The baculovirus Lymantria dispar nucleopolyhedrovirus (LdNPV) is a natural pathogen of gypsy moth caterpillars. Effectiveness of this biological control agent may be limited by a phenomenon referred to as “intrastadial developmental resistance,” meaning there is an increase in systemic resistance to viral infection as larvae age within a stadium. The largest difference in susceptibility is observed between fourth instar caterpillars inoculated with virus immediately after molting (designated 40) and those inoculated at 48-hours post-molt (designated 448). A LD80 in 40 larvae is approximately equal to a LD40 in 448 larvae. We hypothesize resistance of the older caterpillars is due to anti-viral defenses. To test this hypothesis, we examined the effect of immunosuppression by a polydnavirus (PDV) from the braconid wasp Glyptapanteles flavicoxis on age-related resistance to LdNPV. PDV virions are produced in the reproductive system of some parasitic wasps in the Braconidae and Ichneumonidae and serve to protect the parasitoid eggs against encapsulation by disrupting the host’s immune response. In our study, 40 and 448 caterpillars were co-injected with LdNPV and G. flavicoxis PDV. All dead larvae were autopsied to confirm the presence of NPV occlusions. PDV significantly increased NPV-induced mortality in 448 larvae; although PDV also increased mortality in 40 larvae, it was not statistically significant. No death occurred in larvae inoculated with media or PDV only and all survivors successfully pupated. While the mechanism whereby PDV increased susceptibility to NPV has yet to be determined, this study indicates PDV can serve as a useful tool for studying the potential caterpillar immune response against NPV.