Common Black Locust Bugs and How to Handle Them

The black locust tree, Robinia pseudoacacia, is known for its resilience and rapid growth. Despite its tough nature, the tree hosts several insect species, often called “black locust bugs.” Understanding the specific insect affecting a tree is the first step toward proper management.

Identifying Common Black Locust Insects

The primary insect pest of the black locust is the locust borer, Megacyllene robiniae. The adult is a distinct longhorned beetle, about three-quarters of an inch long, with a black body marked by bright yellow, V-shaped bands. A recognizable marking is a “W” shape formed by the yellow bands at the base of the wing covers. Its legs and long antennae are a reddish-yellow color. These adult beetles are often seen in late summer and early fall, feeding on goldenrod pollen.

The larval stage of the locust borer causes the damage. The larva is a creamy-white, legless grub with a brown head, growing to about one inch in length. These grubs tunnel beneath the bark and into the wood of the tree. Evidence of their activity is the primary sign of an infestation.

Another frequent pest is the locust leafminer, Odontota dorsalis. The adult is a small, flat beetle, about a quarter-inch long, with an orange-red body and a prominent black stripe down its back. These beetles are most visible in spring when they feed on new leaves.

The small, yellowish, and flattened larvae feed between the upper and lower surfaces of the leaflets. This feeding creates discolored, blotchy “mines” within the leaves. While other insects can be found on black locusts, the borer and leafminer are the primary pests of concern.

Understanding the Damage Caused

The damage from the locust borer is more severe than that of the leafminer and can affect the tree’s structural integrity. Borer larvae tunnel deep into the tree, feeding on the inner bark, sapwood, and heartwood. This activity disrupts the tree’s ability to transport water and nutrients, weakening it. The tunneling creates extensive galleries that make the trunk and branches susceptible to breaking in high winds.

Visible signs of an infestation include weeping sap or wet spots on the bark in the spring. As larvae feed, they push out coarse, sawdust-like frass, which can accumulate in bark crevices or at the base of the tree. Over time, the tree may develop knotty swellings on the trunk as it grows over damaged areas. In severe cases, a locust borer infestation can lead to the death of young or stressed trees.

In contrast, damage from the locust leafminer is almost entirely aesthetic. The larvae mine the tissue between leaf surfaces, causing the foliage to turn brown with a scorched appearance. While a heavy infestation can make a stand of trees appear brown from a distance, it rarely causes lasting harm to mature trees. The damage is most noticeable in mid to late summer.

Pest Life Cycles and Seasonal Activity

The locust borer completes one generation per year. Adult beetles are most active in late summer and early fall, from late August through September, when they feed on pollen and mate. Females lay their eggs in bark crevices or around wounds on the trunk and large branches.

After the eggs hatch, small larvae burrow into the inner bark to overwinter. In spring, as the tree’s buds begin to swell, the larvae resume their activity, tunneling from the sapwood into the heartwood. They feed and grow throughout the summer, pupating within their tunnels before emerging as adults to start the cycle again.

The locust leafminer has two generations per year in many regions. Adult beetles overwinter in leaf litter at the base of host trees or within bark crevices. They emerge in the spring as leaves unfold to feed and lay eggs on the undersides of the leaflets.

Upon hatching, the larvae mine the leaves, often starting in a collective mine before separating. They pupate inside these mines and emerge as a new generation of adults in July. This second generation of adults will also feed on the leaves and lay eggs, leading to more larval damage before those adults seek overwintering sites.

Management and Control Strategies

For an active locust borer infestation, management begins with cultural practices like pruning and destroying infested branches to remove larvae. This is best done in late spring after the larvae have become active but before they mature.

Natural predators provide biological control. Woodpeckers are significant predators of locust borer larvae, and the wheel bug preys on adult beetles. Maintaining a diverse landscape can encourage these natural enemies.

Chemical controls are an option for severe infestations. Systemic insecticides injected into the trunk or soil can target the tunneling larvae in the spring. Another approach is applying a protective insecticide spray to the trunk and large limbs in late summer to target egg-laying adults and newly hatched larvae. These treatments require precise timing and are best handled by a professional arborist.

Management for locust leafminer is often unnecessary, as the damage is primarily cosmetic. Raking and removing fallen leaves in autumn can help reduce overwintering adult beetles. Parasitic wasps and predatory insects also help keep leafminer populations in check. If chemical intervention is desired for aesthetic reasons, a contact insecticide may be used in the spring to target adult beetles, but this is rarely needed for the tree’s long-term health.

Preventing Future Infestations

The most effective strategy for preventing insect problems from the locust borer is to focus on the tree’s overall health and vigor. Borers are strongly attracted to trees that are stressed, wounded, or in decline. Ensuring the tree has adequate water, especially during periods of drought, is fundamental, as a healthy tree is more capable of defending itself.

Avoiding physical injuries to the trunk and root zone is also important. Wounds from lawnmowers, string trimmers, or construction create easy entry points for borers to lay their eggs. Preventing soil compaction around the base of the tree is also important, as this can stress the root system.

Proper site selection can prevent many future issues. Planting black locust trees in environments where they can thrive is the best long-term defense. Healthy trees larger than eight inches in diameter are less susceptible to successful borer attacks.