“A good winter freeze is good for controlling pest populations,” says Brian Wolyniak, Penn State Extension urban forester at Pittsburgh, Pa. And a deep freeze is even better. A low temperature of -10 degrees Fahrenheit is good to wipe out about 33% of emerald ash borer populations.
The extreme temperatures of the latest arctic blast may kill off a significant percentage of emerald ash borer larvae, according to University of Minnesota forestry expert Lee Frelich. Of course, the deepest cold peaked (bottomed) somewhere between -20 and -26 degrees F early in the week in the Twin Cities.
EAB’s winter mortality is temperature dependent. The larvae can supercool to a certain point. But they die if they freeze. A recent study from the Forest Service in Minnesota showed that 5% of the insects die at 0F, 34% at -10F, 79% at -20F and 98% at -30F.
However, there is the question of what temperatures the insects actually experience, since they spend winter under the bark of trees, and some of them close to the ground, where they may be insulated by the bark itself and possibly by the snow.
This insulation effect can have a substantial effect if overnight minimum temperatures take a brief plunge and recover quickly. In such cases minimum temperatures under the bark can be 2 to 7 degrees warmer than air temperature. With prolonged cold that insulating effect of bark becomes minimal.
But, keep in mind that there’s huge variability in minimum temperature across the landscape, on south or north facing slopes, hill tops and valley bottoms.
Survival of the fittest phenomenon
However, with warmer winters likely to occur in the future, this could change, suggests Frelich. And, the few insects that do survive cold spell might be more resistant to cold than an average insect, and give rise to a new generation of more cold-tolerant insects. But he concedes that entomologists don’t know much about this type of selection in emerald ash borer.