Oak Pest Problem – Two-lined Chestnut Borers

The two-lined chestnut borer was once a principal pest of American chestnut trees. Now that most American chestnuts have died due to chestnut blight, it has become a common secondary pest of oaks. It is likely one of the most important insect causes of oak mortality.

Outbreaks usually follow extended periods of drought or defoliation by insects such as spongy moths. Other factors such as soil compaction, construction damage, trunk or root injury, or chlorosis can also predispose trees to invasion by this insect. The impact of the two-lined chestnut borer can range from scattered branch dieback to the eventual death of the tree. Larval feeding in the inner bark of branches and the main trunk impedes water and nutrient transport. This girdling results in the death of infested portions of the tree. While trees can be killed in the first year of attack, it more commonly takes two to five years as the tree gradually weakens.
Infested oaks often display a distinctivepattern:
• Dead, leafless branches in the upper crown
• Partially alive branches with red or brown leaves in the middle crown
• Healthy, green-leaved branches in the lower crown

What should you do if your tree is infested?
Preserving tree vigor is essential in preventing two-lined chestnut borer damage. Prune out dead or dying branches to help reduce infestation. The most effective way to eradicate an active infestation is by using a trunk injection of an insecticide. After the approximately two year protection period provided by the injection, follow up with a soil-applied systemic insecticide to help deter future attacks. Water trees during periods of drought, about 1 inch per week, to promote sustain-able growth. Improving the root system is also important. Practices such as mulching, applying mycorrhizal treatments, using com-post tea or root biostimulants, and performing soil aeration can all aid in recovery.
If your oak tree shows signs such as branch dieback, small or yellowing leaves, or premature leaf drop, it is time for an evaluation. Treatment options will depend on the stage of infestation, the tree’s location, and its overall health.
Contact a Wachtel Tree Science Certified Arborist to help determine whether your oak tree is a good candidate for treatment.

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