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Megacyllene robiniae

Adult locust borer. Photo: Steven Valley, Oregon Department of Agriculture, Bugwood.
Scientific Name: 
Megacyllene robiniae
Common Name: 
Locust Borer
Growing Degree Days (GDD's): 
2271–2805, Base 50F, March 1st Start Start Date (Source: Cornell Cooperative Extension; Robert Childs, UMass Extension.)
Host Plant(s) Common Name (Scientific Name): 
Black locust (Robinia pseudoacacia)
Insect Description: 

The locust borer is native to North America and found in most geographic locations where black locust occurs. This insect can be particularly damaging to its host plants in the Northeast and central US. Adult beetles are seen in early September until the first frost, often found feeding on goldenrod. Adult beetles are 1/2-1 inch in length, and black with a series of yellow stripes on the head, thorax, and elytra (hardened forewings). Adult female beetles lay one to several eggs in the cracks and crevices of the host plant bark. Eggs are sometimes laid in the holes of where other adult beetles from previous generations of the insect emerged. Eggs are white, oval, and approximately 3 mm in length. Eggs hatch in approximately 1 weeks and the tiny, new larvae bore into the bark until they reach the phloem. Larvae are white, legless, and cylindrical. Larvae feed within the tree and complete a gallery that reaches the heartwood of the tree by mid-June of the next year. Larvae clear their tunnels of their frass, which leads to piles of sawdust like excrement on the ground beneath bark openings in the tree. Larvae can sometimes be heard feeding on their host. Larvae overwinter. At maturity (approx. 25 mm. long), the larvae pupate behind a plug of wood chips and frass. Adults emerge from the opening of the larval gallery. One generation occurs per year.

Damage to Host: 

This insect attacks the host plant trunk and branches as small as 45 mm. in diameter. The locust borer preferentially attacks trees that are at least 4 years old, however once the trunk reaches 15 cm. in diameter, it is attacked much less often. Trees grown on poor sites with drought conditions may also be more prone to attack. The adult insect is not the most damaging life stage. As with most wood borers, it is the larvae that feed on and tunnel in the host plant wood. Trunks and branches may have holes, sap staining/leaking, pushed-out frass and wood chips, and dieback. While damage can be extensive, often the tree is not killed outright. However, because the larvae bore into the heartwood of the host plant, trees may be prone to breakage during periods of strong winds. 


Visually monitor the sunny side of the host plant. Look at the bark for holes or areas leaking frass/wood chips or sap. Young trees grown in full sun showing signs of dieback should be monitored. 

Cultural Management: 

Resistant cultivars exist. A strategy to make ornamental trees less attractive for egg laying by the adult beetles may be to remove goldenrod from the area, which is a source of pollen and food for the adult beetles. Any cultural management practices to maintain tree vigor and reduce additional stress may be helpful in preventing wood boring beetle attack. In particular, grow this tree on the correct site to promote its success, and water during periods of drought. 

Natural Enemies & Biological Control: 

Few parasitoids of the locust borer have been identified, and none have been studied (Johnson and Lyon, 1991). Woodpeckers may provide minimal impact on their populations.

Chemical Management: 

Acephate (NL)

Acetamiprid (L)

Azadirachtin (NL)

Beauveria bassiana (NL)

Carbaryl (L)

Chlorpyrifos (N)

Cyantraniliprole (NL)

Dinotefuran (NL)

Emamectin benzoate (L)

Flonicamid+cyclaniliprole (N)

Imidacloprid (L)

Isaria (paecilomyces) fumosoroseus (NL)

Neem oil (NL)

Permethrin (L)

Pyrethrins + piperonyl butoxide (L)

Spinosad (NL)

Zeta-cypermethrin (L)


Active ingredients that may be applied systemically include: azadirachtin (injection, soil drench), cyantraniliprole (soil drench, soil injection), imidacloprid (soil drench), and neem oil (soil drench).

When used in a nursery setting, chlorpyrifos is for quarantine use only.

Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.

Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.

Read and follow all label instructions for safety and proper use. If this guide contradicts language on the label, follow the most up-to-date instructions on the product label. Always confirm that the site you wish to treat and the pest you wish to manage are on the label before using any pesticide. Read the full disclaimer. Active ingredients labeled "L" indicate some products containing the active ingredient are labeled for landscape uses on trees or shrubs. Active ingredients labeled "N" indicate some products containing the active ingredient are labeled for use in nurseries. Always confirm allowable uses on product labels. This active ingredient list is based on what was registered for use in Massachusetts at the time of publication. This information changes rapidly and may not be up to date. If you are viewing this information from another state, check with your local Extension Service and State Pesticide Program for local uses and regulations. Active ingredient lists were last updated: January 2024. To check current product registrations in Massachusetts, please visit: .