Can low concentration flufenoxuron treatment increase the pathogenicity or production of nucleopolyhedrovirus occlusion bodies in Spodoptera exigua (Hübner) or Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae)?

Abstract

Nucleopolyhedroviruses (Baculoviridae) are lethal pathogens that naturally infect lepidopteran larvae. The chitin synthesis inhibitor flufenoxuron has been reported to potentiate the insecticidal properties of nucleopolyhedrovirus occlusion bodies (OBs) by degrading the integrity of the peritrophic matrix in lepidopteran larvae. In the present study we examined the effect of flufenoxuron treatment on susceptibility of Spodoptera exigua (Hübner) and Spodoptera frugiperda (JE Smith) fifth instars to their homologous nucleopolyhedroviruses, SeMNPV and SfMNPV, respectively. Preliminary bioassays indicated that a 1 mg/l concentration of flufenoxuron resulted in 6.6 - 13.3% mortality of larvae, which could be used in combination with OB inoculum concentrations of 1.5 × 104 OBs/ml of SeMNPV OBs or 1.3 × 10⁶ OBs/ml of SfMNPV OBs. Treatment of larvae with flufenoxuron did not significantly affect virus-induced mortality of either S. exigua (16.7 - 17.2%) or S. frugiperda (28.9 - 34.4%) and had no significant effect on the mean yield of OBs from virus-killed insects of either species. The identity of both SeMNPV and SfMNPV in experimental insects was confirmed by examination of the restriction endonuclease profiles of each virus. These findings contrast with the results of previous studies in which high concentrations of flufenoxuron (100 mg/l) greatly potentiated virus-induced mortality in the silkworm Bombyx mori L., although insect mortality due to flufenoxuron treatment alone was not reported, for reasons that are unclear. We conclude that flufenoxuron (1 mg/l) failed to potentiate the insecticidal activity of SeMNPV or SfMNPV in their respective hosts.