Two leaf-feeding pests on black locust were observed by BYGLers this week: the LOCUST LEAFMINER BEETLE (Odontota dorsalis) and the BLACK LOCUST BUG (Lopidea robiniae). Damage caused by the beetle is often a familiar sight to travelers motoring on Ohio's interstate highways. Larval and especially adult feeding activity produces a captivating reddish-brown leaf coloration that highlights infested trees allowing black locusts to be positively identified at highway speeds! "Flamed" trees will not become apparent until later in the season.
The adult beetles are less than 1/4" long. They have a flattened appearance and are orangish-red with a median black line down the center of the back. Overwintered beetles emerge in the spring to feed as skeletonizers on the leaves of their namesake host as well as several other tree species including beech, cherry, crabapple, dogwood, elm, hawthorn, and yellowwood. They may also be found on a number of herbaceous plants such as soybeans. The early season feeding activity of the overwintered adults usually causes little obvious damage.
Eventually, the beetles begin laying eggs. While black locust is the preferred larval host, the beetles will also select yellowwood. Eggs are laid in clusters of three to five in late May or early June on the undersides of leaves. The larvae hatching from these eggs work their way into the leaf through one entrance hole and live in common blotch mines. Later, they go to other leaves and make separate mines. Larvae feed for about three weeks, pupation takes place in the mine, and beetles emerge a week or 10 days later. Larval leafmines, coupled with the leaf-feeding activity of beetles that emerge from this season's mines, produces the most obvious damage caused by this insect - the flame-orange black locusts.
BYGL readers are probably less familiar with the black locust bug. The locust bug is slightly larger than the leafmining beetle, but both share similar color patterns. The adult bugs are elongate in shape and have a median longitudinal black line down the center of the back that is flanked by two longitudinal orangish-yellow lines. Their legs and antennae are black. The nymphs are orangish-yellow with black legs, antennae, and wing pads. Both the adults and nymphs use their piercing-sucking mouthparts to extract juices from black locust leaves and their feeding activity produces small, yellow leaf spots.
Research conducted at Illinois State University and published in 2004 revealed that black locust bugs secrete chemical compounds that were implicated in defending the bugs against bird predation. When attacked, the bugs discharged liquid from metathoracic glands that contained a chemical brew that gave the bugs a strong and distinct odor. Birds were observed ejecting the bugs out of their mouth after biting them . . . suggesting the chemical discharge served as a feeding deterrent. One can only imagine the birds making a "pa-toowie" sound.