Helicoverpa armigera granulovirus interference with progression of H. zea nucleopolyhedrovirus disease in H. zea larvae

Capsular proteins from Helicoverpa armigera granulovirus (HaGV) have previo usly been shown to enhance H. armigera nucleopolyhedrovirus (HaSNPV) infect ion in H. armigera larvae, Yet, HaGV and HaSNPV, as viable viruses, interfe red with one another. In our study, we have examined the effects of co-infe ction of the slow-killing virus HaGV with the fast-killing virus Helicoverp a tea NPV (HzSNPV) on H, tea larvae. The mortality parameter measured was s urvival time. Virus stocks had 50% lethal concentrations of 3.2 x 10(-9) g HaGV-infected cadavers (GVC) (HaGV) and 32 occlusion bodies (HzSNPV) per cu p. Average survival times were 16.8 and 5.5 days for larvae treated with Ha GV and HzSNPV, respectively; death of HzSNPV-treated larvae was as early as 72 h posttreatment, In co-infection experiments in which larvae were treat ed concurrently with both viruses, the viruses competed in typical fashion for host resources, However, interference with disease progression in HzSNP V-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 ho st larvae. At death, co-infected larvae were observed microscopically to be filled with HaGV granules rather than HzSNPV polyhedra. The time study res ults imply that HaGV might be outcompeting HzSNPV by inhibiting its replica tion. We also observed that H. tea larvae treated with high dosages of HaGV (greater than or equal to 3 x 10(-5) g GVC) were initially stunted but had survival times similar to those of larvae treated with lower dosages.