All insects are involved in symbiotic relationships with microbes. Particularly interesting are insect-fungal symbioses where the insect actually 'farms' fungi for food. How these interactions evolve and how stable they are through evolutionary time is not well known. The western pine beetle is a serious pest in western North America and is involved in a mutualism with two species of fungi that it transports in special structures surrounding its head. After the beetle attacks a tree, the inoculated fungi grow throughout the tree's tissues and the beetle feeds heavily on these fungi while developing in the bark. The beetle and its symbionts are in the early stages of diverging into different species, making it an ideal system for understanding the role of symbiosis in the evolution of new species. Our research combines field surveys, laboratory experiments, and population genetics and genomics, to understand how this interaction functions, the evolutionary history of these relationships, and the consequences of the symbiosis on the genomes of the symbiotic fungi.