Winter survivorship of insects is determined by a combination of physiological, behavioral, and microhabitat characteristics. We characterized the cold tolerance of the walnut twig beetle, Pityophthorus juglandis Blackman, a domestic alien invasive bark beetle that vectors a phytopathogenic fungus. The beetle and fungus cause thousand cankers disease in species of Juglans and Pterocarya. The disease is spreading in the United States of America (USA) and Italy. Contact thermocouple thermometry was used to measure the supercooling points of adults and larvae and lower lethal temperatures of adults from a population from northern California. Supercooling points ranged from -12.2 °C to -25.0 °C for adults and -13.6 °C to -23.5 °C for larvae; lower lethal temperatures of adults ranged from -14 °C to -23 °C. We found seasonal changes in adult supercooling points in fall, winter, and spring. The supercooling point for males was 0.5 °C colder than for females over all months and 1 °C colder in the winter than in other seasons. The cold-tolerance strategy shifted in P. juglandis adults from freeze intolerance (December 2013 and January 2014) to partial freeze tolerance (February 2014). An intermediate level of cold tolerance with a plastic response to cold partially explains survival of P. juglandis outside of its native range in the southwestern USA. In addition, we characterized the relationship between minimum air temperatures and minimum phloem temperatures in two Juglans spp. in northern California and Colorado and characterized portions of the native geographic range of eastern black walnut, J. nigra L., that may be too cold currently for this insect to persist.