Ecology of Mountain Pine Beetle (Coleoptera: Scolytidae) Cold Hardening in the Intermountain West

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Abstract
The mountain pine beetle, Dendroctonus ponderosae Hopkins, spends the majority of its life cycle within the phloem of pine trees, experiencing exposure to temperatures below −30°C in many parts of their expansive range. To better understand cold tolerance capabilities of this insect, seasonal patterns of cold-hardiness, as measured by supercooling points in the laboratory, were compared with seasonal patterns of host tree phloem temperatures at several geographic sites for 2 beetle generations. Larvae were found to be intolerant of tissue freezing, and supercooling points measured appear to be a reasonable estimate of the lower limit for survival. Of the compounds analyzed, glycerol was found to be the major cryoprotectant. No differences in supercooling points were found among instars or between larvae collected from the north and south aspect of tree boles. Both phloem temperatures and supercooling points of larvae collected from within the phloem were found to be different among the geographic sites sampled. Mountain pine beetle larvae appear to respond to seasonal and yearly fluctuations in microhabitat temperatures by adjusting levels of cold hardening.