I was listening to a story on NPR last week about how the unusually cold winter we’ve been having here in Virginia is likely to damp down populations of invasive exotic insects, such as the emerald ash borer (EAB), as well as some of our homegrown invertebrate pests, such as ticks. While this is likely to be the case, we need to keep in mind that insects are highly adaptable . . . and can be surprisingly hardy.
In general, populations among most insects and their kin tend to boom and bust easily because of their short life spans. Many invertebrates produce more than one generation in a single year. Extreme cold, heat, damp or drought can knock them back.
However, over the billions of years invertebrates been around, they have developed a wide variety of strategies that have enabled them to fill diverse ecological niches within varying climates, including extreme ones. While their relatively short lifespans can make individuals vulnerable, populations can also easily rebound when conditions improve and adapt to extremes in fewer generations than large animals.
I mentioned in my Jan. 9 column that Lepidoptera (butterflies and moths) generally use the strategy of diapause, in which they delay the development from larva (caterpillar) to adult during the winter, spending that time in the dormant phase as pupae in a chrysalises (cocoons). This is not true of all Lepidoptera, however. Some have evolved to survive the winter in the larval phase through adaptation of their body structure and internal processes. An extreme example of this is the Gynaephora groenlandica, or Arctic woolly bear moth, which lives near the North Pole.
I’ve been skipping around in Bernd Heinrich’s book “Winter Word: The Ingenuity of Animal Survival,” and after I heard the NPR report, I went straight to a chapter that featured this insect and a woolly bear that lives here in Virginia, although it is in another moth family.
While most of Virginia’s native caterpillars take only a year or two to cycle through their life stages of egg to caterpillar to pupae to adult, Heinrich explains that the Arctic woolly bear has a brief stretch during Arctic summers where it can be active enough to feed and mature. According to varying sources, these brief intervals delay maturation anywhere from to 10 to 14 years. Once the caterpillar is mature enough, its pupation, emergence as an adult and breeding happens rapidly.
Arctic woolly bears thus spend about 90 percent of their lives in the larval stage. They keep their fluids from crystallizing in the cold, which would lead to death, through combining two cold-survival strategies: Supercooling their fluids and breaking down food into sugar that serves as antifreeze. (For more information on the Arctic woolly bear, including photos, visit arcticcaterpillars.org.)
Here in Virginia, the banded woolly bear (Pyrrharctia Isabella), which morphs into the Isabella tiger moth, looks similar to the Arctic species despite their disparate lineage. The banded woolly bear has been spotted as far north at Manitoba, Canada, according to ButterfliesAndMoths.org, so it’s no slouch when it comes to surviving cold weather.
As Heinrich writes in his book, he had heard that our local species could survive extreme cold — as far down as negative-22 degrees Fahrenheit — by using the Arctic woolly bear’s double strategy of supercooling and antifreeze. Heinrich decided to put some banded woolly bear caterpillars to the test, albeit at temperatures more likely to occur in the Northeast. First he tried freezing pupae at 6.8 degrees but they died. Then he decided to try freezing a few of the caterpillars at the same temperature and got a surprise: “Two hours later they were indeed quick-frozen into blocks of ice. They were solid. I could tap the table with them. When I thawed them out an hour later, they were alive and well!”
Such survival of freezing temperatures generally requires the frozen state be reached slowly, as Heinrich goes on to explain, and he had flash-frozen his caterpillars — a more “severe” test.
“Not believing my senses,” he goes on to write, “I immediately repeated the experiment with the same two caterpillars. The result was the same.” Woolly bear caterpillars do survive freezing — even multiple freezing, he concluded, whereas the pupae don’t. “No wonder my caterpillars had waited so long to pupate after coming out of hibernation,” he writes.
This underestimation of invertebrates is a chronic problem with our species. As Heinrich writes in the introduction to the caterpillar story, we withhold respect for these tiny survivors because they are “animals are so different from us that it’s as if they were from an alien world.”
So how will some of the pests noted in the NPR story truly fare this winter? We might want to think about the following before we break out the party hats:
A recent study from the U.S. Forest Service showed that EAB larva had an average supercooling point of negative-13 degrees and that 5 percent of the insects die at 0 degrees, 34 percent at minus-10 degrees, 79 percent at minus-20 degrees and 98 percent at minus-30 degrees. As I wrote about in an article last year on stink bugs, Virginia Tech entomologist Thomas P. Kuhar found that the bug survived temperatures down to minus-4 degrees. According to a 1999 article in the journal Physiological Entomology, the deer tick (Ixodes scapularis) is more vulnerable to cold, but also uses supercooling to survive temperatures ranging from 18 degrees to as low as negative-7.6 degrees.
During the recent cold stretch here in the Blue Ridge Mountains, the temperature didn’t go below 2 degrees — and that’s the ambient air temperature. When we take into consideration that most insects seek shelter, such as under leaf litter (ticks and some caterpillars) or even in our houses (BMSB), we shouldn’t be overly optimistic about huge declines in their populations next year. And we must not forget that populations of “beneficial” invertebrates — those that prey on these pests, including some spiders, mantises, robberflies and wasps — are also likely to be adversely affected by unusually cold weather.