Pest profile

Locust Borer

Megacyllene robiniae · Coleoptera: Cerambycidae[3]

The locust borer is a native longhorned beetle, black with bright yellow stripes, often seen on goldenrod in early fall; its larvae tunnel into the trunks and branches of black locust.[1][2]

Locust borer beetle with black and yellow stripes feeding on goldenrod flowers
Locust borer beetle with black and yellow stripes feeding on goldenrod flowers. (c) Peter Chen 2.0, some rights reserved (CC BY) — via iNaturalist, CC BY 4.0

How to identify it

  • Adults are 1/2 to 1 inch long, black with a series of yellow stripes on the head, thorax and wing covers.[2]
  • Larvae are white, legless and cylindrical, about an inch long when mature.[2]
  • Signs include yellow stains and oozing sap from borer holes, wet spots on the bark and coarse sawdust-like frass.[1][2]

Where it lives

It occurs wherever black locust grows, in eastern Canada and most of the United States; it is particularly damaging in the Northeast and central states and has damaged locusts in most northern Utah counties.[1][2]

US map: public records from 38 states since 1990 (GBIF).

Biology and life cycle

Adults appear in early September until frost, often feeding on goldenrod; peak egg laying is in late September.[1][2]

Females lay eggs in bark crevices and around wounds; eggs hatch in about a week, and young larvae spend the winter in the inner bark.[1][2]

In spring, larvae resume feeding and bore into the sapwood and heartwood, making tunnels 3 to 4 inches long.[1][2]

Damage and signs

  • Black locust is the host (Utah State also lists honeylocust).[1][2]
  • Larvae push frass out of their tunnels, leaving sawdust piles under bark openings.[2]

Health risk

None to people or pets.[1][2]

Control

Prevention

Keep trees healthy and vigorous; vigorous spring growth and sap flow kill many young larvae.[1]

Pesticides

If needed, wet the trunk and larger limbs thoroughly in mid to late August and repeat a month later to target egg-laying adults and newly hatched larvae.[1]

Common questions

What is the black and yellow beetle on my goldenrod?
In early fall it is often the locust borer, which feeds on goldenrod as an adult.[2]
What is oozing from my black locust trunk?
Sap and yellow stains around holes, with sawdust-like frass, point to locust borer larvae.[1][2]
How do I protect a black locust?
Keep it vigorous, and if needed spray the trunk in mid to late August and again a month later.[1]

Sources

  1. Utah State University Extension (IPM), “Locust Borer.” extension.usu.edu/planthealth/ipm/notes_orn/list-treeshrubs/locust-borer.php (accessed 2026-10-15)
  2. UMass Extension Landscape, Nursery and Urban Forestry Program (Landscape Insect & Mite Guide), “Megacyllene robiniae (Locust Borer).” www.umass.edu/agriculture-food-environment/landscape/publications-resources/insect-mite-guide/megacyllene-robiniae (accessed 2026-10-15)
  3. iNaturalist, “Megacyllene robiniae taxon page (order and family).” www.inaturalist.org/taxa/83802 (accessed 2026-10-06)

Updated 2026-10-15. Built from sourced records. Spot an error? Tell us.