A negative effect of a pathogen on its vector? A plant pathogen increases the vulnerability of its vector to attack by natural enemies.

Plant pathogens that are dependent on arthropod vectors for transmission from host to host may enhance their own success by promoting vector survival and/or performance. The effect of pathogens on vectors may be direct or indirect, with indirect effects mediated by increases in host quality or reductions in the vulnerability of vectors to natural enemies. We investigated whether the bird cherry-oat aphid Rhopalosiphum padi, a vector of cereal yellow dwarf virus (CYDV) in wheat, experiences a reduction in rates of attack by the parasitoid wasp Aphidius colemani when actively harboring the plant pathogen. We manipulated the vector status of aphids (virus carrying or virus free) and evaluated the impact on the rate of attack by wasps. We found that vector status did not influence the survival or fecundity of aphids in the absence of parasitoids. However, virus-carrying aphids experienced higher rates of parasitism and greater overall population suppression by parasitoid wasps than virus-free aphids. Moreover, virus-carrying aphids were accepted as hosts by wasps more often than virus-free aphids, with a greater number of wasps stinging virus-carrying aphids following assessment by antennal palpations than virus-free aphids. Therefore, counter to the prevailing idea that persistent vector-borne pathogens enhance the performance of their vectors, we found that infectious aphids actively carrying a plant pathogen experience greater vulnerability to natural enemies. Our results suggest that parasitoids may contribute to the successful biological control of CYDV by disproportionately impacting virus-carrying vectors, and thus reducing the proportion of vectors in the population that are infectious.