Published by Utah State University Extension and Utah Plant Pest Diagnostic Laboratory
ENT-245-23
August 2023
Locust Borer Marion Murray, Extension IPM Specialist and Ryan Davis, former USU Arthropod Diagnostician
Quick Facts • The adult locust borer is a 3/4 inch long black beetle with yellow stripes. The legs and antennae are yellow to reddish. • Black locust is the only host, and trees are damaged when larvae bore into the wood. • Symptoms include yellow to brown bark staining, oozing sap, wet spots, yellow sawdust-like material, and holes. • Healthy trees are less susceptible. Infestations may warrant an insecticide bark spray or injection.
The locust borer, Megacyllene robiniae, belongs to the longhorned beetle family (Cerambycidae), referring to the long antennae of most of the species in this group. This insect occurs in Eastern Canada and in most of the U.S., wherever its host, black locust, grows. In Utah, locust borer has damaged black locust trees in most northern Utah counties. The only host is black locust; honeylocust and other trees are not affected by this pest.
DESCRIPTION AND LIFE CYCLE Adults are black beetles, 3/4 inch long, with a series of bright yellow bands running across the entire body (Fig. 1). On the elytra (wing covers), the bands are V-shaped, and on the base on the wings, they form a “W.” The legs and long antennae are yellow to reddish.
Locust Borer
Fig. 1. Locust borers are commonly seen in late summer collecting pollen and nectar from plants like goldenrod and gray rabbitbrush.
Adults emerge in late summer or early fall and feed on the pollen of goldenrod and other flowers during the morning hours. They are most abundant in Utah during September. During peak egg-laying in late September, females deposit 100-200 small, white, oval eggs singly or in groups of six to eight in bark crevices and around wounds on the trunk and larger branches. Eggs hatch in 5 to 8 days and the small white larvae tunnel into the inner bark where they spend the winter as first instars. In spring, larvae resume feeding about the time of bud swell. They bore into the sapwood and eventually into the heartwood, producing a tunnel 3 to 4 inches long (Fig. 2). The cream-colored larvae are about 1 inch long when mature, distinctly segmented, and legless.
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