Thermal biology of the meadow grasshopper, Chorthippus parallelus, and the implications for resistance to disease

1. The thermal biology of the meadow grasshopper, Chorthippus parallelus, a common, habitat generalist acridid species found in the U.K., was characterised and the influence of thermoregulatory behaviour for resistance against a temperate (Beauveria bassiana) and tropical (Metarhizium anisopliae var. acridum) fungal pathogen was determined. 2. Chorthippus parallelus was found to be an active behavioural thermoregulator, with a preferred temperature range of 32-35°C. 3. Both pathogens proved lethal to fifth instar and adult grasshoppers. No evidence of behavioural fever in response to infection by either pathogen was found, but normal thermoregulation was found to reduce virulence and spore production of B. bassiana. Normal thermoregulation did not appear to affect M. anisopliae var. acridum. 4. These results suggest that the effects of temperature on host resistance depend on the thermal sensitivity of the pathogen and, in this case, derive from direct effects of temperature on pathogen growth rather than indirect effects mediated by host immune response. 5. The implications for possible risks of exotic pathogens and influence of climate change are discussed.