We carried out a 2-year study to elucidate the biology of the gregarious, idiobiont ectoparasitoid Elachertus cacoeciae (Howard) by placing (implanting) laboratory-reared spruce budworm larvae [Choristoneura fumiferana (Clemens)] on current-year balsam fir (Abies balsamea L.) shoots in the field, simulating low (endemic) densities of the budworm. Spring female E. cacoeciae attacked fourth-, fifth-, and sixth-instar budworm larvae, beginning near the predicted peak of the fourth instar and ending about 10–12 days after the predicted peak of the pupal stage of the wild budworm population. The mean (±SE) brood size of spring females was 2.9 ± 0.3 E. cacoeciae pupae per host. The proportion of females increased during the season, with many broods consisting of 100% females late in the season. In 1994 and 1995, the mean proportion of females was 0.74 ± 0.05 and 0.79 ± 0.05, respectively. In the laboratory, development time from eggs to adults was approximately 20 days at 20.6 °C. Adult males provided with honey water lived 43.6 ± 3.2 days, whereas females provided with hosts and honey water lived 90.1 ± 6.6 days. Spring females had a pre-oviposition period of 11.5 ± 1.3 days, resulting in a generation time (egg to egg) of ~31 days. The oviposition period lasted 76.3 ± 7.7 days during which time spring females parasitized 19.2 ± 1.9 hosts, and produced a clutch size of 4.9 ± 0.4 eggs per host, for a lifetime fecundity of 96.8 ± 14.7 eggs. The post-oviposition period was 18.5 ± 3.7 days. Throughout their lifetime, spring females host fed only (host feeding without oviposition) on an additional 9.3 ± 1.9 hosts. Approximately 2% of pupae developing from spring females overwintered, whereas approximately 95% of pupae developing from summer females overwintered. Laboratory results for summer females suggest that they may be adapted to parasitizing alternate host(s) rather than spruce budworm.