Issue: October 11, 1999

Fall web worm problems

All over the valley large healthy trees are being attacked and destroyed by web worms. At first limited to Lombardy Poplars and cottonwoods, they can now be found on sumac, pecans, fruit trees, ashes, and in general most broadleaf deciduous trees. Some trees (poplars) are completely shrouded with webs, and there is probably little chance of those trees surviving. Most affected branches are very high up on the trees and hard to reach with sprayers or pruners. What is the best way to control web worms?

The ability of trees to survive difficult conditions is amazing. The insect you described is the fall web worm which appears to be doing tremendous damage. I was in Las Cruces and saw first hand what you have described. It does indeed look bad; however, but after talking with entomologists, I am assured that the damage will be minimal. Dr. Carol Sutherland, NMSU Extension Service Entomology Specialist, assured me that the web worm only feeds on leaves and doesn't harm the bark or the buds.

Since this is the end of the growing season, the leaves have completed the greatest part of their work; that is, the production of food to sustain the growth of the tree. While there is a little growing season left, early loss of leaves at this time does not harm the tree excessively. Web worms in the spring and early summer can be much more damaging, but the fall web worm seems to have a "contract" with the tree not to attack until the damage is minimal.

The level of infestation in Las Cruces is extreme and suggests that next year you should prepare to treat. It is too late to treat with chemicals now. Chemicals will have very little effect. Next year in August, as the caterpillars are just developing and before there is much development of webbing, you can apply any of several insecticides labeled for control of moth and butterfly larvae. This even includes some organic insecticides. Early in the life of the larvae, they are very susceptible to insecticide and may be controlled.

You alluded to physical removal of the caterpillars by pruning in addition to spraying. If it is possible to reach them, if the pruning will not damage the form of the tree, and if you will not remove too much of the tree by pruning, then pruning may be an effective means of reducing the problem. Often, however, we do more damage by pruning than the caterpillars have done. Burning the webs from the tree will definitely do more harm than the caterpillars and is not recommended. Besides, this can cause damage to homes and other structures if the fire gets away. The best thing to do is wait and tolerate the fact that the web worm is a nuisance.

Ponderosa Pines turning brown

My ponderosa pine trees are turning brown. They have looked good for over 10 years, but now they are very brown toward the trunk. The outside needles are still green, but there is so much brown that I am afraid they are dying.

This has been a common question recently. In most cases what you are observing is the natural drop of needles three or four years old. Even though the ponderosa pine tree is an evergreen tree, the needles don't live forever. After three or four years, the needles fall. If three or four years ago the tree grew vigorously and produced many leaves, then a large number of needles will be dying this year and the tree will look very brown. This is nothing to be worried about.

However, if the needles at the ends of the branches are also brown and dying, then there is a problem which must be investigated. If this is the case, take a sample to your local nursery or Cooperative Extension Service office. They are familiar with the environment, the soil, and common problems in your area and should be able to help you.

If there are no disease, insect, or environmental problems which are obviously the problem, then consider planting problems. Was the tree planted with burlap and chicken wire holding the root ball? This often causes the tree to die about 10 years after planting. Is the soil a heavy clay soil that may have been compacted outside the planting hole, discouraging extension of the roots from the trunk of the tree? Circling roots caused by such conditions can also result in trees which grow well for a few years then cease to thrive. Local experts at the Extension Service or nursery should also be able to help with this problem.

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!