1. Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, P.O. Box 3800, Sainte-Foy (Québec), Canada G1V 4C7.Corresponding author e-mail:
2. Research Institute for Plant Protection, P.O. Box 9060, 6700 GW Wageningen, The Netherlands; present address: Plant Protection Service, P.O. Box 9102, 6700 HC Wageningen, The Netherlands.
3. Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, P.O. Box 3800, Sainte-Foy (Québec), Canada G1V 4C7.