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Abstract
Only about ten years ago the first paper was published demonstrating that heritable bacterial endosymbionts can protect aphids against parasitoids. An explosion of research since has shown that symbiont-conferred resistance is widespread in insects. This challenges existing theory on host-parasitoid interactions. Using models that incorporate effects of symbionts, we identify several factors that are important in determining the dynamics of host-parasitoid coevolution when mediated by symbionts, such as the specificity of symbiont-conferred protection, the costs associated with harbouring symbionts, or their rate of horizontal transmission. We are in the process of studying these factors empirically in a model system comprising the black bean aphid, Aphis fabae, its defensive symbiont Hamiltonella defensa, and the parasitoid Lysiphlebus fabarum. The results so far indicate that symbionts behave such that they do indeed play a crucial role by altering the reciprocal selection between hosts and parasitoids, leading to coevolutionary dynamics that are not observed in their absence. I discuss the implications of these findings for biological control of pest aphids with parasitoids and I derive specific recommendations as to how biological control of symbiont-protected pests may be improved.