Pathogen incidence varies with warming and moisture: The effects of abiotic conditions on a fungal entomopathogen-grasshopper system

Mounting scientific evidence indicates that climate change may decrease fungal entomopathogen incidence which in turn may release insect populations from these environmentally sensitive pathogens. However, the outcomes of host-pathogen interactions are often context-dependent, with the pathogen responding differently to abiotic conditions than its host. In a field experiment, we examined the effect of temperature and moisture on the survival of a grasshopper pest in both fungal pathogen-exposed and pathogen-free populations. Moisture supplementation benefited the pathogen by increasing fungal spore levels (i.e. pathogen abundance) by eight-fold (F_1,318 = 32.52, P < 0.001). Consequently, moisture supplementation resulted in a three-fold increase in total pathogen mortality (X² = 14.57, df = 1, P < 0.001) and shortened pathogen-exposed grasshopper survival time by half. In contrast, warming reduced fungal spore levels by half (F_1,318 = 32.52, P < 0.001). As a result, total pathogen mortality rate decreased by 67%. However, this release from the pathogen did not significantly increase overall grasshopper survival time (X² = 0.47, df = 1, P = 0.49). Pathogen-free grasshopper survival time was unaffected by moisture supplementation while warming decreased survival time by 43% as a result of reduced grass biomass (i.e. food abundance). Our results indicate that a fungal pathogen and its grasshopper host respond differently to environmental conditions with changes in daily temperature and relative humidity altering host-pathogen and host-environment interactions. Therefore, both host and pathogen responses to abiotic conditions may need to be considered when predicting host-pathogen outcomes under ongoing global change.