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 said. “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 said. “I think it does tamp down some of these things that we’d rather not be seeing more of.”