emerald ash borer

Emerald ash borer, adult female.

The emerald ash borer is gorgeous. Tiny, bright green but shimmering with shades of gold, she quietly munches on a few leaves in the spring. Then she begins her devastating work, depositing tiny translucent eggs on the trunks of ash trees. Larvae soon hatch, eat their way through the bark and then settle in for a season of feasting on the tender tissues within – the cambium, phloem and young xylem. A tree cannot survive without these tissues and by next spring the tree will be dead above where the larvae were feeding. It may take only one year or up to five, but the tree is doomed.

The emerald ash borer, Agrilus planipennis Fairmaire in the arcane language of biology, arrived in or near the port of Detroit around 1990. It began killing ash trees in the Detroit area, though it was not noticed for a few years. Left to its own resources, the emerald ash borer would still be hanging around near Detroit. The beetle is tiny, and although females are skilled flyers, they can only fly a short distance during their life span of 2 to 3 weeks.  A female may fly half a kilometer, a really athletic one may go 1.5 km. And that’s it. This beetle only produces a single generation of adults each year.

map of emerald ash borer

Emerald ash borer status in 2008 and 2016. Colored states are infested; red dots are counties of first report. Click for larger map.

The beetle is thoroughly established in my home of Lexington, KY. It is also in New York City and Atlanta. The distance from Detroit to each of these cities is about 550 km, 1000 km and 1160 km respectively. Emerald ash borer would, on its own, take 300 to 1300 years to reach Lexington, 600 to 2500 years to get to New York, and 700 to 3000 years to reach Atlanta.  In fact, the emerald ash borer only needed ten to fifteen years to reach those cities.

Consider the map of emerald ash borer in 2008 and 2016. The rate of spread is astonishing. Instead of taking decades, the insect is leaping from state to state, from city to city, like a wildfire that leaps over its boundaries and is carried by wind. But it is not the wind that carries emerald ash borer. It is us.

If you live in the eastern US or Canada, you’ve seen the billboards on the highway or the posters in campgrounds: Don’t Move Firewood.  The emerald ash borer has taken advantage of the behavior of its major vector, the American camper. We love to camp. We take our trailers or RVs or tents and wander across the landscape. Everywhere we go, we cannot resist sitting in front of a lovely campfire, flames dancing, smoke rising, wood crackling. And we bring the wood with us. Not content to forage for wood near our campsite, or to buy wood from a local vendor, we insist on loading up our trailers with enough wood to carry us through our vacations. In that wood are beetle larvae, quite happy to hitch a ride to a new home where ash trees wait.

Nothing that we do seems to discourage our firewood obsessions. The US Department of Agriculture (USDA) began with quarantines, forbidding the movement of wood out of infested zones. That didn’t work. Then state and federal authorities turned to public relations, putting up billboards and signs, handing out flyers at campgrounds. It is not working. Instead of taking thousands of years to spread throughout the range of ash trees, our firewood obsession has allowed the emerald ash borer to sprint across the continent, spreading death wherever it goes.

Ash trees killed by emerald ash borer

Beetle-killed ash trees in an urban area.

The mortality rate is astonishing – the beetle kills 90-100% of ash trees in infested areas within a few years. Ash is not a minor tree. It represents about 11% of the standing biomass of eastern forests, and roughly the same proportion of many urban forests.  Forests are beginning to look forlorn, the landscape strewn with standing dead trees and the corpses of the fallen.

The situation is not without hope.  Scientists from the USDA and several universities, in collaboration with Chinese scientists, have been catching and breeding predators and parasites of emerald ash borer in China, the original home of the insect. These are now being released in large numbers in the US and Canada. There are also native predators and parasites of closely related beetles in North America. Slowly, given enough time, the emerald ash borer may begin to succumb to their own enemies.  It is also possible that there are resistant ash trees that will survive to reproduce. For any of this to happen, we need time. It will take decades, maybe centuries, for emerald ash borer and their enemies, and our native ash trees, to settle into a sustainable relationship.  But as long as we are unable to slow our obsession with moving firewood, we limit the possibilities for nature to take its course.

If we fail and some day ash trees are only a memory consisting of some furniture and a few old baseball bats, historians will look back and say that it wasn’t the beetle that killed the trees, it was us.

Previous stories: Dead trees and the Great Curve, Tree Volatiles Help Create Clouds

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