Introduction: The gypsy moth, Lymantria dispar, and the nun moth, Lymantria monacha, are closely related species (Lepidoptera, Lymantriidae), co-seasonal and economically important forest pests on broadleaf and coniferous trees. In Central Europe, gypsy moth larvae are frequently parasitized by the gregarious, endoparasitic wasp Glyptapanteles liparidis (Hymenoptera, Braconidae). At oviposition, the female wasp injects between 10 and up to 100 eggs into the hemocoel of a single host larva, together with venom and calyx fluid containing polydnavirus (PDV) particles that subsequently play a critical role in suppressing the host immune response so that successful development of the parasitoid can proceed (Schopf 2007). These viruses, which are integrated in the genomic DNA of the wasp and undergo replication only in the female’s ovary, rapidly enter host hemocytes, fat body, and nervous system following parasitization, and viral genes are expressed. In L. dispar larvae parasitized by G. liparidis, the host’s hemocytes alter their behavior, fail to spread properly (thereby inhibiting the encapsulation response) and partly undergo programmed cell death (apoptosis), resulting in a dramatic drop in the host’s total hemocyte number (Schafellner and Schläger 2009).