This column is in response to a number of questions I have received about treating ash trees for emerald ash borer. This accidentally introduced pest has been present in the Pittsburgh area since 2007, and now many untreated ash trees are dying.
The emerald ash borer was first discovered in the United States in 2002 near Detroit. Entomologists believe this pest was accidentally introduced in wood crates or pallets that were used to ship products to the United States from its native Asia. The borer's larvae feed just under the bark on the vascular system of host plants. If larvae were present when the tree was felled, they could survive and hatch out as adults as long as they were not injured in the process of turning the tree into lumber. It is likely that the infestation was present for 10 to 20 years before the insects were identified.
Despite quarantines on the movement of ash nursery stock, logs and wood chips, emerald ash borer has been confirmed in 19 states as well as Ontario, Canada. The first infestation in Pennsylvania was identified in 2007, in Cranberry. Currently, 34 counties in Pennsylvania have confirmed infestations, with more likely to join the list through the growing season. The borer has killed more than 40 million ash trees to date. Only trees in the genus Fraxinus are susceptible to damage from this pest.
Entomologists from Michigan State, Ohio State, Purdue and University of Wisconsin have been working together since emerald ash borer was first identified to develop treatment protocols and management plans to help communities deal with the aftermath of losing so many trees in a short period of time. One of the group's recommendations is that treatment of valued ash trees should begin when the emerald ash borer has been discovered within 15 miles.
The Pittsburgh area passed that threshold five or six years ago. Trees that have not been treated all along should be carefully evaluated to make sure they are worth treating before spending the money.
The systemic insecticides that provide the most effective control of emerald ash borer move in the tree's vascular system. If larval feeding has damaged the vascular system too much, the insecticide cannot get to the crown of the tree where it is needed. Treating trees that are too far gone will not save them and is a waste of money.
Trees are worth treating if more than half the crown is still alive and the tree appears to be healthy with few signs of borer infestation. Not all trees are worth treating, even if they do not show symptoms of heavy infestation. Treatment is an ongoing expense and should be reserved for those that significantly enrich the landscape or have sentimental meaning to the owner. Trees are not worth treating if less than half the crown is still alive, the tree has significant woodpecker damage, or it has started to send out new shoots low on the trunk (epicormic sprouts).
Homeowners can successfully treat trees that are less than 20 inches in diameter at breast-height (dbh) with products that contain 1.47 percent imidacloprid as their active ingredient. These are applied as a soil drench. Calculate the dbh by measuring the circumference of the trunk 4 1/2 feet above the ground, then dividing the result by 3. Soil drenches are most effective when applied mid- to late spring.
Larger trees -- those more than 20 inches dbh -- should be left to professional arborists because they can use products such as emamectin benzoate and dinotefuran that are not available to home gardeners. These materials have shown improved efficacy in treating very large trees. Certified arborists also have the training and experience to inject insecticides through the bark of the tree using specialized equipment. Trunk injections are best made from early May to mid-June.
If treatment is not an option, be prepared to remove infested trees as soon as possible because they become hazardous very quickly once they have been killed by emerald ash borer.
For more information on treatment options and assessing the viability of infested ash trees, visit http://emeraldashborer.info. For help locating a certified arborist, visit the International Society of Arboriculture at www.treesaregood.org/findtreeservices/FindTreeCareService.aspx.
Send questions to Sandy Feather by e-mail at firstname.lastname@example.org or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.