Upper lethal temperatures in three cold-tolerant insects are higher in winter than in summer.

Upper lethal temperatures (ULTs) of cold-adapted insect species in winter have not been previously examined. We anticipated that as the lower lethal temperatures (LLTs) decreased (by 20-30°C) with the onset of winter, the ULTs would also decrease accordingly. Consequently, given the recent increases in winter freeze-thaw cycles and warmer winters due to climate change, it became of interest to determine whether ambient temperatures during thaws were approaching ULTs during the cold seasons. However, beetle (Coleoptera: Pyrochroidae) larvae had higher 24 and 48 h ULT (the temperature at which 50% mortality occurred) in winter than in summer. The 24 and 48 h ULT for in winter were 40.9 and 38.7°C, respectively. For in summer, the 24 and 48 h ULT were 36.7 and 36.4°C. During the transition periods of spring and autumn, the 24 h ULT was 37.3 and 38.5°C, respectively. While in winter had a 24 h LT range between LLT and ULT of 64°C, the summer range was only 41°C. Additionally, larvae of the beetle (Coleoptera: Cucujidae) and the cranefly (Diptera: Tipulidae) also had higher ULTs in winter than in summer. This unexpected phenomenon of increased temperature survivorship at both lower and higher temperatures in the winter compared with that in the summer has not been previously documented. With the decreased high temperature tolerance as the season progresses from winter to summer, it was observed that environmental temperatures are closest to upper lethal temperatures in spring.