NC State Extension Publications

General Information

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The locust borer, Megacyllene robiniae, is a common and widespread longhorned beetle pest of black locust. It has black and yellow bands across the thorax and zigzag yellow bands on the forewings. The legs and antennae are reddish yellow. The 3/4 inch long beetles are often found on goldenrod flowers. From August through October, females lay eggs in crevices and under scales of the bark of black locust. About a week later, tiny grubs bore into the inner bark and hollow out a small hibernation cell. The following spring, wet spots appear on the bark caused by the young grubs tunneling in the inner bark as they feed. In late spring or early summer, wood dust dust is pushed out of holes in the bark by the developing grubs boring into the wood. By this time, the grubs are an inch long, whitish, elongate, and shaped somewhat like the rattles of a rattle snake. Grubs have six tiny legs near the dark brown head. Sometime in mid summer, the grubs pupate inside the tunnel after stopping it up with a plug of wood fibers. A few weeks later, new adults chew through the plugs and emerge through the holes in the bark left by the grubs. There is one generation per year.

Locust borer

Locust borers are longhorned beetles often found on golden rod.

new adult locust borers

Here are new adult locust borers just before exiting an infested black locust.

Locust borer grub

Locust borer grubs bore right into the heartwood of black locust.

pupa

Locust borer grubs molt into pupae inside their tunnels.

Host Plant

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The locust borer attacks only black locust and perhaps cultivars of other Robinia. Dead and broken limbs and knotty swellings on the trunk are a sign a black locust is infested with locust borers. Damage from borer tunneling may lead to wind breakage, deformed trees, and clumps of sprout growth. The locust borer is probably the chief reason black locust is not used more as a shade tree and for lumber.

Locust borers sometimes ruin the appearance of black locust tree

Locust borers sometimes ruin the appearance of black locust trees in landscapes.

locust borers tunneling into branches and trunk

Black locust trees can be weakened by locust borers tunneling into branches and trunks.

Residential Recommendations

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It has been reported that trees in optimal vigor are less susceptible to damage by locust borers. Anything within reason that can be done to promote healthy growth should be done before resorting to pesticides (ie mulching to conserve moisture and keep roots cooler, watering in periods of prolonged dry weather, and fertilizing according to a soil test). To help protect a valuable black locust shade tree, spray the trunks and the larger limbs with a pyrethroid insecticide, several of which are available in the garden section of most big box stores. It would be a good idea to treat the tree in early August and again in mid August. Pyrethroid insecticides are toxic to fish so avoid contaminating pools, ponds, and streams.

Other Resources

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For assistance with a specific problem, contact your local Cooperative Extension Center.

Authors

Professor and Extension Specialist
Entomology & Plant Pathology
Professor Emeritus
Entomology & Plant Pathology

Publication date: April 3, 2017
Revised: Oct. 1, 2019

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