Effects of temperature and sunlight on mycosis (Beauveria bassiana) (Hyphomycetes: Sympodulosporae) of grasshoppers under field conditions

The influence of environmental conditions on mycosis of grasshoppers caused by Beauveria bassiana (Balsamo) Vuillemin, was investigated. Despite the deposition of considerable quantities of conidia onto grasshoppers (6.7 x 10(3) colony-forming units [CFU] per nymph), B. bassiana did not significantly reduce held populations nor did it affect specific grasshopper taxa. Conditions were warm and sunny during the trial, and slopes of conidial persistence were equally poor on both grasshoppers and grass leaves. Small numbers of conidia (<2 CFU per grasshopper) were recovered from surface-sterilized grasshoppers 5-15 d after application indicating that if infection occurred, B. bassiana did not proliferate in the hemocoel. Considerable mycosis was observed in grasshoppers placed in cages in the greenhouse, but not in grasshoppers confined in cages adjacent to the field plots. Furthermore, the prevalence of disease in the greenhouse cages decreased with the sampling date but the onset of mycosis always occurred 3-4 d after collection, suggesting that environmental conditions in the greenhouse were responsible for the increased susceptibility of grasshoppers. Higher prevalence and more rapid development of disease were observed in grasshoppers placed in shaded cages (83-89%) than in cages exposed to full sunlight (0-15%) or protected from UVB radiation (1-43%); conidial survival was equally enhanced in the shaded and UVB-protected environments. Our results indicate that the poor efficacy of B. bassiana against rangeland grasshoppers was a result of conditions of temperature and light exposure (reduced grasshopper thermoregulation), and not the result of inadequate host targeting or pathogen virulence.