Abstract
Cocoons of the European pine sawfly were exposed in rodent-proof trays to determine what parasites attack this pest in southern Ontario. Exposures at 17 localities showed that two introduced parasites, Dahlbominus fuscipennis (Zetterstedt) and Pleolophus (= Aptesis) basizonus (Gravenhorst), are firmly established throughout the distribution of the host. The former parasite has spread more than 30 miles and the latter at least 70 miles in approximately 20 years. The number of trays found by each species varied considerably with locality. In general, the number of native parasitic species obtained tended to increase with age of infestation, with Mastrus aciculatus (Provancher) being the most prevalent.