Locust Borer Beetle – Megacylene robiniae

Locust Borer Beetle – Megacyllene robiniae
Order Coleoptera / Family Cerambycidae
— capricornes, long-horned beetles, longicornes
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Live adult beetles photographed at Winfield, Illinois, USA. Shown with Goldenrod (Solidago sp.)

The locust borer beetle is native to North America. It attacks only black locust trees of the genus Robinia, which originally grew only in the Allegheny and Ozark mountain regions. Due to its ability to thrive in poor soils, the black locust has been widely used as a shade tree and in reclaiming land damaged by farming and strip mining. The locust borer beetle has extended its range as a result. It is now found over most of the U.S. and southern Canada.

This is one magnificent insect – about 1" long, stately and impressive. I'd never seen one before I stumbled across this guy. The whole time I was shooting, I was saying, "What the hell is this thing!? What a COOL bug!" (Yes, I frequently talk to myself and my subjects when photographing them. Passersby must think me crazy).

Unfortunately, locust borer larvae tunnel into a tree's trunk and branches, weakening the tree and making it susceptible to wind breakage. The damage from borer tunneling and wind breakage often results in deformed trees or clumps of sprout growth.

With over 20,000 species described, Cerambycidae is a large family. Many are serious pests, with the larvae boring into wood, where they can cause extensive damage to either living trees or untreated lumber. A number of species mimic ants, bees, or wasps as this specimen most emphatically does. Mimicry of the vespid wasp's aposematic coloring (see paper wasp below) is a common survival ploy for various families of insects, eg. the flies in Syrphidae.

Most Cerambycidae larvae feed within dead, dying or even decaying wood, but some taxa are able to use living plant tissue. Girdlers (adults of the Onciderini, larvae of genera in the tribes Methiini, Hesperophanini and Elaphidiini) sever living branches or twigs, with the larvae developing within the nutrient-rich distal portion. The larvae of a few species move freely through the soil, feeding externally upon roots or tunneling up under the root crown. [3]

Habitat and Life Cycle: The brightly colored adults appear when goldenrod (Solidago sp.) is in bloom, generally mid-August through October here in the midwest. I took photos of this specimen feeding on goldenrod pollen in the midst of a large mown field, hundreds of yards from trees of any variety, and so it's obvious to me the beetles will range far from their host plants in search of food. Females start laying eggs in the early afternoon and continue until dusk, August – October. Eggs are laid under the bark, around pruning wounds and wind breakage and in cracks in the bark – rarely where they can be easily detected. About a week later, the eggs hatch and small white larvae bore into the inner bark, where they make a small hibernation cell and overwinter. In the spring, the larvae bore into the woody part of the tree, eventually enlarging their tunnels until they are 3-4 inches long and 1/4 inch in diameter. By mid-July, most of the larvae begin the pupal stage, which lasts about 2 weeks. The mature adult beetles emerge through the holes made by the larvae.

Mature larvae are white, about 1 inch (2.5 cm) long and one-quarter of an inch (0.6 cm) in diameter. Newly formed pupae are creamy white and about three-quarters of an inch (1.9 cm) long. Both the larval and pupal stages are spent inside the tree [1].

Paper wasp
Paper wasp Polistes dominula: same food source, same aposematic coloring

Black locust shade trees or lawn specimens can be protected from borers by spraying the trunks and the larger limbs with a lindane emulsion. To prepare small amounts of the spray, add 2 tablespoons (30 ml) of a 20-percent emulsifiable concentrate to 1 gallon (3.8 1) of water. For larger amounts, add 3 pints (1.42 1) of the concentrate to 50 gallons (190 1) of water. In the spring at the time the buds are opening, spray trunks and limbs until wet. Repeat the application in 10 to 14 days. This treatment kills active young larvae that are enlarging their galleries just under the bark.

The same lindane spray also may be applied in late summer to protect the trees from newly hatched larvae attempting to penetrate the bark. Spraying may be done anytime from late August through September, since the female beetles continue to lay eggs during this period.

Locust Borer Beetle

Spraying with chemicals is not considered practical for the protection of black locust in a forest. Severely injured forest stands can be regenerated by clearcutting during the dormant period. The sprouts that follow clearcutting should be thinned by removing all but the most vigorous in each group. This procedure has resulted in a good second crop of trees with very light subsequent injury.

Moderately to lightly injured stands on medium-to-good sites benefit from thinning. In such stands, injury is confined mainly to overtopped, intermediate, or decadent trees. Removal of these trees should reduce the borer population and thereby help protect the more desirable trees.

Borer injury is usually less serious when black locust is grown with other tree species. Mixed stands usually produce denser shade and more leaf litter than do pure stands of locust. Trees are more vigorous when nutrients from decomposed leaf litter are available. In pure locust stands, the addition of several inches of hardwood leaves results in accelerated growth for several years after the treatment and should reduce chances of serious borer damage.

Old black locust trees with dying tops serve as brood trees for the borer. Removing these trees from the vicinity of planting areas should be helpful in reducing damage to the young planted trees. These large brood trees should be cut during the dormant period and either peeled or burned to destroy the borer larvae. [1]


  1. Jimmy R. Galford, Entomologist, Northeastern Forest Experiment Station, Forest Service, U.S. Department of Agriculture, Forest Insect & Disease Leaflet 71, The Locust Borer
  2. Josef Nissley Knull, The long-horned beetles of Ohio: (Coleoptera: Cerambycidae)
  3.,  Locust Borer Beetle – Megacyllene robiniae
Order Coleoptera: Beetles are the dominant form of life on earth: one of every five living species is a beetle. Coleoptera is the largest order in the animal kingdom, containing a third of all insect species. There are about 400,000 known species worldwide, ~30,000 of which live in North America.  Beetles live in nearly every habitat, and for every kind of food, there's probably a beetle species that eats it.
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