Current Issue

Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Beautiful in yellow and black: The locust borer, Megacyllene robiniae


Black and yellow bands on the beetle mimic bands on the stinging wasp next door, warning birds and other predators to stand down.


One of the hallmark events of late summer and autumn is the blooming of goldenrod. Spectacular arrays of brilliant yellow flowers attract throngs of insects intent on collecting and consuming the nutritious pollen found in hundreds of small flowers on each plant. Goldenrod gets a bad rap each fall as people with allergies complain about pollen produced by goldenrod. The truth is that the pollen of goldenrod is quite large, rather sticky, and adapted for transport from plant to plant by the activities of insects rather than the wind. The most likely way for someone to get a dose of goldenrod pollen would be to stick their nose into the small flowers like a bee. Don’t do that. The real culprits for many of the seasonal allergies at this time of year are the ragweeds with their airborne pollen.

Larvae of the locust borer are known as round headed borers. Their galleries structurally weaken trees and can kill branches and sometimes entire trees.

Earlier this season we met other goldenrod visitors, including blister beetles and ambush bugs, but this year is especially good for viewing one more member of the goldenrod gang, the locust borer, Megacyllene robiniae. The adult stage of the locust borer is a spectacular insect called a longhorned beetle, so named for the very long antennae adorning its head. The yellow lines found on its back are a beautiful match to the brilliant yellow of the goldenrod blossom. On sunny days, goldenrods are abuzz with many kinds of stinging insects such as paper wasps, bumble bees, digger wasps, and potter wasps. The yellow and black pattern on the back of the locust borer is a fine match to the pattern of some of these stinging insects. Birds that might like to eat a tasty beetle probably think twice when they see a yellow and black insect that might deliver a memorable sting. This is one way the locust borer likely gains protection from its enemies.



Yellow and black coloration mimics patterns of stinging insects and probably helps the beetle avoid attack by predators while it double-checks each floweret for nutritious pollen.

Large oval holes and frass-packed galleries are a sure sign of locust borers in a black locust tree.

After dining on goldenrod pollen and finding a mate, the female locust borer beetle flies to the trunk of a black locust tree and lays eggs in crevices or wounds in the bark. These eggs hatch into larvae called locust borers that tunnel beneath the bark of the tree, destroying vascular and structural tissues beneath. In this way, they harm black locusts much like the emerald ash borer and Asian longhorned beetle we met in previous episodes. If trees are small or the borers numerous, branches or entire locust trees may be killed when attacked by the locust borer. However, it is not unusual to see veteran black locust trees with numerous locust borer wounds surviving nicely along hedgerows, lanes, and trails throughout eastern forests. Now until the first hard frost is an excellent time to spend a few moments in the goldenrod patch to try to catch a glimpse of these magnificent creatures.     



We thank the Robinson Nature Center for providing habitat for these beetles and inspiration for this episode.