The extreme cold has the potential to beat back some of the invasive insects threatening treasured local tree and plant species.
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," said 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."
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.
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.