Borers in ash tree
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Issue: July 22, 2000

Borers in ash tree

Question:

The leaves on my ash tree have turned a yellow-green color. I consulted with a person at a nursery who informed me that I probably have borers. There are several holes in the tree, and I am afraid that this tree will be lost. Several other ash trees in our neighborhood have been destroyed by some type of infestation. The nurseryman suggested a Dursban(R) based product. Will this take care of the problem? In addition, the tree seems to be slanting more than I remembered last season. Does this have anything to do with the borers?

Answer:

Borers and ash bark beetles are candidates for the source of the problem you have described, but there are other factors to consider as well. The holes in the trees may be due to these pests, but they may also be due to other factors.

The ash borers tend to attack the trunk and, sometimes, the larger branches. After the larvae have finished feeding, they pupate and emerge from the tree, leaving holes in the trunk. They tend to leave their pupal cases extending outward from the tree at the point from which the adult moths emerged. These pupal cases are tan or brownish and papery. This occurs in late summer and is characteristic of the borers. Have you observed these pupal cases remaining on the tree trunk?

The ash bark beetle is most commonly seen in the twigs, though they may move into the larger branches once the tree is heavily infested. Evidence for the bark beetle is small, pin-head sized holes in the twigs. It is often possible to very easily snap the twigs at points where the bark beetle, or other small beetle, has girdled the twig by eating the cambium layer just below the epidermis layer of the twig.

The insecticides mentioned below, and others labeled for use on ornamental or shade trees, can be effective in limiting the damage caused by these insects. It is important to apply chemicals at the time when they will be effective. Check with your County Extension Agent for the proper time of year to apply these products once you have determined whether it is borers or bark beetles doing the damage. Whether or not the pesticides will be effective also depends on the extent of the damage which has already occurred.

As I said, there are other factors which could also cause these problems. Accidents with or misapplication of lawn weed control chemicals could cause these problem. A very common problem that may cause the symptoms you described is the "southwest injury" so common on young trees in New Mexico. This is damage to the bark caused by alternate freezing and thawing of the bark on these trees in the late winter. In this case, the sun warms the south and southwest side of the tree, thawing the bark each day. The bark then refreezes at night. Repeated freezing and thawing can damage the bark. This damage is revealed later in the year by patches of dead and flaking or peeling bark on the southwest or south side of the tree. If this is the case, there may be borers exploiting the damage, but the real problem is the freeze/thaw damage. To prevent such injury next year, paint the trunk of the tree or other areas that are exposed at a direct angle to sunlight, with white water-based paint in late fall. This reflects the sunlight, reducing the winter heating of the bark. Use of insecticides would not be beneficial in this case unless there is evidence of insects exploiting this injured area. If the area of the tree trunk injured by southwest injury is extensive, the tree may be slow to resume normal growth or may never recover.

The leaning is probably not directly associated with the other problems. It may be the result of poor root development on one side of the tree or due to our winds.

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Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.

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