Symbiont-conferred resistance to parasitoids in aphids - Challenges for biological control

Parasitoid wasps are frequently employed to control pest aphids in greenhouse crops. However, aphids can be infected with heritable bacterial endosymbionts that strongly increase their resistance to parasitoid wasps. These defensive symbionts have the potential to compromise the effectiveness of biological control. Unfortunately, we still lack the necessary studies to assess their effects at the scale of greenhouses, but laboratory experiments indicate clearly that defensive symbionts represent a problem. Selection by parasitoids results in the rapid evolution of symbiont-conferred resistance, to the point that symbiont-protected aphid populations can escape control by parasitoids. The limited evidence available from field experiments is slightly less disquieting, suggesting that even though it is operational under natural conditions, protection by defensive symbionts does not always provide a net fitness benefit to the aphid hosts. Factors that can limit the success of symbiont-protected aphids are physiological and ecological costs of harboring defensive symbionts, as well as counteradaptations by parasitoids. Based on a review of the pertinent literature, I derive recommendations for mitigating the challenges resulting from the presence of defensive symbionts for the biological control of pest aphids. These include selective breeding of parasitoids as well as clever release strategies, potentially in combination with other biocontrol agents.