Fall webworms are always interesting in that some years it seems like they are everywhere and other years they are pretty sparse. Fortunately, this year seems like one of the sparse years, but none of that matters if you have them in your trees. Since there are certain areas of town that seem to be getting hit pretty hard with fall webworms, let’s talk about what they are and what you can do about them.

Like a lot of insects that we call worms, they are not worms at all, but instead are caterpillars that will eventually become a moth. Since moths are primarily nocturnal, we likely don’t see many adult fall webworm moths. While they are found in most of North America, Asia, and Europe, in Oklahoma the seem to particularly enjoy the eastern part of the state, as do we.

As is true with quite a few insects, fall webworms overwinter as pupae with each female laying between 400 and 500 eggs typically on the underside of leaves. These eggs are a light green in color.

Here in Oklahoma, we have two different varieties of fall webworms with the first ones beginning to hatch in late April and May. These black or brown webworms pupate and mature in late June. In Oklahoma, we typically get 2 to 3 generations of this particular variety. But here’s where a little math shows us how quickly they can get out of control. Let’s assume one female lays 500 eggs and then assume half of those are female. So, 250 females who then a few weeks later can each lay another 500 eggs. That’s potentially 125,000 webworms. This makes it pretty clear how their numbers can increase rapidly.

We also have another variety of fall webworm that is an orange headed caterpillar. This particular variety emerges in July and follows the same life cycle, producing 2 generations per year.

One thing I find particularly fascinating about fall webworms is how that all seem to work together when threatened. If you or a bird or something disturb their webs, you just might see all of the caterpillars begin to move in unison, rocking the web back and forth. This appears to be a defensive strategy by making them seem larger and more threatening. If you’ve never seen this happen, it’s pretty impressive.

While fall webworm caterpillars appear to prefer pecan, black walnut, and hickory trees, they are not really that picky having been documented on at least 88 species of trees throughout the United States.

In reality, we are mostly unaware of their presence until they start building those webs at the ends of branches on our trees. A large group of fall webworms can defoliate a tree. While this sounds terrible, the reality is that we are not that far away from the seasonal leaf drop of most trees anyway. Losing their leaves now, is not necessarily a fatal blow. That being said, younger trees can struggle with such a defoliation. Older trees can look pretty bad at the end of webworm season, but they typically leaf back out in the spring without any sign of damage. So, in most instances, we don’t really need to do anything about these fall webworms other than just grin and bear it, as we wait for cooler weather to arrive.

If you are a person who is not inclined to live and let live with our fellow insects, there are some things you can do. If the fall webworms are on a smaller tree that allows you to be able to spray the leaves with a home sprayer, you can use an organic pesticide called Bacillus thuringiensis (bT). This is a very effective pesticide with a benefit in that it is only effective on caterpillars so there is no collateral damage to other insects. Before spraying with bT, it’s best to break open the web so your spray can reach inside. Also spray the leaves around the web so that as they expand the web, they will come in contact with the bT. If a rain occurs during your treatment period, you’ll need to re-apply.

With a large infestation in a big tree, you will probably need to contact a professional since as homeowners, we typically don’t have the necessary equipment to spray high into the trees.

If you are not a pesticide person, you can rip open the webs in hopes that this will give birds access to the webworms, providing the birds with a good source of food. I had a conversation with another Master Gardener once who said they would stick a broom into the web and twist it around like they were making cotton candy. Then they would just deposit their creation into the trash. To each their own.

The bottom line is that even though the webs can be unsightly, we don’t really need to do anything about them since we’re approaching the end of the growing season anyway. 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. Photo: Tom Ingram