Two introduced hymenopterous parasites of the European pine shoot moth, Rhyacionia buoliana (Schiff.), are established and widely distributed in southern Ontario: the braconid Orgilus obscurator (Nees) and the ichneumonid Temelucha interruptor (Grav.). Though these two species are economically important in the control of the shoot moth in Europe, they have never become efficient in Canada. A possible cause of their lack of efficiency may be their susceptibility to low temperatures.

This paper is a report on an investigation of the ability of these parasites to withstand low winter temperatures. Laboratory experiments were conducted to determine the resistance of the overwintering larval stages of the parasites to low temperatures. Field observations on the mortality of host and parasites were made at the Waterloo County Reforestation Area, Elmira, near Guelph, Ontario, during the winter of 1958–1959.