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What's bugging you? Feb. 6, 2019

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Photo submitted Thousands of monarch butterflies overwinter on a eucalyptus tree on the California coast.

Have you ever wondered where and how insects survive our cold winters and then magically appear the following spring or early summer?

Insects have a variety of methods for surviving the coldness of winter. Migration is one strategy for escaping the killing temperatures. Migrations of monarch butterflies from northern regions to Mexico and the California coast are good examples of this maneuver. Many crop pests use this technique to escape winter's chill as well. But these same insects migrate back into northern areas from the southern states in the spring.

Many insects successfully pass the winter as immature larvae. The protection of heavy covers of leaf litter or similar shelters protect the woolly bear caterpillar, while other insects replace the water in their bodies with glycerol, a type of antifreeze! Some grubs simply burrow deeper into the soil to escape the cold.

Not many insects are active in the winter, but the aquatic nymphs of dragonflies, mayflies and stoneflies live in waters of ponds and streams, often beneath the ice. They feed actively and grow all winter to emerge as adults in early spring.

Lesser numbers of insects lay eggs which survive the winter. The most prominent insects in this category would be praying mantids, and the destructive corn rootworms that also engage in this strategy.

Still other insects overwinter in the pupal stage, then emerge as adults in the spring. Moths in the silkworm Family, Saturniidae, may be found attached to food plant branches as pupae in the winter.

There are many insects that hibernate as adults. Lady bird beetles are a well-known example, and are sometimes seen in great numbers in the fall as they congregate in the mountains. Many large wasps seek shelter in the eaves and attics of houses or barns. Tree holes, leaf litter and under logs and rocks are common shelters for overwintering adult insects. The mourning cloak butterfly is usually the first butterfly that is noticed in the spring, and this is because it hibernates in tree holes or other shelters during the winter. It reduces the water content of its body, and builds up glycerol which acts as an antifreeze.

Honey bees stay in hives during the winter, and form clusters when temperatures fall. They also are able to raise the temperature in the hive by vibrating wing muscles. The consumption of up to 30 pounds of stored honey during the winter months makes this possible.

In general, insects are able to survive cold temperatures easiest when the temperatures are stable, not fluctuating through alternate thaws and freezes. Many insects can gain shelter and nourishment through the winter in a variety of micro-habitats. Among these niches are under the soil, inside the wood of logs and trees, and even in plant galls.

Blankets of snow benefit insects by insulating the ground and keeping the temperature surprisingly constant. Insects that are inactive during the winter months undergo a state in which their growth, development, and activities are suspended temporarily, with a metabolic rate that is high enough to keep them alive. This dormant condition is termed diapause. In comparison, vertebrates undergo hibernation, during which they have minor activity and add tissues to their bodies.

So no, insects don't just magically appear after winter has passed through spontaneous generation, although it appears so sometimes. Because of their very high reproductive rate, a single adult can give rise to many in short order. From one comes many!

Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.

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Jim Leser, What's Bugging You
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