By Anand Persad, PhD
Urban treescapes are under attack. Seven billion ash trees, the dominant species of urban American canopies, are at risk of being destroyed by the invasive emerald ash borer (EAB) if not treated.
Since its first detection in Michigan in 2002, EAB has spread to 25 states and killed more than 50 million trees. Already, entire cityscapes have been destroyed. In a review published last year, scientists called it “the most destructive and economically costly forest insect to ever invade North America.”
Known for its metallic green wing color, EAB is hard to see with the untrained eye. Even more difficult to detect are the larvae that burrow into the bark and feed on tissue, eventually starving and killing the trees. Signs of infestation include thinning and yellowing leaves, D-shaped holes in the bark, and canopy and bark loss.
Scientists are working to find ways to stop the beetle. It’s been proven that efforts to save trees can be improved by identifying infested trees in their first year. Host trees are still healthy upon initial colonization.
Based on our research, early detection of EAB is critical to managing this invasive species and saving billions of ash trees. Traditional detection methods through sampling and debarking are extremely labor intensive. Monitoring through deployed traps is not sensitive enough to detect initial EAB populations.
Today, most infestations are several years old by the time they are detected. This is often too late for treatments to be effective. If the tree has lost more than 50 percent of its canopy due to EAB infestation, treatments are unlikely to save the tree.
Once discovered, the spectrum for managing EAB infestations varies from “no action” to “aggressive management.” Aggressive management removes dead and declining ash while treating remaining trees.
To discover an early EAB detection method, we investigated and documented symptoms associated with the EAB ash tree complex in urban sites for four years. Trees at the study site in Ohio ranged from those with no visible evidence of EAB infestation to those that were infested for more than two years.
Visual, non-destructive surveys were conducted from late June, after EAB emerges, through August each year between 2009 and 2012. The surveyed trees had not been chemically treated for EAB.
What we discovered is that EAB-positive trees tend to have branch fractures within the upper canopy, specifically located closer to the union of the trunk and the stem. Trees infected with EAB break first in the top of the tree canopy, which can be hard to see from the ground. Additionally bark cracks generally around the first scaffold branch/stem union area are highly correlated with presence of EAB in the tree.
These discoveries allow for EAB to be detected a full year earlier than previously possible, allowing for early treatment that potentially can save the tree. Homeowners can make an early diagnosis by looking for cracks near the lowest branch, and limbs that break closer to the trunk in the upper to middle of the tree canopy.
Multiple treatment options are available to help save trees from EAB. Plus, treating a tree is a much less expensive option than replacing it with one of a similar size. Treatment over the course of several years may be a fraction of the cost of removal, and the tree will continue to provide benefits for years to come.
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Anand Persad, PhD, is manager of arboriculture and plant sciences for The Davey Tree Expert Company, which provides tree care, grounds maintenance, and environmental consulting services throughout the U.S. and Canada.