Diatomaceous earth has been promoted as an alternative to chemical insecticides for a variety of stored grain insects, including grain weevils. Diatomaceous earth and other inert dusts cause mortality by abrading the insect cuticle and causing a lethal loss of water. However, it is not known how this practice might affect associated parasitoids and predators in stored grain systems.
The pteromalid wasp parasitoid, Anisopteromalus calandrae, was very sensitive to direct contact with Protect-It, an inert dust formulation containing 90% diatomaceous earth, and 10% silica aerogel. At room temperature and humidity, 50% mortality occurred after 49 and 72 minutes, for male and female wasps, respectively, when exposed to 2.5 mg dust per sq. cm. Complete mortality occurred in <90 minutes for males, and 120 minutes for females.
Under the same conditions, adults of the rice weevil, Sitophilus oryzae, a host of the wasp, were much less sensitive. After 24 hours, 50% of female and 62% of male weevils died. In laboratory bioassays, winter wheat infested with rice weevil immatures dusted with label rates of Protect-It (200 and 400 ppm) reduced the longevity and reproductive potential of wasp adults, and reduced the parasitization of the immature weevils within grain. Given a choice between treated and untreated grain, wasp females showed a strong avoidance of Protect-It treated grain, and a significant preference for parasitizing weevil larvae in untreated grain. In separate tests, there was no significant difference in weevil survival to adulthood in treated and untreated grain.
Like many chemical insecticides, effective formulations of inert dusts are nondiscriminatory in their action against both pest and beneficial insects. These studies suggest that use of diatomaceous earth products would adversely affect natural biological of rice weevils by A. calandrae, and would interfere with attempts to use this wasp for augmentative releases in stored grain.
J. Perez-Mendoza, J. E. Baker, F. H. Arthur and P. W. Flinn. 1999. Effects of Protect-It on efficacy of Anisopteromalus calandrae parasitizing rice weevils in wheat. Environ. Entomol. 28:529-534.
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