FIELD AND LABORATORY EVALUATION OF 2 CONIDIAL BATCHES OF BEAUVERIA-BASSIANA (BALSAMO) VUILLEMIN AGAINST GRASSHOPPERS

The efficacy of two production batches of conidia of Beauveria bassian a (Balsamo) Vuillemin that showed differential field efficacy in 1992 (GHA 92) and 1994 (GHA 94) were compared against grasshoppers in the l aboratory and field. Conidia of GHA 92 and GHA 94 exhibited good germi nation (>92%) by 24 h, but the rate of germination was slower for GHA 94 than for GHA 92. Although both conidial batches were highly virulen t (LD(50) < 6 x 10(3) 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 fi eld populations of grasshoppers in a 1.5% emulsifiable oil-emulsion am ended 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 x 10(4) to 4.1 x 10(4) cfu/cm(2)). 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 x 10(3) to 4.3 x 10(3) cfu/nymph. Conditions were hot, dry, and sunny, and regardless of the batch, persistence of conidia was equally short on both leaves and gra sshoppers. Neither treatment of B. bassiana significantly reduced fiel d populations nor did either impact differentially on specific grassho pper taxa. However, among grasshoppers collected immediately after con idial 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 samplin g date but the onset of mycosis always occurred 3-4 days after collect ion, This study indicates that environmental conditions in the field a nd not pathogen virulence or targeting were responsible for the poor e fficacy of B. bassiana against grasshoppers.