As a frigid winter takes a toll on the United States and Canada, invasive insect populations are also taking a hit. The U.S. Forest Service estimates that up to 80 percent of emerald ash borers, which have been decimating ash tree populations, were killed by longstretches of bitter cold in the the upper Midwest this year. Several other insect pests, many of which have migrated northward because of milder winters in recent years, also are faring poorly this winter, including corn earworms and gypsy moths. Researchers remain skeptical, however, that the die-offs will have lasting effects on pest populations. Emerald ash borers in Chicago, for example, survived the sub-zero weather because Chicago temperatures fell only to -17 degrees F, rather than Minnesota’s -20 degrees F, which seems to be a critical temperature threshold for the pests.