Phytoremediation of selenium-contaminated soils may be influenced by higher trophic levels including insects. We examined how selenium affects the behavior, survival, and development of the wasp parasitoid Cotesia marginiventris, parasitizing its natural host, the beet armyworm Spodoptera exigua, feeding on alfalfa, Medicago sativa, irrigated with water containing selenate. X-ray absorption spectroscopy was used to quantify the selenium chemical forms in each trophic level. Alfalfa partially transformed selenate to organoselenium. S. exigua contained only organoselenium, both directly absorbed from M. sativa and transformed from selenate. C. marginiventris cocoons collected shortly after larval emergence contained only organoselenium derived from the host. The surprising finding of trimethylselenonium-like species in adult parasitoids and the cocoons from which they emerged suggests that adults and pharates can detoxify excess selenium through methylation and volatilization. Adult parasitoids do not discriminate against selenium-containing alfalfa, even though alfalfa generates selenium volatiles. Parasitoids raised on selenium-fed larvae emerged later and pupae weighed less than their selenium-free counterparts. We conclude therefore that C. marginiventris can be used to control S. exigua damage to M. sativa being used to remove selenium from soils. Moreover, the presence of such insects may improve phytoremediation by increasing biotransformation of inorganic selenium and release of volatile selenium species. Biochemistry; Nutrition; Diet; Life cycle and development; Behaviour; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Population dynamics Spodoptera exigua (Noctuidae); Inorganic substances; Selenium; Food plants; Medicago sativa; Hymenopteran parasites; Cotesia marginiventris; Effects on parasitoid development, behaviour & survival, phytoremediation implications; Trophic structure; Trophic transfer Cotesia marginiventris (Braconidae); Inorganic substances; Selenium; Development; Developmental rate; Behaviour; Lepidopteran hosts; Spodoptera exigua; Life history responses to selenium in parasitoids & implications for phytoremediation; Trophic structure; Trophic transfer; Survival; Survival rate none