Influence of larval age and virus concentration on mortality and sublethaleffects of a nucleopolyhedrovirus on the western spruce budworm (Lepidoptera : Tortricidae)

Mortality and sublethal effects of different doses of the multicapsid nucle opolyhedrovirus that infects the spruce budworm, Choristoneura fumiferana ( Clemens), on the western spruce budworm, Choristoneura occidentalis Freeman , were investigated. C. fumiferana multicapsid nucleopolyhedrovirus (CfMNPV ) is known to be more highly virulent to C. occidentalis than to its own ho st. Larval and pupal mortality were dose-dependent regardless of the age of the larvae challenged. Larval mortality was higher when younger larvae wer e challenged, whereas pupal mortality among the survivors was higher when o lder larvae were challenged. Older larvae were less susceptible to CfMNPV t han younger larvae treated with the same virus concentration. Sublethal eff ects were also greater in the insects treated as older larvae. Sublethal ef fects of CfMNPV oil C. occidentalis infected as sixth instars included prol onged developmental time of males to pupation, decreased male pupal weight, decreased longevity of both male and female adults, and a reduced proporti on of females among the survivors, even though proportionally more males th an females died in the pupal stage. Some of these changes were significant and were age, dose and sex-dependent. Sublethal effects of CfMNPV on surviv ors challenged as fourth instars were significant in reducing the proportio n of females at the highest virus concentration. Fecundity and hatching suc cess of C. occicdentalis was not affected by CfMNPV treatment. These findin gs suggest that CfMNPV should be applied to fourth instars in the field to achieve high larval mortality. Implications of both the combined larval and pupal mortality and sublethal effects of viral application on the populati on dynamics of C. occidentalis are discussed and compared with previously r eported results on other insects.