Tree Facts: The borer problem in green ash trees

Farm Forum

Green Ash trees make up the majority of the woody vegetation on many drainage ways and creeks throughout the western Dakotas. They have also been planted along roadsides, in parks, on campuses, home sites and farmsteads because of their adaptability to difficult growing sites, and their showy yellow, purplish or burgundy foliage in the fall. Generally, ash trees are tolerant and/or resistant to insects and diseases.

However, ash trees do have insect pests that are drawn to them, especially during drought years when they are under stress. The most common pests of ash trees are carpenter worms and common ash borers. Both insects are native to America and have coexisted with green ash trees for thousands of years. Moths of the carpenter worm and ash borer are noticeable from May to August. They lay eggs in cracks, crevices of rough bark and borer holes left from previous year infestations. Infested trees tend to remain that way year after year and they serve as brood trees.

Newly hatched carpenter worm larvae begin their burrows in cracks and crevices and wounds in the bark. Carpenter worm larvae range from two to three inches and the ash borer larvae are about one inch in length. Young larvae are present on infested trees throughout the summer months and are well established in burrows by the end of the first summer. The burrows are enlarged and extended during the second and third years. Borer larvae feed on the cambium tissue around the burrow openings and make galleries in the heartwood for protection.

Green ash trees should be examined frequently and thoroughly for past and current evidence of borers. Carpenter worm infestations older than one year can be detected from the volume of chewed wood, bark bits, frass and webbing. It may not always be possible to determine which borer is the present. The insects if observed are easily separated simply by their sizes, for example the pupal cases are left in the burrows when the moths emerge. The borers would be about an inch and carpenter worm’s two to three inches in length. The larvae of both the ash borer (after three years) and carpenter worm (after four years) pupae and change into moths in May.

Once the moths emerge they do not damage trees but feed on flower nectar. Females attract males for mating and then they lay their eggs. A couple 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.

Infested trees may be cut, removed and burned or buried deeply. The cut should be as close to the ground as possible and allowed to regrow. The sprouts will grow profusely and should be thinned back to several main stems in the short-term and single stems in the long-term. It is possible to take action to minimize tree stress and borer injury in young trees. This can be done by keeping lawn equipment away from trunks, mulching the root zone and watering during dry periods.

Insecticides are not recommended for carpenter worms since the moth flight period is long. A wire can be inserted into the hole to kill the larva if few trees are infested. Woodpeckers are attracted to infested trees for larvae.

Permethrin insecticide sprays can prevent common ash borer attack, but sprays do not control those that are already inside the trees. 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 insecticide is permethrin. 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 USDA Forest Service and the South Dakota Cooperative Extension Services. If you would like more information about “The borer problem in green ash trees,” contact Bob Drown @605-244-5222 Extension 109 or by e-mail at