- What are these creatures?
- Where do they come from?
- Are they harmful or poisonous?
- What can you do to get rid of fall webworms?
In this article, we address these and other questions to help you deal with a fall webworm infestation. Read on to learn more.
In late summer and early into the fall, you may notice large, sack-like clusters of webbing at the ends of the branches of some types of trees, especially fruit and nut trees.
Inside these sacks, you’ll find bustling congregations of fall webworms (Hyphantria cunea).
There is also a sod webworm which lives in or under grass. Hyphantria cunea Drury lives in trees.
What Are Fall Webworms?
The fall webworm is the larvae of the Drury moth, so they are caterpillars.
They are similar to eastern tent caterpillars, which build massive webs in tree crotches, but not at the ends of tree branches.
What Do Fall Webworm Caterpillars Eat?
This insect pest has a very wide range and can be found throughout Europe, North America and southern Canada.
It is easy for this species to spread because it is highly adaptable and feeds happily on about ninety different types of host plant deciduous trees.
The fall webworm is especially fond of:
Even though they prefer fruit trees and nut-bearing trees, these pests will make do with any tree they can find.
The female moths’ deposit eggs on just about any type of tree or shrub.
Non-fruit or nut-bearing trees that seem to be favored include:
When fall webworms emerge from the eggs, they begin spinning webs right away.
The objective is to surround a group of leaves to feed upon.
The caterpillars stay together in large numbers, and the webbing gives them protection from predators.
What Do Fall Webworm Caterpillars Look Like?
The young larvae are light yellow and have two rows of black markings on their sides.
Full-sized caterpillars are marked with orange and black warts from which long, white hairs sprout.
At their largest, the caterpillars are pale yellow and approximately an inch long.
As the caterpillars get bigger and bigger, they outgrow the nest.
The webbing eventually breaks, and the caterpillars fall to the ground.
What Do Drury Moths Look Like?
Drury moths are pretty little white creatures, some are white with brown or black spots.
They have a wingspread of about 1.25 inches.
You will see them flying around fruit and nut-bearing trees mating and looking for places to lay their eggs.
These caterpillars are very active in the summertime.
What Do The Eggs Look Like?
Female Drury moths lay their eggs on the undersides of tree leaves.
They lay several hundred pale yellow eggs at a time.
The masses of eggs are covered in a light layer of hair.
What Is The Life Cycle of The Fall Webworm Caterpillar?
The webworms larvae feed for about a month and a half as they progress through 5 instars (phases of development) to become fully grown caterpillars.
During this time, they continue to expand their webbing to encompass more and more leaves to eat.
By the time they are done, their web sacks can be larger than three feet across.
Inside the large nests, you will find large numbers of caterpillars, dead leaves, partially eaten leaves and caterpillar droppings.
The larvae hatch in about a week. As soon as they hatch, they start spinning their nest around the foliage they will eat.
As they become bigger and bigger, so does the web nest. When they finish one section of leaves, they simply expand to encase more.
The larvae stay together through several molts as they grow bigger and bigger. After the last molt, they may separate and begin feeding independently.
Within about a month and a half, the larvae are mature.
At this point, they leave the web (or it simply breaks and disgorges them) and they move on to pupate in the soil beneath the tree.
During the winter months, the pupa stays safe overwintering on the ground, hidden by leaf litter or secreted into a crevice or a crack.
At the pupal stage they are brown, tear-drop-shaped cases. You may encounter them as you dig in your garden late in the fall or during the winter.
In the springtime, the fall webworm moth emerges as adult moths and flutter about mating and looking for trees to lay their egg masses.
It is not unusual for stragglers to continue emerging throughout the warm months of summer.
How Many Generations Per Year Are Possible?
Depending on the environment, one or two generations are possible annually.
Do Webworms Kill or Harm Trees?
It is possible for a major infestation of webworms to completely defoliate an entire tree, but this is rare.
Unless the tree is already in poor health, it is likely to survive a webworm infestation without lasting harm.
When the caterpillars infest a fruit or nut tree, they seem to appear in large numbers.
On a shade or ornamental tree, they tend to set up shop on just a few branches.
During the six week period while the caterpillars are attaining their full growth, they can eat a lot of leaves.
Sometimes a tree may be nearly covered with their nests.
Even when webworms eat all the leaves off a tree, it doesn’t usually cause lasting damage.
Remember, they appear in early fall, so the tree will have already had a good spring and summer run.
These pests like deciduous trees, and they attack just before the tree would lose its leaves, anyway.
What To Do With Ugly, Unsightly Nests
The main problem is the webs look so ugly while the caterpillars are on board.
A heavy infestation definitely mars the aesthetics of a tree.
If the nests are not taken down, they may flutter raggedly on the tree branches all winter long.
Web nests may be two or three feet across and cover a significant section of a tree branch.
If you can’t stand the sight of the webs, manually remove them.
Treatment and Disposing Of The Webs and Nests
Be sure to dispose of the webs and nests properly getting rid of the pests for good.
Put the nests in a sealed trash bag or toss them in your burn pile if you are permitted to burn debris.
In addition to simply knocking down the nests, you may want to give your tree a good pruning to simply cut off the affected branches and burn them or otherwise dispose of them appropriately.
If there are only a few nests, way up high, you may decide to just leave them. They’ll break apart and fall down on their own eventually.
You can treat the soil under the tree with a Neem oil drench to prevent the pupae from maturing into moths and laying more eggs.
If you have a good population of natural predators in your yard, and the nests are accessible, you may not even need to do that.
Just make holes in the webs so the predators can get in and eat the webworms.
Birds, paper wasps and yellow jackets all enjoy a hearty meal of webworms.
How To Control Webworms Naturally
Natural organic caterpillar control is really the simplest and most effective pest management.
Lots of different types of natural enemies eat the webworm eggs, larvae, pupae, and the moths.
Set up your yard and garden as a welcoming habitat for beneficial insects and friendly fauna.
There are many different types of parasitic wasps (e.g. Braconid wasps), predatory insects, birds and even lizards that prey on these and other types of caterpillars.
Plant sunflowers and other members of the daisy family to attract beneficial wasps.
Keep a slightly wild yard to create a sustainable habitat for other natural garden helpers.
Bacillus thuringiensis (Bt) And Neem Organic Treatments For Fall Webworms
There are a number of safe, organic ways to deal with these unsightly pests. The easiest way is simply to leave them alone until they fall to the ground.
Then treat the soil with a Neem oil drench to eliminate the pupa and improve the quality of your soil.
You can also treat them as you would any pest caterpillar with biological control by applying Bacillus thuringiensis (Bt) early on.
Bt is a natural bacterium only hazardous to caterpillars.
When you notice webworms beginning to make nests, spray the leaves inside the nests with Bt.
The caterpillars will eat the leaves and their intestines will explode.
Be sure to apply Bacillus thuringiensis (Bt) judiciously.
It is just as dangerous to desirable caterpillars (e.g. butterfly caterpillars) as it is to pest caterpillars, so broadcasting it is not a good idea.
Avoid using it in flower gardens where butterfly caterpillars are likely to live.
You can also spray webworm nests with a Neem oil solution. Neem oil is a natural substance very effective against a wide variety of pest insects.
As with Bt, Neem can also have a negative effect on beneficial insects, so use it with care.
Used as a spray, apply Neem oil solutions weekly or bi-weekly.
Prepared as a soil drench, Neem oil is very effective against soil-dwelling pests and has much less effect on good bugs and other beneficial garden fauna.
It is also longer lasting when used as a soil drench and need only be reapplied once every 6 weeks.
What About Chemical Insecticides?
If you have a lot of webworms and not many natural predators, you want to use natural or chemical insecticides in a very measured and controlled manner.
Apply your chosen product lightly to the outside of the nest, or poke a hole in the nest and spray the product in.
Either of these methods ensures that you target the pest and pose minimal impact to beneficial fauna in your garden.
If using a chemical insecticide, it is best to apply while the larvae are still quite small.
Treat the webs and the foliage inside them during the month of July for best results.
Remember to be very careful with chemical pesticides. Read all the instructions and follow the label exactly.
Store chemicals safely and keep kids, pets, and livestock away.
Don’t use chemical pesticides near open water or pour leftover product down the drain. Dispose of containers safely and correctly.