Webworms

Pest Expert Overview: Fall Webworm in Bryan & College Station

Fall webworms are caterpillars that build large, unsightly webs around the leaves and branches of trees.

They are sometimes called silkworms, but that name is not accurate. True silkworms are an entirely different insect. The pest commonly found covering pecan and other shade trees with webs in Bryan and College Station is the fall webworm, Hyphantria cunea.

Fall webworms are in the Order Lepidoptera and the Family Erebidae.

They feed on more than 100 species of broadleaf trees and shrubs. Their preferred hosts vary by location, but pecan trees are among the most commonly infested trees in Central Texas.

Other hosts include:

• Mulberry
• Persimmon
• Walnut
• Hickory
• Sweetgum
• Elm
• Maple
• Willow
• Poplar
• Oak
• Cherry
• Various fruit and ornamental trees

The large webs are usually more noticeable than the caterpillars themselves. Webworms feed together inside the protective web and expand it over additional leaves and branches as they grow.

Although the webs can make a tree look terrible, fall webworms rarely kill a healthy, established tree by themselves. Repeated or severe defoliation can still weaken a tree, particularly when it is already stressed by drought, disease, construction damage, poor soil, or other insects.

Identifying Fall Webworms

Fall webworm caterpillars vary considerably in appearance.

They are generally pale yellow, tan, greenish, or gray and are covered with long hairs. Rows of dark spots are usually visible along the body. Mature caterpillars may reach approximately one inch long.

Two color forms occur. Caterpillars may have either a red or black head, and the color and markings on the body can also vary.

Adult fall webworms are white moths with a wingspan of approximately one to one  and  a  half inches. Some adults are completely white, while others have small dark spots on their wings.

Most homeowners never notice the adult moths. The large webs created by the caterpillars are the obvious sign of an infestation.

Fall webworm webs normally begin around leaves at the ends of branches. The caterpillars remain inside the web while feeding, and the web is enlarged to include additional foliage.

The caterpillars, leaves, droppings, shed skins, and partially eaten branches are all enclosed within the web. Older nests can cover several feet of a branch and may eventually merge with nearby nests.

Fall Webworms Versus Eastern Tent Caterpillars

Fall webworms are often confused with eastern tent caterpillars, but their nests and feeding behavior are different.

Fall webworms build webs around the leaves they are eating. Their nests are most commonly seen during summer and fall and usually begin near the ends of branches.

Eastern tent caterpillars build a more compact nest in the forks of branches during spring. They leave the nest to feed elsewhere on the tree and return to it for shelter.

In simple terms:

• Fall webworms enclose the leaves inside their web.
• Tent caterpillars build a tent and leave it to feed.
• Fall webworms are generally noticed later in the year.
• Tent caterpillars are primarily a spring pest.

The distinction matters because the insects differ in life cycle, timing, and management.

Life Cycle

Fall webworms are caterpillars that build large, unsightly webs around the leaves and branches of trees.

They are sometimes called silkworms, but that name is not accurate. True silkworms are an entirely different insect. The pest commonly found covering pecan and other shade trees with webs in Bryan and College Station is the fall webworm, Hyphantria cunea.

Fall webworms are in the Order Lepidoptera and the Family Erebidae.

They feed on more than 100 species of broadleaf trees and shrubs. Their preferred hosts vary by location, but pecan trees are among the most commonly infested trees in Central Texas.

Other hosts include:

• Mulberry
• Persimmon
• Walnut
• Hickory
• Sweetgum
• Elm
• Maple
• Willow
• Poplar
• Oak
• Cherry
• Various fruit and ornamental trees

The large webs are usually more noticeable than the caterpillars themselves. Webworms feed together inside the protective web and expand it over additional leaves and branches as they grow.

Although the webs can make a tree look terrible, fall webworms rarely kill a healthy, established tree by themselves. Repeated or severe defoliation can still weaken a tree, particularly when it is already stressed by drought, disease, construction damage, poor soil, or other insects.

Identifying Fall Webworms

Fall webworm caterpillars vary considerably in appearance.

They are generally pale yellow, tan, greenish, or gray and are covered with long hairs. Rows of dark spots are usually visible along the body. Mature caterpillars may reach approximately one inch long.

Two color forms occur. Caterpillars may have either a red or black head, and the color and markings on the body can also vary.

Adult fall webworms are white moths with a wingspan of approximately one to one  and  a  half inches. Some adults are completely white, while others have small dark spots on their wings.

Most homeowners never notice the adult moths. The large webs created by the caterpillars are the obvious sign of an infestation.

Fall webworm webs normally begin around leaves at the ends of branches. The caterpillars remain inside the web while feeding, and the web is enlarged to include additional foliage.

The caterpillars, leaves, droppings, shed skins, and partially eaten branches are all enclosed within the web. Older nests can cover several feet of a branch and may eventually merge with nearby nests.

Fall Webworms Versus Eastern Tent Caterpillars

Fall webworms are often confused with eastern tent caterpillars, but their nests and feeding behavior are different.

Fall webworms build webs around the leaves they are eating. Their nests are most commonly seen during summer and fall and usually begin near the ends of branches.

Eastern tent caterpillars build a more compact nest in the forks of branches during spring. They leave the nest to feed elsewhere on the tree and return to it for shelter.

In simple terms:

• Fall webworms enclose the leaves inside their web.
• Tent caterpillars build a tent and leave it to feed.
• Fall webworms are generally noticed later in the year.
• Tent caterpillars are primarily a spring pest.

The distinction matters because the insects differ in life cycle, timing, and management.

Why They are Pests?

The most immediate problem caused by fall webworms is their appearance.

Large webs filled with caterpillars, droppings, dead leaves, and bare branches can make a valuable shade or ornamental tree look neglected or severely damaged.

The caterpillars feed on the leaves enclosed within the web. As they grow, they expand the nest and consume additional foliage. Heavy populations can cover substantial portions of a tree and cause significant defoliation.

A healthy, mature tree can normally tolerate some leaf loss, especially later in the growing season after it has already produced and stored much of the energy it needs.

However, defoliation is still a form of stress.

Trees use their leaves to produce energy. When a large amount of foliage is removed, the tree must use stored resources to replace it. Repeated infestations over several years can weaken the tree and make it less able to tolerate drought, disease, root damage, or attacks from other insects.

Young trees, recently transplanted trees, fruit and nut trees, and trees already under stress deserve more attention than large, healthy trees with only one or two webs.

Fall webworms can also affect nut and fruit production when substantial foliage is lost. Even when the long  term health of the tree is not threatened, homeowners may reasonably want the webs removed because of their appearance.

The caterpillars do not normally bite or sting. Their hairs may cause mild skin irritation in sensitive individuals, so gloves should be worn when handling caterpillars, webs, or infested branches.

Natural Enemies of Fall Webworms

Fall webworms have numerous natural enemies.

Birds, predatory insects, spiders, parasitic wasps, and other organisms feed on the eggs, caterpillars, or pupae. Disease can also reduce webworm populations.

The silk web provides the caterpillars with some protection from predators. Opening or tearing the web can expose them and make it easier for birds, wasps, and other predators to reach them.

Natural enemies often prevent fall webworms from causing serious, long  term damage. Broad  spectrum pesticide applications can kill these beneficial organisms along with the webworms.

For that reason, treatment should be directed toward active colonies whenever practical rather than spraying the entire landscape without determining where the caterpillars are feeding.

Fall Webworm Prevention and Control 

Fall webworm control is most effective when infestations are found early.

Small webs containing young caterpillars are easier to remove and treat. Once a web covers several feet of a branch, the silk may prevent treatments from reaching all of the caterpillars.

Inspect susceptible trees periodically during the warmer months, particularly pecan, mulberry, persimmon, walnut, hickory, sweetgum, and other trees with a history of infestation.

Look for small groups of leaves covered with fresh silk near the ends of branches.

Opening the Web

Small, reachable webs can be torn open with a pole, rake, or other suitable tool.

Opening the web exposes the caterpillars to birds, wasps, heat, weather, and pesticide applications. It may not immediately eliminate every caterpillar, but it removes much of the protection provided by the silk.

Avoid damaging healthy bark or breaking large branches while opening the web.

Do not attempt to reach webs near electrical lines. Those situations should be left to qualified professionals.

Pruning Infested Branches

Small webs located at the ends of reachable branches can sometimes be removed by pruning.

Place the cut web and branch into a sealed bag or crush the caterpillars before disposal. Simply dropping an active nest beneath the tree may allow surviving caterpillars to crawl away.

Avoid excessive pruning. Removing a large branch may injure the tree more than the caterpillars would have.

Pruning also does not guarantee that every infestation has been removed. Egg masses and small developing colonies may be present elsewhere in the canopy and can be difficult to see from the ground.

For that reason, the entire tree should be inspected rather than removing only the largest and most obvious web.

Do Not Burn the Webs

Do not use a torch, lighter, or other flame to burn webworm nests.

Fire can damage leaves, bark, and branches. It may also ignite dry vegetation, roofing materials, fences, or nearby structures.

The caterpillars are rarely dangerous enough to justify setting part of the tree on fire.

Opening the web, pruning a small infested branch, or applying an appropriate treatment is safer and more effective.

Biological Treatments

Products containing Bacillus thuringiensis subspecies kurstaki, commonly abbreviated Btk, can be effective against young fall webworm caterpillars.

Btk must be eaten by the caterpillars. It is most effective when:

• The caterpillars are still small.
• The web has not become excessively dense.
• The foliage inside the web is thoroughly covered.
• The application is made according to the product label.

Btk becomes less dependable against large, mature caterpillars because older larvae consume less foliage before leaving the web to pupate.

Spinosad products may also be effective when labeled for use on the affected tree or shrub.

Although these products are generally more selective than many conventional insecticides, they can still harm the caterpillars of butterflies and moths that contact or consume treated foliage. Applications should be directed toward the infested area rather than made indiscriminately.

Conventional Insecticide Treatments

Several insecticides are labeled for caterpillar control on ornamental trees and shrubs.

The choice of product depends on:

• The tree species
• The size of the tree
• The age of the caterpillars
• The size and density of the web
• Whether the tree produces edible fruit or nuts
• The available application equipment
• The instructions and restrictions on the pesticide label

The treatment must reach the caterpillars or the foliage they are consuming. Spraying only the outside of a dense web may provide poor control.

Opening the web before treatment can improve penetration.

Applications to pecan and other food  producing trees require special attention to label restrictions, harvest intervals, and whether the product is approved for use on that crop. A pesticide labeled for ornamental trees is not automatically labeled for pecan trees grown for nut production.

More pesticide is not better. The objective is to deliver a properly labeled treatment to the active feeding area without creating unnecessary exposure or harming beneficial organisms throughout the landscape.

Can Fall Webworms Kill a Tree?

Fall webworms rarely kill a healthy, mature tree from a single infestation.

Their damage often looks much worse than it is. The silk, dead leaves, droppings, and bare branches remain visible long after the caterpillars have stopped feeding.

The timing of the damage also matters. Trees defoliated late in the growing season have already completed much of their annual growth and are often better able to tolerate leaf loss than trees defoliated early.

That does not mean every infestation should be ignored.

Treatment may be appropriate when:

• A young or recently planted tree is heavily infested.
• A large portion of the canopy is being defoliated.
• The same tree has been heavily attacked in consecutive years.
• The tree is already stressed by drought, disease, construction, or root damage.
• Nut or fruit production is important.
• The webs create an unacceptable appearance.
• Infested branches are close to a house, patio, driveway, or other frequently used area.

A few webs high in a large, healthy tree may be primarily an aesthetic issue. Treating a mature tree solely because one small nest is visible may cost more and create more disruption than the webworms justify.

Preventing Future Webworm Infestations

There is no practical way to guarantee that adult moths will not lay eggs on a tree.

The best prevention is early detection.

Inspect trees that have been infested previously and watch for small webs during spring and summer. Removing or treating colonies while they are small is much easier than dealing with large webs later.

Maintaining tree health also improves its ability to tolerate feeding damage.

Helpful practices include:

• Watering appropriately during drought
• Protecting the root zone from construction and soil compaction
• Avoiding unnecessary injury to the trunk and branches
• Pruning damaged or diseased limbs correctly
• Maintaining an appropriate mulch layer
• Avoiding excessive or unnecessary pesticide applications
• Addressing disease and other insect problems promptly

Do not assume that fertilizing a stressed tree will automatically help it. Excessive nitrogen can promote soft new growth and may create additional problems. Fertilization should be based on the needs of the tree and soil rather than used as a general response to insect damage.

Professional Fall Webworm Control

Fall webworms would be a One-Time Service.  While they can be relatively easy to manage when the webs are small and reachable, control becomes more difficult when colonies are high in the canopy, the webs have expanded over several branches, or multiple egg masses and young colonies are scattered throughout a large tree.

A professional inspection can help determine:

• Whether the insects are fall webworms or another caterpillar
• How much of the canopy is affected
• Whether the health of the tree is at risk
• Whether pruning or treatment is appropriate
• Which treatment is labeled for the affected tree
• Whether the web must be opened to obtain adequate coverage
• Whether the tree can be treated safely from the ground

The best results often come from combining methods. Reachable, heavily infested branches may be pruned while smaller colonies elsewhere in the tree are treated.

Simply removing the largest web may leave behind egg masses or less obvious colonies that continue developing.

Large trees require equipment capable of delivering the treatment into the canopy. Homeowner sprayers may reach only the lowest branches and can leave most of the infestation untouched.

If you are dealing with fall webworms, Venus Pest Company can help. Founded by Aggie Entomologist Dr. Scott Lingren, we provide expert, eco-friendly Integrated Pest Management solutions in Bryan and College Station. To get started, fill out the contact form, call/text 979.774.0486 or email [email protected]. Don’t let nuisance pests take over your home!