Key points
- The locust leafminer (Odontota dorsalis) is a serious pest of black locust (Robinia pseudoacacia).
- Black locust is the favorite host for the adult beetles but will occasionally attack apple, birch, beech, cherry, elm, oak, and hawthorn.
- Heavily infested trees appear scorched or burned in mid-summer. These trees will drop their leaves and will refoliate during the summer.
Description and life cycle
- The adult locust leafminer is a small beetle, about 6mm long.
- Adults are elongate and somewhat flattened. The head is black, and the thorax and most of the wing covers (elytra) are orange. The inner edges of the elytra are black, widening at the posterior end. The elytra are also pitted and ridged on the surface. The full grown larva is creamy colored, flattened, and slightly longer than the adult. Mature larvae have six short black legs and a black head.
- Adult beetles overwinter in bark crevices, under litter, etc.
- The adults emerge in the spring about the same time that the black locusts are leafing out.
- Adults chew holes in the leaves and skeletonize the lower leaf surfaces.
- After mating, the female beetle lays her eggs on the lower surfaces of the leaves in groups of 3-5. The eggs are overlapped, glued together, and covered with excrement.
- Upon hatching, the larvae from a given group of eggs bore into a leaf and feed in a common mine eventually separating and feeding in individual mines which form irregular blotches. The larvae tend to prefer the terminal portion of the leaves and pupate in the mines. Adults emerge and repeat the cycle again.
- There are two generations of locust leafminer in Maryland. The first is in the spring, and the second begins with the appearance of adults in July
- Outbreaks of the locust leafminer occur practically every year, and thousands of black locusts turn brown and are defoliated. They are commonly seen along the highway in natural areas.
- Trees that are defoliated seldom die. However, if the damage occurs during a year of drought, or other poor growing conditions, the trees may be killed.
- If the trees grow two sets of leaves in one growing season, the combined feeding of larvae and adults may destroy both sets.
Management
- To help trees better withstand the effects of defoliation, provide adequate water during the summer. Healthy, vigorous trees are better able to withstand the effects of defoliation that are stressed trees.
- Direct control of locust leafminer in forest situations is not practical and seldom attempted. Control of this pest on large landscape trees would best be accomplished by a licensed and certified Maryland arborist.
Rev. 2020