Under strong pathogen pressure, insects often evolve resistance to infection. Many insects are also protected via immune memory (‘immune priming’), whereby sub-lethal exposure to a pathogen enhances survival after secondary infection. Theory predicts that immune memory should evolve when the pathogen is highly virulent, or when pathogen exposure is relatively rare. However, there are no empirical tests of these hypotheses, and the adaptive benefits of immune memory relative to direct resistance against a pathogen are...