The basic biology, seasonal ecology, and hymenopteran parasitoid community of Plutella armoraciae Busck, 1912 (Lepidoptera: Plutellidae) was investigated. The study area was a dry grasslands region of interior British Columbia, Canada, where P. armoraciae feeds on the introduced mustard Sisymbrium loeselii L., 1755 (Brassicaceae) in partial sympatry with its congener, the diamondback moth Plutella xylostella (L., 1758) (Lepidoptera: Plutellidae). Sampling was conducted over five years, including one year of repeated, intensive sampling over a large region. The two Plutella species can be differentiated with morphology alone and we provide comparisons of features of the eggs, larvae, pupae and adults of each species. Plutella armoraciae was found to overwinter as eggs or early-instar larvae in S. loeselii rosettes, with adults emerging in April and May, and the first full generation each year taking place in May and June. Larvae fed within webbed flowers and buds of S. loeselii plants, giving rise to a second autumn generation of adults that laid eggs of the overwintering generation. The early summer generation of P. armoraciae larvae peaked earlier than, but partially overlapped with, the peak occurrence of P. xylostella larvae. Both Plutella species were relatively common across all surveyed sites, but not highly abundant at any site. At least two species of larval parasitoids, Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) and Diolcogaster claritibia (Papp) (Hymenoptera: Braconidae), were shared between the two Plutella species, and a community of eight other species of parasitoids and hyperparasitoids were reared from the pupae of one or both Plutella species. The observations of D. claritibia emerging from P. armoraciae are the first records of this parasitoid from a host species other than P. xylostella. It was also discovered that one of the most important parasitoids of P. xylostella worldwide, D. insulare, appears to use P. armoraciae as an overwintering host. We hypothesize that in areas where P. xylostella is a seasonal migrant, year-round resident Plutella species such as P. armoraciae could provide important overwintering hosts and reservoirs for shared parasitoids that help to reduce populations of P. xylostella.