Common name: Locust borer
Scientific name: Megacyllene robiniae
Hosts: The locust borer (Megacyllene robiniae) is a serious pest of black locust (Robinia pseudoacacia). It does not attack honey-locust (Gleditsiatriacanthos).
Younger trees and trees under stress are particularly susceptible. Healthy trees can often tolerate some damage from the borer.
The locust borer is native to parts of North America (central Pennsylvania south to Alabama, as well as parts of Missouri, Arkansas, and Oklahoma). It can, however, be found wherever black locust trees are planted.
Damage to black locust trees can be quite extensive, and trees infested with the insect are more prone to storm damage.
Damage
Locust borer larvae tunnel under the bark into the vascular tissues. The larvae feed on the vascular tissue, chewing on the tissue and producing cavities under the bark.
This damage can girdle a tree, leading to dieback of branches.
From each cavity, the larva tunnels into the heartwood of the tree producing galleries that weaken the wood and can lead to breakage of branches.
The larvae clear their excrement from the tunnels they produce, so it is common to find piles of sawdust-like excrement at the base of the tree.
Life Cycle
The locust borer overwinters as young larvae in the inner bark of the tree. When temperatures warm in the spring, the larvae become active again, boring into the tree and continuing their development. Pupation occurs around mid-summer, with adults beginning to emerge from the tree in late summer.
The adult borer is a beetle about ½ to 1 inch long. It has a black body with yellow stripes running across the entire length of the body. It has very long antennae and belongs to the group of beetles known as “long-horned beetles.”
Newly emerged beetles will feed on the pollen of goldenrod from late summer until frost. Mating will also occur during that time.
The female lays her eggs in the crevices of the bark, under bark scales, and even in old borer galleries. The eggs hatch into white larvae. The newly hatched larvae bore into the bark to reach the vascular tissues.
There is one generation per year.
Cultural Management
Trees and shrubs 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 black 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. Plant resistant cultivars where available.
Biological Management
There are naturally occurring enemies of this native pest, primarily woodpeckers. They can help reduce borer populations, but will not always give complete protection.
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.