Overwintering eggs of Porthetria dispar increase their capacity to supercool during exposure to non-lethal conditioning temperatures. Measurements of supercooling points of eggs collected in Quebec and Massachusetts indicate that the gradual movement of the insect north and west from the initial point of introduction into North America has not been accompanied by natural selection for a more cold-hardy population. The protection afforded eggs by cold-hardening is, however, greatly enhanced by the placement of egg masses, typically at or near ground level, where they are subject to maximum insulation by snow cover.

Examination of climatic regimes within the current area of distribution of the gypsy moth in Eurasia indicates that temperature may not be a key factor limiting its northward spread. In central Canada, in the presence of abundant snow cover, the insect may spread as far north as suitable food plants exist. This would favour extension of the range of the gypsy moth throughout the Great Lakes – St. Lawrence Forest Region and much of the Boreal Forest Region of Ontario and Quebec south of James Bay.