Host condition and pathogen identity influence bacterial infection survival in the common eastern firefly, Photinus pyralis

The degree to which host, pathogen, and environmental characteristics determine fitness outcomes in response to infection across the tree of life is a key question in ecoimmunology. Because infection experiments require large sample sizes and careful controls, studies have generally been limited to lab-adapted organisms. Photinus pyralis is a charismatic firefly that is abundant during its annual summer emergence in the Eastern United States. Here, we assess the importance of host condition, pathogen identity and dose, and pre-infection environment on survival outcomes in response to bacterial infection in wild-caught adult male P. pyralis. Although survival after sterile saline injection was not statistically different from survival of uninjected controls, survival kinetics support sterile saline injection as the most appropriate control. Serratia marcescens and Providencia rettgeri infections increased mortality, especially at high doses. In contrast to fruit fly studies, Providencia sneebia, Pseudomonas aeruginosa, and Enterococcus faecalis infections did not. Infection with higher doses of P. rettgeri had an increased risk of death for fireflies caught later in the season than those captured earlier, but seasonality did not significantly interact with dose to impact mortality with any of the other bacterial species. These results highlight the importance of considering host, pathogen, and environmental factors when investigating infection outcomes in wild-caught, non-model systems. The differences in survival between fireflies and fruit flies emphasise that further studies in a broad array of organisms are needed to explore the diversity of infection responses across the tree of life.