Effects of a nucleopolyhedrovirus in mixtures with azadirachtin on Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) larvae and viral occlusion body production

The combined and alone effects of azadirachtin (AZA) and Spodoptera frugiperda multicapsid nucleopolyhedrovirus (SfMNPV) on the mortality of S. frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) were evaluated in the laboratory. For this, diet surface contamination bioassays were performed on S. frugiperda in the third instar. LC50 values for SfMNPV alone were determined to be 430 and 373 viral occlusion bodies (OBs)/mm(2) at 192 and 216 h after treatment, respectively. LC50 values for AZA alone were estimated for two periods of continuous exposure (4 or 5 days). In this case, LC50 values were 45.5 and 16.8 mg L-1 at 216 h after treatment (4 or 5 days of larval exposition to insecticide, respectively). We observed that although the interaction of AZA with SfMNPV increased viral pathogenicity, such improvements were of greater magnitude and more consistent at the lower OB concentration used (177 OBs/mm(2)). Application of SfMNPV (430 OBs/mm(2)), AZA (26.4 mg L-1) or SfMNPV-AZA mixtures resulted in a significant reduction in the mean weight of larvae treated in the third instar across the experiment, by 23-41%, 17-95% and 26-97%, respectively, compared to control. The duration of larval development during the third and fourth instars increased significantly in larvae exposed to SfMNPV-AZA mixtures and AZA alone compared to SfMNPV alone and control treatments. The yield of OBs/mg weight of larvae treated with SfMNPV alone was 1.8-fold higher than OB yields from insects inoculated with SfMNPV-AZA mixtures. We conclude that AZA + SfMNPV mixtures are unlikely to be useful for the mass production of this virus and laboratory observations on the value of AZA + SfMNPV mixtures as a potentiator of biological insecticides require validation in field studies under commercial growing conditions.