Desiccant dusts synergize the effect of Beauveria bassiana (Hyphomycetes: Moniliales) on stored-grain beetles

Diatomaceous earth (DE) is a desiccant insecticide and most efficacious in low humidity. It acts on insect cuticle by absorbing lipids, and perhaps by cuticular abrasion. Beauveria bassiana (Balsamo) Vuillemin, an entomopathogenic fungus, is most efficacious in high humidity and has a complex interaction with cuticular lipids. Interaction between these materials may enhance insect control performance. Assays with stored-grain beetles were conducted with B. bassiana at rates of 11, 33, 100, and 300 mg of conidia per kilogram of grain with and without single rates of DE that killed 10% or less of the target beetles. The assays revealed synergism in effects on adult Rhyzopertha dominica (F.) and Oryzaephilus surinamensis (L.) at all doses. There was statistically significant synergism for adult Cryptolestes ferrugineus (Stephens) and larval R. dominica but at only one B. bassiana rate for each target. Both amorphous silicon dioxide, a sorptive dust, and diamond dust, all abrasive, showed synergistic interaction with B. bassiana on adult R. dominica. These results may provide a basis for a least-toxic approach to control of stored-product beetles and for efficacy enhancing formulation of entomopathogenic fungi.