Emerald ash borer treatment timing is a judgement call

October 9, 2018

With the recent discovery of emerald ash borer (EAB) in Fremont, some ash tree owners might be considering having an injection treatment done to their ash tree.

The Nebraska Forest Service’s recommendation of waiting to treat until EAB has been found 15 miles from a tree is still in place, however, some people prefer not to wait.

If this is the case, know that trunk injections done during fall do not work as well as spring injections. At the very least, wait until next spring before treating an ash tree.

There is no harm in waiting as most borers have stopped feeding for this year and additional damage would not occur before next summer.

Fall treatments can require double the amount of insecticide to provide the same level of control; and with fall applications, treatment may need to be done once a year rather than every other year.

Treatment does not make a tree immune to EAB. Treatment needs to continue for a tree to be saved. And while injection treatments by a professional are considered most effective; there are drawbacks to injections.

To inject the product, a hole is drilled into the trunk. This wound opens the trunk to insect pests and decay. Drilling may also break through internal barriers the tree has developed within the trunk to wall off internal decay, causing decay to spread.

The pesticide itself can cause internal damage that may accumulate over years of repeated injections and possibly kill the tree, even if the pest is controlled. Deep injection holes, and the more product applied, increases internal damage to trees.

There are good reasons to wait until EAB has been found 15 miles from a tree before starting treatment. This recommendation strikes a balance between protecting valuable trees and limiting negative effects of unnecessary treatments.

Also, emerald ash borer does not kill a healthy tree quickly. It takes a few years of infestation for trees to begin to decline, and about five years for an otherwise healthy tree to be killed.

It has also been shown if treatment of an ash tree begins when 30 percent or less of the canopy has dieback from EAB, an otherwise healthy tree can recover. Trees with over 50 percent canopy dieback are much less likely to recover.

There are good reasons to wait. But if you decide not to wait until EAB has been confirmed 15 miles from your tree, and you plan to have your ash tree treated, at least wait until spring.

April through June is the key time for treatment as this is when trees most readily take up a systemic insecticide.

For information on emerald ash borer, go to www.nfs.unl.edu/Nebraska-eab or type Nebraska + EAB in your search engine.

Kelly Feehan is a community environment educator for Nebraska Extension.