The locust borer, Megacyllene robiniae (93968 bytes)

The Locust Borer

By Judy Sedbrook, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver

The locust borer, Megacyllene robiniae , is an insect native to North America and can be found over most of the United States. The adult locust borer is large, nearly an inch in length, and very colorful. Both males and females have black bodies with bright yellow markings. These markings are most notable in the "W" on the first pair of wings.  Legs are red and antennae are black. These long antennae place it among the "long-horned" beetles. These adults are seen predominantly in the fall and can be found feeding on goldenrod.

larvae boring tunnels (25191 bytes)Eggs are laid in cracks in the bark of the tree. They hatch and the larvae bore into the inner bark where they overwinter. This causes bleeding that results in wet spots on the bark. In the spring, thegalleries carved by larvae, pupae (25148 bytes) larvae move into the sapwood of the tree. Their feeding during this process produces a whitish-colored wood dust that can be seen on the bark. The gradual enlarging of their tunnels, until they are 3" to 4" long and " to " inch wide, moves them into the heartwood and the wood dust becomes more yellow. With large infestations, this dust may accumulate at the base of the tree.

By mid-summer, the larvae have matured to the pupal stage. This stage is completed in about 4 weeks and the adult beetlesknotty swelling on truck (26500 bytes) twig and branch dieback (61464 bytes)then exit the tree through the openings made by the larvae. The timing of these stages varies from one part of the country to another, depending on each area's climate.

The damage inflicted by these borers results in twig and branch dieback, weakening the tree and leaving it susceptible to wind breakage. Heavily infested trees will be found to have many broken limbs and knotty swellings on the trunk and larger branches.

Healthy trees, and trees with a greater than 6" diameter, are more resistant to borer infestation. Trees weakened by drought are especially susceptible to invasion by borers. These borers attack only black locust (Robinia pseudoacacia) and its cultivars. The honey locust (Gleditsia triacanthos) is not affected.


Heavily infested trees should be removed and burned to destroy larvae. Borers are difficult to control with insecticides because they are protected within the trees. Insecticides, permethrin or carbaryl, are most effective when used as sprays on trunks and larger limbs during those times when adults are active and laying eggs. Lindane is also effective but has come to be increasingly restricted and may not be available. These pesticides should be applied in the spring to control larvae before they bore into the sapwood, and again from late August through September. This will catch the unprotected adults, when the female borers are laying eggs, and also protect the trees from newly hatching larvae.

Photo of larvae: CSU

Other Photos: Judy Sedbrook


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Date last revised: 01/05/2010