While some people were cursing a canceled flight or wishing they had donned an extra layer on Tuesday, when temperatures in the region took a deep dive, entomologists, foresters and naturalists were rooting for the mercury to drop even lower. That is because the extreme cold has the potential to beat back some of the invasive insects threatening treasured local tree and plant species.

“You do think, ‘Oh great, maybe some of those nasty insects are going to get zapped today,’” says Mark Fisher, director of conservatories and horticultural programs at Brooklyn Botanic Garden. “It’s Mother Nature’s way of dealing with this issue.”

The insects, whether introduced pests like the hemlock woolly adelgid or native ones like the southern pine beetle, have weakened forests from Cape May, N.J., to Litchfield County in Connecticut. They are uncannily adept at surviving the winter, but most have a breaking point. And this week, that point was nigh.

“The lethal temperature for the woolly adelgid is minus 4 or 5 degrees Fahrenheit,” says Richard S. Cowles, a scientist with the Connecticut Agricultural Experiment Station, a state research center. “I was cheering a couple of days ago because most of the adelgids will be dying from the temperatures we saw.”

Dr. Cowles, an entomologist, admitted to rejoicing that temperatures for the early morning hours of Saturday had ranged from 3 degrees to minus 9 in the state. Then came another deep freeze on Tuesday. “At that temperature, ice crystals start forming in the woolly adelgid’s body, and it kills them,” he says.

But entomologists cautioned that once an invasive species has arrived, it is almost always a matter of managing the population, not eradicating it. Some will inevitably survive. Dr. Cowles said that the adelgid population could still rebound within two years.

An aphid-like insect, introduced to the United States in the 1950s from Japan, the woolly adelgid has killed hundreds of thousands of Eastern hemlocks in Connecticut alone since arriving there in the 1980s. The pest, about the size of a period, can pierce the base of needles and suck out the tree’s nutritional supply. The adult can survive the winter on a branch.

Extreme cold is a fortuitous management tool. But with a warming climate, it is one that scientists cannot count on. “The weather will give them a temporary setback, but as soon as the weather warms up, they will take off again,” says Jan Nyrop, a professor of entomology and senior associate dean of Cornell University’s College of Agriculture and Life Sciences.

In the Pinelands in southern New Jersey, state foresters have been battling the southern pine beetle. The beetle can tunnel through a tree’s bark, eating a layer of tissue that supplies the tree with critical nutrients. Until recently, the beetles, which are native to the southern United States, did not survive north of Delaware, because of the cold. But that has changed as winters have turned milder.

The past century in New Jersey has seen a warming trend of 2.3 degrees Fahrenheit. More important than average temperatures in the beetles’ spread is the lack of periodic cold snaps in which the temperature plunges to minus 8 degrees.

The Pinelands have not experienced that kind of cold since 1996, and the first southern pine beetles were detected several years later. Whether the recent deep chill was enough to thwart their progress remains to be seen. The temperature for the past week in Chatsworth, N.J., in the heart of the Pinelands, reached a low of minus 7 on Saturday.

Ron Corcory, the project coordinator for the southern pine beetle with the New Jersey State Forestry Services, said that “sustained, very cold” weather was a powerful weapon. “We’re certainly optimistic that this will have some impact, but we won’t know until the spring,” he says. “We’d want to see a lot of green tops of trees.”

Emerald ash borers need even colder temperatures to succumb. The insects were first detected in 2002, after they arrived on wood pallets from China, and have since killed tens of millions of ash trees in more than 20 states. Studies suggest that temperatures must plummet to minus 30 degrees in order to achieve widespread mortality, and foresters and scientists in Minnesota and Illinois, where it was that cold this week, were hoping for a die-off.

But in New York, scientists were setting their sights on other targets. Amy Berkov, an assistant professor of biology at City College of New York, said she was hoping for some downward pressure on ticks, some of which spread Lyme disease. She recalled a field trip during the unusually mild winter two years ago when one of her students came back bearing a tick.

Dr. Berkov, who specializes in tropical ecology, said that she was warmed by this week’s cold. “Even though I work in the tropics, I like a seasonal climate,” she says. “I think it does tamp down some of these things that we’d rather not be seeing more of.”

Source: NY Times