Insects feed on a variety of dietary nutrients, antioxidants, co-factors, and plant secondary metabolites which may alter their response to microbial infections. We have previously found that dietary Selenium (Se) impacted the growth and development of Trichoplusia ni reared for many generations on diet containing extremely low levels of Se. Larvae had an elevated resistance to per os infection with a baculovirus. In this study we examine how dietary Se affects the growth, development, and Se content of Heliothis virescens that have been laboratory reared for less than two years. Larvae fed a commercial tobacco budworm diet supplemented with greater than 25 ppm Se grew at a much slower rate than insects fed lower levels of Se. However, pupal weights remained constant for all treatments. When 5th instar plasma was mixed with Helicoverpa zea single nucleopolyhedrovirus (HzSNPV) budded virus and the mixture titered on H. zea cells, the antiviral activity in the plasma was altered depending on the amount of dietary Se. This antiviral activity is due to the level of phenoloxidase in the insect hemolymph. Addition of Se directly to H. zea cell culture medium or to control plasma did not impact the level of antiviral activity. This study lends support to the hypothesis that dietary Se may alter the immunocompetence of larval lepidopterans by modulating the expression or activity of plasma phenoloxidase, which in turn acts as an antiviral enzymatic activity directly upon HzSNPV budded virus within the hemocoel of infected insects.