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Locust Borer

Megacyllene robiniae (Forster)

Pest description and crop damage. The adult locust borer resembles a wasp or hornet being black with bright yellow markings. There is a yellow “W” across the wing covers (Elytra). Adults are ca/ 1 inch (50 mm) in length. Their legs are reddish.
The larvae are white legless with a large prothorax typical of Cerambycidae beetle larvae. Eggs are laid in the fall and over winter under shallow bark. Early springs causes the larvae to begin boring to the heart wood and down the branches. The larvae damage locust trees by weakening the branches of the tree with numerous large galleries that either weakens the tree so that it cracks or breaks off or cuts of nutrient and water supply to new growth. Pathogens can enter cracked trees and infect the xylem/phloem layers. Feeding is through the summer with pupation in August and flying adults emerging ca. September. The adults are very active fliers and often visit Goldenrod flowers although they do not have to feed as adults.

Management The only registered product for locust borer is Carbaryl (Sevin) applied in a single dose when adults are active. Be sure to check labels before using any pesticicde Drought stress weakens trees, and these trees are highly susceptible to attack. We have had several years of drought in SE Washington. Supplemental watering of home garden trees will help keep trees vigorous during dry years. Once a large older tree is infested and full of galleries it will die and is a hazard depending on location.