Time, temperature, and population affect the seasonal cold-hardening response of the adult stage of the mountain pine beetle (Coleoptera: Curculionidae)

Cold winter temperatures affect the distribution and abundance of the mountain pine beetle, Dendroctonus ponderosae Hopkins, an eruptive tree-killing bark beetle native to pine forests of western North America. The cold-tolerant larval stage often overwinters but adults also enter winter in some regions or years yet little is known about their ability to survive winter. Here we report on experiments to assess the cold tolerance of mature, brood adults. Adult mountain pine beetles exhibited a strong cold-hardening response over a period of weeks to months that intensified at colder temperatures; however, adult beetles from Cranbrook, which experiences cold winters, were more cold hardy than adults from Victoria, which has mild winters. These results are consistent with a seasonal cold-hardening response that is affected by both environmental factors and local adaptation to climate. The lowest mean and minimum lethal temperatures recorded for preconditioned adults during the study were ‒26.8 and ‒32.2 °C, respectively. Preconditioned adults from Cranbrook and Victoria challenged with prolonged exposure to a cold temperature (‒18 °C) suffered negligible mortality after 8 d; almost half of the Victoria adults and three-quarters of the Cranbrook population were still alive after 28 d. Adults had a limited ability to rapidly cold harden but possessed substantial basal cold tolerance. Our results on the cold tolerance of the adult stage of the mountain pine beetle are expected to contribute to models of forest disturbances that incorporate the population dynamics and climatic suitability of forests for tree-killing bark beetles.