How many of these little nuisances survived the Polar Vortex?
Photo courtesy of Leah Bauer, USDA Forest Service Northern Research Station; Bugwood.org

Alas, the answer to that question may be … maybe.

In the February issue, we reported on the effect that this winter’s brutal cold has had on insect pests; rather, there was speculation that the subzero temperatures might have devastating effects on critters such as emerald ash borer (EAB), gypsy moth and mountain pine beetle. You may recall that one U.S. Forest Service biologist predicted that up to 80 percent of EAB might already have perished in areas of Minnesota where temps bottomed out at 20-something below zero.

Only time—and spring—will tell. Research conducted by Robert C. Venette and Mark Abrahamson, “Cold Hardiness of Emerald Ash Borer, Agrilus Planipennis: A New Perspective,” showed, basically, that a prolonged spell of subzero temps has the potential to kill off a significant number of EAB. Venette is a research biologist with the U.S. Forest Service in Minnesota, and it was his prediction that we cited in February’s Plant Health. Here’s an excerpt from their report:

“From our simple model that related the coldest temperature experienced by emerald ash borer larvae to the extent of mortality, we predicted that when larvae reach -17.8°C (0°F), 5 percent will die; at -23°C (-10°F), 34 percent will die; at -29°C (-20°F), 79 percent will die; and at -34°C (-30°F), 98 percent will die. …

“Minnesota winters, especially in the northern part of the state, may cause substantial mortality of emerald ash borer larvae. However, even with the extreme cold air temperatures that were experienced near Grand Rapids, Minn., some emerald ash borer larvae survived. Thus, cold temperatures may not completely eliminate the insect. However, cold temperature may help to keep populations from building quickly and may give ash trees some time to recover from initial attacks.

“We have also learned that air temperatures, recorded at standard meteorological weather stations, are not necessarily the most reliable measure of the temperature experienced by overwintering emerald ash borer larvae. Trees warm considerably on sunny days through radiant heating. Larvae that are able to form a pupal cell in the outer sapwood may be afforded some protection against brief drops in temperature.

“These results have significant implications for predictions of the future range of emerald ash borer, spread rates of the insect in areas with a harsh winter climate, and the time required for these insects to kill a tree.”

Mature fourth-instar larvae or “J-larvae” of emerald ash borer (EAB) are found in pupation cells in the outer sapwood of an ash tree. For EAB with a one-year life cycle, J-larvae are found in fall, winter and early spring in the outer sapwood or outer bark. Such protection may-or may not-have saved them from freezing temperatures during this extreme winter.
Photo courtesy of Houping Liu, Michigan State University; Bugwood.org

Temperatures this winter were well within the killing range, but as the report states, “trees warm considerably on sunny days through radiant heating.” This fluctuation in temperatures may have provided enough respite from the extreme cold that EAB populations could survive. (Of course, EAB are not affected by wind chill; protected as they are beneath the outer bark, the little nippers are not directly exposed to frigid zephyrs.)

Timing also may play a part in how cold affects EAB. Reports from other states and from Canada lower the expectation of a massive die-off. In London, Ontario, provincial forest entomologist Taylor Scarr said that the cold snap may have occurred too late to make a significant impact. Scarr, who works for the Ministry of Natural Resources, claimed that this winter’s chill “didn’t get cold enough, soon enough, fast enough. It has to happen early, it has to happen fast, and it has to happen deeply.” Perhaps half of the population will perish – which is better than none.

In Iowa, the cold may not have been prolonged enough to make a dramatic difference. The actual temperature may have been sufficient, but according to horticulturists with Iowa State University, a 36-hour period of subzero temps (as low as -27°F) probably wasn’t long enough to cause “significant mortality” to EAB larvae. That opinion was echoed along Chicago’s North Shore suburbs, where last year Lake Forest identified more than 1,400 infested ash trees on city property and 450 privately owned trees. “The weather will not have a huge impact on EAB,” stated Peter Gordon, Lake Forest’s arborist. “It will result in some reduction of EAB larvae, but not to the point that it will be noticeable.”

And in Michigan, ground zero for the U.S. invasion of emerald ash borer, Michigan State University professor—and EAB expert—Deborah McCullough stated that EAB’s ability to acclimate is key to its survival. And it appears that these pests are good at producing a kind of antifreeze that helps to prevent their cells from freezing.

If the beasts have been exposed to severe cold and survived—if they’ve managed to acclimate to the frigid temperatures—we’ll have to count on other means of control. It’d be nice to believe that there’s an upside to this winter’s fury, but we’ll have to see what the warmer temperatures of spring produce.