Author
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Lord, Jeffrey |
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Submitted to: Biological Control
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 5/13/2005 Publication Date: 7/1/2005 Citation: Lord, J.C. 2005. Low humidity, moderate temperature, and desiccant dust favor efficacy of Beauveria bassiana (Hyphomycetes: Moniliales) for the lesser grain borer, Rhyzopertha dominica (Coleoptera: Bruchidae). Biological Control 34: 180-186. Interpretive Summary: Fungi that are pathogenic for insects are excellent candidates for controlling pests of stored products, but they are often considered to be efficacious only with high ambient moisture. Tests that were carried out under various conditions of temperature and moisture with a commercial insecticidal fungus (Beauveria bassiana) against a primary pest of stored grain, the lesser grain borer, showed the greatest efficacy at the lowest moisture level. At 26ºC (79ºF) and 43% RH, when the fungus was combined with diatomaceous earth, another non-toxic insect control, reductions reached 98.8% compared with untreated borers. A temperature of 34ºC (93ºF), in the upper range for stored grain in Kansas, had a negative impact on efficacy at 75% RH, but not at 43% RH. The germination rates of fungus spores declined more rapidly at the greater moistures, thereby reducing residual activity, but there are indications that the stress exerted on the insect by desiccation is a more important factor. This work helps to correct a misconception that impedes the acceptance of fungi as safe and reliable alternatives to traditional chemical insecticides. It also provides guidance for using fungal insect controls in a manner that achieves maximum benefit. Technical Abstract: 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 34ºC, 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 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 at 43% RH and only 8-76% reductions at 75% RH. When adult beetles were exposed to B. bassiana at 30ºC 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 less 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 34ºC had a negative impact on efficacy at 75% RH, but not at 43% RH. Preincubation of conidia at 30ºC and 75% RH, but not 43% RH, for five 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 fewer surviving progeny at 43% RH than at 75% RH, indicates that stress contributes to the greater fungus and DE effects at the lower humidity. |
