You’ve heard people say: It’s not the heat, it’s the humidity. Well, for insects: It’s not the cold, it’s the humidity.

There’s a reason that household flea problems spike in late summer… that fruit flies are hovering over the sink in fall … that wood-infesting insects do most of their feeding in fall … and that some species of subterranean termites and ants swarm in late summer. It’s because the relative humidity (RH) is high.


For insects it is all about conserving water—their own water. The entire design of insects, from their outer covering to their size (it’s a volume-to-surface area thing), is about not losing water.

Over-wintering insects—such as stink bugs, caterpillars, house flies and many, many others—have several ways of staying alive:

• Some insects reduce their feeding and/or foraging activity. Wood-infesting beetles are a good example of this
• Many insects have freeze-resistant body fluids. The blood may contain some alcohols that act as an anti-freeze
• Some species can reduce the water content in their cells, as it is the water crystals in cells that are the cause of freezing to death.
• Many over-wintering insects stay warm by burrowing just a few inches below the soil surface. In spite of the air temperature, soil temperature four inches deep rarely goes below freezing. But the key is that the soil remains wet—and it’s all about moisture.

Humidity is a key factor in the survival of insects. For example, for wood-boring beetles, it is the wood-moisture content that is the key factor.

The beetles don’t stop feeding in attics and crawlspaces because it is cold (the wood acts as a very good insulator); they stop because the wood moisture drops, and the air moisture drops.

This also can be seen in the way that fruit flies hover over the sink when the humidity in a house drops, and the RH drops in the fall. There’s a reason fleas are not such a problem in winter—the dog or cat still has them—but the RH indoors is tough on the larvae. Or in the fact that American cockroaches in basements shut down foraging activity in winter. It has nothing to do with temperature, because the temperature of basements really doesn’t change that much—but the air humidity drops.

When you watch TV weather reports in September and October, pay attention to the RH. You’ll notice that it will drop as the weeks progress. You may even notice the reduced humidity as your own skin becomes drier and requires more lotion or moisturizing soap in the fall and winter. Insects are very aware of this drop as well, and they become less active or die.

The reverse also is true and helps to explain why insects become more active in the spring and summer. As you watch the RH climb during these seasons, you can assume that house flies, fruit flies, bed bugs, and most other insects, are going to be doing very well.

A common misconception is that a hard freeze in the winter will wipe out many of the insects. In fact, it takes much more than this to make a difference to insect populations. To severely influence an insect pest population it may require 35 days at or below 14ºF—and that is not easy to do in most of the U.S.

And that is exactly the point—insects “know” that too, and they are not the least bit worried.