Résumé
I present evidence that gregariously feeding species of lepidoptera have evolved resistance to infection by their viral parasites, in terms of the larger amounts of virus needed to cause an infection as larvae age. First, I show a significant positive association between age-related resistance to infection by viruses and gregarious feeding behaviour in 13 case studies involving temperate lepidoptera. Since pronounced levels of resistance to viral infection are not found in solitary species it can be inferred that this resistance has evolved in gregarious species at a cost. Further, that gregariousness itself often breaks down with larval age, suggests that the host responds to the selection pressure of the virus through not only physiological adaptations, but also behavioural ones. Second, I present evidence from a laboratory experimental system (Pieris brassicae and a granulosis virus) showing that observed age-related resistance is sufficient to explain the insensitivity of the per capita risk of infection to the number of larvae in the brood. The findings presented here are relevant to the use of pathogens for the biological control of insect pests.