Damage

Two-lined chestnut borer adults feed on leaves, but this damage is minor. The real damage is done by the larvae which tunnel under the bark, damaging the water-conducting tissues of the tree. This leads to wilting of leaves and dieback of branches.

This insect attacks weakened trees. Two-lined chestnut borer is not an aggressive pest targeting healthy trees and is not usually the primary cause of decline of a tree. It is an opportunistic pest that takes advantage of a tree that is already under stress from some other problem. On healthy trees, the borer’s activity is usually restricted to branches that died from some other cause.

Life Cycle

Two-lined chestnut borer (TLCB) overwinters under the bark of the tree as a larva. The larvae pupate in spring and emerge as adults by boring D-shaped exit holes through the bark.

Adult emergence occurs in late spring, around 500-550 growing degree days (base 50), when arrowwood viburnum (Viburnum dentatum) is beginning to flower and black locust (Robinia pseudoacacia) is in full flower.

The adult beetle is ½-inch or less in length, narrow, and black with a blue-green tint and two yellow lines down the back.

Once they emerge, the borers fly to the tops of oaks or other host trees to feed on the leaves, and then will eventually move down the tree to the branches and bark where they mate.

The females lay their eggs in cracks in the bark and the larvae hatch in one to two weeks. The larvae then bore through the bark into the cambium layer, where they feed and create galleries. This damages the sapwood, which conducts water throughout the tree.

At maturity, the legless larvae are white, slender, up to an inch long, and have a wider flattened head. At this point, usually in late summer or early fall, the larva creates a small cavity in the outer bark or sapwood and overwinters.

The TLCB typically has one generation per year.

If eggs are laid in the late summer and the larvae are not fully mature when winter arrives, they remain in the cambium layer through the winter and resume feeding the following spring. They feed throughout the summer until they are mature enough to create pupal cells for overwintering. In this case, it takes two years for TLCB to complete its life cycle.

Management

Cultural Management

Preventing a two-lined chestnut borer infestation is the best approach. This can be accomplished by reducing stressors on potential host trees.

Water trees with an inch of water each week during hot, dry periods to reduce drought stress. Do not change the flow of drainage around the tree and do not compact the soil.

If there is defoliation by other insects, manage those insects to reduce tree stress. If two-lined chestnut borers are present, prune out the infested parts of the tree before the adult borers emerge in the spring. Practice good sanitation by properly disposing of infested wood by burning or chipping to avoid beetle spread.

The borers can be transported in firewood, so precautions should be taken when handling and transporting oak wood and related products.

Chemical Management

Systemic insecticides may be applied as soil drenches, trunk injections, or bark sprays for management of this pest.

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. Use pesticides safely and wisely; read and follow label directions. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For more information, contact The Morton Arboretum Plant Clinic at 630-719-2424 or plantclinic@mortonarb.org.