The emerald ash borer (Agrilus planipennis) is an invasive insect species which was brought to North America in 2002. Since it has no natural predators capable of controlling it in North America, it is now threatening to wipe out all its ash trees 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.
The emerald ash borer is native to northeast Asia. There it attacks not only the local ash species, but also elm, walnut, and Japanese wingnut. However, in its native environment, biological checks and the trees’ own chemical defenses provide a natural balance. Even where these other tree species are available in North America, the emerald ash borer still prefers to attack local ash trees exclusively, which have no natural chemical defenses.
The earliest discovery of the emerald ash borer in North America was in June 2002, in Canton, Michigan. It is believed to have stowed away in improperly-treated wooden cargo crates carried by Asian ships. Backtracking what is known of the infestation now suggests that the emerald ash borer had been present in North America for nearly a decade before its discovery. Immediate investigation by the Canadian Forest Service in the regions downwind from Canton discovered the ash borer already well established in Essex County. At the time of discovery, it did not even have a common English name.
Spread has been rapid, made faster by the fact that the emerald ash borer is capable of infecting healthy trees. Within five years, the ash borer has spread to ten states (Illinois, Indiana, Maryland, Michigan, Missouri, Ohio, Pennsylvania, Virginia, West Virginia, Wisconsin) and two provinces (Ontario, Quebec), with suspected sightings in Minnesota. Because the confirmed Quebec infestation lies directly north of New England, it is now believed the infestation may already be established in New England.
The U.S. Department of Agriculture originally tried to eradicate the emerald ash borer, but that genie was already out of the bottle at the time of first discovery. Today the U.S. Department of Agriculture, the Canadian Forest Service, the Ontario Ministry of Natural Resources, and the Canadian Food Inspection Agency are working together to quarantine infected areas to limit spread until another solution can be found. Road signs in these regions remind travelers that the transport of firewood in these areas is forbidden.
At present, there are no effective chemical protections against the emerald ash borer. One pesticide which has been used with limited success against the emerald ash borer is imidacloprid. 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.
Biological attempts at limiting spread include the Michigan release of three different species of parasitic wasps known to feed on emerald ash borer larvae. The long-term ecological effects of this release remain unknown.
Many major cities within the current region of infestation have placed a moratorium on further planting of ash trees. Several state and provincial have tried to create ‘firelines’ against the ash borer by removing every ash tree in a four-mile wide strip beyond 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.
