Mortality and sublethal effects of different doses of the multicapsid nucleopolyhedrovirus 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 host. Larval and pupal mortality were dose-dependent regardless of the age of the larvae challenged. Larval mortality was higher when younger larvae were challenged, whereas pupal mortality among the survivors was higher when older larvae were challenged. Older larvae were less susceptible to CfMNPV than younger larvae treated with the same virus concentration. Sublethal effects were also greater in the insects treated as older larvae. Sublethal effects of CfMNPV on C. occidentalis infected as sixth instars included prolonged developmental time of males to pupation, decreased male pupal weight, decreased longevity of both male and female adults, and a reduced proportion of females among the survivors, even though proportionally more males than females died in the pupal stage. Some of these changes were significant and were age, dose and sex-dependent. Sublethal effects of CfMNPV on survivors challenged as fourth instars were significant in reducing the proportion of females at the highest virus concentration. Fecundity and hatching success of C. occidentalis was not affected by CfMNPV treatment. These findings 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 population dynamics of C. occidentalis are discussed and compared with previously reported results on other insects.