It doesn’t seem as bad as last year, but the fall webworms are back on my trees. What should I do? SF

It’s always a bit of a mystery why some years the fall webworms seem to be everywhere and other years they are pretty scarce. Fortunately, we are not seeing a lot of them this year. But if they are in your trees, the degree of concern is a little different.

Fall webworms are found in most of North America, Asia, Europe, and in northern Mexico. In Oklahoma, they seem to be more of an issue in the eastern part of the state.

While they are called webworms, they are not worms, but caterpillars on their way to becoming a moth. Adult fall webworm moths are white with a wingspan of about 1.5 inches. As a moth, they are primarily nocturnal.

Overwintering pupae emerge as adults, each mated female can lay between 400 and 500 eggs. These eggs are laid in masses, typically on the underside of leaves and tend to be light green color.

Oklahomans get to experience two different varieties of fall webworms. The first ones begin to hatch in late April to May. These caterpillars are usually black or brown. Once hatched they begin making the webs we are familiar with. This serves to protect them as they feed within the web. As the caterpillars get bigger, they expand the webs to make more food available. These black or brown fall webworms mature and pupate in late June. Here in Oklahoma, we usually have about three generations per year of this variety. With each female being able to deposit 400 to 500 eggs you can see how their numbers can increase rapidly with each generation.

The other variety is an orange headed caterpillar. These emerge in July and engage in the same lifecycle. We typically have 2 generations of this variety per year.

One kind of creepy and yet interesting aspect of these insects is that if you disturb one of their webs, they can begin to move in unison, causing the web to rock back and forth. Apparently, this is some kind of defensive strategy to make them seem more imposing. Either way, it can be a bit disconcerting if you are seeing it for the first time.

Fall webworms have been documented on at least 88 species of shade, fruit, and ornamental trees in the United States. In Oklahoma they seem to prefer persimmon and pecan with black walnut and hickory commonly playing host as well. But with large infestations, they aren’t quite as picky when choosing a host.

The most challenging part about their lifecycle is that many people don’t like the look of their webs. Yes, they can defoliate a tree, however this damage usually happens close to when the trees were going to be dropping their leaves anyway. Plus, by this time of year, most trees have stored up all the nutrients they need for the winter so losing their leaves a little early is not a fatal wound. However, if a young tree or a tree that is already in distress suffers a significant infestation, there can be permanent damage. If you have a pecan tree in your yard or neighborhood, it’s likely you have seen it looking pretty bad in the fall but the leaf out again in the spring like nothing ever happened.

If you are going to attempt to exert some level of control over your fall webworms, you start by keeping an eye out. They are much more vulnerable in the early stages than after they have spread throughout your tree or trees.

Depending on when and where you find them you can spray with an organic pesticide called Bacillus thuringiensis (BT). This pesticide is only effective on caterpillars and must be applied to areas where the caterpillars are feeding. So, you will need to either split open the webbing and spray inside, or spray around the webbing so that as they expand their web, they will come in contact with the spray. After a rain, you will need to re-apply this pesticide.

If your infestation is in a large tree with high branches, you will probably need to hire a professional because, as homeowners, we really don’t have the equipment to spray up to the treetops. Plus, the higher-powered sprayers of the professionals will be able to penetrate their webs.

Yes, their webs can make your trees look pretty pitiful, but since they don’t do any permanent damage, there’s always the option of just grinning and bearing it since your tree will be back next year with very little chance of diminishment.

If that’s not enough to encourage you to just live and let live, birds love those juicy caterpillars. So, if we put up with a little unsightliness, we can help provide food for our feathered friends. See you in the garden!

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org.  James B. Hanson, USDA Forest Service, Bugwood.org