Influence of Beauveria bassiana strain GHA on nontarget rangeland arthropod populations

Arthropod abundance on mixed-grass rangeland was determined before and after spray treatments for grasshopper suppression. These evaluations were made in field plots (16.2 ha) treated with either Beauveria bassiana (Balsamo) Vuillemin strain GHA at 1 x 10(3) conidia in 1.89 liter oil per 0.405 ha, carbaryl at 559.88 g (AI) in 0.59 liter total volume per hectare, or no insecticide (untreated controls). Infectivity of B. bassiana was determined by capturing grasshoppers from treatment plots 7 d after application and monitoring them for external growth of the fungus. Major predators, parasitoids, and pollinators were captured with pitfall and malaise traps weekly throughout the summer season. About one-half of the grasshoppers collected from B. bassiana treatment plots exhibited external growth of the fungus. Formicidae, Araneae, and Carabidae decreased in all plots during periods of heavy precipitation after the treatment date, but these declines were not caused by mortality as a result of B. bassiana or carbaryl spray treatments. Ground-dwelling arthropod abundance rebounded to pretreatment levels approximately equal to 2 wk after treatments. No statistical differences in the abundance of aerial insects were detected with respect to treatment effects; however, natural increases in abundance in all plots were observed as the season progressed.