Abstract The great web‐spinning pine‐sawfly A cantholyda posticalis M ats. ( H ymenoptera: P amphiliidae) is one of the major defoliating insects of pine forests. Mass outbreaks of this species have occurred in S cots pine ( P inus sylvestris L .) stands in C entral E urope and in the southern parts of E astern E urope and S iberia. R ecently, the outbreak range has shifted northward and new outbreaks have been recorded in E stonia and F inland. The dynamics of the supercooling capability of pre‐imaginal stages of A . posticalis (eonymphs, pronymphs, pupae) collected from the soil in a pine forest on the I sland of S aaremaa, E stonia, were determined. The supercooling points ( SCP ) of overwintering eonymphs and pronymphs changed during the winter, although there were no significant differences between the mean SCPs of either larval stage on any measuring date. The mean SCPs decreased from O ctober (−8 to −10 ° C ) to J anuary and M arch (−15 to −17 ° C ) and increased once again in M ay (−9 to −11 ° C ). The average SCP of pupae was considerably higher (−5.3 ° C ). Overwintering larvae in the soil are potentially covered by an insulating layer of forest litter, moss and snow, thus avoiding direct exposure to low temperatures. Cold‐hardiness in both overwintering larval stages of A . posticalis is sufficient for successful survival during winter in the northern B altic region. At present, outbreaks of A . posticalis in higher latitudes are concentrated in milder maritime regions near the coast of the B altic S ea, although these may expand in the future as a result of climate warming, with the sawfly possibly becoming a serious pest of pine forests of northern E urope.