Will We Kiss Our Ash Goodbye?

American Forests, Winter 2013

An ecological catastrophe is unfolding across the upper Midwest and is spreading outward, as scientists struggle to find solutions to the latest insect invasion: emerald ash borer.

By Dr. Deborah G. McCullough

Ash trees in a Toledo, Ohio neighborhood in June 2006. Credit: D. Herms

Toledo, Ohio trees after emerald ash borer in August 2009. Credit: D. Herms.

It’s not like we haven’t seen this sort of thing before. In the early 1900s, people who lived in the eastern U.S. watched chestnut blight, an exotic pathogen, roll through, killing large and small trees and altering the hardwood forest forever. A few decades later, Dutch elm disease, an exotic pathogen carried by an exotic bark beetle, came through, killing majestic American elms along city streets and in forests. Today, more than 450 species of nonnative forest insects and at least 17 significant forest pathogens are established in the U.S. Most go unnoticed, but about 15 percent have had major consequences. And it’s starting again.
Emerald ash borer, an Asian insect first identified in Detroit, Mich., in 2002, has become the most destructive forest insect to ever invade the U.S. Tens of millions of ash trees have already been killed in forests and swamps, along waterways and in urban, suburban and rural neighborhoods. Populations of emerald ash borer, commonly known as EAB, have been found in 18 states, along with Ontario and Quebec. And almost assuredly, there are more populations, simmering away, that haven’t yet been discovered.

BEAUTIFUL KILLERS 

An adult emerald ash borer feeding on a leaf. Credit: David Cappaert.

Adult EAB beetles are beautiful insects and amazingly good at finding and colonizing ash trees. Unlike many insects, EAB does not appear to produce any long-range pheromones to attract potential mates. Instead, the beetles use their vision and the mix of chemicals emitted by ash leaves, bark and wood to find their host trees and each other. They are particularly attracted to the blend of compounds given off by stressed or injured ash trees and to specific shades of purple and green. Once beetles find an ash tree, they nibble along the margins of leaves throughout their three- to six-week life span. Leaf feeding is important for the beetles to mature, but it has virtually no effect on the trees. After 15 to 20 days of leaf feeding, the females begin to lay a few eggs at a time, tucking them beneath bark flaps or in bark crevices. Many beetles mean many eggs — bad news for the tree when they hatch. The tiny, cream-colored EAB larvae hatch from their eggs in mid-summer and chew through the rough outer bark to reach a layer of inner bark, called phloem. Phloem is the tissue used by trees to transport carbohydrates and other nutrients from the canopy down to the roots. The larvae feed in s-shaped tunnels, called galleries, for several weeks in summer and early fall. As the larvae grow, the galleries increase in size. Galleries often etch the outer ring of sapwood, which ash trees use to transport water up from the roots to the canopy. A few larvae feeding in a large branch or on the trunk of an ash tree have little effect on the tree. Over time, however, as the density of larvae builds, the ability of the tree to transport nutrients and water is disrupted by the galleries. The canopy begins to thin, and large branches may die. Eventually, the entire tree succumbs.

Once EAB populations begin to build, nearly all ash trees in the forest, swamp or urban area are likely to become infested and die — often within a time span of only a few years. Read more....