Properties of nucleopolyhedrovirus occlusion bodies from living and virus-killed larvae of Spodoptera frugiperda (Lepidoptera: Noctuidae)

Abstract

Previous studies report that the pathogenicity of nucleopolyhedrovirus occlusion bodies (OBs) is reduced if OBs are collected from living infected insects compared to virus-killed insects. We compared the production and characteristics of SfMNPV OBs collected from living and virus-killed fourth instars of Spodoptera frugiperda at 3–4 days, 5 days and 6–7 days post-inoculation. Cumulative virus-induced mortality increased from 10.5 % at 3 days to 99.6 % at 7 days post-inoculation. The production of OBs/mg larval weight increased significantly between the first and second samples, but did not increase thereafter. The total number of OBs produced in living and virus-killed larvae increased at each sample time. The size (cross-sectional area) of OBs increased significantly during the experiment, probably due to OB maturation. The median prevalence of immature OBs decreased over time from 25 to 44 % in the first samples to 9 % in the final sample. OB pathogenicity was compared in second instars that consumed a discriminating concentration of inoculum (2 × 104 OB/ml). Virus-induced mortality increased markedly in larvae that consumed OBs collected later (6–7 days) compared to OBs from earlier samples, but was similar for OBs from living or virus-killed insects at each time point. We conclude that the pathogenicity of OB samples was sensitive to the prevalence of immature OBs rather than the viral genome content of OBs, which did not differ over sample times or in OBs from living or virus-killed insects. These findings can inform decisions on the production of biological insecticides and laboratory studies on the insecticidal properties of OBs.