Cottonwood Borer

Cottonwood Borer, (Plectrodera scalator Fabricius) is found in the eastern US, New York. Larvae and adults feed on willow and poplar, with cottonwood being the most preferred. Adult beetles can be found on and around host plants during the summer. They are large 1 1/4 inch long, robust longhorned beetles with black antennae as long or longer than the body.

The body is beautifully marked with a bold pattern of black rectangular areas on a creamy white to yellow background. Larvae are legless, cylindrical oval in cross section, creamy white bodies and brown to black headed, growing to 1 ½ inch long.

There are a great many other cerambycid beetles that attack trees and other plants, but few others have larval stages that feed on roots or are larger than the cottonwood borer.

Life Cycle: Adults begin to emerge in late spring late May or early June in Oklahoma and feed on the tender shoots of young trees. Mating and egg laying occur over an extended period during the summer. To oviposit, the female digs away the soil at the base of the tree, cuts a niche in the bark, and deposits one or more eggs. Upon hatching, the larvae mine downward in the Inner bark, penetrating into the wood at the approach of cold weather.

The second winter the much larger larvae occupy large tunnels at the base of the tree. Pupation occurs within the gallery the second spring, the entire life cycle extending for a two-year period. The new adult chews through the pupal chamber and digs its way to the soil surface to escape.

Habitat: These beetles generally infest cottonwoods, but can also occur on poplar and willow trees. Larvae of the beetles chew tunnels around roots and form galleries at or below, the soil line. Larval tunnels can be as long as 8 inches or as wide as 3-inch diameter oval areas. The size of larval tunnels is dependent on the tree size and infestation site. These tunnels are often packed with wood shavings and frass, which is one of the signs of damage in infected trees. Adults live on the same trees, but can be found higher on the host plant during the daytime.

Diet: Adult beetles feed on the tender shoots of young cottonwood, poplar, and willow trees. This can cause shoots to shrivel and break off, heavily damaging host trees. Larvae feed on the inner phloem of trees, specifically at the root collar and below.

Damage: Damage occurs primarily at the bases of infested trees and consists of sawdust-packed tunnels up to ½ inch in diameter bored into the wood, which weakens the trees and interferes with the translocation of water and nutrients. Damaged trees may blow over, breaking off at the base. Suckers and smaller branches near the base may be killed. Coarse sawdust and excrement frass is pushed outside of the burrows and can be seen at the bases of trees and suckers which grow from the main trunk.

Controls: Periodically examine cottonwood, poplar and willow trees for evidence of borer activity, checking for the beetles, emergence holes in bark, galleries with sawdust push through holes in the bark and dead or dying branches near the bases of tree trunks.

Proper management by removing and destroying infested trees or locating new nurseries away from infected sites can possibly minimize damage by larvae. Additionally, three weekly applications of insecticide to the lower trunk and surrounding soil soon after adult emergence can control and reduce adult populations.