Low humidity, moderate temperature, and desiccant dust favor efficacy of Beauveria bassiana (Hyphomycetes: Moniliales) for the lesser grain borer, Rhyzopertha dominica (Coleoptera: Bruchidae)

Entomopathogenic fungi are often considered to be efficacious only with high ambient moisture. When adult female Rhyzopertha dominica were introduced onto wheat kernels with 200 mg/kg of Beauveria bassiana and/or 100 mg/kg of diatomaceous earth (DE) and incubated at 26, 30, 32.5, or 34 °C, there was significantly lower emergence of adult progeny at 43% RH than at 75% RH. On grain that was treated with both the fungus and DE, there were 95-97% reductions in adult progeny vs. controls at 43% RH and 22-91% reductions at 75% RH. On grain that was treated with fungus alone, there were 82-90% reductions in progeny adults at 43% RH and only 8-76% reductions at 75% RH. When adult beetles were exposed to B. bassiana at 30 °C and RHs of 43, 56, 75, or 85%, the mortality means were not significantly different among humidities, but when the beetles were exposed to B. bassiana from egg to adult under the same regimes, the number that survived was lower for B. bassiana-treated beetles than for control beetles at 43, 56, or 85% RH, but did not differ significantly at 75% RH. A temperature of 34 °C had a negative impact on efficacy at 75% RH, but not at 43% RH. Preincubation of conidia at 30 °C and 75% RH, but not 43% RH, for 5 days resulted in reduced mortality of adult beetles. The germination rates of B. bassiana conidia declined more rapidly at the greater RHs and when mixed with wheat than when unmixed, thereby reducing residual activity. The lower progeny adult production at 43% RH than at 75% RH, suggests that stress contributes to the greater fungus and DE effects at the lower humidity.