Megacyllene robiniae

Distinguished from the closely related hickory borer (Megacyllene caryae) by season of activity (fall vs. spring), complete yellow elytral bands that extend fully across the elytra (vs. often broken and incomplete in caryae), and the yellow anterior pronotal margin (vs. black in caryae). The W-shaped third elytral band is diagnostic. Distinguished from Megacyllene decora by narrower, less bold banding and smaller size.

Adults measure 11–28 mm in length with a narrow, elongated body. The elytra display alternating black and yellow transverse bands in a zigzag pattern; the third band forms a distinctive W-shape. The pronotum has a narrow yellow margin along its anterior edge. Antennae are dark brown to black, reaching two-thirds body length in males and one-half in females. Legs are reddish-brown. Full-grown larvae are legless, white, robust, and approximately 25 mm long.

Found wherever unprotected black locust trees (Robinia pseudoacacia) grow, particularly in uncultivated fields and meadows where goldenrod is present. Adults are strongly associated with flowering goldenrod for feeding. The species has expanded its range following the ornamental planting of black locust but remains confined to North America.

Endemic to eastern North America. Native range coincides with black locust distribution; expanded westward to Idaho and Colorado through ornamental tree plantings. Present in Canada (New Brunswick, Nova Scotia, Ontario, Prince Edward Island) and throughout the eastern and central United States. Not established outside North America despite black locust introductions elsewhere.

Adults emerge in late August and are active through September and October. Peak activity occurs from late day to dusk. Females seek black locust trunks in fall to locate wounds for egg-laying. Larvae overwinter within bark, resume feeding in spring, and pupate in late July to early August.

Adults feed on pollen of goldenrods (Solidago spp.). Larvae feed within the wood of living black locust trees (Robinia pseudoacacia), tunneling through heartwood and creating galleries approximately 10 cm long by 7 mm wide.

• Robinia pseudoacacia - larval hostLiving trees; larvae tunnel in trunks and branches
• Solidago - adult food sourcePollen feeding; strongly associated with goldenrod flowers

Females lay eggs in crevices and near wounds on black locust trunks in fall. Eggs hatch and larvae overwinter beneath bark. In spring, larvae burrow into wood and create extensive tunnels. Larvae overwinter a second time within the tree, then pupate in late July–early August. Adults emerge late August–September. Development spans approximately one year with overlapping generations possible.

Adults are diurnal with peak activity from late afternoon to dusk. They are strong fliers and readily visit goldenrod flowers for pollen feeding. Females actively search tree trunks for wounds suitable for oviposition. Adults are not particularly wary and can be approached for observation. The species is a classic "fall borer" with adult activity timed to coincide with late-season goldenrod bloom.

As a specialist herbivore, larvae significantly impact black locust populations by weakening trees through tunneling. Larval galleries serve as primary infection sites for wind-borne spores of Fulvifomes robiniae, a fungus causing heart rot disease in Robinia species. This beetle-fungus interaction substantially reduces black locust timber value and increases tree susceptibility to wind damage. Adults contribute to pollination of goldenrod through pollen feeding.

Major pest of black locust timber and ornamental trees. Historically caused severe economic damage; by 1900, black locust timber value was reported as "practically destroyed" across much of the United States outside mountain forests. Currently managed with carbaryl insecticide applied during peak adult activity (August/September). Adults occasionally cause concern when mistaken for structural pests, though they do not infest buildings. Frequently encountered by entomologists and naturalists as a conspicuous fall insect on goldenrod.

• Megacyllene caryaeNearly identical black and yellow banding; distinguished by spring adult activity, broken/incomplete elytral bands, black anterior pronotal margin, and often reddish legs. Breeds in dead hickory wood rather than living black locust.
• Megacyllene decoraLarger, chunkier body with wider, bolder black and yellow bands. Less common, associated with false indigo (Amorpha fruticosa) rather than black locust. Adults occur on goldenrod and snakeroot in fall.

• longhorn beetle
• pest
• mimicry
• fall active
• wood borer
• goldenrod associate
• black locust pest
• North American endemic

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
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