Many insects reduce the water content of their bodies and produce glycerol and other protective compounds that lower the freezing point of the body fluids. Butterfly boxes also are known as butterfly roosts and hibernation houses. Advertisements claim these bird house-sized structures will encourage butterflies to stay in your garden by providing necessary shelter. It is tempting to dismiss these well-intentioned accessories as worthless, but that's not quite true. But the butterflies couldn't care less.
Several studies show that the boxes are virtually never used except by spiders, wasps and mice.
The few butterflies that do remain for the winter appear to find sufficient natural hiding places under dead leaves, twigs and wood piles, where they hang upside down with their wings folded together. The best encouragement for butterflies is to diversify your landscape. Plant a wide variety of flowers and plants that are attractive to butterflies and that will feed the hungry caterpillars. And remember to reduce insecticide use as much as possible.