Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/23/2003
Publication Date: 3/1/2004
Citation: Akbar, W., Lord, J.C., Nechols, J.R., Howard, R.W. 2004. Diatomaceous earth increases the efficacy of beauveria bassiana against tribolium castaneum larvae and increases conidia attachment. Journal Of Economic Entomology 97(2): 273-280. Interpretive Summary: The red flour beetle is a notoriously difficult to manage pest of grain and grain products. Our previous studies identified synergism in the combination of the desiccant dust, diatomaceous earth, and an insect pathogenic fungus for controlling several other stored-product beetles. Now we have found that similar synergism occurs with the red flour beetle. Only adult red flour beetles had been previously tested with fungi or desiccant, and we confirmed that they are highly tolerant of both. Against larvae, however, diatomaceous earth at the suggested use rate enhanced the efficacy of the fungus, Beauveria bassiana, across a wide range of doses. The presence of diatomaceous earth resulted in 17- and 16-fold decreases in the amount of fungus needed to kill 50% of the beetles at selected humidities. We identified two factors that contribute to the synergism. Spore attachment to larvae was significantly greater in the presence of diatomaceous earth than without it, and the lipids that were taken up by diatomaceous earth included phospholipids, confirming that damage to the insect cuticle makes subcuticular nutrients available to conidia for their germination. These results will help us to develop a strategy for using a combination of environmentally benign materials to control insect pests of stored-products.
Technical Abstract: Studies were conducted to test the suppressive ability of Beauveria bassiana (Balsamo) Vuillemin alone and in combination with diatomaceous earth against the red flour beetle, Tribolium castaneum (Herbst). Adults did not show a dose response to B. bassiana, and the addition of diatomaceous earth did not result in a significant increase in mortality. Against larvae, however, diatomaceous earth at 190 mg/kg of grain enhanced the efficacy of B. bassiana at all concentrations ranging from 33 to 2700 mg of conidia/kg of grain. The presence of diatomaceous earth resulted in 17- and 16-fold decreases in the median lethal concentration of B. bassiana at 56 and 75% RH, respectively. No significant differences in larval mortality in response to B. bassiana and diatomaceous earth alone or in combination were found between 56 and 75% RH. Conidial attachment to larvae was significantly greater with 190 mg/kg diatomaceous earth than without it. The partial analysis of lipids taken up by DE from the larvae revealed the removal of phospholipids and long-chain fatty acids. These results support the hypothesis that diatomaceous earth enhances the efficacy of B. bassiana against T. castaneum, at least in part, by damaging the insect cuticle, thus increasing conidial attachment and making nutrients more available to conidia for their germination.