A major outbreak of the hemlock looper, Lambdina fiscellaria (Guenée), predicted for 1997 on the Gaspé Peninsula in Quebec, collapsed due to egg parasitism by Telenomus spp. Only 3,857 ha of balsam–spruce forest were defoliated from the forecasted 130,000-ha outbreak area. This represents only 3% of the anticipated outbreak area. We present data that indicate highly efficient natural control by Telenomus spp. The L. fiscellaria population also collapsed due to egg parasitism by Telenomus spp. on Anticosti Island in the same year. In the past, L. fiscellaria egg parasitism has usually been estimated from egg samples collected in the fall or early spring surveys and was based mostly on the eggs’ black coloration. However, our observations indicate that entirely black and opaque eggs result from parasitism by Trichogramma spp., those attacked by Telenomus spp. varying from translucent to dark brown but always showing a single dark spot on the chorion. Moreover, our data on seasonal egg parasitism showed important and rapid increases in parasitism by Telenomus spp. in late spring. Therefore, similar unexplained L. fiscellaria outbreak collapses that occurred in the past might have also been caused by egg parasitism by Telenomus spp. There are several species in the Telenomus complex attacking L. fiscellaria and the most abundant one might be a “keystone species” in the population dynamics of this defoliator. The spring activity of Telenomus spp. may lead to errors when forecasting L. fiscellaria populations based only on fall egg surveys.