Bacterial symbionts can act to protect insects from the depredations of parasitoids, as occurs in the partnership between the pea aphid, Acyrthosiphon pisum and the bacteria Hamiltonelladefensa, which, when infected with the bacteriophage APSE, saves the aphid from the endoparasitoid wasp Aphidius ervi. While it is known that aphids harbor diversity in H. defensa symbiont defensive capacity, strongly associated with symbiont and phage genotype, the phenotypic conditions showing this diversity in defense (i.e. failure of parasitism) are underexplored. In order to better understand the processes reflected by this diversity, we tested whether or not measures of aphid and developing wasp health are differentially affected by symbiont strain. Pea aphids infected with a variety of known symbiont genotypes were experimentally parasitized and dissected to reveal the condition of the aphid and developing wasp, particularly the aphid embryos, the wasp instar, and the parasitism arsenal consisting of specialized cells called teratocytes. Particular strains of symbiont-protected aphids are capable of reducing the number of teratocytes in internally developing wasps compared to aphids devoid of symbionts. However, such symbiont-mediated effects on parasitoids neither significantly alter larval development nor do they save developing aphid embryos from being destroyed by parasitoid wasps. Our results suggest that symbiont-mediated anti-parasitoid defenses are effectively useless subsequent to particular developmental stages despite overall success in parasitism defense, with defensive capacity being developmentally precise.
