Poplar and Willow Borer

Problem type: Insect

Name of problem:
 Poplar and Willow Borer

Plant name(s): Willow and poplar species; occasionally alder and birch species

Symptoms / Characteristics:
Cracks and holes in the bark of branches and trunk. Larval feeding weakens tree stems. Upper portion of the tree, or possibly the whole tree may be broken or dead. Trees that are 2 to 3 years old appear to have the most damage.

Damage begins by adults drilling into the bark to lay eggs. The eggs then hatch into white, legless, C-shaped larvae that burrow just under the bark, then move deeper into the tree where they excavate tunnels. This results in splitting stems and openings. Sawdust-like frass is pushed out through the holes as tunnels are made. Sap and red-brown and white shavings may also ooze out of openings commonly found around the base of the stems. Larvae often bore around a branch or stem, girdling it and preventing the transport of water and nutrients beyond that point. The larvae pupate in cells at the end of the tunnel and emerge through a round hole cut in the bark as an adult.

The adult is a weevil (snout beetle) with a long curved snout. They are generally black in color with small black, gray and/or pink scales on their backs. Adults feed on young bark but do not cause much damage. If infested stems are spit or cut open, larvae and their tunnels will be visible. Old, infected stems will have circular emergence holes, scarring and calluses over injured areas. Sprouts may develop as a reaction to severe injury by the borer. Infected trees may be killed or may lose their form and become bushy due to sprouting.

Control / Preventions:
If the number of infected trees is small, larvae can be removed by probing in the excavated tunnel holes with a flexible wire. A registered insecticide may also be injected into the holes to kill larvae. The holes then need to be plugged to prevent the fumes from escaping. Control of larvae should be done when sawdust is evident. A residual insecticide can be sprayed on the bark of tree trunks and branches. One application should be made in early June and a second application in early September. This can control the adults, killing them as they crawl over the surface before they lay eggs. The poplar and willow borer may require 2 to 3 years to complete its life cycle. The timing of developmental stages may differ and overlap from year to year.