Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [85]
Overwintering in the supercooled state can be a dangerous gamble, because contact with ice crystals can provide nucleation sites (places for ice to start forming) and the whole animal can then freeze solid in seconds, meaning certain death. Many insects survive the entire winter while supercooled, but in order to do so they require overwintering sites where they can avoid any contact with ice. For example, the temperature at which Alaskan green stinkbugs crystallize into ice (and die) is near -2°C when they come into contact with snow, but they remain unfrozen and alive down to near -18°C when kept dry (Barnes et al. 1996). The overwintering queens of yellowjacket wasp (Vespula vulgaris) in interior Alaska also supercool. The wasps isolate themselves from contact with ice by attaching their mandibles to the undersides of leaves or leaf litter and then hang suspended through the winter. The supercooling points of these free-hanging wasps as measured in the laboratory decreased from near -10°C at the beginning to near -16°C in late winter, whereas temperatures in their hibernacula were always higher. Thus, these freeze-intolerant insects suffer little winter mortality from freezing. Curiously, the queens of another wasp variety, the white-faced hornet (Vespula maculata), from South Bend, Indiana, are freeze-tolerant, and for overwintering they produce ice-nucleating factors in the blood that promote freezing, to prevent supercooling (Duman and Patterson 1978). Unlike those of the yellowjackets, their nests are not underground. In Maine and Vermont, where I live and work, their big gray paper nests are suspended in trees, but these are empty in winter. I’ve not yet found any of their overwintering queens.
Alaskan wasp queen hibernating by hanging to avoid ice crystals and maintain supercooling.
There are those insects that can’t avoid moisture and therefore can’t supercool. They include the grubs of long-horned beetles (Cerambycidae) that feed in live, moist wood. These “sawyers,” so-called because they leave telltale piles of sawdust wherever they burrow out of a log, chew through wood in the summer with their hard sclerotized mandibles, making a sound like someone using a cross-cut saw. (The adults, curiously, make a squeaky cry when you hold them, using a little scraper in the head-thorax junction.) Larvae that I dug out of the wood in summer and placed in -10°C conditions quickly froze into solid blocks, and they were then dead. The dead white grubs, when thawed, turned brown in a day. By winter the adults have long died, and the overwintering larvae are immobile and silent in their wood galleries. They now survive the low ambient air temperatures of winter, well below -20°C, and they now do not freeze into solid blocks because they then have antifreeze in their blood.
The logs of our cabin do not contain any sawyer grubs because when I built it I peeled the fresh logs so that they would dry quickly and become unsuitable food for them. However, the logs did eventually contain a large colony of carpenter ants. Bad news, because they are permanent residents, once ensconced. They produced huge piles of sawdust in far greater amounts than beetle grubs would have produced. I feared they would hollow out the logs and cause them to collapse. After a few years of their presence there, with no end in sight, I was, in desperation, about to hire an exterminator when, in the summer of 2001, a huge phalanx of red ants (Formica subintegra—which are normally slave-raiders of the Formica ants) came in, waged war, and within one week heaps of still-wiggling dismembered carpenter ants (Camponotus) and carpenter ant