The coldest air to reach Minnesota in decades was probably deadly to pests such as the emerald ash borer, which forest officials say has killed more than 10 million trees.
U.S. Forest Service biologist Robert Venette said that 80 percent of those insects could have been exterminated by temperatures plunging to between minus 22 and minus 26 degrees Fahrenheit.
Venette explained that it takes that kind of cold to reach beneath the bark of infected trees, where the pests winter in a larval state.
The cold is also believed to have killed off a large number of gypsy moths, which eat the leaves of more than 300 species of trees, shrubs and other plants.
But the extreme cold also split or cracked the limbs and stems of some trees, leaving them vulnerable to insects and other threats.
Photo: Natural Resources Canada