Laboratory assessment of forest microhabitat substrates as sources of the gypsy moth nuclear polyhedrosis virus

Various bioassays were used to determine which forest microhabitats contained the most nuclear polyhedrosis virus (NPV) of the gypsy moth ( Lymantria dispar) and which of these microhabitats most readily led to infection of newly hatched gypsy moth larvae. Soil just under the leaf litter layer and gypsy moth pupal exuviae on tree trunks had significantly more virus than tree bark. In the laboratory, neonates became infected by walking over field-collected pupal exuviae, but contaminated larvae did not transmit the virus to other larvae when confined for a day with them in the same container. Washing larvae with disinfectant immediately after they walked over pupal exuviae reduced the rate of infection, suggesting that polyhedra from exuviae adhere to caterpillars' integuments and are ingested later. However, less than 7% of larvae that walked over contaminated soil became infected, even though 60% of caterpillars died from NPV in bioassays of water extracts of soil, demonstrating the abundant presence of viable virus in soil. Thus, of the environmental surfaces tested, pupal exuviae were the most important source for horizontal transmission of virus to newly hatched caterpillars.