We have shown that when Heliothis virescens larvae were reared on diet containing excess ascorbic acid (Vitamin C), their development slowed markedly. Additionally, plasma melanization and virucidal activity of plasma against a baculovirus were completely inhibited. We used an ascorbate-free diet to evaluate ascorbate deficiency on larval H. virescens immunocompetence. Ascorbate-free diet slowed the growth rate of larvae, reduced pupal weights significantly, and severely inhibited adult emergence. Larvae did not develop at normal rates when reared upon diets containing less than 0.7 g/L ascorbate. The effect of ascorbate deficiency on baculovirus resistance was especially pronounced. Larvae developing on ascorbate deficient diet experienced far higher levels of mortality following per os infection. For example, a viral dose causing 85% mortality in larvae reared on asorbate-free diet caused 40% mortality in larvae fed diet supplemented with 3.5 g/ml ascorbate. Viral infection, as monitored by epifluorescence microscopy, in insects fed ascorbate or an ascorbate-free diet was also markedly different. In summary, we have shown ascorbate, which is known to be a free radical scavenger, to be important not only in insect development but also in larval resistance to baculovirus infection.