AbstractThe cabbage maggot, Delia radicum (L.), is a serious pest of cruciferous crops in temperate regions of North America and Europe. The effects of undersowing rutabaga, Brassica napus L. subsp. rapifera Metzg. (Brassicaceae), with white clover, Trifolium repens L. (Leguminosae), on second-generation cabbage maggot and its natural enemies were studied in Newfoundland in 1997 and 1998. In 1997, totals of 1311 and 724 eggs were recovered from bare and undersown plots, respectively. More eggs were present in bare plots than undersown plots on various specific dates. In 1997, rutabagas from bare plots weighed more than those from undersown plots, although damage ratings were similar, suggesting that competition, not cabbage maggot feeding, caused the yield differences. In 1998, there were few cabbage maggots present and little damage or yield reduction in either treatment. Similar numbers of cabbage maggot pupae were extracted and reared from each treatment in each year. In 1997, of the pupae reared from undersown plots, 48% produced cabbage maggot flies, 14% produced parasitic Hymenoptera, and 8% produced Aleochara bilineata Gyllenhal (Coleoptera: Staphylinidae); 19% of the pupae from bare plots produced cabbage maggot flies, 8% produced parasitic Hymenoptera, and 36% produced A. bilineata. More A. bilineata were captured in pitfall traps in bare plots than in undersown plots. The effect of clover on carabid beetles was species specific. There were more Bembidion lampros (Herbst) and Amara bifrons (Gyllenhal) in bare plots in 1997, and more Pterostichus melanarius (Illiger) in undersown plots in both years. Despite consistently lower egg numbers in undersown plots than in bare plots, the numbers of pupae in the two treatments were similar at the end of the season. We speculate that this may be due to differential, density-dependent mortality of immature stages of cabbage maggot caused by predators and parasitoids.