The natural enemy complex of the gypsy moth, Lymantria dispar (Lep., Lymantriidae) in different phases of its population dynamics in eastern Austria and Slovakia – a comparative study

Abstract

Parasitism and pathogen mortality of Lymantria dispar were compared between host populations of different densities in Slovakia and Austria. Over a period of 4 years, L. dispar eggs, larvae, and pupae were collected in a stage‐specific manner at three mixed oak stands in each country and reared in the laboratory to assess parasitoid and pathogen mortality. At sites with low host densities, L. dispar abundance was artificially augmented by exposing egg masses and young larvae. We ascertained marked differences between the natural enemy complexes of L. dispar populations in Slovakia and Austria. Overall, pathogens caused highest mortality in the former and parasitoids in the latter. Moreover, the species composition differed significantly between both countries. High variation was also observed between years and host populations at different densities. The investigations revealed that egg parasitization was low in Slovak L. dispar populations, and no egg parasitoids were found in Austria. Larval and pupal parasitism was low at sites with outbreak populations, but higher at the Austrian than at the Slovak site. The tachinid Parasetigena silvestris was the dominant species at elevated host densities. Nuclear polyhedrosis virus also caused significant mortality in these cases. An increase in parasitism by several species was noticed after the breakdown of L. dispar populations. The ichneumonid Phobocampe spp. caused high levels of parasitism during the post‐culmination years. At sites where host abundance increased in the first year of the study, significant differences between the Austrian and the Slovak locality could be ascertained. Mortality was low in the population at the latter site, and host abundance increased to outbreak levels the year after. The population at the Austrian site suffered high parasitism by the tachinids P. silvestris and Blepharipa pratensis and this might have contributed to the prevention of a further increase in L. dispar abundance in this locality. Braconids showed a strong reaction to the local, artificial increase in host density at study plots with innocuous L. dispar populations. Particularly high parasitism by Glyptapanteles liparidis was observed at the Austrian site, but G. porthetriae and Cotesia melanoscela also accounted for significant mortality in both low‐density populations. The artificial L. dispar populations were usually eradicated by natural enemies before pupation of the hosts.

Species diversity and niche separation of cocoon parasitoids in different forest types with endemic populations of their host, the Common Pine Sawfly Diprion pini (Hymenoptera: Diprionidae)

“…Though spatially and temporally restricted, they parasitized a high proportion of 221 above-ground cocoons in summer, indicating that Monodontomerus may be able to limit the population growth of sawflies that results from a bivoltine development. Hoch et al (2001) observed a rapid reaction of the parasitoid Glyptapanteles liparidis to artificially increased populations of Lymantria dispar. Such spatial and/or temporal density-dependent responses of natural enemies may prevent the development of outbreaks from "hot spots" of increased herbivore density.…”

“…Parasitoids are generally considered to be key natural enemies determining these population dynamics (Kidd & Jervis, 1997). On the other hand, it is suggested that strong fluctuations in host density may also create profound changes in the species composition of a parasitoid community due to the occurrence of high vs. low density specialists (Pschorn-Walcher, 1977;Mills, 1990;Hoch et al, 2001). Furthermore, varying environmental conditions may increase or restrict the abundance of parasitoid species by determining the presence or absence of other essential resources (Simmons et al, 1975;Roland et al, 1997;Lewis & Whitfield, 1999;Quayle et al, 2003).…”

Quantifying horizontal transmission of Nosema lymantriae, a microsporidian pathogen of the gypsy moth, Lymantria dispar (Lep., Lymantriidae) in field cage studies

“…(as reviewed in McManus and Solter, 2003). More typically, however, microsporidia occur at lower enzootic levels (Novotny, 1989;Hoch et al, 2001;Pilarska et al, 1998) and are detected in most populations in Europe. They must, therefore, have effective paths for disease transmission.…”

First record ofEntomophaga maimaiga(Entomophthorales: Entomophthoraceae) in Slovakia

“…It was also often observed in mixed infections together with bacteria or fungi. Mortality by microsporidia was usually low (<5%); however, species of Entomophthorales have never been reported (Fuester, Drea, Gruber, Hoyer, & Mercadier, 1983;Hoch, Zúbrik, Novotný, & Schopf, 2001;Solter et al, 2010;Weiser, 1998). …”