The bacterium Hamiltonella defensa provides protection to the aphid Acrythosiphon pisum from parasitoid wasp predators by killing the wasp eggs before they can develop. H. defensa is often infected by the bacteriophage APSE, which is linked to this protective phenotype. The study aims to separate the protective effects of H. defensa and APSE by generating APSE-cured and APSE-free H. defensa strains to determine each component's individual contribution to aphid protection. The relationship between the bacterium, phage, and host evolved because their combined defenses benefited all parties against their shared predator.