Locust Borer, Megacyllene robiniae
Locust borer adults have brightly colored yellow markings and are about 19 mm (0.75 inch) long. The beetles also have reddish legs in addition to a “W” shaped mark on the back of their wing covers.
Larvae are white and do not have legs. Around mid-July, fully mature larvae are about 25.5 mm (1 inch) long.
• The locust borer is a type of beetle. Beetles are the most diverse insect group with over 350,000 known species worldwide, however only a fraction of these are considered insect pests.
• The locust borer is native to eastern North America. Its host plant is the black locust tree, but it does not damage other closely related locust trees. Black locust trees are used in reforestation plantings throughout the United States. The transport of infested trees has aided in dispersing this insect to most regions of the country.
• Drought stressed trees and trees lacking sufficient soil nutrients are more susceptible to locust borer attacks.
Locust borer adult
Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org
Locust borer larvae feed on the wood of black locust trees, forming galleries that cut off the tree’s ability to transport water and nutrients. Drought stressed and poorly nourished trees are more susceptible to locust borer infestations. Cracks in trees provide a route of entry for other pathogens and can result in a secondary infection.
Lacy L. Hyche, Auburn University, Bugwood.org
Locust borer larva in wood
James Solomon, USDA Forest Service, Bugwood.org
Locust borer pupating in wood. The adult beetle will exit through the hole on the right, which was created by the larval stage.
James Solomon, USDA Forest Service, Bugwood.org
Locust borer feeding injury to wood
Life History and Habits
All beetles undergo complete metamorphosis, and have four life stages: egg, larva, pupa, adult.
Adult beetles can be found feeding on goldenrod pollen early in the day during the late summer and fall. They are also found on black locust tree trunks searching for egg laying spots, which are deposited in crevices in the bark and around wounds of living trees.
After hatching, larvae bore deeper into the bark where they hibernate throughout the winter. As leaf buds swell the following spring, larvae bore deeper into the wood where they continue feeding and maturing. Maturing larvae enlarge the holes and push out frass (dried insect feces).
Trees infested with borer larvae are structurally weakened, making them more susceptible to wind breakage. This feeding injury can cause further damage through deformation or the growth of sprouts in clumps. However, the presence of dead and broken limbs as well as knotted growths on trunks are the most obvious symptoms of severe infestations.
There are other symptoms of locust borer attack depending on the season. Wet spots are present on bark in early spring due to tunneling of larvae. Later in the spring and early summer, white wood dust may be visible as developing larvae continue to feed and enlarge the entry hole. This dust appears yellow in late summer when larvae burrow deeper into the tree’s heartwood. In heavily infested trees, wood dust can accumulate around the tree’s base in a circular shape.
In forests, spraying should be avoided. Areas with severe infestations should be clearcut during the dormant period. All sprouts that develop after clearcutting should be thinned, leaving behind only the most vigorous sprouts in the group.
Thinning is an option in stands with only moderate to light injury. Removal of overtopped, intermediate, or decadent trees can help reduce the pest population and protect other healthy trees. Mixed stands are usually less prone to serious injury.
Since poorly nourished trees are more susceptible to borer infestations, several inches of decaying leaves (or mulch in residential landscapes) should be added to stands with only locust trees. This can provide extra nutrients, increasing growth for several years and reducing the likelihood of serious borer damage. Maintaining tree health by supplementing water in periods of drought can also aid in managing these pests.
When buds are opening in the spring, certain sprays (see references below) can be applied to trunks and larger limbs until wet, with a repeat application in 10 – 14 days. This kills larvae as they enlarge galleries under the bark. Old black locust trees should be removed if they have dying tops, as these can serve as a reproductive hot spot for the borer. After removal, these trees should be cut when borers are dormant and peeled or burned to kill locust borer larvae.
Galford, J. (1984). The Locust Borer. Forest Insect & Disease leaflet 71 – US Department of Agriculture: Forest Service. Available https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev2_043731.pdf
Washington State University. (n.d.). Locust Borer. Washington State University – Department of Entomology. Available https://entomology.wsu.edu/outreach/bug-info/locust-borer/