Five chemical insecticides, methomyl (Lannate), phosmet (Imidan), carbaryl (Sevin-4-oil), permethrin (Pounce), sulprophos (Bolstar), each applied at 3 dosages, were field tested in Idaho and Montana (USA) for efficacy against the western spruce budworm, C. occidentalis (Freeman) (a defoliator of Douglas fir, Pseudotsuga menziesii (Mirb.) Franco, and grand fir, Abies grandis (Dougl.) Lindl.). As a companion study, parasite populations were monitored for the immediate effect of the insecticides on apparent parasitism. Reductions of C. occidentalis populations in treated plots ranged from 39.4% (Bolstar, 29.96 g/l) to 94.1% (Pounce, 11.98 g/l). Twenty-one species of parasites (Hymenoptera and Diptera) were reared from C. occidentalis collected in Montana and 16 spp. in Idaho; species recovered are listed. Unlike earlier reports, significant increases in apparent parasitism occurred in only 2 instances, i.e., after the application of Lannate and Pounce, and involved the braconid, Apanteles fumiferanae Viereck.