PHYSIOLOGICAL AND CHEMICAL ECOLOGY Cold Tolerance of Four Species of Bark Beetle (Coleoptera: Scolytidae) in North America

We investigated the overwintering biology of four temperate-latitude bark beetles: Dendroctonus frontalis Zimmermann, Ips pini (Say), I. grandicollis (Eichhoff), and I. perroti Swaine. All four species were freeze-susceptible. However, there was variation within and among species in overwintering biology that related to their geographic distribution. D. frontalis and southern populations of I. grandicollis continued to reproduce and develop under the bark of their host plants throughout thewinter and did not showany seasonal adjustments in their lower lethal temperatures: mean supercooling point 6 SD 5 212.15 6 4.02 and 212.25 6 2.508C. In contrast, northern populations of I. grandicollis and I. pini employ a behavioral strategy in which adults migrate to the forest soil, where they are insulated from temperature extremes by litter and snow. Furthermore, adult supercooling points of both northern populations declined from about 2138C in summer to about 2178C in early winter. A concomitant decline in lipid content suggests that lipid metabolism may be involved in seasonal adjustments of cold tolerance in I. pini. An assortment of temperature manipulations failed to provide any evidence of cold tolerance acclimation. Immatures, which remain in the inner bark of their host trees, have lower lethal temperatures of 25 to 2128C, and are especially vulnerable tomortality from freezing. I. perroti, a northerly distributed species, had similar cold tolerance and overwinter behavior as northern populations of the other two Ips species. Winter mortality fromfreezingcouldbean importantdeterminantofpopulationdynamics inall four species. Understanding variations in cold tolerance and overwinter behavior among insects species may help predict population dynamics and distribution of potential pests.