By PAUL HETZLER
In states like Indiana, Illinois and Ohio, “emerald cities” are becoming all too common. Unlike the fabled city in the Land of Oz, their “emerald” designation refers not to their hue, but to the fact they've suffered through an emerald ash borer (EAB) infestation, losing thousands of ash trees in the span of a few years. Since its discovery in 2002, EAB has drastically altered the look of many communities as tree-lined streets are stripped bare by the emerald invader, leaving entire neighborhoods bereft of trees.
For such a destructive pest, this Asian beetle is quite beautiful. It’s small (3/8” to ½”), bullet-shaped and would be easy to overlook if not for its bright, metallic, emerald-green color with shiny copper highlights. The beetles themselves do little harm, but their immature stage (larvae) feed on the cambium, the layer of live tissue just beneath the bark, which girdles and thus kills ash trees.
Since the EAB kills only true ash in the genus Fraxinus, mountain ash is safe. But unless an effective biocontrol (a disease, parasite or predator that kills EAB) is found soon, New York State’s 900 million ash trees will vanish. On average, ash represent about 7 percent of northern New York's forests, but certain habitats have much higher concentrations. The small (in relative terms) number of landscape ash trees treated with insecticides (at considerable expense) through the 15-year EAB infestation period will survive.
With EAB closing in on the North Country from the west, south and north, there’s no way to stop it. In 2013 it was confirmed in multiple sites in southern Ontario, Canada, including Cornwall, just across the St. Lawrence River. EAB are quite capable of flying over the river, and you can bet they won’t register with the Border Patrol. It would seem the EAB’s arrival in northern New York is close at hand.
However, a significant bright spot has developed that should warm our hearts— this winter’s frigid weather and wild temperature swings. A study conducted by the USDA Forest Service and Minnesota Department of Agriculture concludes EAB are not as cold-hardy as previous research suggested.
According to authors Robert C. Venette and Mark Abrahamson, “…when larvae reach -17.8°C (0°F), 5% will die; at -23°C (-10°F), 34% will die; at -29°C (-20°F), 79% will die; and at -34°C (-30°F), 98% will die.”
Furthermore, the study found that when cooled quickly, EAB don’t have time to acclimate and will die at much “warmer” temperatures. In other words, this is a great winter for delaying the arrival of EAB in northern New York. But the study cautions 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…[they] may be afforded some protection against brief drops in temperature.”
This winter will buy us time, but emerald ash borers will get here eventually. It’s important to learn the signs of infestation and scout frequently. When an adult EAB emerges from a tree it makes a distinctive D-shaped exit hole, with the straight part of the ‘D’ on the bottom. The hole is tiny—1/8” across—and hard to see.
Sprouts growing out of a trunk or large branch are called epicormic shoots, and are a good indicator of EAB infestation. Another conspicuous sign, one that’s easiest to see in winter, is woodpecker activity on ash bark (not deep cavities). Report all suspected cases of infestation to the New York State Department of Environmental Conservation or your Cooperative Extension office.
It’s suggested homeowners not treat ash trees until EAB is confirmed within 10 miles. Keep in mind that even trees with early infestations can be saved through treatment. Insecticide options can be found at www.emeraldashborer.info or call Cornell Cooperative Extension for details.
Planning, cooperation and community involvement are the keys to weathering the inevitable EAB storm. EAB-ravaged cities are already replacing trees (employing a far greater diversity of species), returning the emerald color to their neighborhoods. Dorothy would approve.
Paul Hetzler is a forester and Cornell Cooperative Extension of St. Lawrence County horticulture and natural resources educator.