By: Jean Ferdinandsen Certified Arborist WI-0149A
Two-lined chestnut borer was a principal pest of American chestnut. Now it is a common secondary invader in oaks. This insect pest only invades weakened trees. It is probably the most important insect cause of oak mortality. Outbreaks usually follow extended periods of drought or defoliation by insects such as gypsy moth or tent caterpillars. Other stress factors such as soil compaction, construction damage, trunk or root injury, or chlorosis can predispose trees to invasion by this insect.
The impact of two-lined chestnut borer can range from scattered branch dieback, to total 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 death of infested portions of the tree.
The infestation first becomes visible in mid-July. Leaves of infested branches turn a uniform brown and are held onto the tree (an important distinction from oak wilt). Borer attacks and dieback occur first in the upper crown and then extend downward to the main trunk. Trees can be killed in the first year of attack, but it usually takes a two to five year period as the tree weakens. Infested oaks often have a distinctive pattern of dead leafless branches in the upper crown, branches in the middle crown are partially alive and have red/brown leaves, and branches in the lower crown are alive and have green leaves. ("Dead, Red, Green "pattern)
Adult beetles emerge in late May from a "D" shaped exist hole. Eggs are laid shortly after emergence. These quickly hatch and bore through the bark into the tree. The larvae overwinter inside the tree. Two years are required for complete insect development.
Maintaining tree vigor is essential to preventing two-lined chestnut borer attacks. Oaks that are maintained through regular pruning, fertilization, mulching, watering, etc. are less susceptible to borer attacks. Nothing can be done to save infested portions of a tree once symptoms are visible, because at that time damage is nearly complete.
Prune out and destroy all dead/dying branches to help reduce an infestation. Water trees 1"/week if dry. Promote sustainable growth. If fertilizing during dry conditions, make sure extra water is available. Avoid construction damage (esp. to the trunk & roots) and compaction. Avoid lawn fertilizers with weed killers (esp. dicamba). Work to enhance the root system via mulch, mycorrhizae inoculations, use of compost tea or root stimulants, and soil aeration.
Trunk injections of insecticide can get rid of borers in the tree. This should be followed up with soil systemic insecticides for a period of time to deter further insect attacks. New oak transplants should be protected with insecticide trunk sprays and systemic insecticide until fully established.
A multi-pronged approach is often needed when two-lined chestnut borer strikes. Care, treatment and pruning may all be necessary. Our Certified Arborists can work with you to determine the best approach. Wachtel Tree Science is your partner for healthy trees.