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When we have a very cold winter, I often hear words of comfort from friends and 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 forecasted doom for a variety of arthropod pests. There is a flip side to this cold winter optimism, though, and that is a prediction of an explosion of ticks, fleas, mosquitoes etc. when the winter is warm.
I wanted to explore these ideas since, as each of us must know by now, this year’s winter has been a very warm one. Globally, in fact, January 2020 was the warmest January in recorded history! Here in Western Pennsylvania the average temperature for our January 2020 was 37 degrees F. This is the warmest January that we have had since 2006 (38 degrees F), and it is nearly 10 degrees above our historical January average (28.5 degrees F). January 1880 was the warmest January ever in our region (average monthly temperature that year was 44.4 degrees F). There are only a handful of Western Pennsylvania Januaries, though, that have been warmer than the one we just experienced!
So, are over-wintering arthropods, especially insects and arachnids, affected by warm (or very cold) winter temperatures? The answer is “yes” and “no.”
There are a number of strategies that insects and arachnids use to survive the winter. Some involve avoiding the cold either by finding natural hibernating spots (like insulated spaces in the soil or protected cavities under the loose bark of trees or even inside the dead bodies of the previous year’s adult life stages) or by finding similar spots within human houses and other buildings. Some of these human habitat hibernators, though, actually can be negatively affected by the consistently warm temperatures of their refuges. Hibernators are well adapted to lowering their basal metabolic rates with decreasing temperatures but may, when the house or building in which they are hibernating is kept uniformly warm, experience elevated basal metabolic rates and as a consequence use their body fat reserves too rapidly and starve before Spring arrives.
Eastern tent caterpillars overwinter as first instar caterpillars still encased in their eggs, and common bagworms overwinter as unhatched eggs These caterpillars and eggs are packed up inside well insulated egg sacs that are attached to tree bark and other types of plant stems. For bagworms, larger egg sacs are better insulated and provide better protection from the cold, and for both species specific location of the eggs sac placement can affect survival. Very cold winters can kill these overwintering organisms, but for eastern tent caterpillars
these have to be temperatures lower that -31 degrees F (University of Kentucky Agricultural Bulletin). For bagworms in large eggs sacs, 50% lethality is seen at temperatures just under 0 degrees F (Canadian Entomologist February 2013). Temperatures warmer than these extreme cold readings allow for the significant survival rates (typically 80%) of the overwintering egg/caterpillars. The latitude line of winter cold lethality changes with changing seasonal temperatures. As climate warms, this line is pushed ever further to the north, exposing higher latitude forests to these defoliating insects. And, if climate cools, this infestation line drops to the south.
Insects can also alter their body fluids with chemicals that act 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 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 the 50 degree days to arrive with very little damage to their tissues or organs.
Most of “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 colder than average and also warmer than average winters. They also survive the occasional cold snaps of spring. The timing of the early spring emerging endemic insects, though, may occur either earlier than normal (if the winter/spring has been warmer than average) or later than normal (if the winter/spring has been colder than average). These insects, then, may emerge ahead of or lag behind their targeted spring plants. Endemic, summer-active insects, though, don’t seem to be greatly affected in any way by prolonged winter cold periods or spring freezes or rising winter temperatures and early spring thaws (data from the Ohio Agriculture Research and Development Center).
Non-endemic arthropods may be affected by our roller coaster Spring 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 and stimulated by warmer weather are the emerald ash borer and the hemlock woolly 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, and, as we discussed above for the common bagworms and eastern tent caterpillars, warmer temperatures may allow these pests to push into increasingly northern regions .
So what about fleas? Cold temperatures (defined as temperature of 40 degrees or less) will kill adult fleas, but the immature life stages of the fleas (larvae, nymphs, or eggs) are extremely tolerant of very cold temperatures. So the adult fleas outside will die in the winter unless the winter is so warm that 40 degree F low temperatures are not reached, but even if these adults life stages are killed 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. A cold spring might delay the flea explosion, but it will eventually occur! An interesting but potentially unpleasant side note to this flea discussion concerns fleas that are living inside of a house. Cold winter temperatures never do kill off the indoor dwelling adults (not even I would set my house thermostat down to 40 degrees!), and these fleas may stay active and continue to infest the resident dog or cat all winter long.
Mosquitoes? Same idea. Endemic mosquito species 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 larva 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 so that they can finish making their eggs. Again, cold spring temperatures may delay their emergence, and warm spring temperatures may hasten their emergence, but neither seems to significantly affect their numbers.
Non-endemic, especially southern mosquitoes, though, like Aedes aegypti (the mosquito that carries Denge fever, yellow fever and the Zika virus) are killed by cold winter temperatures. These mosquitoes are intruding into our region during the summer but are not yet setting up locally persisting, reproducing populations. Rising winter temperatures, though, may soon allow these dangerous mosquitoes and several other warm-weather species to become permanently established here in Western Pennsylvania.
And, what about black-legged ticks? These are the arthropods that can transmit the bacterium that causes Lyme Disease, and these ticks for a variety of reasons (see June 2, 2016, Signs of Summer 2 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 most brutal of winter temperatures to have any effect on them at all. Warmer winter temperatures, though, may allow an earlier Spring emergence and more rapid advancement of the ticks through their reproductive cycles which could lead to larger and larger populations of this species.
Another tick of great interest that may be expanding its range northward because of warmer winter temperatures is the Lone Star tick (Amblyomma americanum )(also called the “turkey tick”). This tick is a vector for a variety of illnesses including ehrlichiosis, tularemia and the southern tick associated rash illness (STARI). They can also transmit the heartland virus. The Lone Star tick also can cause a human host to become severely allergic to dietary meat by the bite injection of an immunologically active, salivary carbohydrate called galactose-alpha-1,3-galactose. This oligosaccharide, which is present in all mammalian meats except those found in primates, stimulates the human immune system to make IgE antibodies which mediate a delayed, but extremely severe, anaphylactic response when the sensitized host eats any mammalian meat.
So if our winters are getting warmer many trees pests may invade into more and more northern forests. Most endemic insect species won’t be greatly affected, but early spring emerging species may get out of synch with their obligatory spring plants. Disease carrying “southern” mosquitoes and ticks may become established in more northern regions, and endemic fleas and ticks may get active earlier in the season and reproduce more rapidly.
Do you miss the good old winters? Let’s all remember January 1977 when our average monthly temperature was 12 degrees F! Take that you bagworms and tent caterpillars!