Effects of protect-it on efficacy of Anisopteromalus calandrae (Hymenoptera: Pteromalidae) parasitizing rice weevils (Coleoptera: Curculionidae) in wheat

The parasitoid Anisopteromalus calandrae (Howard) was very sensitive to direct contact with Protect-It, an inert dust formulation containing 90% diatomaceous earth and 10% silica aerogel. LT(50)s at room temperature and humidity were 49 min (95% CL = 41-51) and 72 min (95% CL = 69-74) for males and females, respectively, in petri dishes containing 2.5 mg dust per square centimeter. Under the same conditions, adults of the rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae), a host of A. calandrae, were much less sensitive. Mortality of S. oryzae after a 24-h contact period was 50% for females and 62% for males. When tested in no-choice laboratory bioassays at 27 degrees C, label rates of Protect-It (200 and 400 ppm) dusted onto hard red winter wheat, Triticum aestivum L. (13.9% moisture content) that was infested with immature rice weevils reduced the longevity of parent A. calandrae females and significantly reduced parasitization of the weevils at 3 tested humidities, 43, 60, and 75% RH. Parasitoid progeny production was also significantly reduced, At 75% RH, 92.8 +/- 2.9 parasitoid progeny were produced in untreated wheat compared with 12.6 +/- 2.6 progeny in the treated wheat. Sex ratio of parasitoid progeny was not significantly affected by the dust treatments at any relative humidity. In two-choice tests in divided petri dish arenas, single A. calandrae females showed a strong avoidance of Protect-It-treated wheat and a significant preference for parasitizing weevils in untreated wheat. However, significantly more parasitoid progeny were produced in dishes in which one-half contained Protect-It-treated wheat and the other half contained untreated wheat compared with dishes in which both halves of the divided dishes held untreated wheat. Reasons for the Protect-It-stimulated oviposition response by A. calandrae are not known, but may be related to stress induced by the dust. In separate tests, there was no significant difference in emergence of weevils from treated or untreated wheat, regardless of the weevil age at time of dusting, or the relative humidity at which the dusted wheat was maintained. Our studies provide evidence that any natural control of pest insects exerted by local populations of parasitoids, or enhanced biological control by augmentative releases of parasitoids, would be adversely affected by the use of Protect-It or other diatomaceous earth products on stored grain.