Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)
SUPERCOOL(ED) HOUSEGUESTS (WITH AND WITHOUT ANTIFREEZE)
The miceapparently get inside through tiny holes and by chewing and pulling out the oakum fiber plugs between logs. In this they may be aided by the stronger red and flying squirrels, which also pull out the oakum for use in their nests. Whether independently or working together the mammals ultimately provide the main means of entrance for a crowd of insects.
The crowd is always snug inside our Maine cabin, come winter. It consists mostly of cluster flies. According to Harold Oldroyd, who wrote the fly bible, the 1964 Natural History of Flies, there are several species of these robust members of the genus Pollenia. Most of them are several times the size of the more familiar housefly. Pollenia are calliphorid, or “flesh” flies. The larvae of North American native species eat the flesh of dead animals, but the most common flies in the cabin, Pollenia rudis, were introduced from Europe and their larvae parasitize earthworms; they eat them alive from the inside out. Big and bristly, these flies are not handsome, like the shiny metallic green-and-blue native Pollenia, which never enter the cabin. Already in the fall the Pollenia rudis perch in crowds on the logs outside the cabin and sun themselves. When it starts to cool, they slip through the cracks. By November most have made their way inside, but at that time they remain unobtrusive unless I build a roaring fire in the woodstove. Then within minutes they come poking out of the cracks and crevices, and if it is still daylight hundreds or thousands gather in buzzing masses at each of the eight windows making a collective hiss. They apparently perceive the warmth as the return of spring and take it as their signal to try to leave by flying directly to the light at the windows. If given a chance to exit, at least they do not try to stay on, unlike some other previously mentioned guests. Even on the coldest days they rush out instantly when I open the windows wide but they can fly only a short distance before the cold grips them and they plummet immobile to the snow. I’ve captured hundreds of the flies at my windows, painted them with dabs of red paint, and then released them outside to see if they return. When I used my Dustbuster a few days later to vacuum them up by the cupfuls at the windows, I got mostly new ones but there were some returnees.
I’ve become curious about their overwintering adaptations because I haven’t found any of them in the wild. Temperatures can dip to -30°C outside (and the cabin is unheated for most of the winter). Some of these flies that I’ve brought into my lab in Vermont and subjected to -20°C were frozen solid and dead in minutes. But at -10°C, still a bitterly cold temperature, others did not freeze and survived; within seconds of being warmed they crawled about and again flew as vigorously as minutes before. I suspect they might survive by supercooling, and the very dry environment in the cabin is ideal for supercooling.
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 parts were strewn inside and outside the cabin. The raiders totally eradicated the carpenter ant colony, and then went back to their huge nest in the nearby field.
The carpenter ants are able to use the dry wood, because unlike the stationary beetle larvae, they are highly mobile and bring water into their nests to moisten the wood, if need be. I’ve found them living in still-upright balsam fir trees in winter and, since ice crystals are mixed in with the masses of comatose ants, they need some other strategy besides supercooling to survive winter.
Both the ants and the beetle larvae spending the winter inside tree trunks endure temperatures close to those of the ambient air. I’ve brought both into the house and warmed them up, but unlike flies, which spring to life almost immediately when warmed, the ants and beetle larvae seem stone dead, even after being warmed. Only after a few days at room temperature do they gradually show movement, eventually resuming full activity. But when I have taken these revived ants and beetle larvae and stuck them out in the cold from whence they came, they died quickly. Clearly their survival is dependent on their antifreeze-induced torpor. The ants and beetle grubs, when cold-adapted, contain large amounts of glycerol or other sweet-tasting antifreeze (I have not tasted the flies), which prevents ice crystals from forming in their bodies and probably pickles them into inactivity. It takes a long time at elevated temperatures to get that antifreeze out of the blood.
The other winter cabin guests—I’m thinking of three species in particular—are beautiful and more benign in habit. When the cabin is heated, for example, they don’t have the annoying Pollenia’s tendency to hover around the bed light and then, when it’s turned off, to dash under the covers and buzz there rudely.
The first of these three species, the mourning cloak butterflies (Nymphalis antiopa), usually remain in crevices outside and only rarely make it into the cabin. In the fall I commonly see one or two fluttering under the cabin roof. The second species, the multicolored Asian ladybugs (or ladybird beetles), has arrived here in numbers only in the past few years. These occupy the cabin by the thousands in some years, and by only dozens in others. First imported from eastern Asia by the U.S. Department of Agriculture in the late 1980s as a biological control on pecan aphids, this species (Harmonia axyridis) was later introduced to other areas, including the Northeast, to control similar pests. It has since spread far and wide on its own, to most areas of the United States and to parts of Canada. Like many species of ladybirds (there may be some four hundred ladybug species in North America and three thousand or so worldwide), these beetles all have handsome black coloration. Those ladybugs endemic to North America species have specific color patterns, but in the Asian species, virtually every individual is uniquely colored. Each bug’s background hue can be deep red, or orange, or yellow, and it may have no spots, tiny black dots, or spots that coalesce into black bands. (Another species that very rarely overwinter in the cabin is uniformly coal black with one red spot on each wing cover.) The colors serve as warning to stay clear; multicolored Asian ladybugs secrete a foul-smelling fluid when crushed. But even certified bug-haters find it hard to want to kill ladybird beetles. Besides being prettified in handsome colors, they have soft rounded curves, little legs, and petite feet. They are about as cute as any bug on the planet. Adorable, even. But cuddly, they are not. We found that out after they moved into our Vermont home in the fall by the tens of thousands (literally), and then stayed for over six months—despite all our best efforts to try to evict them.
A sample of multicolored Asian ladybird beetles from my window.
Our family made a most intimate acquaintance with these beasts during the winter of 2001-2002. A few pesky forerunners had appeared in our Vermont home in previous years, when they were an almost welcome diversion from the cluster flies. But that winter, the cluster flies paradoxically were almost absent, while the beetles staged an invasion. I don’t know how, or why. I tested one batch in the freezer compartment (at -14°C) of the refrigerator, where they readily (well, 61 of 148 subjects) survived.
Spiders are fortunately not alive in my winter cabin. Here is “Charlotte” dead and shriveled on a beam, next to her eggs, which do survive.
I had over the winter opened the bedroom window on numerous occasions to then brush them off the panes and the walls and throw them out by the hundreds. Still there were always plenty left at night congregating on the reading lamp. After we shut it off, the bugs then settled into our beds. Being dehydrated by the dry indoors, they not infrequently tried to crawl into our eyes at night, looking to suck up some of our moisture or to nip at our skin. By Easter, they started to come out of hibernation in force. We then had an infestation by the thousands, and as a last resort my wife Rachel tried to vacuum them up with a Dustbuster. Like riled-up skunks, they released their toxic chemical defensive secretions. Rachel is a biologist and the paragon of tolerance toward creatures. She is enthusiastic when the kids bring in spiders, earthworms, slugs, millipedes, sow bugs, and centipedes. But by April 2001, after another go at them with the Dustbuster, she declared Harmonia axyridis “disgusting.” I didn’t argue.
My grudging tolerance for the beetles stems mostly from their predatory habits. In addition to pecan aphids, the multicolored Asian ladybugs and their larvae feed on plant-sucking insects such as the woolly adelgid, which is decimating hemlock stands from Virginia to New England. Over its lifetime, a single multicolored Asian ladybug can devour an estimated 600 to 1, 200 aphids. In the 1960s the adelgid, too, was introduced from Asia (although it arrived inadvertently, probably on a nursery plant). Twenty years later it had become a serious problem. A hemlock tree attacked by adelgids invariably dies. So far, I have not had a problem with adelgids on my hemlocks.
The third insect species that is a regular if not abundant visitor is the green lacewing (family Chrysopidae). The light, bright green of the lacewing extends to its four large wings, delicate membranes stretched between a network of veins. Lacewings have a certain aura. But this is lost on aphids. Lacewing adults as well as their larvae, commonly called aphid lions, are ferocious predators like their relatives the antlions, whose larvae build deadly sand traps that capture ants. In contrast to all of the other house and cabin winter crowd, I seldom find more than a half dozen, but I commonly see them (unlike any of the other cabin occupants) under loose, dry bark of trees in the winter woods. They are rare enough guests in the cabin to be a treat.
Some deliberately bring houseguests into their dwellings for the winter. Our son Eliot and I found some tiny pale yellow ants overwintering in nest chambers under stones. In these chambers, attached to roots and rocks, we saw chalk-white blobs that, on close inspection, revealed themselves as aphids. Of course these aphids, close relatives of the dreaded adelgids, are there because they make themselves useful (to ants). They secrete sweet honeydew in the summer when the ants put them back out to pasture. And when they have finished milking them of honeydew in the fall, they tuck them safely back underground for winter storage into their chambers where we found them. They perch there all winter without, most likely, ever moving and being a nuisance.
The diversity and abundance of winter wildlife in my cabin is not necessarily enviable. Anyone can be similarly blessed. By allowing a few openings, I now play host to the good, the bad, the beautiful, and sometimes to the useful.