Emerald ash borer (EAB), an invasive insect native to Asia, was first identified in Michigan in 2002 and has since been observed in other Great Lakes states and Canada. All types of true ash (Fraxinus) are susceptible to borer damage. Mountain ash (Sorbus) is not. Adults are emerald green beetles; larvae are white borers that tunnel in a serpentine pattern under the bark. Damage to water-conducting tissues can be so severe that the tree dies within 2-3 years. EAB has the potential to be devastating to ash trees in the landscape and in native forests throughout North America. For now, containment is the primary control strategy of foresters. When an infestation is discovered, all ash trees nearby are destroyed in an attempt to prevent the borers from spreading from tree to tree. Various systemic insecticides are also being tested. Many work well, but none is 100 percent effective. There is no need to apply insecticide to your ash trees if you don't live near an infection zone.
If you're thinking of planting a new shade tree and live in the Great Lakes region, consider planting something other than an ash. Containment may eradicate the emerald ash borer, but if it fails to do so you may be setting yourself up for yearly insecticide treatments to protect your ash tree. Initial symptoms of EAB are general yellowing and thinning of the foliage. Branches begin to die back. Often you'll see woodpeckers working on the tree, extracting the larvae that live just under the bark. If you look closely, you may see D-shaped exit holes in the bark (where larvae emerged), serpentine tunneling under the bark, or adults and larvae. If you suspect EAB damage, check with your city forestry department or county cooperative extension office to confirm the diagnosis and to find out about the most current control measures.