The emerald ash borer (Agrilus planipennis) is an invasive insect species that is threatening to wipe out all the ash trees of North America in the same way as the elm trees once fell victim to Dutch elm disease. Susceptible species are the white ash (Fraxinus americana), green ash (Fraxinus pennsylvanica), and black ash (Fraxinus nigra). Other, rarer, ash species are affected as well, such as the pumpkin ash (Fraxinus profunda), but because these are further apart in the affected region, the infestation has not yet been able to spread as widely among these species.
Upon landing on the tree, a female ash borer lays her eggs in crevasses in the bark. These eggs hatch into larvae that will grow up to an inch long by feeding on the tree’s phloem and xylem, thus destroying its circulatory systems. These feeding tunnels are visible just beneath the bark as vertically-oriented S-shaped tunnels. In younger trees, the presence of such feeding tunnels may be signaled by vertical splits in the bark, although in older trees these splits may be disguised by normal bark expansion and growth.
When the ash borer is fully mature, it will leave the tree by chewing a distinctive D-shaped hole out through the bark, which may also contain some sawdust. These holes will be most frequently visible in June and July. The adult ash borer eats only the foliage, which is much less damaging to ash trees.
Because ash borer larvae are contained within the stem of the tree, just under the bark, it can be up to a year before any surface damage is visible. By the time mature insects emerge onto the outside of the tree and the infestation becomes visible, the damage has already been done and the mature insects are already able to fly, surviving just long enough to find a new tree and lay their eggs. The full life cycle of the emerald ash borer lasts one year, perhaps two years in colder climates.
Early symptoms of a weakened circulatory system which may indicate ash borer infestation include generalized crown dieback and opportunistic leaf diseases, and new foliage may be yellowed and wilted. In a severely-attacked tree, up to the top third or half of all branches may die in a single year. After a year or two of infestation, the under-crown will also begin to die. It is also common for young shoots to spring from the base of the tree, below the region of major circulatory disruption.
Increased interest by woodpeckers indicates a high local population of insect species, some of which may include ash borers. However, the crevasses created by woodpeckers also make it easier for ash borers to lay eggs.
There are native predators for the ash borer in North America, but none capable of keeping it in check. Three different species of parasitic wasps known to feed on ash borer larvae have been released in Michigan, but the long-term ecological effects remain unknown.
Because the emerald ash borer feeds on the circulatory system of the tree and its parent cambium, any pesticide must become systemic within the tree to be effective. One pesticide under consideration for use against the emerald ash borer is imidacloprid, which is applied by spraying. However, imidacloprid has been linked to behavior disruptions in honeybees, which may be related to colony collapse disorder.
Another pesticide which has shown some promise is TreeAzin, a derivative of the neem tree, but it must be injected into individual trees, and is only effective before the tree starts showing symptoms of infestation. This treatment is prohibitively expensive on a forest-wide basis, and the need for injection also increases the tree’s vulnerability to other agents. Because TreeAzin research has only begun in 2008, it remains unknown whether TreeAzin only delays the death of the tree.
Many major cities within the current region of infestation have placed a moratorium on further planting of ash trees. Ash borer quarantines are currently in place in ten states (Illinois, Indiana, Maryland, Michigan, Missouri, Ohio, Pennsylvania, Virginia, West Virginia, Wisconsin) and two provinces (Ontario, Quebec), with suspected sightings in Minnesota. Road signs in these regions remind travelers that the transport of firewood in these areas is forbidden. Because the confirmed Quebec infestation lies directly north of New England, it is now believed the infestation may have already spread to New England.
Several state and provincial have tried to create ‘firelines’ against the ash borer by removing every ash tree in a four-mile wide strip past the last identified point of infection. Many of the removed trees have themselves proved to be infested with emerald ash borer larvae, requiring the quarantine area to be extended. So far, the ash borer has managed to spread past every one of these attempted barriers. Since ash borers are incapable of extended flight, it is believed that much of this spread has been due to the transport of infected firewood.