Blue Ridge Naturalist: Winter Temps and Insect Numbers


© Marlene A. Condon

The author has two small artificial ponds on her property that teem with numerous kinds of critters (such as this Green Frog), many of which feed upon mosquito eggs and larvae. (Photo: Marlene A. Condon)

People have the mistaken idea that cold winter weather kills insects and other invertebrates, thus limiting the numbers of these animals by the time spring arrives. But if it were true that harsh winter temperatures kill these critters, there wouldn’t be any of them at all in areas north of Virginia, where it typically gets much colder every year than it does here.

These animals need to make it through freezing conditions to perpetuate their kind. If they hadn’t figured out how to survive such conditions, they would have gone extinct by now. Therefore it’s actually adverse spring and summer conditions, such as drought, that are more likely to negatively impact the number of invertebrates each year.

Conversely, folks tend to think that mild winter temperatures will increase invertebrate numbers, but, in fact, this situation can be deadly. Many kinds of hibernators, such as insects, may die if they become active during the winter in response to warm temperatures because there simply isn’t going to be much food available for them. The lack of food at this time of year is one of the reasons they need to hibernate.

In 2012 the Centers for Disease Control blamed mild winter temperatures for the faster spread of West Nile Virus by mosquitoes. However, in a naturally functioning environment, such a scenario would be highly unlikely to happen.

If it’s warm enough for mosquito eggs to hatch or mosquito larvae to become active in ponds or still areas of streams, it’s also warm enough for their aquatic predators to be actively feeding upon them.

The result is that few mosquito larvae would be able to survive to adulthood, only enough of them to maintain the proper functioning of the environment. And, of course, adult mosquitoes would also be taken by predators, reducing the numbers of mosquitoes available to reproduce. The same holds true for artificial ponds in your landscape, as long as you allow them to work naturally.

If, however, you instead treat a pond as an aquarium that gets cleaned out every year and perhaps has chemicals added to it, wildlife will have difficulty surviving within it—and that means you won’t have your natural system of checks and balances to keep the pond (and yard) functioning properly. Under these circumstances, of course, you may indeed help mosquitoes to proliferate.

Other common ways in which people create breeding habitats for mosquitoes is by leaving standing rain water within gutters that need maintenance, kiddy pools, toys left outside, and tarps over outdoor furniture. Water features that function as gardens only (i.e., they are used only for growing plants instead of functioning as genuine ponds full of life) are problematic as well. These areas will usually be devoid of animals that feed upon mosquito eggs and larvae.

As for bird baths, they should be emptied every day and fresh water put in. It should be obvious that you need to replace the water daily because birds leave behind visible waste and debris. Yet retailers advertise mosquito dunks to use in birdbaths, even though this pesticide is totally unnecessary. It’s also not as harmless as many folks believe.

Mosquito dunks are composed of Bacillus thuringiensis subspecies israelensis, a bacterium that specifically kills mosquitoes (and their close relatives). Although this pesticide is touted as nontoxic to humans, if bacterial spores are inhaled or rubbed onto the skin, they act as foreign proteins and can cause allergic reactions. Thus Bti should be handled with care.

Additionally, studies have shown that Bti, which is used in spray programs, could be more persistent in the environment than previously believed, with the potential for bacterial proliferation and thus an increased accumulation of these bacteria in mosquito habitats. Such Bti persistence would lengthen the amount of time that organisms are exposed to the insecticide, increasing the risk that target insects could acquire resistance to it.

Bti spores have also been found in untreated areas, raising the concern that microbial insecticides can spread, causing ecological harm.

If homeowners were better about correcting the conditions on their properties that allow mosquitoes to increase in number, localities could do away with large-scale pesticide-spray programs that many citizens and all health departments demand, but which pose threats to the environment.

Additionally, if most folks weren’t constantly trying to banish practically all wildlife (except perhaps birds and butterflies) from their yards, they would not be faced with the need for pesticide usage in the first place. This is exactly the wrong course of action. Without a variety of organisms in your environment to keep populations balanced, you end up with overpopulations that can’t help but be pestiferous—to themselves as well as to people!

The reality is that we cannot change the way the natural world works. Instead we must change the way we live.