Cross-infectivity of a Putative Spodoptera picta Nucleopolyhedrovirus to Spodoptera litura Fabricius (Lepidoptera: Noctuidae)

Abstract

A putative nucleopolyhedrovirus (NPV) from a field-collected local Spodoptera picta population was isolated for the first time. Its pathogenesis was described in S. picta larvae and was tentatively assigned the name Spodoptera picta nucleopolyhedrovirus or SppiNPV. With the objective of utilizing this SppiNPV as a biological control agent, its cross-infectivity to S. litura – a very important agricultural pest in the country – was evaluated. Third instar S. litura larvae were infected with approximately 100 occlusion bodies (OBs) of SppiNPV using artificial diet- incorporation. SppiNPV infection in S. litura was clearly observed as the 4th instar larvae exhibited pinkish coloration, a characteristic sign of NPV-infection. In addition, the integument of the mildly-infected larvae easily twisted when touched and often expelled slimy fluid from their mouth. At the advanced stage of infection, larvae became extremely fragile resulting in cuticles that were easily ruptured. At 6 d post-infection (dpi), 20% mortality was observed in the SppiNPV-treated S. litura. At 16 dpi, all the cutworms in the control set-up emerged as normal adults, while only three cutworms (10%) in the NPV-treated larvae emerged as adults but with short malformed wings and abnormally soft and fragile abdomen. The effect of SppiNPV on the weight gain of S. litura larvae was also monitored at 4 and 6 dpi. Although significantly higher weight (F = 5.09; df = 58; P > 0.0278) was recorded in the untreated cutworms than the NPV-treated cutworms at 4 dpi, weight gain did not significantly differ in SppiNPV-infected and untreated larvae. Overall, a significantly higher mortality of 46.67% (F = 25.37; df = 58; P > 0.0001) was recorded in SppiNPV-infected cutworm larvae. Further studies to characterize this novel SppiNPV, increase its virulence to S. litura through the continuous passage in S. litura, and optimize the amount of OB to be applied to ensure 100% mortality are recommended so that it can be used as a biological control agent with expanded host range.