Locust Borer Attacks Black Locust Trees

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

The locust borer (Megacyllene robiniae) only attacks black locust trees.  It’s a native insect and was first found in the eastern part of the United States in natural stands of black locust trees.  The locust borer caused problems in colonial times by rendering the durable black locust wood unsuitable for use as fenceposts.  As settlers moved west in the country, they brought black locust trees... and the borer with them.  The borer is now found throughout North America.

The adult locust borer is one of the “long-horned beetles” with antennae almost as long as its body.  This 3/4 inch black, slender, elongated beetle is sometimes mistaken for a wasp because of the distinctive bright yellow markings on its back.  Most characteristic is the “W” shaped band across the wings.  The legs are reddish.

As an adult beetle, this pest feeds on goldenrod and other flowers in late summer and early fall.  After feasting on flowers and mating, the female adult beetles lay eggs singly or in small groups in bark crevices, cracks, callus tissue around wounds, and other hiding places on the bark of black locust trees.  Interestingly, this egg laying activity usually takes place from early afternoon to late evening.

In about a week, the eggs hatch into tiny larvae which bore directly into the bark until they reach living tissue.  They spend the winter in the inner bark where they make a small hibernation burrow about one inch in length.  In the spring, they start feeding on the tree in earnest, boring upward and inward towards the center of the trunk.  At some point they make a sharp turn and descend about 3 inches down the trunk within the heartwood.  

They’re apparently good housekeepers, pushing frass and sawdust out of their entrance holes and other openings made for clearing out clogged tunnels.  In the spring, visible clues to their presence are moist areas on the bark, which is caused by sap coming from these opening.  The frass and sawdust materials pushed out of the tunnels and collecting beneath the trees is also a good clue.  They continue boring until the tunnels are three to four inches long and about 1/4 inch in diameter.  The tunnel is oval-shaped to accommodate their round head and the way it eats the wood.  It’s such a good eater, you can actually hear it feeding as it chomps away on the wood.

In mid-summer, the larvae, which have grown to approximately one inch in length, are ready to pupate... the stage where they change from a larva into an adult.  They emerge about a month later as adult beetles.  They exit through the openings they made as larvae.  There is one generation per year.

This past week I have been receiving samples and calls from numerous tree owners and cities in the region who are concerned about the damage from the borer that they’re finding in their black locust trees.  The city of Kahlotus has a great number of infested trees and the city of Richland is faced with the predicament of having 600 to 800 black locusts within their entire park system, with 60 or more in Howard Amon Park alone.  A number of these are badly infested and will probably need to be removed for safety reasons.

What type of actual damage does the borer do?  The beetles primarily attack the trunks and branches of black locust (Robinia psuedoacacia) trees, which are 1½ inches in diameter up to seven to eight inches in diameter, occasionally attacking larger wood.  It usually waits to attack trees until they’re four years of age or older.  Heavy attacks by the locust borer will weaken trees and retard growth.  Severe infestations or repeated attacks can leave wood virtually “honeycombed” and prone to limb breakage during windstorms.  During our last windstorm, a number of badly infested trees lost limbs and branches.

The borer tends to attack stressed trees more heavily.  The first line of defense against this pest is to keep the trees in as good health as possible with adequate watering and fertilization practices.  Usually older trees are not attacked, but when there is a high population of borers or the trees are stressed, the tops of older trees become infested too. 

While the locust borer doesn’t generally kill trees, trees can be killed when the population becomes epidemic in proportion.  Drought weakened trees are especially susceptible to attack.  Soil compaction also contributes to borer attack.  Even pruning creates favorable sites for infestation, since the adult females like to lay their eggs in callus tissue on the edge of wounds.

When the borer is already in the wood there isn’t much one can do in the way of “control” other than pruning out badly infested wood.  Chemicals applied to the bark will not penetrate and kill the larvae.  Systemics applied to the roots for uptake into the plants also don’t get into heartwood and older sapwood tissues where the larva does most of its feeding. 

Any chemical insecticides used for control are applications made to trunks and main branches to prevent reinfestation by young borers in the fall.  Appropriate applications of pesticides labeled for borer control are made in late summer or early fall.  Sprays are targeted at the bark of the trunk and larger branches (greater than one inch in diameter).  The spray applications should thoroughly wet the bark surface.  Special attention should be given to wounds and callus growth where the borers like to lay their eggs.

Heavily infested trees with dying tops serve as “brood” trees for the borer.  It would be advantageous to remove these trees from an area where other, healthier black locusts are growing.  However, the trees should be removed and destroyed during the dormant season when they contain the larvae.  Limbs and branches should also be removed if an attack leaves their wood structurally weak.  These should be removed as soon as the hazard is detected.  When replacing these trees, a community should consider planting a mix of species.  Pure stands or large groups of any one species invites devastation by an insect population or an attack by disease, like the locust borer or Dutch elm disease.