Susceptibility of Heliothis armiger (Lepidoptera: Noctuidae) on Sorghum to Nuclear Polyhedrosis Virus

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Abstract
Applications of a commercial nuclear polyhedrosis virus were made to sorghum at 100% flowering (at which stage most eggs of a natural infestation of Heliothis armiger [Hübner] had hatched), or at 2 or 4 days thereafter. The treated larva populations were characterized in terms of instar and size (length), density, and estimated susceptibility to the virus. The dosage-mortality responses were similar on all three occasions despite differences in rated susceptibility based on mean larva sizes. This was explained by laboratory data on relationships between larva susceptibility to the virus and daily food consumption (= virus acquisition). We concluded that the larvae of instars one through three had a nearly equal probability of becoming infected because the increase in tolerance to the virus with larva age wascounteracted by an associated increase in virus acquisition (larva feeding). Therefore, timing of application is not critical as long as early instars are treated. The relative incidence of the virus disease and parasitism by a Microplitis sp. parasitoid suggested that prior parasitism suppressed infection of H. armiger by the virus. Conversely, the level of parasitism was significantly lower in sprayed larvae than in unsprayed larvae when the timing of the virus application coincided with the highest rate of parasitization.

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