Locust Borers

Locust Borers are flightless as they lack significant wings and shell covering that appears as though a series of overlapping plates. Antenna are visible on the head. The insect can appear as a dull black or in some cases a shiney black or dark blue. These particular beetles do not fly and are slow movers.

Locust borer larvae weaken trees, rendering them susceptible to wind breakage and retarding growth. Severe infestations of locust borer result in many dead and broken limbs, along with swollen areas on the tree trunks. In some cases, wet spots appear on the bark in early spring, about the time of bud swell–the result of young larvae tunneling into the inner bark.

Life Cycle: Locust Borer are active in the late summer and fall, late August through October. They are commonly seen feeding on the pollen of goldenrod blossoms during the morning hours. Later in the day, sometimes well after sunset, they are seen on the trunks of black locust trees searching for egg laying sites. Eggs are usually deposited in rough bark crevices and around wounds on the trunks of living trees.

Newly hatched larvae bore into the inner bark and construct small hibernation cells in which they spend the winter. Activity is resumed in the spring when the leaf buds begin to swell. At this time, oozing sap may be seen around larval entry holes in the trunk. The larvae soon bore into the wood where they continue to feed until mature, around mid-July. As the larva grows, it enlarges its tunnel to the exterior, through which it pushes its granular frass to the outside and through which the adult eventually emerges. There is one generation per year.

Damage: The boring activity of the larvae starts when the tree becomes active in the spring. The larvae bore up and in towards the heartwood, frequently returning to the opening to enlarge it. After reaching the heartwood the larvae bore down staying near the margin on the trunk and in the center of the branches.

Larval tunneling in the trunk and limbs results in broken and dead limbs, weakened trees, excessive sprout production, and even death of the tree.Low vigor trees, damaged trees, and trees under environmental, drought, or nutritional stress are most likely to be infested. Trees of less than six inches diameter are most often attacked, while those larger than eight inches diameter are generally safe from attacks.

Controls: Spray trunks and larger limbs with a pesticide from the table below. The first application should be made about mid August and a second 2 or 3 weeks later. Insecticides are generally not very effective, however. If the borers are already inside a tree, fumigate using aromatic solvent such as fingernail polish remover. Put the solvent into an oiler or some device that will squirt the solvent into the grub holes. After fumigation, sealing the tunnels with wax or putty helps to hold in the fumes. Remove and destroy badly infested trees in late fall or early spring. Select a good site for black locust trees and keep them healthy. Do not allow livestock to graze in locust plantings.