In some areas of the state, black locust trees have taken on a brown color (Figure 1). This is due to activity of locust leafminer, which results in trees that appear scorched. The adults feed on lower leaf surfaces while larvae feed between leaf layers forming hollow pockets within leaves. In some years, this can be a serious pest of black locust. Damaged trees may drop their leaves and become defoliated (Figure 2) but will usually put out new leaves in midsummer. While black locust is the preferred host, locust leafminers will also feed on apple and soybean.
Insect Biology and Damage
Locust leafminer overwinters as an adult (Figure 3). Adults begin to skeletonize leaves in May and lay eggs for first generation larvae that mine leaves. Larvae pupate within the leaves and emerge as adults in July. There are two generations and the second may attack the new leaves that grew after the first defoliation. Generally, locust leafminer outbreaks are common and occur every few years. Defoliated trees usually refoliate after losing leaves.
While long-term health of trees is rarely threatened, trees may be killed if they are stressed due to drought or if defoliated several years in a row. To assist trees to better withstand attack by locust leafminer, provide adequate water during dry periods.
Ric Bessin, Extension Entomologist