Systemic component to intrastadial developmental resistance in Lymantria dispar to its baculovirus

We investigated intrastadial developmental resistance of the gypsy moth, Lymantria dispar, to its host-specific baculovirus, L. dispar multinucleocapsid nucleopolyhedrovirus (LdMNPV). Susceptibility of the gypsy moth to LdMNPV decreased markedly as the insect aged within the fourth instar and this resistance was systemic because it could not be overcome by bypassing the midgut and injecting the virus directly into the hemocoel. An LD₈₈ dose of polyhedra delivered orally to newly molted fourth instars produced 74, 37, 29, 27, 38, and 60% mortalities in larvae that were orally inoculated at 12, 24, 48, 72, 96, or 120 h post-molt to the fourth instar, respectively. An LD₇₇ dose of budded virus delivered intrahemocoelically to newly molted fourth instars produced 84, 54, 29, 48, 59, and 46% mortalities in larvae that were injected at 12, 24, 48, 72, 96, or 120 h post-molt, respectively. Developmental resistance was also observed in fourth instars fed on oak foliage, no matter whether they were inoculated orally or intrahemocoelically, suggesting that intrastadial developmental resistance in gypsy moth larvae has a systemic component. Also, the host plant did not appear to affect systemic resistance, although it did increase midgut-based resistance. The degree of developmental resistance by intrahemocoelic inoculation was equivalent in oak- and diet-fed insects. In contrast, the degree of resistance by oral inoculation was much greater in oak- than in diet-fed insects, probably because of the combined effects of host-plant inhibition and increasing resistance to viral disease as the larvae aged within the instar.