Helicoverpa armigera Granulovirus Interference with Progression of H. zea Nucleopolyhedrovirus Disease in H. zea Larvae

Abstract Capsular proteins from Helicoverpa armigera granulovirus (HaGV) have previously been shown to enhance H. armigera nucleopolyhedrovirus (HaSNPV) infection in H. armigera larvae. Yet, HaGV and HaSNPV, as viable viruses, interfered with one another. In our study, we have examined the effects of co-infection of the slow-killing virus HaGV with the fast-killing virus Helicoverpa zea NPV (HzSNPV) on H. zea larvae. The mortality parameter measured was survival time. Virus stocks had 50% lethal concentrations of 3.2 × 10 −9 g HaGV-infected cadavers (GVC) (HaGV) and 32 occlusion bodies (HzSNPV) per cup. Average survival times were 16.8 and 5.5 days for larvae treated with HaGV and HzSNPV, respectively; death of HzSNPV-treated larvae was as early as 72 h posttreatment. In co-infection experiments in which larvae were treated concurrently with both viruses, the viruses competed in typical fashion for host resources. However, interference with disease progression in HzSNPV-fed larvae occurred even when HaGV was fed to larvae up to 36 h after the NPV, a time at which NPV infection should have been well established in host larvae. At death, co-infected larvae were observed microscopically to be filled with HaGV granules rather than HzSNPV polyhedra. The time study results imply that HaGV might be outcompeting HzSNPV by inhibiting its replication. We also observed that H. zea larvae treated with high dosages of HaGV (≥3 × 10 −5 g GVC) were initially stunted but had survival times similar to those of larvae treated with lower dosages.