Cold tolerance of four species of bark beetle (Coleoptera : Scolytidae) inNorth America

We investigated the overwintering biology of four temperate-latitude bark b eetles: 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 bi ology that related to their geographic distribution. D. frontalis and south ern populations of I. grandicollis continued to reproduce and develop under the bark of their host plants throughout the winter and did not show any s easonal adjustments in their lower lethal temperatures: mean supercooling p oint +/- SD = -12.15 +/- 4.02 and -12.25 + 2.50 degrees C. In contrast, nor thern populations of I. grandicollis and I. pini employ a behavioral strate gy in which adults migrate to the forest soil, where they are insulated fro m temperature extremes by litter and snow. Furthermore, adult supercooling points of both northern populations declined from about -13 degrees C in su mmer to about -17 degrees C in early winter. A concomitant decline in lipid content suggests that lipid metabolism may be involved in seasonal adjustm ents of cold tolerance in I. pini. An assortment of temperature manipulatio ns failed to provide any evidence of cold tolerance acclimation. Immatures, which remain in the inner bark of their host trees, have lower lethal temp eratures of -5 to -12 degrees C, and are especially vulnerable to mortality from freezing. I. perroti, a northerly distributed species, had similar co ld tolerance and overwinter behavior as northern populations of the other t wo Ips species. Winter mortality from freezing could be an important determ inant of population dynamics in all four species. Understanding variations in cold tolerance and overwinter behavior among insects species may help pr edict population dynamics and distribution of potential pests.