Emerald ash borers arrive in the West. How far will they go?


In today’s edition of the HCN Death and Disease Report: The emerald ash borer, a half-inch long, iridescent green beetle that’s decimated Eastern and Midwestern hardwood forests, has touched down in the West for the first time. Foresters in Boulder County, Colo., noticed a “suspect tree” on Sept. 23, and now it’s official: the emerald ash borer has joined spotted knapweed and zebra mussels among the state’s least-welcome visitors, right up there with people who stop dead in traffic at the sight of an elk.

Experts say it’s only a matter of time before the pest, which is native to Asia, continues to spread westward, threatening native trees and urban forests alike.

The emerald ash borer was first discovered in Michigan in 2002. By 2011, it had spread to 15 states, mostly through the transportation of infected firewood. Today, it’s reached 22 states and two Canadian provinces, and more than 50 million ash trees have been killed by the larvae, which feed on the inner bark, disrupting the tree's ability to transport water and nutrients. Regular applications of insecticides can help, but only if the infestation is caught early. Most trees simply die.

Colorado marks the farthest West the emerald ash borer has yet been detected, and though the state Department of Agriculture is taking steps to quarantine Boulder County, a friend of mine who works full time battling invasive beetles back East told me, simply, that the emerald ash borer “can’t be stopped.”

The deceptively beautiful emerald ash borer, which has the potential to devastate urban and native forests in the West. Courtesy USDA.gov.

While Colorado is home to some 2 million ash trees, though, they aren’t native to the state. Most were planted in the 1970s and ‘80s after Dutch elm disease wiped out the then-popular elms. (Sense a pattern here?) Now, ash comprise up to 80 percent of the canopy in some urban subdivisions, and 15 percent of all trees along the well-developed Front Range.

One might imagine that foresters wouldn’t be terribly upset about one non-native species getting rid of another. Homeowners, however, are already shelling out thousands of dollars to try to protect their trees, and the local news is full of lamentations for the beloved ash. Like Californians who rose up earlier this year to oppose the thinning of introduced eucalyptus in the Bay Area, Coloradans’ — and other Westerners’ — impassioned response to the emerald ash borer shows just how important the urban forest has become in the increasingly urbanized West.

According to Friends of the Urban Forest, urban trees increase property values, decrease air conditioning costs and pollution, and reduce flooding. They’re even credited with calming road rage, increasing spending and reducing crime. And especially for urbanites in historically shade-free regions like the Front Range, they’re pleasant to have around.

Ash are also popular in urban and suburban environments in Wyoming, Montana and other Western states, and foresters there are alert for signs of the ash borer. One city arborist in Oregon told a blogger that he stopped using ash as a street tree three years ago in anticipation of the ash borer’s arrival.

Yet though ash are beneficial to urban landscapes – not to mention expensive to replace ­­– they are replaceable, and their ecological importance is arguably slim. Not so in parts of Oregon, Washington, California and even Arizona, where millions of native Western ash species (Fraxinus velutina, the Arizona ash, and Fraxinus latifolia, the Oregon ash) that can live to 250 years make up unique riparian forests so far unharmed by the destructive beetle that every year creeps closer. No one seems to know for sure whether Western ashes are susceptible to the ash borer, but given that they’re in the same genus as other ash species, it seems likely.

Krista Langlois is an editorial intern at High Country News. She tweets @KristaLanglois2.

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