Fungal Infections Lead to Shifts in Thermal Tolerance and Voluntary Exposure to Extreme Temperatures in Both Prey and Predator Insects

Abstract Pathogens can alter the thermal tolerances of insects; these changes can have cascading impacts across trophic levels in terrestrial food webs. However, the effects of fungal infections on thermal tolerances and behavioral responses to extreme temperatures (ET) across trophic levels have rarely been studied. We examined how a fungal pathogen, Beauveria bassiana, affects the thermal physiology and behavior of an herbivorous insect, Acyrthosiphon pisum, and a predator beetle, Hippodamia convergens. We compared thermal tolerance limits (CTMin and CTMax), thermal boldness (voluntary exposure to ET), energetic cost (ATP) posed by each response (thermal tolerance and boldness) between healthy and infected insects with two fungal loads. Results showed that fungal infection reduced upper thermal tolerance (CTMax) of aphids and beetles, as well as lower thermal tolerance (CTMin) of beetles. CTMin of aphids was not altered by infection. Healthy aphids and beetles crossed warm ET zones (ETZ). Fungal infection modified the tendency, or boldness, of aphids and predator beetles to either cross warm or cold ETZ. ATP levels increased with pathogen infection in insect species, and the highest ATP levels were found in individuals that crossed cold ETZ. Fungal infection narrows the thermal tolerance range and inhibited thermal boldness behaviors to cross ET which may have implications for predator-pay interactions, food web structures, and species distributions. As environmental temperature rises, response to thermal stress will be asymmetric among members of a food web at different trophic levels.