The toxicities of residues of carbofuran (Furadan®), methyl parathion, methoxychlor, and methidathion (Supracide®) were evaluated in the laboratory againstMicroctonus aethiopoidesLoan, an alfalfa weevil parasite. Adult parasites were exposed at different time intervals to alfalfa plants previously treated with insecticides. Applied dosages approximated field application rates. Carbofuran was the most toxic insecticide through day 7 but induced only 10% mortality by the 10th day. Methidathion was highly toxic to the parasites 2 days postspray and almost totally dissipated after 1 wk. Methoxychlor residue was the least toxic initially but most persistent, inducing ca. 10% mortality 2 wk later. Methyl parathion residues, although causing 100% mortality immediately postspray, declined sharply 24 h later and caused 0% mortality by day 5.