The discovery of holes in a tree trunk or its branches usually means it has been the victim of a boring insect attack. While it is alarming to discover an increasing number of trees with significant borer damage, it is not unexpected. Most borers take advantage of trees weakened by drought stress, injury, insects or disease. Several years of extremely dry winters along with last summer’s extreme heat has stressed local trees and shrubs, making them vulnerable to attack. Some of the dastardly culprits attacking local trees include:
• Ash Borer (aka lilac borer): The adult is a moth that looks like a yellow jacket. It primarily attacks ash, lilac and privet. The moth lays its eggs on the bark. They hatch into small larvae that tunnel into trunk and branch wood, weakening it. The larvae pupate and emerge from the tree in May or June, leaving noticeable one-fourth-inch exit holes.
• Redheaded Ash Borer: The adult is a “longhorned” reddish brown beetle with an elongated body and long antennae. It also resembles a yellow jacket because of the yellow to white horizontal bands on its back. While called an ash borer, it attacks a variety of trees, including ash, linden, oak and others. Like many other borers, it lays its eggs on the bark of stressed or dying trees. These hatch and then eat their way under the bark and tunnel into the wood as they mature. There may be more than one generation of these borers a year with adult beetles emerging from spring through summer and leaving 1/4” exit holes.
There are different types of bark beetles, and one or more of these are attacking local stressed arborvitae and other evergreens. Typically, the adults are small, brown beetles. What they lack in size they make up in number. They feed directly under the bark, creating serpentine paths as they eat. Their feeding can girdle trunks and branches, cutting off the tree’s access to water and nutrients. Their exit holes are pencil-point sized.
Other borers that commonly attack landscape trees in this area are the bronze birch borer, the peach borer and the locust borer.
Unfortunately, pesticide applications are not that effective for borer control in attacked and dying trees. Sprays made to the bark surface will not kill any borers residing under the bark or within the wood. For sprays to be effective, they would need to be applied when the adults emerge. Timing of sprays is critical, and they may need to be reapplied if the insect emerges over a span of several months, or has several generations a year.
There are some systemic insecticides that are applied as a drench to the base of trees and taken up into plant tissues. These are only effective on some flatheaded borers, like the bronze birch borer, that spend most of their time feeding in tissue just beneath bark. They are not effective in controlling borers that eat mostly in tree wood.
Washington State University Extension experts indicate that the best control for any borers is to keep trees healthy and vigorous to prevent attack. This is sage advice, but too little too late for attacked and dying trees.