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. In southeast Michigan, where EAB was first established, scientists have documented 99 percent mortality in forest stands dominated by green ash (Fraxinus pennsylvanica), white ash (Fraxinus americana) or black ash (Fraxinus nigra). More than 60 million ash trees, ranging from one inch to five feet in diameter, have been killed by EAB in this area alone.
We know most adult EAB stay within about a half mile of where they emerge. In any population, however, at least a small proportion of beetles seem to fly farther — for reasons that are as yet unknown. Adult EAB are relatively good fliers; they’re much more agile and streamlined than bark beetles, for example. Mature females are probably capable of flying three miles. Unfortunately, EAB has been moved across longer distances by people who unknowingly transported infested ash trees from nurseries or recently cut logs or firewood. Once an ash tree dies or is cut, the phloem dries out, and it will not be re-infested, but any larvae already under the bark can complete their development and emerge as adults. Federal and state quarantines have been imposed to regulate the transport of ash trees, logs, wood and related materials to reduce the risk of additional EAB introductions.
In fact, accidental transportation of infested ash is probably how EAB got to North America in the first place. The introduction of EAB into the U.S. and Canada almost certainly occurred when infested wood crating or pallets originating in China arrived in the U.S. In its native range in China, EAB functions as a secondary pest, colonizing only severely stressed or dying Asian ash trees. In North America, however, native ash trees have no co-evolutionary history with EAB and have few defenses to resist this pest. While EAB beetles still prefer to colonize stressed ash trees, they will also readily infest — and eventually kill — healthy ash trees.
THE HIGH COSTS OF INFESTATION