Field and laboratory evaluation of two conidial batches of Beauveria bassiana (Balsamo) Vuillemin against grasshoppers.
The efficacy of two production batches of conidia of Beauveria bassiana that showed differential field efficacy in 1992 (GHA 92) and 1994 (GHA 94) were compared against Melanoplus sanguinipes in the laboratory and field (in Alberta). 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 M .sanguinipes in the laboratory, GHA 92 was slightly more virulent than GHA 94. Conidia were applied to field populations of M. sanguinipes in a 1.5% emulsifiable oil emulsion amended with 4% clay at a volume of 112 litres ha-1. 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 (2.4-4.1 × 104 cfu cm-2). All nymphs of M. sanguinipes collected from plots sprayed with conidia of GHA 92 and GHA 94 were equally infested with B. bassiana; conidial populations averaged 3.5-4.3 × 103 cfu/nymph. Conditions were hot, dry, and sunny, and regardless of the batch, the persistence of conidia was equally short on both leaves and M. sanguinipes. Neither treatment of B. bassiana significantly reduced field populations nor did either impact differentially on specific grasshopper [Orthoptera] taxa. However, among other species of grasshopper 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 indicated that environmental conditions in the field, and not pathogen virulence or targeting, were responsible for the poor efficacy of B. bassiana against M. sanguinipes.