Many people wonder what to do about webworms. When controlling fall webworms, it’s useful to analyze what exactly they are. Webworms, or Hyphantria cunea, usually appear on trees in the fall (while tent worms appear in spring), causing unsightly nests and severe leaf damage. Let’s learn more about fall webworm control.
Webworms are caterpillars that weave loose webbing around the tree’s foliage whilst munching on leaves, resulting in plant stress and leaf loss. This larval “nest” may cover single leaves or leaf clusters, but more often entire branches covering several feet (1 to 2 m.) across.
Webworm treatment options have to do with the life cycle of the critter. Webworms overwinter as pupae in cocoons found in the bark of the tree or amongst leaf litter. In the spring, adults emerge and deposit eggs, often creating large numbers of these caterpillar laden webs in a single tree. These caterpillars may go through as many as eleven growth stages (instars) before leaving the web to pupate and multiple generations occur per year.
The webworm caterpillar is about an inch (2.5 cm.) long with a black to reddish head and light yellow to greenish body with a mottled stripe of two rows of black tubercles and tufts of long whitish hairs. Adults appear as white moths with dark spots on the wings.
What to do about webworms? There are several schools of thought on the best way to kill webworms. Fall webworm control runs the gamut from insecticides to burning the nests. Yes, webworm treatment may extend to the lengths of burning the nests, so read on.
Controlling fall webworms may be difficult due to their sheer large numbers and the variety of trees which they attack. Damage to such cultivars of hickory, mulberry, oak, pecan, poplar, redbud, sweet gum, willow and other ornamental, fruit and nut trees may require a specific webworm treatment as the best way to kill webworms.
A webworm treatment for control of fall webworms that is highly recommended is the use of dormant oil. The best way to kill webworms with dormant oil is in the early spring while the tree is dormant. Dormant oil is preferable due to its low toxicity and easy availability; any local garden supply store will have it. Dormant oil attacks and kills the overwintering eggs.
The control of fall webworms also includes the more toxic varieties of insecticides, such as Sevin or Malathion. Sevin is a webworm treatment which kills the webworms once they are outside of the nest. Malathion works in much the same manner; however, it will leave a residue on the tree’s foliage. Orthene is also an option for fall webworm control.
And the last, but certainly not the least dramatic method, is to burn them out. Some folks utilize a propane torch attached to a long pole and burn out the webs. I can name a couple of sound reasons for the insanity of this method of fall webworm control. Controlling fall webworms via this route is dangerous due to the flaming webs one must dodge, the probability of making a conflagration of the entire tree and not least, the difficulty in hanging onto a stepladder with a flaming 20 foot (6 m.) pole! However, to each their own.
The safest and most effective method of what to do about webworms is as follows: Prune the tree in the spring and spray with a lime-sulfur and dormant oil spray. As buds begin to break, follow up your webworm treatment by spraying Sevin or Malathion and repeat in 10 days. Also, make sure to clean up any leaf debris to remove overwintering pupation populations.
Ever notice a big mass of webbing on a tree? It’s most likely made by a tent caterpillar or Fall Webworm. Fall Webworms are not to be confused with the eastern tent or the forest tent caterpillars. The main difference between them is the time frame in which they are active. According to Jerome F. Grant (Professor-Entomology and Plant Pathology), the eastern and forest tent caterpillars are only active in the spring. The fall webworm, however, is active from in the spring through fall with three generations occurring each year here in Tennessee. Read on to learn how to control Fall Webworms.
The fall webworm forms large web nests that cover the ends of the branches. They enlarge the nest as they continue to feed. Each generation can feed four to eight weeks. You may see two races of fall webworms here in the Knoxville/Oak Ridge area. The Blackheaded Fall Webworm is yellowish or greenish with two rows of dark bumps that border a dark stripe. The Redheaded Fall Webworm is brownish orange or yellowish tan with orange red bumps.
Fall webworms can attack many types of trees including but not limited to persimmon, pecan, hickory, black walnut, sweetgum, American elm, maples, and sourwood. Although they are not considered a serious forest pest, fall webworms can completely defoliate small trees in the landscape.
To control Fall Webworms and prevent defoliation, it is probably best to use a chemical control on smaller trees and shrubs that are infested. Willow Ridge recommends using Thuricide® at the rate of 4 teaspoons per gallon of water. Using Ortho® Dial N Spray hose end sprayer with the setting on 4 teaspoons and the spray nozzle in the jet position. You can penetrate the web nest with the jet spray. This will thoroughly saturate the leaves on which the worms are feeding. On larger trees, remove the web nests that can be reached. Don’t worry too much about the nests high up in the tree.
As always, read instruction label on any chemical product you use and call Willow Ridge Garden Center at 865-481-3825 with any questions you may have.
The fall webworm ( Hyphantria cunea ) is a moth that is native to North America. Moths belong to the insect order Lepidoptera which also includes butterflies. Insects are animals with an exoskeleton, a three-part body (head, thorax, and abdomen), six jointed legs, compound eyes, and a pair of antennae. Insects that belong to the order Lepidoptera (mainly butterflies and moths) have a characteristic life cycle familiar to many of us that involves a complete metamorphosis (called a holometabolous life cycle). The adult butterfly or moth mates and then lays eggs (usually on a host plant). The eggs hatch into the larval form of the organism, commonly known as a caterpillar . Caterpillars are very different physically from the butterfly or moth form, and caterpillars are usually herbivorous and quite a nuisance to many gardeners (for example, cabbageworms, cabbage loopers, hornworms, armyworms, leafrollers, corn borers, and webworms). These voracious feeders can chew their way through your landscape as they grow through a series of stages called instars . Once fully matured, the larva change appearance as it develops into a pupa. A cocoon and a chrysalis are well-known pupal coverings of some butterfly and moth pupa. The pupa is covered with a hard skin, and the insect within is undergoing its metamorphosis into the adult form. When the metamorphosis is complete, a sexually mature adult butterfly or moth emerges and takes flight.
Fall webworms are best known for their larval stage, a caterpillar that builds webbed nests on the tree limbs of many hardwoods in the late summer and fall. While the webworms are considered to be a pest species, they rarely do lasting damage to the trees they inhabit. Nevertheless, most gardeners don’t want webworms in their beautifully manicured landscape. Webworm larvae typically appear in late summer through early fall. The adult moth lays her eggs on the underside of leaves, and they hatch in about a week. The larval stage lasts four to six weeks, and the each caterpillar can consume many whole leaves. In this species, the larvae feed in colonies and produce the characteristic “webbing” that gives them their name. The webs are enlarged to enclose additional leaves as the larvae feed and grow. The webworm pupa overwinters in the bark of the tree (and sometimes in the leaf litter at the base of the tree). Adults emerge the following year to repeat the cycle. The fall webworm caterpillar feeds on many different types of deciduous trees. In severe cases, an entire tree can be defoliated.
There are many ways to go about combatting webworms. In the early spring while the tree is still dormant, you can use dormant oil which can kill the overwintering pupa before the adult emerges to continue the life cycle. You can also clean up leaf debris in the winter to remove overwintering pupae. Another approach is to use Trichogramma wasps, tiny predators that will eat the webworm eggs before they hatch. These wasps work best when released in mid- to late- summer where they can attack and eat the eggs before they hatch. It is recommended to release three rounds of wasps (spaced two weeks apart) starting in early August. Once the webs appear, use a long rake or pole to pull them down and destroy them by hand. In larger trees, you can also prune out any affected branches. Throw the nests away to prevent any of the insects from overwintering somewhere in your yard. You could also just use a pole to tear a hole in the web sack which will allow webworm predators (yellow jackets, paper wasps, and birds) easier access to their prey.
The suggestions given above are the safest and healthiest for your garden. I don’t typically recommend the use of insecticides (organic or otherwise) because they will also kill many beneficial insects that you want to keep in your garden. (And as a neuroscientist I also know that most insecticides are neurotoxins, designed to attack the nervous system of insects, which isn’t that different from our own, meaning that they can affect you, too!) I only recommend light use of dormant oil to minimize effects on beneficial insects. Also keep in mind that webworms are typically only an “eyesore” – they don’t usually cause lasting damage to your hardwoods, except in rare years when an outbreak is severe. So the most organic approach would be to leave them alone, or to encourage beneficial insect predators to take care of them for you. If you are intent on using an insecticide, the use of Bacillus thuringensis (Bt, a bacterium that infects and kills many caterpillar species) or Neem (derived from a tropical tree) usually does the trick for webworms. Spray Bt or Neem sparingly on the webs (or into a small hole you make in the webbing), and the caterpillars will come in contact with the insecticide while crawling around within the web. But keep in mind that Bt and Neem both target and kill beneficial insects as well.
A raised garden bed is filled with cool-season vegetables, including (from front) broccoli, Swiss chard, onions, kale, purple cabbage and red lettuce.
Photo by: Jane Colclasure/P. Allen Smith
Jane Colclasure/P. Allen Smith
Don't stop vegetable gardening just because it's cold out. There are plenty of lettuces and greens that will mature quickly, before intense winter cold sets in (and some even taste better with a touch of frost, like kale). Try planting lettuces, turnips, mustard greens, collards and kale and be sure to mix a rich compost into your vegetable garden to give those edibles a good head start. In very cold regions, frost blankets can protect lettuces and tender greens even into colder weather.
Alternatively, if you don't want to plant edibles, consider planting a cover crop (winter rye, field peas, clover) in your vegetable beds to enrich the soil over the winter.
Have you noticed large, messy webs on trees? You may have seen a colony of fall webworms. These caterpillars hatch in mid-July but tend to become more noticeable as the summer progresses. They often eat branches bare of leaves but are they a threat to tree health?
What do they look like?
Fall webworms are small, fuzzy pale-yellow caterpillars (figure 1) that build large, conspicuous white webs in trees in the late summer (figure 2). Their webs stretch over tree branches and grow over the course of the summer. When disturbed, the caterpillars will violently thrash back and forth in a bid to ward off predators.A colony of fall webworm caterpillars feeding on a leaf. Note that the web covers the leaves they are currently eating. Photo by Judy Gallagher. Trees will often have multiple fall webworm webs on them. This photo shows a typical number of webs for a large tree. Notice that the webs tend to be on the ends of branches and that the leaf damage is concentrated close to each web. Photo by Ken Gibson.
What kind of damage do they cause?
Fall webworms eat the leaves of many species of deciduous trees and bushes. This damage occurs late in the summer shortly before the trees normally drop their leaves for fall. Therefore, fall webworms very rarely do serious damage to trees. In most cases the trees will grow their leaves back the following spring. On rare occasions, a tree that is already highly stressed may be further weakened by fall webworm damage. However, most trees, even heavily infested trees, are minimally affected and show no signs of damage the following spring.
Do they need to be managed?
Fall webworm damage generally looks much worse than it is. In general, trees only need to be managed for fall webworm if the owner is concerned about aesthetics. In that case, the easiest means of management is pulling the web off the tree by hand and putting it in a bucket of soapy water or freezing it. Some people may be sensitive to the caterpillars’ hairs so gloves should be worn to prevent contact.
In cases where the webs are too high up to be reached, they can be managed through insecticides. Further instructions can be found here.
Cover image by Photo by msumuh on flickr.
On my commute into the office this morning, I couldn’t help but notice how the fog and sunshine made the webby nests in all the trees glisten. Those webs, which cause homeowners much grief, are caused by the fall webworm. These builders of web houses are often confused with bagworms or the eastern tent caterpillar. But each of these creatures makes a different and very distinct home for their caterpillars. Let’s focus on the fall webworm.
Fall webworms can be up to one inch long. They come in two color forms. Those with black heads are yellowish white while those with red heads are brown. They are covered with long, soft gray hairs. Fall webworms will feed on more than 100 types of trees but they prefer trees like pecans, black walnut, mulberry, elm, sweetgum, willow, apple, ash and oak. They are most often seen in pecan trees.
The caterpillars form fine silken webs on the ends of the branches around the leaves they feed on. They will enlarge the webs if they need more leaves. They feed on the leaves for a couple of weeks before they leave the trees to become pupae. These pupae eventually turn into a white moth. This moth can fly away to lay eggs on trees to start another generation of webworms. There can be up to four generations of fall webworms in a year. The webworms survive the winter as pupae in cocoons in protected places.
Although these caterpillars do eat the leaves, healthy trees are able to withstand a great deal of insect damage to their leaves without lasting injury. One of the easiest things to do may be to do nothing. Treatment requires direct contact of an insecticide with the caterpillars inside the web, which would usually mean a ladder and a high pressure sprayer.
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In late summer, the outside branches of many of the more commonly planted deciduous trees can become covered with webs produced by fall webworm caterpillars. These white to tan fuzzy caterpillars have black dots on their back and can be up to 1.5 inches long. When unchecked by the wide variety of insects and birds that feed on them, caterpillars can defoliate entire trees in late summer. Although this late season defoliation is too close to the time of leaf drop to harm plant health, most homeowners prefer to keep unsightly webs off their trees.
Large webs of fall webworm on branch edges of tree.
Two races of fall webworms, the red headed and the black headed, are present in Indiana. The black headed race has a lighter body color and emerges from a single layer of eggs deposited on the undersides of leaves near the branch tip. The red headed race can have more of a tan color and emerges from a double layer of eggs.
As early as May and into July, white adult moths of both races may emerge from cocoons in the soil and leaf litter to lay eggs on the tips of branches. Caterpillars feed for 6 weeks on leaves while surrounding themselves with webs that protect them from predators. In most of Indiana, south of U.S. 30, a second generation of moths will emerge and continue to feed into September.
Caterpillars feed on leaves and live inside webs.
Close-up of redheaded race of fall webworm.
Adult fall webworm moth and eggs.
Control of fall webworm caterpillars is best achieved if actions are taken before the tree is covered with webs from either the first, or the second generation. Small webs can be simply pruned off and destroyed if easy to reach, and only a small proportion of the tree is affected. Pesticides (Table 1) should be used when the extent of the webbing or number of webs is too large to make pruning practical. Applications of the biobased insecticides acelepryn and Bacillus thuringiensis, which conserve the webworm’s insect natural enemies, are especially effective when applied early in the infestation process. Although the biorational insectides indoxacarb, spinosad, and tebufenozide can kill wasp parasites of fall webworm, it does not affect the predatory insects that contribute to control of this pest. Further, like the other materials listed, it can be used to rescue trees from caterpillars on extensively webbed trees. Pesticides alone are not enough to remove the webs from the trees. Trees with heavy webs can remain unsightly well into the winter. Webs will be removed if pesticides are applied with a high pressure sprayer by a professional applicator. Homeowners seeking to remove webs after caterpillars have been killed could use a strong stream of water from a garden hose that is fitted with a spray nozzle.
Small webs can be pruned off and destroyed.
Scientific Name: Mimosa webworm - Homadaula anisocentra Meyrick
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This work is supported in part by Extension Implementation Grant 2017-70006-27140/ IND011460G4-1013877 from the USDA National Institute of Food and Agriculture.