The Black Locust Borer, Megacyllene robiniae, is a native insect and a significant pest of the Black Locust tree, Robinia pseudoacacia, throughout North America. This longhorn beetle is the primary reason for the tree’s decline and structural failure in many planted areas. The borer’s range has expanded considerably from its original native habitat due to the widespread planting of its host tree for landscaping and land reclamation. Focusing its entire life cycle on the Black Locust, the insect is directly responsible for damage that weakens and can ultimately destroy the tree.
Identifying the Adult Black Locust Borer
The adult Black Locust Borer is distinctive and easily recognizable, often mistaken for a wasp due to its bright coloration. It is a slender, cylindrical beetle, typically measuring about three-quarters of an inch (19 mm) in length, sometimes reaching one inch (25 mm). The insect has a jet-black body covered with a pattern of bright yellow bands that encircle its thorax and abdomen.
A distinguishing feature is a W-shaped yellow band that extends across the insect’s elytra, or wing covers. The beetle belongs to the long-horned beetle family (Cerambycidae), named for its prominent, long black antennae. Its legs are generally reddish, complementing the vivid yellow and black striping that provides protective mimicry.
Adult borers emerge in late summer and early fall, often coinciding with the bloom of goldenrod flowers. They are commonly observed during the day, feeding on the pollen and nectar of goldenrod (Solidago species) and other late-season flowers. This feeding behavior is an important marker for timing control measures, as the adults are actively searching for mates and egg-laying sites on the Black Locust bark.
The Destructive Larval Stage
The Black Locust Borer’s life cycle spans one year, with the destructive phase occurring entirely within the tree’s wood as a larva. Females lay 100 to 200 small, white, oval eggs from late August through early October, depositing them singly or in small clusters in bark crevices or under bark scales on the trunk and larger branches. The eggs hatch within five to eight days, and the newly emerged larvae immediately tunnel into the inner bark where they form a small hibernation cell to overwinter as first instars.
In the spring, around the time the tree’s buds begin to swell, the larvae resume feeding and begin boring deeper into the tree. They first mine the sapwood, the living vascular tissue just beneath the bark, and then penetrate the heartwood. This creates extensive tunnels, or galleries, that can reach three to four inches long. This tunneling severely disrupts the tree’s vascular system, hindering the flow of water and nutrients, which can lead to branch dieback and overall decline.
As the larvae feed and enlarge their tunnels throughout the spring and summer, they expel a sawdust-like excrement called frass, often visible on the bark surface or accumulating at the tree’s base. Early-season feeding causes sap to ooze out, leaving wet, dark spots on the bark. Later tunneling into the heartwood produces a coarse, yellowish frass. The sustained internal damage structurally weakens the wood, making the tree susceptible to wind breakage and causing gnarled, knotty swellings on the trunk as the tree attempts to seal off the injury. Larvae reach their full size by mid-July, pupating within their tunnels before mature adults emerge in late summer to begin the cycle anew.
Protecting the Black Locust Tree
Maintaining the tree’s overall health is the most effective defense against the Black Locust Borer, as healthy, vigorous trees are less susceptible to attack. Cultural control methods focus on reducing stress factors that attract the beetle. This includes ensuring proper watering during drought and providing appropriate fertilization to support strong growth. Trees planted in poor soil conditions or stressed by environmental factors are the most likely targets for egg-laying females.
Pruning and mechanical control are helpful in managing localized infestations, but they must be timed correctly. Infested wood, identifiable by frass or knotted swellings, should be pruned and removed before the mature larvae pupate. This removal should ideally occur between the fall and the following spring when the larvae are dormant or newly active. Destroying the infested wood prevents the new generation of adults from emerging and continuing the cycle.
For high-value ornamental trees, chemical control can be used, but timing is important due to the borer’s life cycle. Preventative treatments, such as systemic insecticides (e.g., dinotefuran or imidacloprid), can be applied as a soil drench early in the season. These treatments are translocated into the tree’s vascular system before the adults emerge. Alternatively, a residual insecticide bark spray can be applied to the trunk and larger limbs in late August to mid-September when adults are active and laying eggs. This topical application targets the egg-laying females and newly hatched larvae before they bore deep into the wood and become protected.