Depending on climate there can be up to 4 generations
By now, you are seeing masses of spidery webs engulfing the tips of tree branches in your yard and around town. Not sure what they are? They’re fall webworms. Many people confuse these with a similar sight in the spring, typically in the crotches of trees, which is the eastern tent caterpillar.
The fall webworm (Hyphantia cunea) is the larval form of the tiger moth. The tiger moth is about 2 inches across from wingtip to wingtip. Its color can range from pure white to heavily speckled with brown. It is native to North America and Mexico and is distributed throughout the United States and Canada. Over wintering in leaf debris and mulch, the adult moth appears in May to July. She lays about 200-500 eggs in a greenish mass underneath the leaves of deciduous trees in late spring through fall. A wooly or scaly coating protects the eggs. Depending on the climate there may be from one to four generations. The eggs hatch in a few days and the larvae begin spinning webs around the leaves as they eat. They do not leave the protection of the web while they are eating and growing. Once they have had their fill, by mid-summer, they form cocoons in the bark of trees and in ground debris. This first generation is small and generally goes unnoticed.
If you can reach the nest open it and give the birds a treat
The second generation that follows is more obvious, larger and more destructive. In much of the U.S., there are two generations. The caterpillars are yellowish to greenish, covered with long silky grey hairs, with a dusky stripe down the back and a yellow stripe on either side. They may have either a black or red head. Full-grown larvae can eventually reach an inch in length.
They only pose a minor threat as a forest pest, but shade trees and ornamentals can be heavily defoliated. Fall webworms feed on about 100 different species of deciduous trees but rarely on conifers. In the Eastern United States, trees that are targeted include: pecan, walnut, American elm, hickory, fruit trees and some maples. In some areas, persimmon and sweet gum are also affected. In the western areas of the country, adler, willow, cottonwood and fruit trees are favorites.
In most cases, the defoliation that occurs will not materially damage the trees because they will loose their leaves anyway. Only in severe cases is the infestation so bad the tree cannot recover. But, in the case of pecan and fruit trees, the leaves are more critical for fruit production and the ripening fruit can become bound up in the webs substantially reducing the yield.
Homeowners have some control over populations of fall webworms. A thorough late fall or early spring clean up of leaf litter and mulch will help eliminate many of the cocoons. Then determine your tolerance for the eventual occurrence of webs. Opening up the webs you can reach makes the larvae available to birds so you can let nature take its course.
Cleaning up leaf litter will destroy many cocoons
A spray or dusting of Bt (Bacillus thuringiensis) while not harmful to wildlife, bees and humans will kill non-harmful caterpillars and butterflies and should be used with caution. The web must be open in order to deliver the spray. Affected branch ends can be pruned and burned or thrown in the trash. Do not burn the nests while on the trees as this can cause more damage than the pests themselves. Diazinon can be applied but be sure to avoid drift and run off into surface waters. Carbaryl (sold as Sevin) is also effective. We encourage you to use the most effective but least toxic method of control.
Our native fall webworm has caused great concern in China where it has no natural enemies. Even some bird species will not touch the worms. They were first found in China in 1979 having hitch hiked into their ports on ships. Recent research has uncovered a type of alkali bee, which is parasitic and kills the larvae. Sometimes we are so focused on the invasiveness of foreign species here we fail to take note that our natives can be as dangerous on other continents.
About Joe Lamp'l
Joe Lamp'l is the Host and Executive Producer of the award winning PBS television series Growing A Greener World. Off camera, Joe dedicates his time to promoting sustainability through his popular books, blog, podcast series, and nationally syndicated newspaper columns. Follow Joe on Twitter
Is burning the web okay? That is always what we used to do, and it takes very little flame to burn enough web so the birds can get them. Usually, the branch is not damaged.
Hey Richard. I’m not familiar with this option. However, seems to me if you can get to the web to burn it, you could just as easily break it up with a long stick or such. And no worms would be harmed in the process. Yes, the goal is that they all become bird food, but hopefully not after burning alive first. While in theory I suspect the flame idea works usually, the flame just seems to add an unnecessary element that has more risk than benefits.
We got home from the weekend to discover, what I am guessing to be web worms all over our patio. What is the best way to get rid of them from the patio/house? I’m concerned about our dogs, are the toxic to them?
I would get a broom or stick or something to open the webs and expose them. Webworms don’t want to live indoors and as long as you can expose them to birds and predators, that is all you need to do. If you’re worried about your dogs, def. don’t spray, nor do you need to. But I don’t know of cases were dogs are attracted to worms and don’t know if they would be harmful if consumed.
Joe: Enjoyed your article on web worms. Last year we had a very large Hickory tree almost completely covered with web worms. The tree is so large my husband couldn’t reach the webs to destroy them. At the end of summer (it could have been earlier), we were inundated with worms all over our house. They were crawling on the walls (exterior), patios, patio furniture, etc. They were so thick you couldn’t find anywhere to step without crunching them. This lasted for about two weeks. We tried killing them on our porch (because we were out there a lot) with a fly swatter. We didn’t want to spray because we felt it would be harmful to us being so close.
My question is: Does a lot of rainfall in the Spring cause severe web worms, and if so, are we going to go through the same thing this year? Also, what kind of tree service (or exterminator) would we call to spray this large tree? Thank you for your time!
Hi Judy. It sounds like conditions were prime from the previous year to promote an explosion of web worms the following year. I suspect this will not be a typical occurrence each year. Since these worms are a major source of food for birds, I don’t encourage or recommend anyone to spray them because of the secondary impact the poison would have to birds eating them.
My approach is to expose the webs as they develop using a large stick or broom handle etc. to open them up for access to the birds. Hopefully they move in and clear out your population before they get out of hand.