Influence of Thermal Ecology on the Mycosis of a Rangeland Grasshopper

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Abstract
The clearwinged grasshopper, Camnula pellucida, exploits incident solar radiation to raise internal body temperatures above ambient levels. Under adequate temperature and light conditions, grasshopper body temperatures may exceed air temperatures by 10°—15°C with both nymphs and adults preferring to thermoregulate near 40°C, their optimal temperature for development. Comparison of age—specific phenology data collected from field sites with projections from computer simulation studies on grasshopper development, indicated that incident solar radiation can highly accelerate maturation of grasshoppers. Also, prolonged exposure to temperatures above 35°C is beneficialto grasshoppers, as it is detrimental to the fungal pathogen Entomophaga grylli (US pathotype I), which is a major cause of natural mortality in many species. E. grylli protoplasts grew little or not at all if incubated at a constant 35°C in tissue culture medium. Studies, conducted in vivo at constant temperatures, verified that E. grylli has an upper thermal limit of survival and development near 35°C. IN nature, however, these organisms are exposed to fluctuating temperatures. Further in vitro studies revealed that protoplast survival and development were significantly affected by incubation at 40°C for as little as 2 h per day and almost totally inhibited by 40°C for 8 h each day. In vivo incubation studies also showed an increase in the incubation period and a decline in the proportion of grasshoppers dying of E. grylli mycosis with increased daily exposure to 40°C. Infected grasshoppers were also held at 25°—30°C and allowed to range freely between shaded and lighted areas, so body temperatures could be raised through normal basking activity. Compared to a control group held in diffuse light, E. grylli mycosis was almost totally eliminated from the basking grasshoppers. The few individuals treated with solar radiation that died from E. grylli infection survived significantly longer then infected individuals not exposed to the solar simulator. Computer simulations using field—collected temperature, moisture, and solar radiation data revealed that behaviorally regulated thermal effects on E. grylli survival and development could limit disease development in some locations and seasons. Although it is unclear that the behavioral trait of basking is an adaptive response to selection pressures caused either by the pathogen or by other selective factors, it is clear that thermoregulatory behavior provides survival advantages to C. pellucida. In addition to their specific biological significance, these results demonstrate that behavioral exploitation of a physical resource in the environment can be used to profoundly alter biological relationships between species in this case, allowing one organism to eliminate an antagonist by causing its mortality.

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