Use of pathogen combinations to overcome the constraints of temperature on entomopathogenic hyphomycetes against grasshoppers

The influences of temperature on the mortality of grasshoppers (Melanoplus sanguinipes) inoculated with the entomopathogenic fungi, Beauveria bassiana and Metarhizium flavoviride, alone and in combination was investigated. Basking by grasshoppers had less influence on the prevalence and timing of mortality in nymphs treated with M. flavoviride than with B. bassiana. In B. bassiana-treated nymphs allowed to bask for only 1 h/day, 44% less mortality was observed, decreasing to 98% less death in nymphs basking for greater than or equal to 4 h/day, In contrast, only a slight decrease in mortality (13 to 23%) was observed in M. flavoviride-treated nymphs permitted to bask for 4 to 6 h/day. For both fungi, more mortality was observed in nymphs exposed to 35 than to 40 degrees C for various durations per day. However, exposure to high temperatures had a greater inhibitory effect on B. bassiana than M. flavoviride. The efficacy of both entomopathogens, alone and in combination, was determined in simulated cool and hot fluctuating temperature environments and at a constant 25 degrees C. The former two environments were derived from weather data on two different days at Lethbridge in July and were adjusted to simulate grasshopper thermoregulation during daylight periods (0600 to 2200 h). At 25 degrees C, there was no difference between treatments in the prevalence of mortality In the hot temperature environment, less mortality was observed for B. bassiana (3%) than for M. flavoviride (52%). Conversely, in the cool temperature environment, less mortality was observed for M. flavoviride (46%) than for B. bassiana (100%). Application of both pathogens simultaneously resulted in a final prevalence of mortality that was greater than that for M. flavoviride in the hot temperature environment and equal to that for B. bassiana in the cool temperature environment. The application of B. bassiana and M. flavoviride in combination may be a way to overcome some of the constraints of temperature on entomopathogenic Hyphomycetes against grasshoppers, especially where temperatures fluctuate or are high for a significant period of time. (C) 1997 Academic Press.