AppearanceFrom a distance, ''M. robiniae'' can easily be mistaken for a wasp or bee. Even at a closer look, it is often mistaken for ''M. caryae'' or ''M. decora''. The adult beetle grows between 12 and 20 mm long and has a W-shaped third stripe on the elytra. The antennae of both sexes are dark brown. The male's antennae are two-thirds its body length, and the female's are one-half. The legs are reddish brown.
NamingThe specific name, ''robiniae'', is derived from the name ''Robinia'', which is the generic name of the black locust tree, ''Robinia pseudoacacia'', on which the larvae feed. The name ''Robinia'' was coined by Linnaeus to honor the royal French gardeners Jean Robin and Vespasien Robin .
StatusIn 1900, the value of ''Robinia pseudoacacia'' was reported to be practically destroyed in nearly all parts of the United States beyond the mountain forests which are its home by ''M. robiniae''. Were it not for these beetles and their larval tunnels promoting fungal infections, it could be one of the most valuable timber trees that could be planted in the northern and middle states; young trees grow quickly and vigorously for a number of years, but soon become stunted and diseased, and rarely live long enough to attain any commercial value. Currently, only one registered product, carbaryl, is effective against ''M. robiniae''. It is applied in a single dose when adults are most active .
HabitatIts geographic range has grown over the years following the expanding range of ''R. pseudoacacia''. As more and more people use the black locust tree as an ornamental, the range of ''M. robiniae'' grows. It can be found almost anywhere unprotected black locust trees grow, and is often more abundant when ''Solidago'', commonly called goldenrod, is also present. Because of the adults' floral preferences, they tend to stay in uncultivated fields and meadows.
FoodThe females are often found running up and down black locust trunks in search of wounds in which to lay their eggs in the fall. Both sexes are most common from late day to dusk. Adults feed on pollen of goldenrods of the genus ''Solidago''. The eggs hatch and the larvae spend the winter hibernating within the bark. Once winter ends, the larvae burrow into the tree trunk and start to tunnel. These tunnels are around 10 cm long by 7 mm wide, and serve as a primary infection site for wind-borne spores of the fungus ''Phellinus robiniae'', which causes a damaging heart rot disease of ''Robinia'' species. The larvae pupate in late July and early August, and adults start to emerge in late August and throughout September.
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