Field studies have shown that increasing concentrations of the linear furanocoumarin xanthotoxin are negatively associated with survivorship and body size of adult male Copidosoma sosares, a polyembryonic parasitoid of the parsnip webworm. As specialist herbivores of Apiaceae, larval parsnip webworm larvae obligately feed on plant tissues containing high xanthotoxin concentrations. Here we examine whether xanthotoxin, incorporated into artificial diet, has sex-specific effects on male and female C. sosares development time, clutch size, body size, and sex ratio (of mixed sex broods). In addition, development time and body size of unparasitized parsnip webworms are measured to determine if xanthotoxin’s effects on C. sosares are prey-mediated. Finally, we examine whether metabolism of xanthotoxin by C. sosares reproductive larvae, precocious larvae, and embryos differs between all-male and all-female broods.