EFFECTS OF TEMPERATURE AND THERMOREGULATION ON MYCOSIS BY BEAUVERIA-BASSIANA IN GRASSHOPPERS

The influence of behavioral thermoregulation by grasshoppers (Melanopl us sanguinipes) on mycosis caused by Beauveria bassiana was investigat ed in controlled environments. The cardinal temperature for B. bassian a conidial germination and hyphal development was approximately 35 deg rees C. A low prevalence of mycosis (less than or equal to 7%) was obs erved in inoculated nymphs exposed to a continuous temperature of 35 a nd 40 degrees C, whereas continuous exposure to 30 degrees C did not h ave a significant effect on disease development. Daily exposures to 35 and 40 degrees C for 1 and 6 h, respectively, decreased mycosis in ny mphs. In both environments, a strong correlation (r greater than or eq ual to 0.95) was observed between hyphal growth on potato dextrose aga r and final mycosis, Although high temperatures delayed conidial germi nation, only conidia exposed to continuous 35 or to 40 degrees C for m ore than 8 h exhibited reduced germination after 24 h, The effects of temperature on conidial germination were poorly correlated with diseas e, and when nymphs were exposed to 35 degrees C for 24 h, less mycosis was observed only in grasshoppers exposed between 1 and 2 days postin oculation. The thoracic temperature of nymphs permitted to bask adjace nt to a heat source ranged from 38 to 42 degrees 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 he at gradient, a higher prevalence of B. bassiana-infected nymphs select ed hotter positions than noninfected nymphs, suggesting a ''behavioral fever'' response to infection. This study indicates that high tempera ture and thermoregulation can adversely affect B. bassiana mycosis of grasshoppers and may explain the poor efficacy of this entomopathogen observed in some held experiments. (C) 1996 Academic Press, Inc.