Three months ago, our fields and forests buzzed and chirped with six-legged life: cicadas overhead, crickets underfoot. Now snow has blanketed the landscape, and they and most other insects are gone and won’t reappear until spring.
How do they survive this weather? What keeps them from freezing to death?
The risk of freezing is, in fact, a greater challenge for insects than for many other creatures in the natural world, and that’s because of their size. The smaller an animal, the more surface area it has relative to its mass and, consequently, the faster it loses heat from that surface/air interface. Even an especially robust bumblebee has much more proportional surface area than, say, an elephant.
The main problem posed by freezing is this: When water freezes into crystals, it expands. If this happens inside living cells, the crystals can squeeze against cell membranes and shred them, destroying tissues from the inside out.
In cold weather, most insects enter a state of diapause similar to – but physiologically distinct from – mammalian hibernation. In diapause they use little or no energy. However, insects, while in that state, still must avoid death from freezing.
Finding shelter is the answer for some. Ladybird beetles, cluster flies, and green lacewings sneak into our warm homes through cracks in walls. Females of the parasitic ichneumon wasps burrow into tree-stumps or clumps of moss to avoid harsh wintry blasts. A bumblebee queen will find refuge in a rotting stump, where she survives long after all the other members of her hive have succumbed to the cold.
Some insects generate high concentrations of antifreeze chemicals, collectively known as cryoprotectants, which prevent tissue from freezing or which limit the amount of damage caused by freezing. One cryoprotectant, glycerol, prevents ice crystals from forming inside the cells of certain insects. The grubs of longhorn beetles are an example. They survive by pumping themselves full of glycerol and then entombing themselves in deadwood.
Other species or the eggs of species, such as those of mantises, will freeze but only after producing other chemicals that will pull moisture from their cells, so that when ice crystals do form, they form in the spaces between cells. This mitigates damage.
Lepidoptera—butterflies and moths—are a diverse group that uses varieties of strategies to survive winter. The wooly bear, the tiger moth caterpillar, winterizes itself and crawls beneath leaf litter or between logs in your woodpile. The eastern tent caterpillar moth lays glyercol-laden eggs on cherry or apple twigs, where they remain throughout winter. The big, boldly-colored cecropia moth overwinters in a cozy cocoon of silk and wrapped-around leaves. Some lepidoptera, such as mourning cloak butterfiles, survive as adults, overwintering between shaggy tree bark or in hollowed-out trees.
One type of Lepidoptera, monarch butterflies, has it all figured out. They just go south. Monarchs in our region fly 3,000 miles to the volcanic mountains of eastern Michoacan in central Mexico each fall, where they join tens of millions of other monarchs from east of the Rockies. Monarchs live only several months, so the returnees to New Hampshire or Vermont belong to a second or third generation.
A few insects maintain a fairly active life throughout winter. A group of noctuid moths maintains body temperature by shivering. They can raise their body temperature by upwards of 50 degrees F. Be on the lookout for them flying around any time the temperature reaches 32 degrees or higher. For perspective, consider this: Temperature fluctuations of even about 5 degrees pose danger for humans.
One of the more elaborate survival strategies is employed by the goldenrod fly. Its life cycle is perfectly atuned to that of their namesake plant. In spring, adults lay eggs on the plant’s fresh green shoots. Chemicals, either delivered with the egg or produced by the larva, co-opt the plant’s growth, and cause the formation of a tumorlike sphere, called a gall, around the baby fly. The larva chews an escape tunnel in the fall but doesn’t use it; instead, it retreats to the protective center of the gall, and awaits spring and its final metamorphosis. It’s easy to collect the growths from any stand of goldenrod in the fall; dissecting them is a staple of field biology classes.
Most insects remain out of sight in winter, but on mild days, you may see ladybugs that have crept from the woodwork to soak up the sun, or on a walk with snowshoes in the woods, you may see snow drifts peppered with tiny snow fleas.
For snow fleas, the snow is not something to flee, rather it is a welcome banquet table at which to dine on such treats as pollen and algal cells. They eat while other insects sleep.
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