Data from the tree ring study showed how the infestation grew and spread. Some of the spread was natural adult beetles flying from one ash tree to another. However, new "satellite" populations were started by people transporting infested ash trees from nurseries or as logs and firewood. By 2003, they had expanded beyond the six counties encompassed by the tree ring study.
Part of the problem was the difficulty of detecting new infestations, said McCullough. Ash trees have no symptoms when they are first infested. By the time a tree is declining, at least three or four generations of beetles have emerged and gone off to colonize new ash trees.
In addition, reports of declining ash trees were not uncommon in Michigan and surrounding states in the 1990s. Problems ranged from road salt to drought to changing water tables. When shiny green beetles emerged from dying ash trees, however, researchers knew it was something out of the ordinary.
Specialists at the Smithsonian Institute and London's Museum of Natural History could not identify the beetles. Eventually, an entomologist in Slovakia, who intensively studied these and similar beetles, was able to identify the specimens. Still, the species had no common name until the MSU entomologists and their colleagues came up with "emerald ash borer."
By the time Michigan identified the invader, ash trees across southeast Michigan were dead or dying, McCullough said. "We think emerald ash borers probably arrived from China, where they attack only very stressed or dying ash trees. Because of that, they are not considered an important pest in China. The Asian ash species have evolved with the beetles so healthy trees there are resistant to them. In North America, emerald ash borers would still prefer to attack stressed trees but it will do fine on healthy trees, too."
|Contact: Layne Cameron|
Michigan State University