When you're out skiing or walking through the woods in winter, keep an eye open for insects.

Insects? In a Yukon winter?

They're out there, says Ed van Randen, Forest Health and Watershed Forester for the Department of Indian and Northern Affairs in Whitehorse. Most insects aren't active in cold weather, but if you know what to look for you can see where some of them have hunkered down for the winter.

The easiest to spot are the insects that form galls on trees. The insect injects a hormone that makes the tree grow tissue to suit the insect. The gall that forms often provides both a refuge and a food source for the insect that triggered it.

"It can be a very specific type of growth," van Randen says.

The galling adelgid, which resembles an aphid, causes spruce trees to grow galls that look very much like spruce cones. You can see the difference, once you know what to look for, says van Randen, but a casual observer will assume the gall is another cone.

One of van Randen's favourite galls is formed on willow trees.

"It looks just like a wooden rose," he says. "Look for it near the top of the tree at a junction between branches or at the end of a branch."

Most over-wintering insects are less easy to find than the galling insects. Some, particularly caterpillars and true bugs, spend the winter in the buds of trees. They drill a hole at the base of the bud and crawl in. In spring, when they revive, the bud becomes a food source.

Other insects are rolled up in leaves, hidden under the bark of trees, or buried in the forest litter under an insulating layer of snow.

The only insects that commonly remain active all winter are aquatic insects, van Randen says. As long as there is water beneath the ice in a stream or lake, aquatic insects like mayflies can survive.

"They just keep trucking along in that environment," says van Randen. "It stays the same, summer and winter."

Because they don't generate their own heat, as mammals do, insects that spend the winter on land have to find a way to survive freezing. As water cools, individual water molecules begin to gather around small foreign particles, such as a grain of dust, and form hard, sharp ice crystals. The ice crystals will eventually expand and rupture the body cells, killing the insect.

Most insects use a form of antifreeze to survive freezing temperatures, van Randen says. As the temperature drops, their cells produce glycerol, which prevents water crystals from forming within the cells.

At the same time, the insect -- and each of its cells -- actively eliminates all waste products or other foreign particles that an ice crystal might form around. This two-step process, called supercooling, allows body fluids to remain liquid at temperatures far below their normal freezing point.

Supercooling is highly effective. In winter, more than 30 percent of the body weight of the Arctic Carabid Beetle, for example, might be composed of gycerol, enabling the insect to survive temperatures of -50 degrees C.

Some insects have another strategy. They can isolate ice crystals between cells and allow them to form without danger of cell damage. These "freezing-tolerant" insects can withstand a limited amount of freezing, van Randen says, but the system is less effective than supercooling in extremely cold winter temperatures.

The ability of insects to survive even the extreme conditions of a Yukon winter astonishes van Randen.

"You couldn't find another body plan that has been so well adapted," he says.

Most insects spend the winter in larval form. Larvae seem to be better at producing glycerol, van Randen says, and therefore can survive through colder temperatures than adults.

In cases where both larvae and adults survive, they often use different strategies, he says. The larvae of spruce bark beetles produce glycerol to protect their cells and spend the winter under the tree bark.

However, the adults are less efficient at producing glycerol and need more protection. As soon as they reach maturity, they burrow out of the tree bark and drop to the bottom of the tree. There they burrow back into the base of the tree and spend the winter under the snow, sometimes in congregations of thousands.

"Almost without fail, if they're not under the snow the adults will die in a normal Yukon winter," van Randen says. "They might have a survival rate of one percent."

For more information about insects in winter, and at other times of the year, contact DIAND Forest Resources in Whitehorse or Environment Canada.