The Pine Processionally Moth, Thaumetopoea pityocampa, has historically been restricted to regions of Europe with a Mediterranean climate. The caterpillars are active in winter. They have adaptations that prevent freezing above -7 degrees C. (Water with no salts or antifreeze will freeze at 0 C.) Between -17 C and -7 C, the caterpillars freeze solid, but are not killed. Their bodies can freeze and unfreeze depending on temperature. Temperatures below -17 C are lethal.
Cold tolerance allows the caterpillars to feed and develop throughout most of the winter. During brief cold snaps the caterpillars can survive freezing and unfreezing. Regions where winter temperatures consistently go below -17 C exclude Thaumetopoea pityocampa. In recent decades, the range of Thaumetopoea pityocampa has extended northward*. This is due to climate change and winters that no longer have temperatures below -17 C in the regions of expansion. As climate change continues, the range is predicted to expand further to the north.
*Andrea Battisti Stig Larsson and Alain Roques. Processionary Moths and Associated Urtication Risk: Global Change–Driven Effects. Annu. Rev. Entomol. 2017. 62:323–42