The Winter: Cold

In years when we have very cold, winter temperatures I often hear words of comfort from acquaintances: “These cold temperatures, at least, will kill the ticks and fleas!” and “The cold, at least, will kill the mosquitoes,” and so on down a long list of arthropod pests. (Photo by D. Sillman)

I wanted to explore this idea since, as each of us must know by now, this year’s winter has been a pretty cold one. The average temperature for Pittsburgh’s January 2014 was 21.4 degrees F. This is the coldest average January temperature in many years but nowhere near our January of 1977 when the monthly average was 11.4 degrees F! At the other extreme, some of you may remember January 2006 when the monthly average was 38.3 degrees F! Very little snow that year!

So, how do over-wintering arthropods, especially insects and arachnids, handle very cold winter temperatures? Very well, apparently.

There are a number of very effective strategies that these insects and arachnids use to survive. Some avoid the cold either by finding natural hibernaculae (like insulated spaces in the soil or the protected cavities under the loose bark of trees).  Many species also enter human habitations (I bet that everyone reading this has lady bird beetles and stink bugs somewhere in their houses!). Insects can also alter their body fluids with chemicals that act like natural anti-freeze. These “supercooled” individuals ride out the cold temperatures in an apparently frozen, inactive state, but readily thaw out and regain full function when the temperatures moderate. Other insects synthesize special proteins (like the protein I mentioned last week in the “snow flea’) that resist freezing. These proteins are concentrated in certain regions of the insect’s body where they protect vital organs and allow less vulnerable parts of the organism to freeze.

Most insects and arachnids require temperatures of at least 50 degrees F for full activity, but all of the behavioral and physiological tools they can employ allow many of them to wait for those great sounding 50’s to settle in with very little damage to their tissues or organs.
Most “endemic” species of arthropods (that is, those species that are native to this area and have evolved to survive in the changeable seasonal climates of our region) very easily survive both brutally cold winters and also the occasional cold snaps of spring. Sometimes the development of early season insect pests is slowed down by cold spring temperatures. These insects, then, lag behind their targeted spring plants because plants can continue to grow and develop at much colder temperatures than the insects. In this situation less damage to the spring plants is noticed. Endemic, summer-active insects, though, don’t seem to be greatly affected in any way by prolonged winter cold periods or spring freezes (data from the Ohio Agriculture Research and Development Center).

Non-endemic arthropods, though, may be affected by our roller coaster weather patterns especially if those exotic, invasive species have come from areas where the evolutionary selection for suitable weather survival mechanisms has not occurred. Two invasive species that may be at least somewhat inhibited by cold weather are the emerald ash borer and the Hemlock wooly adelgid. Cold northern winters may also push the invasion line of the southern pine beetle (an endemic species of the southern United States that has spread into the north) further back to the south. Most ecologists, though, think that with the overall trend to warmer and warmer average global temperatures these occasional cold winter set-backs in the spread of these tree destroying pests are just transient victories in an on-going losing war.

So what about fleas? (And here I am talking about fleas that are ectoparasites of dogs and cats, not those wonderful, collembolan snow fleas). Cold temperatures (defined as temperature of 40 degrees or less) will kill adult fleas, but cold temperatures will not kill immature life stages of the fleas (larvae, nymphs, or eggs). So the adult fleas outside will die in the winter, but there will be significant numbers of the cold resistant immature life stages to repopulate the ecosystem and infest our pets once warm weather has returned. At least if there is a cold spring, though, the flea explosion might be delayed a bit, but it will eventually occur! An interesting but potentially unpleasant side note to this flea discussion concerns fleas that are living inside of our houses. The cold winter temperatures never do kill off the adults (not even if I would set my house thermostat down to 40 degrees!), and these fleas may stay active and continue to infest the resident dogs or cats all winter long.

Mosquitoes?   Same idea. Mosquitoes in their overwintering life stages don’t seem to be affected by very cold winter temperatures. For some species of mosquitoes the overwintering life stage is a cold resistant egg, while for other species it is a larvae that lasts through the winter. There are even some mosquitoes in which mated adult females are the overwintering life stage. These adult females, when they emerge in the spring, are ready right away for a blood meal to enable them to lay their fertilized eggs. Again, cold spring temperatures may delay their emergence, but the cold does not seem to significantly reduce their numbers.

And finally, what about black-legged ticks (also called “deer ticks”)? These are the arthropods that can transmit the bacterium that causes Lyme Disease, and these ticks for a variety of reasons (see August 24, 2013 blog for a discussion) have in recent years greatly increased in numbers throughout the eastern United States (including Western Pennsylvania). A study published in 2012 in the Journal of Medical Entomology clearly showed that in spite of “common knowledge” to the contrary, cold winters (and they used Upstate New York as their cold winter site!) do not reduce the numbers of overwintering black-legged ticks. The ticks just have too many adaptations for cold tolerance and too many protected microhabitats available for even the brutal winter temperatures of New York State to have any effect on them at all. Something very interesting to me personally about this tick study was the fact that these researchers used the same SUNY-ESF field lab site for their study that I used for my PhD research! Small world!

So, what is silver lining about having such a cold winter? Do we stay inside more and get more work done? Do we get to eat more high-calorie foods without significant weight gain? Or do we just appreciate the spring when it finally does come? I am personally anticipating a long bout of Spring Fever this year.