Locust borer

Locust borer - Adult at rest

Latin name: Megacyllene robiniae (Forst.)
French name: Cyllène du robinier
Order: Coleoptera
Family: Cerambycidae

Distribution

Canada

Canada, United States

Micro-habitat(s)

Bark, sapwood

Damage, symptoms and biology

The locust borer is a native insect that attacks black locust throughout its range in Canada and the United States. Trees growing in the open (e.g. along roadsides) or planted in badly eroded soils or in nutrient-deficient soils (e.g. in land reclamation and landfill sites) provide an ideal environment for the locust borer and pave the way for subsequent severe attacks. Drought-weakened trees are especially subject to attack. Injury is caused by the larvae mining in the inner bark and sapwood, and later burrowing into the solid wood. Swellings form on the trunk and branches, the bark cracks, the wood takes on a characteristic blackish colour, and the trees become gnarled and scarred. Often they become stunted and worthless as shade trees, and severe attacks may kill them. The presence of larvae in a tree may be detected by yellowish "sawdust-like" borings that accumulate in bark crevices on the trunk.

The mature larva is about 25 mm long, sub-cylindrical, and widest just behind the head. The adult beetle is about 20 mm long and black with yellow cross bands on the thorax and wing covers. The adult emerges from late August to mid-September and is often found on the flowers of goldenrod where it feeds on pollen. Soon after emergence, it lays white eggs in groups of six to eight in bark crevices on the larger branches and on the trunk of black locust. The eggs hatch about 10 days later and the larvae bore through the bark to the cambium where they form cells and pass the winter. The following spring, they become active again and bore deeper into the tree trunk as development progresses. The larvae mature by mid-August and transform into pupae at the end of their tunnels. The adults emerge 10 to 15 days after pupation through holes maintained for the exudation of larval excrement.

Other information

Maintaining black locust trees in good vigour by proper watering, pruning and feeding will greatly reduce their susceptibility to borer injury.

Canadian Forest Service Publications

Locust borer

Diet and feeding behaviour

  • Xylophagous: Feeds on woody tissues (wood).
    • Borer: Bores into and feeds on the woody and non-woody portions of plants.

Main host(s)

Black locust