By Caleb O’Brien
COLUMBIA, Mo. — Imagine it’s winter. Imagine you’re outside, without central heating, without clothes, without matches. Now imagine you’re the size of an insect larva.
Every year, insects in colder climes face the challenge of surviving winter’s freezing temperatures. They deploy both behavioral and biochemical strategies to better their chances of survival.
These innovative approaches showcase the wonder and diversity of the insect world. Let’s take a closer look at each approach.
Behavioral strategies: Mind over matter
Perhaps the simplest way to avoid freezing to death is to avoid the cold. Like some birds and many retirees from the Northeast, some insects undertake seasonal long-distance migrations. The monarch butterfly’s annual trek from Canada to California or Mexico and back is a particularly famous example; they are known travel up to 3,000 miles in an annual pilgrimage occupying the lives of a entire generation of the species. Other insects — such as some species of dragonfly — migrate as well.
There are also ways to avoid the cold while staying closer to home. For example, some insects seek haven from freezing temperatures by hibernating in protected shelters called hibernacula. Rob Lawrence, forest entomologist for the Missouri Department of Conservation, says insects may tunnel into plant tissues or find shelter under leaves on the forest floor, where the temperatures are generally warmer than at the exposed surface.
But there is only one species of insect that overwinters in North America without hibernating: the honeybee. The keys to the bee’s success are equal measures foresight and cooperation. Bees spend the summer stocking their hives with honey and pollen. They store their hard-earned provisions in small hexagonal chambers made of wax. Then, in the bleak and flowerless months of winter, the colony dines exclusively on the surplus.
Food stores alone are not enough to ensure the survival of a colony, however. Like humans, honeybees need to maintain a constant temperature to survive. Bees will strive to keep the hive’s temperature near 96.8 degrees during every season. In the summer, worker bees cool the colony by fanning their wings near the entrance and circulating air throughout the hive, whereas during the cold of winter they resort to what beekeepers call “the cluster.”
As the temperature outside plummets, honeybees form a spherical mass deep within the heart of the hive. The size of the cluster varies in response to the ambient temperature. Much like the mercury in a thermometer, the cluster will loosen when the temperature outside increases and contract when it gets colder, as the bees snuggle together for warmth.
Biochemical strategies: To freeze or not to freeze
Insects have evolved two primary biochemical strategies for winter survival: freeze avoidance and freeze tolerance.
Freeze-avoiding insects are capable of withstanding subzero temperatures by producing antifreeze proteins. One insect that uses antifreeze proteins to survive the winter is the spruce budworm. Adult spruce budworms are drab and innocuous moths, but as larvae they dine voraciously on conifers. The antifreeze proteins in the spruce budworm lower the freezing point of the liquid inside the insects’ bodies, thus allowing them to endure low temperatures without experiencing the cell damage that can result from freezing, according to an article by John Duman in the 2001 Annual Review of Physiology. When a liquid fails to freeze at its normal freezing point, it becomes supercooled. Many scientists believe that antifreeze proteins work by bonding to the sites where ice crystals first form. They change the shape of the nascent ice crystals, thereby lowering the temperature at which freezing can continue and allowing the liquid to become supercooled.
Freeze-tolerating insects employ a far different technique: They corral ice crystals into the spaces between their cells using what are called protein ice nucleators, or PINs. The larvae of the cranefly and the goldenrod gall fly, for example, deploy ice nucleators between their cells, according to David McMullen’s 2004 paper “Molecular and Biochemical Adaptations Conferring Cold-Hardiness in Two Gall Insects.” PINs have nearly the opposite effect of antifreeze proteins: Rather than inhibiting ice, scientists think PINs provide a convenient structure upon which water molecules can organize into ice crystals.
For a cold and rapidly stiffening cranefly larva, ice nucleators can be lifesaving. Not only do they act as hitching posts for ice crystals, but by confining ice to the space between the cranefly’s cells, ice nucleators suck water from inside the insect’s cells. This makes the cellular liquid more concentrated, lowers the cell’s freezing point and reduces the likelihood of cellular damage from ice crystals.
But as Lawrence points out, it’s important to note that most insects rely on a combination of behavioral and biochemical techniques to weather the winter. When just a few degrees can make the difference between life and death, it’s imperative to utilize every tool at your disposal.