Gd. Inglis et al., FIELD AND LABORATORY EVALUATION OF 2 CONIDIAL BATCHES OF BEAUVERIA-BASSIANA (BALSAMO) VUILLEMIN AGAINST GRASSHOPPERS, Canadian Entomologist, 129(1), 1997, pp. 171-186
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.