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 c aused by Beauveria bassiana (Balsamo) Vuillemin, was investigated. Des pite the deposition of considerable quantities of conidia onto grassho ppers (6.7 x 10(3) colony-forming units [CFU] per nymph), B. bassiana did not significantly reduce held populations nor did it affect specif ic grasshopper taxa. Conditions were warm and sunny during the trial, and slopes of conidial persistence were equally poor on both grasshopp ers and grass leaves. Small numbers of conidia (<2 CFU per grasshopper ) were recovered from surface-sterilized grasshoppers 5-15 d after app lication indicating that if infection occurred, B. bassiana did not pr oliferate in the hemocoel. Considerable mycosis was observed in grassh oppers placed in cages in the greenhouse, but not in grasshoppers conf ined in cages adjacent to the field plots. Furthermore, the prevalence of disease in the greenhouse cages decreased with the sampling date b ut the onset of mycosis always occurred 3-4 d after collection, sugges ting that environmental conditions in the greenhouse were responsible for the increased susceptibility of grasshoppers. Higher prevalence an d more rapid development of disease were observed in grasshoppers plac ed in shaded cages (83-89%) than in cages exposed to full sunlight (0- 15%) or protected from UVB radiation (1-43%); conidial survival was eq ually enhanced in the shaded and UVB-protected environments. Our resul ts indicate that the poor efficacy of B. bassiana against rangeland gr asshoppers was a result of conditions of temperature and light exposur e (reduced grasshopper thermoregulation), and not the result of inadeq uate host targeting or pathogen virulence.