Toxicity of Five Insecticides to the Alfalfa Weevil, Hypera postica1 , and Its Parasites, Bathyplectes curculionis2 and Microctonus aethiopoides34

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Abstract
The LD50's of carbofuran, carbaryl, malathion, phosmet, and methoxychlor were determined by topical application in the laboratory on adult female Bathyplectes curculionis , adult female Microctonus aethiopoides , 3rd-instar larvae of Hypera postica , 3rd-instar larvae of H. postica parasitized by B. curculionis , adult overwintered H. postica and adult overwintered H. postica parasitized by M. aethiopoides . Selectivity ratios, calculated as the LD50 of the parasite divided by the LD50 of the weevil larva or adult, indicated that each of the insecticides was more toxic to the 2 parasites than to the host larvae, but, comparatively, carbofuran and carbaryl were 2–3 times safer to the parasites than malathion, phosmet, or methoxychlor. Carbofuran was selective in favor of both parasites over the adult weevils. Parasitized larvae and adults of H. postica were more susceptible to the insecticides than were their nonparasitized counterparts and most of the parasite larvae died within the susceptible hosts. Larvae were more tolerant to all the insecticides than were the adults. The usefulness of the insecticides in an integrated control program is discussed on the basis of this laboratory study.

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