Insects and other animals are frequently infected with maternally inherited symbiotic bacteria, which can benefit their animal hosts by providing nutrition or host protection. Many beneficial symbionts are themselves infected with viruses called bacteriophages, which have the capacity to impact host biology in critical ways, yet their roles in animal-bacterial symbioses are poorly understood. In the pea aphid, Acyrthosiphon pisum, the bacterial symbiont Hamiltonella confers protection from attack by parasitic wasps, but only when bacteriophages called APSEs are also present. Outside of being required for aphid protection, however, little is known about the influence of APSEs on H. defensa and aphid biology, including specifically how APSEs contribute to the demise of parasites and its potential role in the regulation of symbiont abundance. This project will use a combination of approaches, including genomics and molecular biology, to characterize the roles of APSEs in the regulation and maintenance of the protective symbiosis and to identify specific factors responsible for wasp mortality. Deciphering phage-symbiont interactions is important to the study of animal-microbial interactions generally and to understanding the biology of many economically important insects like aphids. The proposed work will yield information of practical importance for the management of pest insect populations and on the role of phages in the establishment and maintenance of bacterial-arthropod associations, which are widespread in the natural world. The interdisciplinary nature of this research is also well suited as a platform to train young scientists at the interface of symbiosis, cell biology, parasitology, and molecular processes. This project will train high school students, undergraduate and graduate students, and post-doctoral researchers.