Effects of Temperature and Thermoregulation on Mycosis byBeauveria bassianain Grasshoppers

The influence of behavioral thermoregulation by grasshoppers (Melanoplus sanguinipes) on mycosis caused byBeauveria bassianawas investigated in controlled environments. The cardinal temperature forB. bassianaconidial germination and hyphal development was approximately 35°C. A low prevalence of mycosis (≤7%) was observed in inoculated nymphs exposed to a continuous temperature of 35 and 40°C, whereas continuous exposure to 30°C did not have a significant effect on disease development. Daily exposures to 35 and 40°C for 1 and 6 h, respectively, decreased mycosis in nymphs. In both environments, a strong correlation (r ≥ 0.95) was observed between hyphal growth on potato dextrose agar and final mycosis. Although high temperatures delayed conidial germination, only conidia exposed to continuous 35 or to 40°C for more than 8 h exhibited reduced germination after 24 h. The effects of temperature on conidial germination were poorly correlated with disease, and when nymphs were exposed to 35°C for 24 h, less mycosis was observed only in grasshoppers exposed between 1 and 2 days postinoculation. The thoracic temperature of nymphs permitted to bask adjacent to a heat source ranged from 38 to 42°C. In nymphs basking for 1 h per day, 46% less mycosis was observed, decreasing to 98% less disease in nymphs allowed to bask for 6 h or greater per day. On a heat gradient, a higher prevalence ofB. bassiana-infected nymphs selected hotter positions than noninfected nymphs, suggesting a “behavioral fever” response to infection. This study indicates that high temperature and thermoregulation can adversely affectB. bassianamycosis of grasshoppers and may explain the poor efficacy of this entomopathogen observed in some field experiments.