Susceptibility to infection is determined by a combination of genetic and external factors, including host diet. Here, we explore the influence of ingested cotton foliage on baculoviral infection. Heliothis virescens larvae fed cotton foliage immediately preceding Autographa californica multiple nucleopolyhedrosis virus (AcMNPV) inoculation, experience significantly less viral mortality than cohort larvae fed artificial diet. The Trichoplusia ni granulovirus (TnGV), described to disrupt the peritrophic matrix of larval lepidopterans, was used to clarify the role of the matrix in this diet-dependent suppression of mortality. Cotton-fed H. virescens larvae that ingested TnGV prior to AcMNPV inoculation experienced a significant increase in AcMNPV mortality, equivalent to mortality of larvae fed artificial diet. Thus, the peritrophic matrix of cotton-fed H. virescens larvae appears to be a major factor in larval resistance to AcMNPV infection.