Keywords: entomopathogens, forest ecology, Lymantria dispar, natural enemies, parasitoids A local population outbreak of the spongy moth, Lymantria dispar L. (Lepidoptera: Erebidae), in Lower Austria, resulted in the defoliation of an oak forest in 2018 and 2019. In the study year 2020, the population density was still high but was expected to decline. In the present work, the role of parasitoids and pathogens in the collapse of the population was investigated. In total, 20 egg masses, 680 larvae and 12 pupae of L. dispar were collected in the field from April to July 2020 and reared until the emergence of adult moths or death. Causes of mortality were determined based on emerging parasitoids and on microscope inspection of non-parasitized cadavers. Stage-specific mortality rates were calculated for eggs, larval stages (L1–L6) and pupae. Seven parasitoid species and three pathogens were responsible for the premature death of spongy moths. The egg parasitoid Anastatus disparis (Hymenoptera: Eupelmidae) emerged from 19% of all eggs. Larval parasitoids caused stage-specific mortality rates from 15% to 61%. The dominant parasitoid of young and intermediate instars was the solitary wasp Glyptapanteles porthetriae (Hymenoptera: Braconidae). Mature larvae were mainly parasitized by the parasitic flies Blepharipa pratensis (Diptera: Tachinidae) and Parasetigena silvestris (Diptera: Tachinidae). Pathogens caused stage-specific
mortality rates from 11% to 41%. The nuclear polyhedrosis virus (LdNPV; Lefavirales: Baculoviridae) was the dominant pathogen in all instars. The introduced fungus Entomophaga maimaiga (Entomophthorales: Entomopthoraceae)—detected for the first time in Austria in 2019—caused low mortality rates in older larvae. Starting from high egg mass densities in spring, the spongy moth population declined to negligible levels until summer.
