Effects of parasitism on a defensive symbiosis

It is currently known that the pea aphid (Acyrthosiphon pisum) sometimes contains a defensive symbiont called Hamiltonella defensa. That this symbiont confers resistance against parasitoid wasps is widely accepted, but how the symbiont responds to parasitism to protect the aphid is not fully understood. Preliminary research shows that defensive symbionts maintain similar levels in parasitized and unparasitized aphid clonal lines. However, parasitoid resistant aphid lines that contain Hamiltonella also harbor a bacteriophage that seems integral in maintaining the defensive phenotype. Phage levels sometimes change in response to parasitism, indicating that parasitoid resistance in the aphids may not be solely constitutive.