Technical Abstract: Greenhouse research by the USDA ARS coauthors and a University of Wyoming/USDA ARS field trial in 2002 indicated that there was potential for certain vegetable oils in an attracticidal formulation to enhance the efficacy of Beauveria bassiana Strain GHA in a strip-treatment. In 2003, results from replicated 16.2 ha plot studies in S.D., against populations of Ageneotettix deorum and Aulocara elliotti, indicated no statistically significant differences in population reductions among the Beauveria bassiana treatments with refined canola oil or Orchex 792 oil carriers, applied in a RAATS approach (50% coverage of a protected area) vs. untreated control populations, even though laboratory evaluations of post-treatment field collections showed there was sufficient grasshopper exposure to conidia to cause >80% infection and death. In 2004, results from replicated 16.2 ha plot studies conducted in S.D. against the same species, indicated there were no statistical differences in population reductions among the Beauveria bassiana treatments with raw canola oil, refined canola oil or soybean oil carriers, vs. untreated controls in a RAATS approach (50% coverage). Considerable background prevalence (>60%) of Beauveria infections obscured differences among treatment-related infection rates in post-treatment grasshopper samples. Both studies strongly suggest that simply replacing paraffinic oil with a vegetable oil may not produce enhanced mortality in unconfined field populations of grasshoppers in typical rangeland situations in S.D., whether it be in traditional broadcast or RAATs applications. Enhanced thermoregulatory behavior ('behavioral fever') among the target grasshoppers was observed to significantly interfere with field efficacy in both years. Field-collected grasshoppers unable to thermoregulate suffered >80% mortality and mycosis within 7 days, while commensurate field populations did not display similar reductions.