July 7, 2000 | By: Mark Eclov

Many of the black locusts in Kentucky are beginning to turn brown. It is probably not a sign of an early fall. Instead the browning is caused by an insect that has apparently set up housekeeping for some time to come.

A close inspection of leaves on the lower branches of damaged trees will bring you into contact with one of the most serious insect pests of black locusts, which is appropriately called the locust leafminer.

"The adult of this insect is a beetle that is about 1/4-inch long and has an orange body with a black stripe down the back and black head, antennae and legs," noted Monte Johnson, Extension pesticide impact assessment specialist in the University of Kentucky College of Agriculture.

The adult beetle hibernates wherever winter protection can be found, often in litter under the locusts. As temperatures warm in the spring, the beetles move to the developing foliage and begin to feed on lower leaf surfaces, leaving speckled, shot-hole type damage.

The beetles then lay eggs and the resulting larvae eat into the inner layer of leaf tissue, forming a mine. Preferred feeding sites are near the tips of leaflets, where the mines take on the shape of irregular blotches.

After emerging in midsummer, adults of this second generation begin feeding on the leaves, subjecting the trees to more damage. This second generation will again lay eggs on the leaves and second-generation larvae will add more damage to the foliage. After adult beetles emerge in late summer and fall, they will look for overwintering sites. It's no wonder that black locusts are turning brown!

Black locust trees may produce a second growth of leaves during a growing season. If damage by the locust leafminer is extensive on this foliage in successive years, trees may die or at least be weakened to the point where they may be susceptible to other pests and diseases.

"In a year such as we're having, drought stress is a real concern as well," said Johnson. "Although black locusts are the most obvious plants host, other plants such as false indigo, bristly locust, Japanese pagoda tree, and golden chain tree are fed on as well. "

The adults are known to feed on include dogwood, elm, oak, beech, cherry, wisteria, hawthorne, and several herbaceous plants. The locust leafminer is found throughout the eastern United States and Canada. Several natural control agents including parasitic wasps and predators such as the wheel bug prey on various life stages of the locust leafminer.

Most black locusts are found in the wild or along roadsides and control measures for this pest are seldom used in this setting. If a tree is part of a residential or recreational area, there are pesticide applications that can be used to reduce the damage.


Monte Johnson 859-257-6693