Locust Borer

Megacyllene robiniae — the locust borer — is one of about 35,000 species in family Cerambycidae (the longhorn beetles) and one of the most striking longhorn beetles in eastern North America. The species is widespread across all of eastern and central North America from southern Canada south through the eastern US to Texas, with distribution closely tied to the geographic range of black locust (Robinia pseudoacacia) host trees. Adults are 12-25 mm long, with the species' diagnostic coloration: bright YELLOW-AND-BLACK banded body — yellow elytra (wing covers) with bold zigzag black bands, plus yellow head and pronotum with similar black markings, and long antennae also banded yellow-and-black. The combination is unmistakably WASP-LIKE in appearance. Adult locust borers are voracious nectar feeders on autumn flowers, especially GOLDENROD (the species often co-occurs with goldenrod blooms in late September-October across the eastern US — peak adult activity coincides with peak goldenrod flowering), where they gather in dense aggregations on the flower clusters. The species is one of the most-photographed BATESIAN-MIMIC INSECTS in NA macro nature photography because the wasp mimicry is exceptional — bird and small-mammal predators that have learned to avoid stinging Hymenoptera also avoid attacking the harmless locust borer. The species' BIOLOGY is closely tied to BLACK LOCUST (Robinia pseudoacacia) host trees — the only known host plant for the species' larvae. Adults mate in autumn on goldenrod flowers and other late-season flowers; mated females then locate black locust trees and lay eggs in cracks in the bark; larvae hatch and tunnel into the inner bark and sapwood of the host tree, feeding on tissue and creating extensive tunnel galleries inside the wood. Larvae develop over the winter and through the following summer, then pupate in autumn and emerge as adults — completing one generation per year. The larval tunneling causes significant damage to black locust wood — weakening the trunk structure and providing entry points for fungal pathogens that further degrade the wood. The species is the single most economically important PEST OF BLACK LOCUST in NA — historically a major problem for black locust used in fence posts, mine timbers, and other utility wood applications (black locust is one of the most rot-resistant NA hardwoods and was historically prized for these uses, but locust borer damage substantially reduces the wood's value). Modern impact is reduced (black locust is less commonly cultivated today), but the species remains a significant problem for ornamental black locust plantings and for natural black locust forest health. The species is harmless to humans (no sting despite the wasp mimicry, no significant bite).