Tree Facts: Common ash borers

Farm Forum

Green ash trees make up a majority of the woody vegetation in many of the tributaries of creeks throughout the western Dakotas. It has been planted along roadsides, in parks, on campuses, home sites and farmsteads because it can adapt to difficult growing sites, and provide showy yellow, purplish or burgundy foliage in the fall. Generally, ash trees are tolerant and/or resistant to insects and diseases.

The most common pest of ash trees is the common ash borer (CAB). The insect is native to America, has been around for thousands of years and is found throughout the Dakotas. It also infests lilacs and privet. The adult stage of this insect looks like a wasp with smoky brown forewings, clear hind wings and a wingspan of 1 to 1½ inches. It has a slender body that varies in color from reddish to yellowish to brownish black with orange or red bands around the abdomen.

CABs develop beneath the tree bark, where they burrow and feed on the live tissues that conduct food, water and nutrients. Burrows may exceed 12 inches long by 1/3 inch wide by two inches deep. Larvae are about an inch long, creamy white with a brown head, have three pairs of legs and several pairs of fleshy pro-legs.

Most CAB infestations occur in the lower portion of the tree trunk, from the ground up to 10 feet high. This insect is drawn to stressed or aging trees and can kill such trees. However, in most cases it will feed on the trees for years without killing them. Young ash trees can be killed by this insect because it weakens the trees and then they break during storms.

CAB moths emerge in the spring usually during early May and live for about a week. They do not damage trees but feed on flower nectar. Females attract males for mating and lay about 400 eggs on rough bark or near trunk wounds. Two weeks later the eggs hatch and the larvae tunnel into the host and feed on the tree beneath the bark. In the late summer or fall they burrow deeper into the sapwood and go dormant for the winter.

The larva undergoes a metamorphosis before pupation in late winter. The larval gallery is extended just under the bark surface, leaving a cellophane-thin window of plant tissue. This allows the new moth to pop through the window and emerge. The brown, brittle, empty pupal cases are sometimes noticeable on branches or trunks. The exit holes are round or oval and ¼-inch diameter. Only one generation occurs each year.

There are several ways to control CAB. Taking action to minimize tree stress and injury in young trees is the best prevention. This is best done by keeping lawn equipment away from trunks, mulching the root zone and watering during dry periods. Insecticide sprays can prevent CAB attack, but sprays do not control borers already inside the plants. Spray in early May when bridal wreath is in full bloom or when lilacs are just finishing bloom and again three weeks later.

The most effective preventive products have permethrin as the active ingredient. Products containing permethrin include Bonide Borer-Miner Killer, Gordon’s Bug-No-More, Hi Yield Garden, Pet and Livestock Insect Control.

My sources for this news release were the South Dakota and Iowa Cooperative Extension Services. If you would like more information about “Common Ash Borers,” contact Bob Drown @605-244-5222 Extension 109 or by e-mail at