Overwintering in yellowjacket queens (Vespula vulgaris) and green stinkbugs (Elasmostethus interstinctus) in subarctic Alaska

Insects overwinter in subfreezing environments by seeking thermal refugia and using physiological strategies of freeze tolerance or freeze resistance. We describe hibernacula locations, thermal conditions, and freeze resistance by supercooling for yellowjacket queens, Vespula vulgaris, and adult green stinkbugs, Elasmostethus interstinctus, overwintering in interior Alaska. Insects were located in leaf litter in open forest understory during mid-September. Yellowjackets were found hanging by their mandibles underneath leaves, while stinkbugs were found scattered in the duff. Both V. vulgaris and E. interstinctus were freeze intolerant killed by slow cooling to an exotherm, when tested after acclimation to 0 degrees C or after natural winter acclimatization. Hibernating insects collected in early fall supercooled to between -9 degrees and -10 degrees C; crystallization temperatures (supercooling points) decreased to -16.4 degrees +/- 1.7 degrees C for V. vulgaris and -16.9 degrees +/- 0.7 degrees C for E. interstinctus collected from the litter in spring. Two weeks of treatment at 0 degrees C infall was sufficient to lower crystallization temperatures in V. vulgaris to spring levels When cooled while in contact with snow insect supercooling capacity was substantially compromised, which suggests that dry conditions, posture, and position of queens are important for winter survival. Insect hibernacula were insulated from winter air temperatures (averaging - 19.4 degrees C, with extremes of -40 degrees C) by the overlaying duff and 60 cm of snow. Thermal conditions measured next to overwintering V. vulgaris averaged -6.5 degrees +/- 1.8 degrees C and cooled no further than -133 degrees C, which suggests that winter mortality due to freezing of supercooled insects is rare.