 2003; Kunca et al. 2005, 2006, 2007, 2008). 300 host plants have been recorded worldwide. It seems Tree species Area damaged (in hectares) % that younger larvae prefer mainly oak, possibly other Quercus spp. 38,099 91.09 trees with soft leaves, and only older instars are more Carpinus betulus 2,492 5.96 Robinia pseudoacacia 743 1.78 polyphagous (Patočka 1970; Novotný 1986). Fagus sylvatica 417 1.00 In Slovakia, larvae cause defoliation mostly in oaks Populus spp. 72 0.17 Tilia spp. 2 0.00 (Quercus cerris L., Q. robur L., Q. petraea (Matt.) Liebl., Acer spp. 1 0.00 Q. pubescens Willd.) (Tab. 1), but they can also feed on Alnus spp. 1 0.00 other trees and shrubs such as Carpinus betulus L., Acer Total 41,827 100.00 spp., Robinia pseudoacacia L., Prunus spp., Crataegus 1985; Novotný 1986). Laboratory experiments showed spp., Malus sylvestris Mill., Pyrus communis L., Tilia spp., that gypsy moth populations developed faster, and its lar- Populus spp., Betula spp. as well as others (Turček 1956; vae and pupae gained more weight on Q. cerris than on Patočka 1973; Stolina 1985; Novotný 1986) (Table 1). In Slovakia, Q. cerris (Fig. 3) seems to be the most pre- Q. petraea. Q. cerris females also laid significantly more ferred food source (Turček 1953; Patočka 1961; Stolina eggs than those from Q. petraea. Field studies demon- 58 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Fig. 3. A) Defoliation of Q. cerris stands by the gypsy moth are often severe; Šenkvice, June 2019. B) Defoliation of the spruce trees in 2018 (Vraná nad Vltavou, Czechia) documents gypsy moth is a highly polyphagous species. C) Unusual phenomenon in Slovakia: defoliation of Vaccinium myrtillus by the gypsy moth in a meadow habitat in 2011–2012 and 2020. strated as well, a much faster development with less mor- defoliated too (Fig. 3). Novotný (1986) reported defo- tality of the caged gypsy moth larvae feeding on Q. cerris liation of Picea abies L., Abies alba Mill., Larix decidua compared to those on Q. petraea (Schopf et al. 1999). In Mill., Pinus sylvestris L., Pseudotsuga menziesii Mirb. and Slovakia, there is about 174,500 hectares of forests with Pinus strobus L. Some conifers, as P. abies or P. silvestris, prevalence of oak (Green report 2019) (Fig. 5). occurred in mixed forests were heavily defoliated by the gypsy moth in 2018 and 2020 in Slovakia and Czechia Some tree species remain completely or partially untouched during gypsy moth outbreaks, like Loranthus (Zúbrik, Liška, personal observation). In Slovakia, beech (Fagus sylvatica L.) was defoliated only on a relatively europaeus Jacq., Fraxinus excelsior L., Fraxinus ornus L., restricted area (Patočka 1967b; Novotný 1986), unlike Ligustrum vulgare L. and Morus alba L. (Turček 1956; neighbouring Hungary, where forests of it had heavy Novotný 1986). This is not the same everywhere, as some local insect pest populations may have different feeding defoliation on large areas in the past (Csóka et al. 2015). preferences (Patočka 1973). In 2011–2012, and again in 2020, approximately Coniferous trees are also usually untouched; how- 0.25 ha of meadows (600 m a.s.l.) near Banská Štiavnica ever, taken individually in oak forests, they can be heavily in Slovakia covered by Vaccinium myrtillus L. (Fig. 3) was 59 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 completely defoliated, along with Sorbus spp., Salix spp., 28 insect species from the orders Hymenoptera and Dip- tera and one nematode were recorded during the investi- Malus sylvestris as well as P. abies and P. sylvestris grow- gation in 1991–1996. Species like tachinids Parasetigena ing solitary in that area (Zúbrik, personal observation). silvestris R.–D. and Blepharipa pratensis Meig., braco- Damages to fruit tree plantations, vineyards and even nids Cotesia melanoscelus Ratz., Phobocampe spp., and agricultural crops (Zea mays L.) were also reported in the Glyptapanteles liparidis Bouché were the most impor- past from Slovakia (Turček 1949a, 1956; Patočka 1973; tant (Hoch et al. 2001). A broad range of insect parasi- Leontovyč et al. 1980; Novotný 1986; Alford 2010). toids was also confirmed by other authors (Čapek et al. 1969; Čapek 1988; Zúbrik 1997; Turčáni et al. 2001). At latency sites, C. melanoscelus was the dominant spe- 5. Natural enemy complex cies, followed by Phobocampe spp. and P. sylvestris. The of the gypsy moth oligophagous tachinids P. silvestris, B. pratensis and braconid G. liparidis were the dominant parasitoids In spring, some egg masses eaten by unknown predators at sites of outbreaks and pro-outbreaks (Zúbrik 1997; are often seen in forests (personal experiences). It seems Hoch et al. 2001). About 20% of larvae investigated that predation may play a meaningful role, especially were killed by parasitoids. Mortality by pathogens was during the latency period. Inversely, density-dependent higher more than 30% (Hoch et al. 2001). That of larvae relationship (between the gypsy moth egg mass density by pathogens during the culmination stage can reach and the predation) was found, but this was not signifi - even 60% (Novotný 1989). The most frequently occur- cant (Turčáni et al. 2003). About 30% mortality in eggs ring pathogen, which has been present in Slovakia, is the is caused by birds as predators, such as Certhia familiaris Lymantria dispar multicapsid nuclear polyhedrosis virus L., Sitta europaea L., Parus major L., Cyanistes caeruleus (LdMNPV). LdMNPV is considered the main reason for L. and Aegithalos caudatus L. (Turček 1949a). The gypsy the collapse of the gypsy moth outbreak in 1949 (Charvát moth larvae may fall into the diet of several bird species in 1967). Other pathogens, such as Bacillus thuringiensis forests. However, they appear to be barely able to reduce Berliner, Beauveria bassiana (Bals.-Criv.) Vuill., Nosema the population density of this pest (Krištín 1999). serbica Weiser and Nosema lymantriae Weiser were also There are two egg parasitoids present on the terri- recorded (Novotný 1989; Zúbrik 1997; Hoch et al. 2001). tory of Slovakia, native Anastatus disparis Ruschka and The exotic entomopathogenic fungus Entomophaga mai- Ooencyrtus kuwanae How (Hymenoptera: Eupelmidae), maiga Humber, Shimazu & Soper was found in Slovakia originating in Asia. Both species were released in that for the first time in 2013 (Zúbrik et al. 2014). Further country at certain sites between 1960 and 1965 (Čapek study revealed that the fungus was relatively widely 1966, 1971). They were successfully recorded again spread here (Zúbrik et al. 2018b). It was suggested as well between 1985 and 1988 (Novotný & Čapek 1989) and that it is spreading from the Balkan Peninsula (Zúbrik et also later, from 1992 until 1995 (Zúbrik & Novotný al. 2016). Field study conducted in Slovakia during the 1997), although, parasitisation rates were low, varying years of 2014–2017 documented a narrow host range of from 1 to 3%. E. maimaiga. Therefore, significant negative effects on The natural enemy complex is much broader in the the native lepidopteran fauna are not expected to occur larval and pupal stages than in the egg stage. In total, (Zúbrik et al. 2018a). Fig. 4. Area damaged by the gypsy moth during the period from 1945 to 2020. Period from 1945 to 1960 is coloured in different shade. In this way we wanted to point out, that information we had available about gypsy moth population density in the period up to 1960 is less accurate and not very detailed. While information after 1960 is more accurate and quite comprehensive. 60 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Predation on pupae was also investigated. It shows al. 2020a). Only 4 outbreaks were included into the data that small mammals may play an important role in pupal analyse in Hlásny et al. (2015), and also outbreak period, mortality (Turčáni et al. 2001). Invertebrate preda- not outbreak phase was used in analyse. In our study, 9 outbreaks and outbreak phase was used. This difference tion on gypsy moth caterpillars and pupae can also be caused, our results slightly differ from those published a major mortality factor, mainly Calosoma sycophanta L. and Calosoma inquisitor L. (Coleoptera: Carabidae) by Hlásny et al. (2015). We tried to construct a trend for the length of the out- frequently seen in forests during an outbreak period break phase and the length of the period between two (Zúbrik, personal observations), but they have not yet outbreak peaks (outbreak frequency) (Fig. 6). The trend been systematically studied as predators of larvae in indicates the period between two subsequent outbreaks Slovakia. seems to be more or less constant and duration of the out- Outbreak situation inu fl ences in a different way forest break phase seems to be gradually shortened during the ecosystems. Structure of natural enemies has changed study period. However, the coefficient of determination (Hoch et al. 2001). Impact of defoliation on the presence is very low in both cases. The shortening of the length of of plant diseases was also conr fi med (Patočka & Novotný the outbreak phase may be related to the appearance of 1985). The number of birds increased 1.5 to 2 times dur- the fungus E. maimaiga. Several circumstances suggest ing the outbreak period (Turček 1956). that E. maimaiga has affected the length of gradation in 2013–2014 and 2017–2020 significantly (Zúbrik, per - sonal observation). 6. Dynamics of the gypsy moth population There are some traditional areas for gypsy moth According to our analyses, there were nine gypsy moth outbreaks on Slovakia’s territory that do not change too outbreaks on the territory of Slovakia since 1945 (Table much over time (Fig. 5). These regions are, such as the 2, Fig. 4). Over the period of 1945–2020, 155,034 ha of following: 1) the western part of the country, around deciduous forests were touched with varying intensity, Pezinok, Modra, Šenkvice, Bratislava; 2) around Nitra, representing an average annual damage of 2,040 ha. The Čifáre, Levice and Nové Zámky; 3) around Veľký Krtíš, strongest outbreak was recorded in 2000–2008. Totally Rimavská Sobota and Lučenec; and 4) eastern Slovakia, 51,479 ha were attacked during that period (Tab. 2). The around Michalovce, Ortov and Sobrance (Pfeffer 1961; year with the greatest damage intensity was in 2004, Patočka 1961, 1973; Charvát 1967; Kunca et al. 2005, when 21,304 ha were attacked. On average, 17,226 ha 2006, 2007; Zúbrik et al. 2019). Gypsy moth outbreaks were affected per one outbreak. We have found outbreak usually start in the western and central parts of the coun- periods in Slovakia that repeat with frequency of 7.8 ± try, and then slowly shift to the East (Turček 1956; Char- 2.2 years and the average outbreak phase lasts 3.1 ± vát 1967; Zúbrik et al. 2019). We tried to confirm this 1.1 years overall. This value agrees with previous stud- statement on an example of the outbreak in 2002–2007 ies of Patočka (1954), Novotný (1986). Novotný (1986) (Fig. 7). Defoliation in 2002 was reported only from was speaking about an outbreak period of three years. western part of Slovakia and defoliation in eastern part Sometimes local outbreaks collapse faster from different clearly occurs r fi st only after 3 year in 2005 and continued reasons, after one or two years (Patočka 1973; Zúbrik et to 2006. Fig. 5. Map showing the distribution of all oak forest types in Slovakia (only those with more than 10% of oak in composition were considered as oak forest). Very schematic indication of traditional areas for gypsy moth outbreaks on Slovakia’s territory (see description in text). 61 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Pezinok and Palárikovo. Also, Patočka (1961) mentioned Outbreak No. 1: this outbreak and dated it as 1954–1957. Years with very About this r fi st, it was a very strong gypsy moth outbreak mild weather in 1952, 1954, 1955 and 1956 could have after World War II, reported Hendrych (1959). Patočka initiated this, which started in stands of Fageto-Querce- (1961) mentioned that there was damage to 30,000– tum and Carpineto-Quercetum under warmer climate. 50,000 hectares during two outbreaks in 1946–1949 and Outbreak culminated in 1956 and then the gypsy moth then in 1954–1957. Charvát (1967) conr fi med that about density was reduced, probably due to the unsuitable 30,000 ha were defoliated and it culminated in 1949 dur- weather in 1957 and 1958. Charvát (1967) dated this ing the outbreak period of 1946–1949. He was speak- oubreak approximately to the years 1953–1958. ing about a “large outbreak” in 1946–1949, comparing it with the one in 1953–1958, which he commented as a Fig. 6. The length of the outbreak phase (blue dots, 1-1948–1950, 2-1955–1957, 3-1963–1965, 4-1972–1975, 5-1984–1987, 6-1992–1995, 7-2003–2006, 8-2013–2014, 9-2019) and the length of the period between two outbreak peaks (red dots, 1-1949–1956, 2-1956–1964, 3-1964–1973, 4-1973–1986, 5-1986–1993, 6-1993–2004, 7-2004–2013, 8-2013–2019) with linear trends (duration of the outbreak phase – blue dotted line, duration of the period between two outbreak peaks – red dotted line). Trend lines were calculated using the method of least squares. “smaller outbreak of local importance”. Patočka (1973) Outbreak No. 3 described outbreaks in 1946–1948 as “massive” and Leontovyč et al. (1980) deeply discussed this period of the subsequent two in 1954–1956 and 1964–1966 as 1963–1967. They reported complete defoliation near “smaller ones”. Turček (1950) and Konôpka (1978) Pezinok, Palárikovo, Levice, Šahy, Lučenec and other also reported about a large, area-wide defoliation dur- areas. It is estimated that there was damage to about ing this outbreak period. It was preceded by extremely 2,000 ha annually during this one. Patočka (1967b) and dry and hot weather in 1947. The defoliation started Čapek et al. (1969) also noted this outbreak and dated it in western Slovakia and moved to eastern Slovak low- as 1963–1965, with an abundance reaching culmination lands in subsequent years (Charvát 1967). Forests were especially damaged near Levice, Nitra, Lučenec, Šahy, in 1964. Čapek et al. (1969) studied larval parasitoids in Rimavská Sobota and Sobrance (Patočka 1953; Charvát the gypsy moth population and he therefore independ- 1967). Turček (1949a) described outbreaks at the local ently monitored the density of the latter on seven study level, reporting total defoliation in 1946–1948. In 1949, plots. The results show that the number of gypsy moth after all primary food sources were consumed; hungry egg masses has already increased in 1963 and their abun- caterpillars caused damages even to agricultural crops dance culminated in 1964. Population was locally high (Patočka 1961). even in 1965, but in 1966, it falls down to a low level, apparently also affected by cold spring weather (Patočka Outbreak No. 2 1967b). In 1968, egg masses only occurred sporadically Shortly about this outbreak, reported Pfeffer (1961), (Charvát 1969). largest-scale defoliation was observed in 1955, near Šaštín-Stráže and Smolenice, as well as in 1956, near 62 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 local outbreak in the area of about 200 ha was recognised Outbreak No. 4 (Zúbrik & Turčáni 1997). Leontovyč et al. (1980) r fi st dated this to the years 1971– 1974. They mentioned a situation where 300 hectares, Outbreak No. 7 even of coniferous trees (Pinus spp., P. abies, Pseudot- The pest population density rose slightly in 2002 followed suga spp.), were damaged. Patočka et al. (1999) recorded by a very fast increase in 2003. The outbreak started in an increase in the gypsy moth density, also in that period, the western part of the country and progress to the East and confirmed the outbreak culminating in 1973–1975, (Fig. 7). In 2004, already 21,304 ha were defoliated by with local occurrences in 1976 and 1977.) A very high the gypsy moth, mainly around Nitra and Levice (Kunca pest density in the whole zone was observed in 1975, but et al. 2005). As for 2005, 13,498 ha were damaged, pri- in 1976 and 1977, only small outbreaks were reported marily in the vicinity of Veľký Krtíš, Krupina, Lučenec and the damaged area was increasing from 145 ha in as well as Bratislava and Nové Zámky (Kunca et al. 1976 to 247 ha in 1977 (Surovec et al. 1989). 2006, 2007, 2008; Zúbrik 2006). Aerial applications of insecticides were realised on a territory of 29,831 ha in Outbreak No. 5 2004–2006. Approximately 7,000 hectares were defoli- The exact data about this outbreak are provided by ated (officially 6,025 ha) in 2006 (Kunca et al. 2006). Surovec et al. (1989). It speaks about an outbreak where Kunca et al. (2007) were speaking about the end of the approximately 2,400–4,500 hectares were damaged on outbreak in 2007, when only a very restricted area was an annual basis, especially in Bratislava, and slightly damaged (45 ha) and no application was made. They less in the Banská Bystrica administrative district. First concluded that this was the largest gypsy moth outbreak defoliations were already reported in 1983 (near Tesárske during the last 50 years. It was exceptionally severe not Mlyňany). In 1984, local occurrences were more frequent only in Slovakia, but also in many other EU countries. in every primary affected zone, nearby specic fi ally Čifáre, Hungary, for example, reported record-breaking dam- Podhájska, Nitra and Žitavany. In 1985 and 1986, there ages to 212,000 hectares starting from 2005 (Csóka & was area-wide defoliation reaching impressive intensity. Hirka 2009), while in Croatia the gypsy moth affected Significant reduction in abundance was found in 1986 33,000 ha in this particular year (Hrašovec et al. 2008). and local outbreaks became scarce in 1987. Outbreak Švestka (2004) observed a large gypsy moth outbreak in period had a low-range peak in 1986. These data were the neighbouring Czechia in 2003 and 2004. He expected also confirmed by analysing the official statistical evi - that defoliation will continue in 2005. dence from the state forest (Zúbrik et al. 2013). Patočka et al. (1999) dated this outbreak to the years 1984–1987 Outbreak No. 8 too. This is very unusual outbreak and after consideration, we decided to keep it here, despite it does not full fi all criteria Outbreak No. 6 for a typical outbreak period. In 2011, in certain areas, a The period that was warm and dry at the beginning of the high number of gypsy moth larvae and adults was found nineties of the last century (especially the year 1992) con- in the oak forests. During autumn 2011, high amount of tributed to the rapid progress of the gypsy moth outbreak gypsy moth egg masses in oak stands was confirmed. A in 1993 and 1994 (Zúbrik 1998). Novotný & Turčáni further signic fi ant rise in population density was expected (1993) reported previously in 1992 some changes in the in 2012. However, this did not occur (Zúbrik et al. 2013). population density. The infested area already reached In 2013, defoliation was recognised on about 200 ha in more than 2,000 hectares in 1992, with a tendency to some isolated “spots” (for example, near Ortov in the further increase. Outbreak culminated in 1993, and eastern part of Slovakia) (Kunca et al. 2014). In 2014, very high level of infestation remains in 1994. The most about 150 ha was defoliated (Kunca et al. 2015, 2016). damaged areas were close to Nitra, Levice, Malé Karpaty, A detailed monitoring of the gypsy moth population den- Košice and Prešov. In 1995, damages were not so exten- sity conr fi med these observations; it slightly increased in sive. In 1996, the gypsy moth population was in latency; a 2011–2013. As for 2014, there was a fast decline into a Table 2. Outbreaks overview in the period 1945–2020. Hectares defoliated Hectares defoliated No. of outbreak Outbreak peak Outbreak Outbreak phase Control methods used during outbreak during outbreak phase 1 1949 1946–1951 29,822 1948–1950 28,000 Dynocid (DDT) 2 1956 1953–1958 10,600 1955–1957 10,000 Dynocid (DDT) 3 1964 1963–1967 6,100 1963–1965 5,500 no data available 4 1973 1971–1978 7,268 1972–1975 6,278 no data available 5 1986 1979–1990 13,588 1984–1987 11,799 B. thuringiensis + growth regulators 6 1993 1991–1999 33,639 1992–1995 32,210 B. thuringiensis + growth regulators 7 2004 2000–2008 51,579 2003–2006 49,758 B. thuringiensis + growth regulators 8 2013 2013–2014 350 2013–2014 350 B. thuringiensis 9 2019 2017–2020 2,053 2019 2,000 no control realised Total (ha) 155,034 145,925 63 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Fig 7. Area damaged by the gypsy moth from 2002 to 2007 in different geographical parts of Slovakia, demonstrating, that the defoliation started in the western part of the country and progress to the East. latency stage (Zúbrik, unpublished data). Despite these (food source, natural enemies, etc.) and some are pest´s changes were only occurring at low levels and were not density-independent (temperature, wind, humidity, rain- recognised so significantly at a “macro” one, we do con- fall, etc.) (Clark et al. 1967; Barbosa & Schultz 1987). sider them as a gypsy moth outbreak. In the same years, Gypsy moth outbreaks come, according to some quite strong-evidence outbreaks were also reported in authors, mostly after years of warm weather, as well some other European countries (Tabaković–Tošić 2015). as balanced climate in May and without late frost dur- ing spring months in Slovakia (Patočka 1973; Novotný Outbreak No. 9 1986; Kunca et al. 2013). We can also discuss how much was the decline in pest population density, affected with In 2018, the gypsy moth population density again has a heavy, late frost occurring in southern Slovakia – on increased significantly in many places and a new out - April 18 2012, was measured −9.4 °C (Kunca et al. break started (Zúbrik et al. 2019). Data from a detailed 2013). Patočka (1967b) stated that spring weather in monitoring conr fi med, that gypsy moth population den - sity was very high in 2018 in an area of 2,418 ha, although April and May could have a very significant impact on no significant damage was reported (Kunca et al. 2019a, pest abundance. However, it seems that most of these 2019b). Zúbrik et al. (2019) expected that about 2,000 statements were just expert estimations and were not to 4,000 ha were going to be damaged in 2019, the out- seriously supported by e fi ld experiments. Not any deeper break will continue to reach a peak in 2020, and gradu- research was done on the territory of Slovakia to assess ally decline in 2021. Despite high defoliation occurring the impact intensity of individual climate factors on gypsy in 2019 in some areas, most of the gypsy moth larvae died moth population dynamics. at the beginning of June 2019 infected by E. maimaiga. We addressed temperature and precipitation trends, Very cold and wet conditions in May can be a predispos- analysing data from 26 meteorological stations in the ing factor for strong activity of fungi (Zúbrik et al. 2020a, area of gypsy moth outbreaks (Fig. 8). So far, we have 2020b). This situation resulted in only 91 ha which were not made a statistical comparison between the trend in ofc fi ially reported as damaged in 2019, but the area defo - population dynamics and that of precipitation and tem- liated by the gypsy moth was in reality definitely larger, perature. It is likely that some deeper analysis is needed estimated at a level of 2000 ha (Zúbrik et al. 2020b). Only to estimate more precisely this influence. It seems that very few local areas of the country are staying with high another factor or more than one could deteriorate the gypsy moth population density in 2020. One of these population dynamics besides these two. Definitely more was a forest near Párovské Háje (Nitra region). Field research and investigation is required in this area. observations confirmed almost no defoliation in Slova - Natural enemy complex plays an important role in kia caused by the gypsy moth in 2020 (Zúbrik, personal population dynamics of pests (Novotný 1989; Hoch et al. observation). In neighbouring Czechia about gypsy moth 2001). In 2013, E. maimaiga was found for the first time outbreak in this period reports Liška (2018). in Slovakia (Zúbrik et al. 2014). The impact of this fungus on the L. dispar population should not be underestimated since that time, especially when certain indications are 7. Causes of the origin and the collapse suggesting it was introduced a few years earlier, as was of gypsy moth outbreaks finally recognised (Zúbrik et al. 2014, 2016, 2018b). There are generally several factors determining outbreaks Zúbrik et al. (2016) stated that interactions between of leaf-eating insects. Some are pest´s density-dependent E. maimaiga and gypsy moth population dynamics can 64 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 be very strong. This entomopathogenic fungus can even Novotný 1986; Novotný & Surovec 1992; Csóka & Hirka lead to the collapse of its outbreak. Further observation 2009). documented that local outbreaks in the country were Novotný (1986) divided the tree species into several very heavily attacked by E. maimaiga (near Šenkvice, categories regarding their responses to the defoliation Párovské Háje, Čifáre, etc.) in 2019 as in 2020, and it caused by the gypsy moth. The most resistant ones, such was clearly responsible for fast collapse of outbreaks as Q. cerris, Tilia spp. or Prunus spp., can completely occurring locally (Zúbrik et al. 2020a; Zúbrik personal replace their foliage within about 60 days. Impacts of observation). It can be assumed that due to E. maimaiga, defoliation on the health of trees used to be more severe gypsy moth outbreaks should reach a lesser intensity in if repeated for two or three consecutive years and if it is the future and be more local compared to the past. combined with dry and hot weather conditions. Genetic predisposition and food quality are also Defoliated trees have reduced annual stem growth; important (Patočka 1973). Patočka (1973) concluded they are less tolerant to water stress and easily attacked that outbreaks in Slovakia will occur only during particu- by secondary pests (Patočka et al. 1999; Nakajima 2015; larly favourable years and in the most suitable areas. In Camarero et al. 2018, 2019). The Turkey oak evidently experiments with manipulated water availability, gypsy had the best recovery potential (Csóka et al. 2015). It moth larvae consumed much more leaves (birch Betula almost totally replaced its lost foliage in four months after pendula) and the food conversion efficiency was lower, severe defoliation by gypsy moth caterpillars in western if food comes from plots with no watering (Castagney- Hungary. The pedunculate oak and beech needed two rol et al. 2018). However, the growth rate of these lar- years to reach the same level of recovery. This r fi st species vae was the same for both types of experimental plots, used to suffer from a heavy infection of powdery mildew watered and non-watered. These results suggest that Erysiphe alphitoides, following defoliation, which may larvae compensated the low quality of leaves from areas slow down tree recovery (Csóka et al. 2015; Zúbrik et with no watering by consuming larger amounts of them. al. 2020a). Defoliation can be fatal in case of seedlings Adaptation to lower food quality in drought conditions (Patočka 1973). generally leads to greater consumption, which is accom- Impact of defoliation on plant production can also be panied with more tree damage (Jactel et al. 2012). very important, mainly in areas where natural regenera- tion is expected. There was almost no crop output occur- ring in heavily defoliated stands (Turček 1956; Novotný 8. Impacts of defoliation on forests 1986). Similar impact of it on acorn production is known Generally, direct defoliation-induced tree mortality is in North America (McConnell 1988). ever lower, even in sensitive forests, compared to that Patočka (1973) was also speaking about reducing caused by wind, snow or bark beetles (Kunca et al. 2019b). the recreational value of forests during gypsy moth Some authors pointed out its impacts on radial growth, outbreaks. The invasion of large, hairy caterpillars in presence of secondary pests (Agrilus spp., S. intricatus, the recreation area of Pata, during the pest outbreak of Armillaria spp., etc.) and the one on coniferous trees, 1972–1973, caused problems to forest visitors and resi- which is typically negative (Turček 1950; Patočka 1974; dents in the area. Fig. 8. Development of mean annual temperatures (°C) and annual precipitation (mm) in the period of 1931–2015 calculated as an average values from 26 weather stations of Slovak Hydrometeorological Institute (SHMI). Mean annual temperatures and precipitations were smoothed using smoothed conditional means – local polynomial regression fitting. Grey background behind smooth represents standard error with 95% confidence intervals. 65 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 about 3,000 ha were planned, while B. thuringiensis Bio- 9. p revious and current methods of control bit XL was proposed exclusively (Zúbrik et al. 2019). Intense applications of Dynocid (DDT) against gypsy They were not realised yet, partially due to a restriction moth larvae were realised in the past in Slovakia. Several from the State Nature Conservancy officials and also to authors reported about large aerial applications during high mortality of the larvae by E. maimaiga (Zúbrik et al. the outbreak of 1946–1949 (Turček et al. 1950; Patočka 2020a). The applications were usually taken place in the 1973; Konôpka 1978). Later on, forests were treated by spring, between May 5 and 20 (Zúbrik 2004). insecticide (DDT), about 5,000 ha were in 1955, 4,000 in 1956, and 6,000 in 1957 (Kudler et al. 1958). Some authors observed negative effects of DDT on natural ene- 10. Conclusions mies and environment (Turček 1949b; Patočka 1973). Patočka (1961) discussed a possible control method This is the most complete and most detailed reconstruc- based on collecting and destroying egg masses. This is tion of the gypsy moth population dynamics on the ter- more ecologically friendly but less effective, compared to ritory of Slovakia, although some partial reconstruction chemical applications against larvae. As a disadvantage, was already compiled and published (Zúbrik et al. 2013; this procedure is considering that part of these masses Kunca et al. 2013, 2014; Hlásny et al. 2015; Zúbrik et al. can be overlooked and many are in inaccessible areas in 2017a). We have included these works in our assessment, the crown. and they have been l fi led with new, until now unanalysed Since the 60’s and the 70’s of the last century, intense data. experimentation with biological agents against gypsy – There were nine gypsy moth outbreaks since 1945. In moth larvae was realised in Slovakia. As the most prom- the period of 1945–2020, 155,036 ha were damaged ising one, B. thuringiensis was tested in several trials with varying intensity. The strongest outbreak was in (Hešková 1978; Novotný 1985, 1988a, 1988b; Novotný 2000–2008 when 51,579 ha were attacked. On aver- & Švestka 1986, Turčáni 2001). Larval mortality was age, 17,226 ha were affected per one outbreak period. high and full efficiency was reached after 2–8 days of We have found outbreak periods that repeat with fre- treatment, depending on the dose (Novotný 1988b, quency of 7.8 ± 2,2 years and the average outbreak 1989). Besides B. thuringiensis, also LdMNPV (Novotný phase lasts 3.1 ± 1,1 years. The outbreak started in 1985, 1989), B. bassiana (Novotný 1989), viruses the western part of the country and progress to the (Švestka & Pultar 1997) and microsporidia (Weiser & East. The period between two subsequent outbreaks Novotný 1987; Hoch et al. 2008; Solter et al. 2010) have seems to be more or less constant and the duration of been tested against the gypsy moth on the territory of the outbreak seems to be gradually shortened during Slovakia. the study period. As a result of this intense research in the field of – There are several factors influencing the gypsy moth biomonitoring, gypsy moth outbreaks were almost population density in Slovakia. For example, weather exclusively managed with biological and biotechnical in May, late frosts, genetic predisposition, natural preparation since 1984–1987. Insecticide formula- enemy complex and food quality are considered by tions based on B. thuringiensis were used on 1,126 hec- many authors as key ones. Unfortunately, most pub- tares (24% of total 5,878 ha sprayed) during those lished statements were just an expert estimation and years (Novotný 1988a). In order to prevent damage were not sufficiently supported by field experiments. in 1992–1995, extensive aerial applications were also – Gypsy moth is extremely polyphagous species in the realised. B. thuringiensis-based bioinsecticides and country; however, Q. cerris seems to be the most pre- insect growth inhibitors (e.g. Dimilin) were applied ferred host. on a territory over 15,000 hectares (Turčáni & Zúbrik – Defoliated trees have reduced annual stem growth; 1997; Zúbrik 1998). In the period of 2004–2006 (three they are less tolerant to water stress and easily years) applications from the air were even more intense. attacked by secondary pests. From the total treated area (29,831 hectares; 8,298 ha – Intense applications of Dynocid (DDT) against gypsy in 2004, 15,955 in 2005, 5,538 in 2006), the biological moth larvae were realised in the past. Gypsy moth preparation based on B. thuringiensis (Biobit XL) was outbreaks were almost exclusively managed with used on 6,637 ha (22.2%). Growth regulators were for biological and biotechnical preparation since 1984. the remaining 23,194 ha (77.8%). Aerial applications – The exotic entomopathogenic fungus E. maimaiga were financially covered by the Ministry of Agriculture was found in Slovakia for the first time in 2013. Cer- and co-financed by State Forest Enterprises. Their cost tain circumstances suggest that, due to E. maimaiga, reached c. 365,000 EUR in 2004, c. 740,000 EUR in gypsy moth outbreaks should reach a lesser intensity 2005, and c. 170,000 EUR in 2006 (Zúbrik & Kovalčík 2005; Zúbrik 2006; Zúbrik et al. 2006). In 2019, applica- in the future and will be more local compared to the tions from the air against the gypsy moth on a territory of past. 66 M. Zúbrik et al. / Cent. Eur. For. J. 67 (2021) 55–71 Csóka, G., Pödör, Z., Nagy, G., Hirka, A., 2015: Canopy Acknowledgements recovery of pedunculate oak, Turkey oak and beech This research was supported by the Slovak Research and Devel- trees after severe defoliation by gypsy moth (Lyman- opment Agency (APVV) via grants No. APVV-0707-12, APVV- tria dispar): Case study from Western Hungary. Cen- 14-0567, APVV-15-0531, APVV-15-0348, APVV-16-0325, tral European Forestry Journal, 61:143–148. APVV-19-0116 and APVV-19-0119, the Scientic fi Grant Agency Čapek, M. et al., 1969: Výskum entomopatogénnych VEGA via grants No. VEGA 2/0012/17, VEGA 2/0032/19 and also by the European Regional Development Fund (ERDF) via článkonožcov kalamitných škodcov lesných drevín. Project No. 313011X531 “Development of biologically and bio- Zvolen, VÚLH, R-VII-3/1, 21 p. technically oriented systems of forest protection against domestic Čapek, M., 1961: Kalamita obaľovača smrekovcov- and non-native (invasive) organisms”. This work also received ého Zeiraphera diniana Guen. na smreku v oblasti support from the Ministry of Defence of the Slovak Republic prašivej. Lesnícky časopis, 7:260–271. via project VLMSR0120 and from the Ministry of Agriculture Čapek, M., 1966: Doterajšie skúsenosti s introdukciou and Rural Development of the Slovak Republic via the project vaječných parazitoidov mníšky veľkohlavej Lyman- SLOVLES. Also supported by Ministry of Agriculture of the tria dispar L. z južnej Európy. In: Možnosti využití Czech Republic via institutional support MZE-RO 0118. The biologického boje v ochraně zemědělských plodin a authors thank Dominique Fournier (Canada) for linguistic and lesních kultur. Souhrn referátů z vědeckého semináře, editorial improvements. 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Journal

Forestry Journalde Gruyter

Published: Jun 1, 2021

Keywords: population dynamics leaf-eating insect; periodic outbreaks; natural enemies; Entomophaga maimaiga

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