September 27, 2014

1 - Fall webworms are attacking trees in New Mexico and it looks like the trees will be killed; don't worry they are doing very little harm.

Yard and Garden September 27, 2014


I have bag worms in my cottonwood tree. Whole branches are coated with their webbing and the webbing is squirming with worms. What can I do?


Santa Fe


What you have in your tree are not bag worms, rather they are fall webworms. These are the caterpillars of a fairly pretty white moth. Bag worms are caterpillars that create a sleeping bag type of structure that they live in and crawl around trees in. That structure is covered with pieces of leaves to conceal them from birds. The large-scale webbing, encompassing whole branches (and sometimes most of a tree) is the webbing of the fall web worm. Be thankful. Bag worms can do much more harm to a tree. The fall webworms are not very particular about which trees they feed on. Many species of trees from cottonwoods to apples to elms and many other species are affected. The caterpillars are eating leaves that have already finished their work for the summer. The leaves have provided the food the tree needs from them. In a very short time the leaves will fall from the tree, so the worms are not doing much harm to the tree.

The webbing surrounding the caterpillars protects the worms and their food (the leaves) from any insecticide you may apply. The caterpillars are large now and preparing to form cocoons for overwintering. These large caterpillars would not be readily killed by insecticides even if you could penetrate the webbing to get the insecticide to them. The general recommendation is not to spray at this time of the year. You can use strong jets of water from a power washer, a long pole, or baseballs and rocks to rip open the webbing so that some of the caterpillars fall out. That may expose some to predation by birds, but is more to make you feel like you are doing something.

The good news is that when the populations of fall webworms increases, the populations of predators that feed on them often increases. There is a good chance that the population of fall web worms will be much less next year. However, just to be safe, begin looking for signs of the fall webworm in late July and early August next year. If you see them developing, there are several insecticides you can use at that time to stop them. One of the effective insecticides is a natural product often called "B.t.", a toxin produced by a bacterium (Bacillus thuringiensis). The toxin from the kurstaki strain of B.t. is specific to larvae of moths and butterflies (that includes fall webworm). It paralyzes the intestines of the larvae causing them to stop feeding and die. It will not harm other insects; especially beneficial insects that help protect your plants from other pests. It will also not harm you or your pets and livestock (and wildlife) when used according to the directions on the label. There are other strains of B.t. that produce toxins to control other insects such as mosquitos or beetles.

A final warning - if the webbing appears in your trees in the spring it will be evidence of different insects feeding in the tree. Damage to the leaves earlier in the year can cause significant harm to the trees. If you observe earlier webbing (in May or June) take samples to your local NMSU County Cooperative Extension Service office to have the insect identified and a proper course of treatment recommended.

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:, office: 505-865-7340, ext. 113.


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

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

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