You probably know of the astounding 3,000 mile annual migration of Monarch Butterflies to spend the winter in Mexico. Every local insect species has a strategy for surviving freezing weather. The Monarch’s incredible journey is one dramatic, but atypical, example. Most insects don’t get to vacation abroad. In fact, most pass the winter within a few feet (or millimeters) of their summer homes.
When the days grow shorter and temperatures drop, insects’ winter survival behaviors are triggered. For most, that means finding someplace warm to avoid the freezing temperatures to come. Some insects, however, are actually able to freeze through and “come back to life” when they thaw. These freeze tolerant insects encourage ice crystals to form within their bodies, but they control the timing. They typically expedite their freezing through a quick-chill adaptation. Just like freezing ice cream, the results are best if you freeze it fast and then keep it from thawing. The banded woolly bear caterpillar is an example of a freeze tolerant insect in our region. Come spring, it thaws and begins to spin a cocoon for metamorphosis.
Most insects need to avoid freezing if they want to survive the winter. Ladybugs, for instance, are notorious for overwintering in the comfort of human homes. Though we love to see them in the garden, we prefer that they stay out of the living room! Other freeze avoidant insects find shelter from external icing by burrowing into the ground, snuggling in beneath the bark of a tree, or hiding in leaf litter. In the case of immature insects, the eggs are deposited in a sheltered place to await the spring.
As you might expect, it still gets pretty cold underneath the bark of a tree during the winter, so these freeze avoidant insects have some biological tricks up their sleeves. The goal is simply not to freeze, but 32° isn’t the limit. Instead, these insects have developed methods to stay unfrozen down to temperatures well below that. Some produce personal antifreeze proteins that keep ice from crystallizing in the bloodstream. Many are able to turn their body fluids into non-freezing liquid by expelling particles from the gut— the fewer particles there are in water, the slower it will change to ice (pure water won’t make this transition until -42°).
Understanding the over-wintering behaviors of insects is fundamental to controlling them. This is why any professional involved in plant health care needs to understand the life cycles of insects. By knowing when an unwanted insect is vulnerable, we can narrowly focus on that window of opportunity. Scale insects, for example, spend the winter as larvae on branches or leaves covered by a hard or waxy shell. Aside from scraping them off by hand, they are virtually impervious to attack. But by understanding the timing of their emergence in the spring, it’s possible to use a foliar spray that targets these colonies as they leave hibernation.
It’s also possible to take action against many insects before, or while, they overwinter. Hemlock Woolly Adelgid is actually more active during fall and winter than in summer, so a horticultural oil treatment in the fall to smother these insects helps to prevent damage that would occur over the winter months. Likewise, many types of mites and scale insects are still vulnerable in October and November. Foliar applications during this time can dramatically curtail their chances of surviving the winter and causing damage in the spring.
Understanding the life cycle of insects allows us to better protect the health of plants, and reduces the amount of pesticides used.