Issue: July 13th, 1998

Ash bark beetles

My ash tree has a lot of dead twigs this year. They are at the ends of branches that still have some healthy growth. I have also noticed that a lot of the twigs have fallen out of the tree when the winds blew. These twigs seemed to break straight across when they fell out. What is wrong with my tree?

It sounds like your tree has been infested by the ash bark beetle. This beetle is especially causing damage to green ash trees in New Mexico. Although, it seems to prefer green ash trees, it can cause damage in other ash trees which are under stress from improper irrigation, pruning, or weather-related injuries.

The ash bark beetle is a small black, or mottled gray beetle in the adult stage, or small, white, legless grubs with brown heads. These larvae will be found in tunnels under the bark of the branches. You may also notice small, "ventilation" holes in the twig due to the bark beetle. The branches seem to break straight across, because the twigs are often girdled so there is less wood at one point and the branch snaps at that point. Once you confirm that ash bark beetles are responsible for the symptoms you described, you may use several techniques to limit the damage due to bark beetles.

Keep your trees healthy so that they will be less attractive to the beetles. Proper pruning, irrigation, and fertilization will help keep the trees healthy. Once they are infested, the insects and diseases that they carry will weaken the trees and lead to further infestation.

Infested branches and twigs should be pruned from affected trees. These twigs and branches must be disposed of in a manner to prevent emergence of the adult beetles which will then reinfest ash trees. Shredded twigs may be placed into a compost pile which, if properly constructed, will generate sufficient heat to kill the bark beetle larvae. Other organisms in the compost pile will also help prevent emergence of the adult beetles to lay eggs and cause further infestation.

Branches and twigs may be solarized. This involves cutting the branches into manageable sized pieces, placed in a sunny location, and covered with clear plastic which is sealed around the edges with soil or other material to contain solar heat under the plastic. In six to eight weeks, sufficient heat should be generated to kill most of the bark beetle larvae. Any adult beetles which emerge will be trapped under the plastic and killed by the heat.

Insecticides labeled for bark beetle control may also be applied in the autumn as adults prepare to hibernate and again in the spring before they emerge to lay their eggs and continue the cycle of infestation and damage.

These insecticides should be applied to the bark of the trunk and larger limbs as a course, low-pressure spray. Spreader-stickers will increase the effectiveness of these products. As with all pesticides, read and carefully follow the label directions. Use all recommended safety equipment and spray when the wind is not blowing.

The information presented here was derived from New Mexico State University Extension Circular 550, Managing Ash Bark Beetle, by Drs. Gary Smith, Charles Ward, and L. M. English. This publication is available from your New Mexico County Cooperative Extension Service office.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!