Jim Edsen, a forester with the Vermont Department of Forests, Parks & Recreation, conducts a woolly adelgid survey in southern Vermont. Photo by Barbara Shultz.
According to the Northeast Regional Climate Center at Cornell University, last winter was among the top five coldest on record in Ithaca, New York, when you consider the number of days the temperature dipped below zero. The cold did a number on our wood piles, but did it affect exotic insects like the hemlock woolly adelgid and the emerald ash borer? Unfortunately, the answer is probably no.
Insects prepare for winter by building up glycerol – basically an antifreeze – in their blood. The accumulation of glycerol is usually gradual in response to environmental triggers. Cold tolerance varies between insect species and according to the season. Typically, insects are the least cold tolerant in summer, gaining tolerance through fall and early winter to a high point in January and February, and then gradually decreasing again in spring.
Many have hoped that the spread of exotic invasive insects would be limited by cold winters in northern New England and New York. Last winter’s very cold temperatures, in particular, fueled speculation that hemlock woolly adelgid (HWA) might be set back. (The adelgid remains in place for life once it settles on a twig and begins feeding, and so it can’t avoid the cold.) A recent laboratory study demonstrated that HWAs in the Berkshire Mountains in Massachusetts suffered 97 percent mortality at 22 degrees below zero and none survived 31 degrees below zero.
The problem is that natural conditions rarely mimic lab conditions. As everyone knows, there are cold spots and warm spots on the landscape. And even when the temperatures do get cold enough to kill a large number of individuals, the remaining HWAs – which are all female and reproduce asexually – can ramp up reproduction and quickly rebound. Population dynamics are a pretty nuanced thing, and while it might seem that 97 percent mortality would be hard to overcome, the reduced density means less competition. The food quality of the hemlock twigs will stay better for a longer time, giving the surviving HWAs a fertile field in which to flourish.
Research indicates that cold tolerance is a genetically linked trait, so progeny of the survivors may also be cold tolerant. Last winter I worked with students in my lab at Cornell University to monitor two sites that have been harboring HWA for a few years: one in Cayuga Lake in central New York, the other in the northern Catskills. At a site near Cayuga Lake, the temperature never got below 8 degrees below zero, yet we found HWA mortality to be 92 percent. On the other hand, at a site in the northern Catskills, temperatures got to 24 degrees below zero and we found only 82 percent mortality. It may be that HWA populations in the northern Catskills have developed better cold tolerance than the adelgids near Cayuga Lake.
The Emerald Ash Borer (EAB) has an advantage over the HWA in that it spends the winter under an ash tree’s bark, which shelters it from winter extremes. The tree’s mass collects solar energy during the day, which moderates low temperatures at night. The few hours of extreme cold experienced in the early hours of a morning won’t be felt under the bark of a tree. So even though your thermometer might indicate an extreme temperature, EABs are not nearly that cold.
Research on the cold tolerance of the EAB has been conducted in Minnesota and Ontario. Lab research in Ontario indicated that the lethal temperature for the EAB was between 9.4 degrees below zero and 15.5 degrees below zero, whereas in Minnesota another lab study found 98 percent mortality at 30 degrees below zero. While 20-degree below zero temperatures were fairly common in many locations last winter, there were also warmer periods between these cold snaps that limited the amount of time a tree trunk actually spent at that temperature. There’s currently no evidence that EAB populations are developing cold resistance, but researchers in Minnesota evaluated mortality in logs placed outside in 33-degree below temperatures, and found that there were still a few survivors.
The take-home message is that cold temperatures are not a “silver bullet” for controlling our invasive forest pests. By enduring the bitter cold last winter we might have bought a year of relief with HWA, but due to their reproductive prowess, they will be back soon, perhaps even stronger than before. In New York, at least, EAB populations seem to hardly have been phased by last winter. We must prepare for the arrival of these insects in order to mitigate their impacts and make plans to preserve the genome of our threatened native ashes and hemlocks through seed collection and treatment of seed trees to keep them alive. We need to act quickly and wisely to conserve what we can of our native forests.