Common name: Honey-locust borer
Scientific name: Agrilus difficilis
Hosts: The honey-locust borer (Agrilus difficilis) is a common species that feeds on honey-locust trees (Gleditsia triacanthos). Trees under stress are particularly susceptible. Healthy trees can often tolerate some damage from the borer. The honey-locust borer is native to much of the eastern half of the United States.
Damage
Honey-locust borer adults will feed on leaves, making notches on the leaf margins, but this damage is relatively minor. The real damage is done by the larvae, which tunnel under the bark into the vascular tissues.
The larvae produce serpentine tunnels, damaging the vascular tissues of the tree. This damage can girdle a tree, leading to dieback of branches and even death of the tree.
Areas where the young larvae bore into the bark may ooze sap. Over time, the sap may harden into a gum.
The borer can attack both weakened and healthy trees. In healthy trees, the larvae may be killed by a heavy flow of sap and this can minimize damage.
Life Cycle
Honey-locust borer overwinters as larvae under the bark of the tree. Adults pupate and begin to emerge from the tree in late spring, with adult emergence continuing throughout summer.
The exit hole made by the adult is shaped like a “D,” similar to those made by related species (bronze birch borer and emerald ash borer).
The adult honey-locust borer is a narrow beetle about ¼ to ¾ inch long, varying in color from metallic greenish-black to purplish-bronze. There are distinctive white or yellow bands present along the sides of the body.
Newly emerged beetles will feed on leaves and then mate. Each female can lay about 30 eggs. The eggs are laid, singly or in groups, in the crevices of the bark. Each egg is covered with froth that later hardens.
The eggs hatch into cream-colored larvae that will be somewhat flattened and will have strongly segmented bodies when mature. The newly hatched larvae bore into the bark or stems. Sap will ooze from the entry sites, and the sap may harden over time.
The larvae spend the season making serpentine tunnels under the bark and feeding on the vascular tissues.
There is one generation per year.
Management
Cultural Management
Trees of low vigor or in a weakened state of health are especially susceptible to borer attack. Prevention is the key in controlling wood-boring insect pests. Grow honey-locust in a site that will keep stress to a minimum. Keep plants healthy and vigorous through proper planting, mulching, watering, fertilizing, pruning, and winter protection practices. Protect trees from injuries. Remove dead limbs or trees promptly to avoid infestations. Remove bark from felled trees if stored for firewood.
Chemical Management
Systemic insecticides and trunk sprays are available to manage this pest.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
For more information, contact The Morton Arboretum Plant Clinic at 630-719-2424 or plantclinic@mortonarb.org.