Emerald ash borers


The tallest tree in my backyard miniforest (center left, not totally leafed out in this photo, which was taken in early May 2008) is a white ash (Fraxinus americana).

Our native ash trees are under severe threat from an Asian beetle, the emerald ash borer, that arrived in Michigan in 2002 and has since spread throughout much of the country (but not yet to Bergen County, although it has been found both north and south of us). Relatively harmless to healthy trees in its native habitat, the beetle kills our native ashes in 3-4 years. Read what to look for and some ways to prevent infestation here.

Today’s Science Times has an article on current efforts to use the trees’ natural defenses to protect them. Plants are always fighting threats from funguses, insects, and other predators, and they have many defenses against attack (ever been stuck by a thorn?). In Asia, ash trees have evolved specific defenses against the emerald ash borer, and scientists in this country have been studying how they work. It turns out that Asian ash trees make specific chemicals to fend off the borers, and although American ashes produce some of the same chemicals, they may not make them fast  enough, or in the right combinations, to defend against the borers.

But researchers have recently found that a plant hormone, methyl jasmonate, acts as a kind of alarm, prompting the entire tree to produce chemicals that kill the borers. And when American ashes are sprayed with this hormone, they too produce chemicals that kill the borers. The effect is as great as a strong dose of insecticides.

The next step would be to develop a line of resistant American ash trees, trees that can mobilize natural biological responses as fast as the Asian trees can. Until that time, monitor your ash trees for signs of damage. The most important and earliest would be dieback in the crown of the tree. If you see that kind of damage, notify the US Forest Service. And don’t move firewood: that’s a major way that the borers are spread.


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