Questions on: Webworms

Ron Smith, Horticulturist, NDSU Extension Service


Q: I have web worms in my pecan tree. My neighbor cut the branches out and burned the branches on his tree. Because my tree is so tall, what else can I do to rid my tree of web worms? What is dormant oil and where can I find it? How do I apply it? Thank you for your assistance. (e-mail reference)

A: Dormant oil is used in the early spring while the tree is dormant. On pecan trees, it should probably be applied by a professional applicator. The oil kills the overwintering eggs, but has a low toxicity. Any garden supply store should have it.


Q: I enjoy reading the good information you provide concerning trees. One method I've found to control web worms is to attach a propane torch to a long pole and burn the nests. I used to have them all over my cottonwood trees, but got rid of them after burning the webs for a couple of years. Someone told me that this disrupts the worm’s cycle. The drawback is dodging the flaming webs as they fall to the ground. Dodging the webs can be hazardous if standing on a stepladder and holding a 20-foot pole with a propane torch taped to the end of it. I know this sounds like something out of a Patrick McManus book, but it sure worked for me. (Bismarck, N.D.)

A: Yes, I have done that, too, but never on a ladder or with a propane burner attached to a long pole. You are a brave man. Your solution definitely will interrupt the worm’s life cycle. Don't try this when you get over 60! Thanks for the nice comments about the column!


Q: What is the best solution for webworm problems? They are the insects that produce webs on trees and destroy the foliage. The person who called me had Sevin on hand, so I recommended that she try it. Is it a strong enough chemical to take care of the problem? (e-mail reference)

A: For the most part, Sevin will kill the webworms that are caught outside of the webbing. Malathion will do that as well, but it will leave a residual on the foliage that it covers. When the insect larvae eat the foliage, they will die. The best control is to spray trees that tend to become infested with dormant oil in the early spring before the leaf buds open.


Q: A woman called about curled leaves on her tree. After she looked closer, she saw shiny cobwebs covering the whole tree. What should she spray on the tree? Can the tree be saved? Can mums be put out now or should she wait until spring? (Wahpeton, N.D.)

A: Tell her to clean up all fallen leaves this autumn. Spray the tree with dormant oil next spring, before the leaves come out. That should take care of any overwintering cocoons or pupae that remain on the tree. She also should be vigilant about monitoring the tree during the summer. If there is any evidence of the webworm returning, she should spray immediately with Orthene. About the mums, many folks grow them in an inconspicuous part of their garden or property and then move them to a more visible area as the summer annuals begin fading.


Q: I’m having a lot of trouble with web worms. I’ve tried spraying with different insecticides, but that doesn’t seem to kill them. The tree has developed only a few blossoms for the last several years. It is approximately 15-years-old and hasn’t grown much. (e-mail reference)

A: In the spring, selectively prune the tree and spray it with a lime-sulfur and dormant oil spray. Follow that up with a spray at bud break with Sevin or Malathion and again 10 days later.


Q. I'm writing concerning the webworms that are very abundant this year. I do spray them and destroy the webs when small. I'm wondering, what lays the eggs and can they be controlled in the fall?

Also I find brown-like clusters on branches, which appear to be dried up. Where do they come from?

How can I control these worms in the fruit trees: plum, currant, gooseberry, chokecherry? (New England, N. D.)

A. Whether you are talking about webworms or tent caterpillar, the egg-laying adult is a moth. The webworm makes nests over the ends of branches, while the tent caterpillar makes them at tree crotches. These can be controlled by spraying with dormant oil and lame sulfur in the early spring, just before leaf-out.

The larvae in fruits is controlled by spraying with Sevin or Malathion just at petal drop. Following good sanitation in the fall—picking up leaf and fruit litter also helps.


Q: I would like to know how webworms get into the trees. Are they laid in egg form by butterflies, or moths, or do they travel from the ground up the tree trunks? I have tried to keep them out of my trees, but my black walnut has gotten so tall that I can no longer reach them to cut out the infested branches (which I have burned immediately in previous attempts). I would like to know if there is some way to spray the tree to discourage future infestations. The neighbors’ cottonwoods also have the same problem. (E-mail reference, Oakes, N.D.)

A: The fall webworm adult is a moth that emerges from pupation during late spring and lays hundreds of eggs on the underside of the leaves. Following a spring spray schedule -- roughly in May and again in June-- should keep them in check. Typically predators keep them in check, with two being especially important - the ichneumon wasp and the braconid wasp. Important also is the leaf litter clean up in the fall, as that is where the overwintering pupation takes place.


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