Over the last few years, I have noticed something quite shocking. Many 100+ year old oaks in the twin cities dying quite suddenly. Symptoms usually start in the sunny upper canopy and progress downwards over time. I see dieback at the end of branches, sparse, small or discolored foliage, or leaves that wilt suddenly, turn uniformly brown and typically remain attached to the branches. This is followed by the entire tree dying with maybe a few green leaves clinging to life in the lower canopy. The culprit is the twolined chestnut borer (A. bilineatus). Although the pest’s common name associates it with chestnut (Castanea) trees, oak (Quercus) trees are in fact its main host plant.

Consider the oak and the twolined chestnut borer old friends who have evolved together for centuries. In the circle of life, twolined chestnut borer beetles are opportunistic members of Mother Nature’s clean-up crew, taking out weak and dying oaks. Under favorable conditions healthy trees can defend themselves. But as temperatures have risen in recent years, the insect’s population and winter survival rate have skyrocketed.

Stresses have also increased for oaks such as drought, flooding, bur oak blight, oak wilt, old age, or defoliators like cankerworms. Trees lose their ability to defend themselves from massive beetle onslaught. Compromised defenses combined with an excessive beetle population act like a pack of wolves attacking a moose. When they gang up on the tree in large numbers, they’re able to overcome its resistance.

Twolined chestnut border larvae feed under the bark and destroy the nutrient and water conducting tissues (i.e., the phloem and xylem). This girdles and kills the branches above the point of feeding. Trees can be killed in the first year of attack; however, death usually occurs after 2-3 successive years of borer infestation. The twolined chestnut borer is sun loving typically attacking the crown during the first year, with the remaining live portions of the branches and trunk being infested during the second and third years.

Symptoms of twolined chestnut borer can resemble those of other issues (e.g., diseases, environmental stress, etc.). To confirm the presence of twolined chestnut borer, branches must be sampled to look for additional evidence. Peeling bark from sampled branches is a standard procedure to check for twolined chestnut borer. Infested branches will have feeding galleries (tunnels) under the bark; fresh larval galleries will be packed with pale frass (sawdust-like material). Larvae may also be present. Adult emergence holes (D-shaped, and 1/8 inch wide) can sometimes be spotted as an additional clue. Samples can be sent to a diagnostic lab.

Because this insect attacks stressed oaks, any action that reduces stress will lower the probability of infestation. This includes:

• Avoid any injury or pruning of oaks from April through July, to prevent oak wilt.
• Avoid soil and ground disturbance within the crown, including digging, equipment and vehicle travel, installing landscaping tiles or stone and changing grade with fill or gravel.
• Maintain adequate soil moisture by watering oaks with a soaker or drip hose for 1 hour per week during dry periods through the growing season.
• For high-value oaks, systemic insecticide labeled for TLCB can protect and help oaks with limited dieback recover. The insecticide treatments for two-lined chestnut borer are similar to those used to protect ash trees from emerald ash borer.

Two lined chestnut borer starts with dead top branches.

Trees at this stage can recover with treatment.

White oak infested by twolined chestnut borer; photo shows branch mortality and faded leaves.

A group of oaks under attack from the twolined chestnut borer, note thinning upper canopies.

A dead bur oak in a stressful environment, killed by twolined chestnut borer.

Adult twolined chestnut borer is about one inch long when fully grown.

Outer sapwood on the tree trunk engraved by twolined chestnut borer larval galleries.