AbstractThe efficacy of two production batches of conidia of Beauveria bassiana (Balsamo) Vuillemin that showed differential field efficacy in 1992 (GHA 92) and 1994 (GHA 94) were compared against grasshoppers in the laboratory and field. Conidia of GHA 92 and GHA 94 exhibited good germination (> 92%) by 24 h, but the rate of germination was slower for GHA 94 than for GHA 92. Although both conidial batches were highly virulent (LD50 < 6 × 103 conidia per nymph) against nymphs of Melanoplus sanguinipes (Fabricius) (Orthoptera: Acrididae) in the laboratory, GHA 92 was slightly more virulent than GHA 94. Conidia were applied to field populations of grasshoppers in a 1.5% emulsifiable oil-emulsion amended with 4% clay at a volume of 112 L/ha. There were no differences between GHA 92 and GHA 94 in the deposition of spray droplets on water-sensitive papers or of conidia on leaves and coverslips (2.4 × 10 to 4.1 × 10 cfu/cm2). All grasshopper nymphs collected from plots sprayed with conidia of GHA 92 and GHA 94 were equally infested with B. bassiana; conidial populations averaged 3.5 × 103 to 4.3 × 103 cfu/nymph. Conditions were hot, dry, and sunny, and regardless of the batch, persistence of conidia was equally short on both leaves and grasshoppers. Neither treatment of B. bassiana significantly reduced field populations nor did either impact differentially on specific grasshopper taxa. However, among grasshoppers collected immediately after conidial application and maintained in cages in the greenhouse, over 80% died of infection with B. bassiana. For both conidial treatments, the prevalence of disease in caged grasshoppers decreased with the sampling date but the onset of mycosis always occurred 3–4 days after collection. This study indicates that environmental conditions in the field and not pathogen virulence or targeting were responsible for the poor efficacy of B. bassiana against grasshoppers.