A limited role in parasitism for Microplitis demolitor polydnavirus

Spodoptera frugiperda larvae stung by Microplitis demolitor undergo physiological alterations characteristic of parasitism. However, despite these physiological modifications, parasitized S. frugiperda larvae never yield adult wasps. Our original hypothesis that unsuccessful parasitism was due to a transcriptionally inactive polydnavirus proved untrue. Microplitis demolitor polydnavirus (MdPDV) successfully infected and expressed, albeit transiently, in S. frugiperda hemocytes. MdPDV expression was most abundant in the first three days of parasitism, then sharply declined on Day 4 post-parasitization and continued to decline for the remainder of the study. During the period of MdPDV expression, S. frugiperda hemocytes were non-adherent, incapable of spreading in vitro and did not encapsulate M. demolitor eggs in vivo. Concurrent with diminishing viral expresssion, S. frugiperda hemocytes regained their ability to adhere and spread in vitro and encapsulated M. demolitor eggs in vivo. Although MdPDV disrupted S. frugiperda's encapsulation response for the first three days post-parasitization, M. demolitor was unable to develop in this noctuid species. Failure to develop was independant of viral activity, all M. demolitor eggs oviposited in S. frugiperda larvae failed to complete embryogenesis and died within 24 hour of oviposition. S. frugiperda larvae infected with MdPDV exhibited alterations in development very similar to other lepidopterans that are permissive hosts for M. demolitor. In addition, MdPDV DNA persisted in Spodoptera frugiperda hemocytes in the absence of viral expression.