How Do They Do It?

Gardeners often talk about how a good, cold winter will kill off the insect populations, yet each spring and summer brings an overabundance of bugs in the yard. Insects are not endothermic – able to generate their own heat internally. Instead they are ectothermic which means that they must rely on external sources to provide the heat they need to survive, so if it’s so cold, how do they do that? How insects survive during the winter months depends on several things.

Some of their survival depends on whether they are of a species that spend the winter above ground as eggs laid on plant material or sheltered in leaves or mulch. Butterflies, lady bugs, and aphids are good examples of these. If the winter is not particularly brutal and the temperatures haven’t been cold enough for a broad kill, the aphid infestation is generally bad the following summer.

If the insect typically overwinters in the ground, some protection is offered initially, but if the conditions aren’t harsh and the soil doesn’t freeze deeply, those infestations can be bad the following year too. Good examples of critters that spend their winters underground are hornworms, some grasshoppers, and grubs.

The bad news is that through the years insects have adapted to their environment and the likelihood of their survival largely depends on whether they have a freeze-avoidant or freeze-tolerant mechanism.
Freeze-avoidant is the most common adaptation. The insects that have developed this system can tolerate cold temperatures and freezing to a certain point, but will die off when temperatures drop below the freezing point of their bodily fluids. For many it will take temperatures in the 0 to -20 range before this starts to happen. Emerald Ash Borers, for instance, can tolerate temperatures up to -30 which is why they’ve been so difficult to manage on the Front Range- populations simply don’t die back very effectively. Many common insects in our area that overwinter here are freeze-avoidant which is how those populations continue to persist. Good examples include aphids, many borers, and pine beetles.
Freeze-tolerant insects are the kings and queens of adaptation and can survive temperatures low enough to cause as much as 65% of their bodily fluids to turn to ice. It’s almost impossible to kill these pests off and even during the harshest winter, they will survive. Most common examples of insects that fall into this category are cockroaches and woolly bear caterpillars.

If that wasn’t complicated enough, there are a few other factors that influence how well insect populations survive the winter too. One is the amount of moisture that is to be found in the soil and in the general area where the bug is overwintering. If it is too dry, that will be effective in drying out their habitat and can cause die off in some species.

Late spring temperatures can play a role as well. A late freeze in May or June can be very effective in killing back the populations that have already begun to emerge and reproduce.

No matter how harsh the winter, insects have become the masters of adaptation and can not only survive, but sometimes thrive, during the following summer. While I applaud the survival skills of friendly bugs like lady bugs, each summer evening when I’m sitting outdoors, I wish the mosquitoes had been a little less adaptive!