Selection for increased cold tolerance in Epiphyas postvittana (Lepidoptera: Tortricidae) late instars

Adaptation to an invaded environment is considered crucial to the success of biological invasions by exotic insects. For temperate areas of North America, cold temperatures prevent invasions by many exotic species. Consequently, the effects of cold on populations of invading insects are often included in pest risk assessments. Current assessments of risk typically assume a species' response to cold is immutable over short to medium time horizons.  In this study, we investigated how short-term cold exposures may impact the cold tolerance of an invasive light brown apple moth (Epiphyas postvittana) population over multiple generations. Late instars of E. postvittana exhibit partial freeze tolerance, such that a small proportion can survive exposure to their supercooling point and develop into reproductive adults. Therefore, we used freezing as a selection pressure; late instars were briefly exposed to their individual supercooling points, survivors were randomly mated as adults, and their progeny were supercooled as late instars. Selection continued in this way for over 20 generations.  While we did not see a consistent response to selection through time, the degree of partial freeze tolerance was more variable than previously observed and cycled in an apparent pattern every six generations, driven by a yet-unknown mechanism. The variability in cold tolerance observed in our study was beyond what typical laboratory studies would capture. Adequately characterizing the variability in cold response may be just as important as knowing the rate of adaptation to characterize future risks posed by invasive insects.