Ecological immunology distinguishes between long-term, evolutionary costs of possessing defences against parasites and the short-term costs of using them. Evolutionary biologists have typically focused on the former in the search for constraints on the evolution of resistance. Here we show in the peach-potato aphid, Myzus persicae, that short term costs may be of equal evolutionary importance. Survivors of more resistant aphid clones suffered a higher reduction of fecundity upon parasitoid attack than survivors of more susceptible clones. This genetically based trade-off between benefits and costs of defence may limit the evolution of increased
resistance and explain the maintenance of genetic variation for resistance under environmental variation in parasitism risk.