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Locust Borer

September 16, 1998

Throughout Illinois, the locust borer (Megacyllene robinae Frost), a longhorned beetle, attacks the trunk and limbs of smaller black locust trees and the branches of larger ones. At one time, this beetle was found only in the Allegheny Mountain region from Pennsylvania to Georgia and the Ozark Mountains. Widespread use of black locust (Robinia pseudoacacia L.) in reforestation, restoration of damaged land, and as a shade tree has expanded the geographical range of the borer to include most of the country. The borer, a native to the United States, attacks only black locusts and its cultivars; it does not affect the honey locust.

The adult locust borer is a conspicuous and brightly colored beetle. Its jet-black body is encircled in yellow bands, with a distinctive W-shaped band extending across the wing covers. The adult is about 3/4 inch long with reddish legs and black antennae. The adults are most abundant in September when they feed on the pollen of goldenrod (Solidago spp.) blossoms.

Females lay eggs on their locust hosts in the early afternoon to late evening from late August to early October. Eggs are deposited in tissues of branch scars, under bark scales, or simply in cracks in the bark. Eggs hatch in about one week, and the tiny larvae bore into the inner bark. Larvae later construct a hibernation cell where they spend the winter.

Locust borer larvae are white and legless, reaching a maximum length of about one inch. In the spring, they begin boring into the woody parts of trees, frequently penetrating the heartwood. Throughout the spring and summer, larvae enlarge their feeding tunnels to three to four inches long and about 1/4 inch diameter. Tunnels are initially formed in an upward and inward direction from the point of entrance, then angle sharply straight down the trunk, resulting in an L-shaped tunnel. By mid-July, most larvae have matured and entered the pupal stage, which is completed by late July or the first half of August. Mature adults then emerge through holes they chewed through the bark as larvae.

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. In early summer, the developing larvae push white wood dust out of holes in the bark as they bore into the sapwood. By late summer, larvae have reached the heartwood and the color of the wood dust changes to yellow.

To minimize losses from the locust borer, you should remove infested trees that harbor the larvae. Such removal may reduce the threat to healthy trees. Borer damage may also be reduced by maintaining tree vigor. There is evidence that borer injury is less serious in stands of mixed tree species where there is denser shade and more leaf litter than in pure black locust plantings. Water-stressed trees may also be highly susceptible to attack; therefore, watering trees during times of drought may increase their resistance.

Finally, spraying the trunk and large branches of black locust trees with persistent insecticides in the fall may protect them from borer attack. However, maintaining tree vigor is the more desirable and environmentally sound method of minimizing the threat from locust borer.

Author: Matthew Ginzel Larry Hanks


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