Sod webworms are ground-dwelling caterpillars, and they are especially fond of digging into the thatch of your lawn and feeding on grass roots and leaves.
This can do significant damage to a lawn.
In this article, we share advice to help you diagnose your sod webworm problem and eradicate these destructive pests.
What Do Sod Webworm Caterpillars Look Like?
These caterpillars are quite small, ranging from a quarter-inch to a full inch in length.
They are dull colored and come in shades of off-white, gray, brown and olive drab.
Sod webworms have four rows of dark spots on the underside and a brown head capsule.
The Adult sod webworm caterpillars rest during the daytime. They hide in burrows just below the surface of the soil in silk-lined tunnels.
They are most active late in the spring and throughout the summer. They are the larval stage of a very plain and dull colored moth.
What Do Sod Webworm Moths Look Like?
There are two species of sod webworm moths:
Crambus and Parapediasia. Both are often referred to as “lawn moths” because they flutter up from the grass soil surface as you walk across it.
You may also hear these moths called “closed-winged” because they rest with their wings closed and wrapped over the abdomen.
They are also nicknamed “snout moths” because they have protruding, siphoning mouth-parts.
The adult moths are brown, slender and very light colored. They have easily recognizable protruding snouts.
On spring and summer days, you will notice them fluttering directly above the grass on your lawn. At night, they are attracted to sources of light.
At rest, they fold their wings up tight, so their bodies look very narrow and stick like. The adult moths range between half an inch to three-quarters of an inch in length.
The moths are harmless on their own. They do not eat anything. They live a short and frantic life focused entirely on reproduction.
The caterpillars are another matter. They feed voraciously on grass blades through spring nights and leave ragged brown patches in their wake.
How Many Generations Can Snout Moths Produce Annually?
In ideal circumstances, these lawn pests can produce two generations a year.
If you are experiencing a heavy infestation, the unrelenting nature of doubled reproduction can do some serious damage to extensive sections of your lawn.
Do Moth Sightings Always Indicate Caterpillar Infestation?
Interestingly, it is quite common to see lots of moths and not have heavy caterpillar infestation.
The reason why is unclear. But it is logical to speculate that steadily improving lawn conditions eventually make the lawn inhospitable as a place for moths to lay eggs and caterpillars to live and grow.
What Is The Life Cycle Of The Sod Webworm?
Late in the springtime and early in the summer, female moths hover over lawns at dusk dropping down occasionally.
Each time the female moth dips into the grass, she deposits a mass of very small, cream-colored eggs.
Each clutch may hold as many as 200 eggs, and a single female can lay multiple clutches.
Survival of the eggs depends on consistently high humidity, so the female moth typically deposits the eggs in low spots where moisture collects.
When the eggs hatch, the larvae construct their burrows in the soil and begin feeding on the surrounding grass.
When the webworm larvae reach a length of about one inch, they abandon their burrows and begin constructing cocoons of soil and silk.
They pupate for a couple of weeks and then emerge as mature moths.
The lifespan of a mature moth is very short (only a few days) and is devoted entirely to reproduction. Mature moths do not eat.
What Kind Of Grass Do Sod Webworms Like Best?
Although sod webworms will eat any kind of grass, their favorites appear to be the cool-season grasses:
- Kentucky Bluegrass
- Perennial Ryegrass
- Fine Fescue
Even if you plant favored varieties, the good news about these pests is that if you maintain your lawn well, you are unlikely to be bothered by them.
Even though they are likely to be present in almost any lawn, their numbers are typically small in a well-cared for, balanced ecosystem.
They are highly subject to naturally occurring diseases and predation by “good bugs” and other beneficial fauna.
Problems arise in non-irrigated lawns during times of drought. Then sod webworms can wreak a great deal of havoc.
NOTE: The tropical sod webworm the common name for (Herpetogramma phaeopteralis) (Lepidoptera: Crambidae) feeds on warm season St Augustine grass.
What Kind Of Damage Do Sod Webworms Cause?
Most sod webworms just eat grass blades, but some species eat the roots (like grubworms) and the crown of the plant as well.
Naturally, this causes much more significant and lasting damage.
If you have a major infestation of these pests, you will notice your grass disappearing and brown ragged patches popping up where green grass once stood.
How Can You Recognize Sod Webworm Damage?
Even though these caterpillars can eat twice their weight in leaves in a single night, initial damage can easily go unnoticed.
Small, immature caterpillars naturally cause less damage. Large, hungry caterpillars can wreak havoc.
If you catch the webworm infestation early, you can avoid a lot of damage.
With a light infestation, you may never notice the damage these pests cause. If this is the case, then you don’t actually have a problem.
However, if you have a heavy infestation and you do not detect the caterpillars before they begin causing damage, you may become aware of their presence because large brown patches begin cropping up in your lawn.
How Do You Diagnose A Webworm Problem?
With a heavy infestation, you may see very large brown spots in your lawn.
It’s important to diagnose the problem accurately before you begin treatment because there are lots of grass diseases that can cause big brown patches.
If you treat for webworms, but the problem is a type of lawn fungus, you will be wasting your time and money.
3 Surefire Ways To Confirm And Determine The Severity Of Your Webworm Infestation:
#1 – On Home Lawns Look For The Worms’ Burrows In The Thatch Of Your Lawn
They are easy to identify because the opening to the burrow is covered with loose dirt.
The burrows are silk lined, and you may also see bits of grass, dirt and droppings woven into the silk as reinforcement.
Worms hide in these cozy burrows during the day and venture forth at night to eat grass and to collect it.
At dawn, they may drag pieces of grass back into the hiding place to keep as a midday snack.
Luckily, determining whether webworms are present and whether or not their presence is a problem is quite easy.
Since they do not burrow deeply into the soil, you can expose them quickly by loosening your grass thatch using a trowel. Keep your eyes peeled for:
- Caterpillars hiding in tunnels
- Cut and missing grass blades
- Green fecal pellets (aka caterpillar droppings)
#2 – Look For Sparkles
Walk through the yard early in the morning when the sun is up but dew is still on the grass.
Look for sparkly spots in the grass. These occur because dew collects on the webs and reflects the sunlight.
#3 – Flush Them Out With The Floatation Test
Another way to tell whether sod webworms are present is to try the flotation test.
This method enables you to force the caterpillars out of hiding in a controlled area so that you can count them and calculate the severity of your infestation.
To do a flotation test, you’ll need three simple tools:
- A gallon can (e.g a coffee can) with both top and bottom cut out (a coffee can or industrial sized veggie can works well).
- An ounce of dish soap
- A gallon jug of water
To perform the test, press the can into the soil in an area where you suspect an infestation.
Combine the water and the dish soap and pour the solution into the can.
The solution will sink into the ground and irritate and flush out any sod webworms that may be hiding there.
If more than a dozen caterpillars emerge in the small space provided by the coffee can, you have a significant problem and need to deal with it.
If there are fewer than a dozen caterpillars, you have a tolerable situation and needn’t take any aggressive action.
Perform this test in early June and again at the beginning of August so that you can detect both possible generations of webworms.
How Can You Prevent Or Control Sod Webworm Populations From Growing?
The best way to control and manage sod webworms is by implementing Integrated Pest Management (IPM) strategies.
This method involves taking regular, small steps in a number of areas that work together to create an inhospitable habitat for sod webworms.
Here are 11 of the most effective IPM techniques for dealing with sod webworms:
#1 – Plant Resistant Types Of Grass
There are quite a few grass cultivars and tall fescue that are not especially attractive to these pests. If you favor Kentucky bluegrass, look for Park and Windsor cultivars.
#2 – Turn To Science
If you are starting a lawn in an area that had significant webworm problems in the past, choose a genetically engineered type of grass known as “endophytic”.
These grasses have beneficial fungi embedded within their tissues.
NOTE: Do not feed these types of grasses to livestock.
#3 – Manage Your Lawn Well
Eliminate potential habitat for webworms by reducing the thatch layer in your yard to no more than 1” inch deep.
#4 – Keep The Soil Airy, Light And Well-Watered
Webworms like to make their homes in hot, dry, compacted soil.
Crowd them out and keep your lawn thick and lush by aerating the lawn and watering regularly. Water is especially important during drought conditions.
#5 – Provide A Habitat For Natural Enemies And Predators
Maintain a diverse and slightly wild landscape to attract and encourage good bugs and beneficial fauna. Some of the most effective webworm predators are:
- Ground spiders (e.g. tarantulas, jumping and wolf spiders) eat webworms and low flying moths.
- Carabid and Rove beetles eat sod webworms.
- Robber flies (Efferia aestuans) eat webworms.
- Ants (esp. Pheidole tysoni) attack webworms.
- Parasitoid wasps lay waste to webworms.
- Macrocheles mites eat webworm eggs.
- Birds eat sod webworms and moths.
- Web-weaving spiders eat moths
#6 – Treat The Infested Area With Insecticidal Soap
Make a water and liquid insecticidal soap solution at a rate of 2 tablespoons of soap per gallon of water. Spray over your lawn in small sections.
When the caterpillars come to the surface, rake them up quickly and toss them into a bucket of soapy water.
#7 – Apply A Neem Oil Drench
Drenching the affected area with a Neem oil solution will have the same immediate effect as treating the area with an insecticidal soap solution.
As an added benefit, a Neem oil drench has a lingering effect and can negatively impact soil dwelling pests for up to six weeks.
#8 – Use Bacillus thuringiensis (Bt) As A Biological Control
The natural bacterium Bacillus thuringiensis (Bt) is a very effective biological control fort killing caterpillars. Just follow packaging instructions to create a spray to disperse over their food source.
When the caterpillars eat the treated food, the Bt will cause their intestines to explode (after a couple of days).
If you see lots of lawn moths fluttering over your grass, you can effectively prevent an infestation by waiting a couple of weeks and then applying a Bt solution.
When the tiny sod webworm larvae eggs hatch, they gobble up the treated grass blades causing the larval feeding to die before they can do much damage.
#9 – Use Beneficial Nematodes
Insect-parasitic nematodes (Steinernema carpocapsae) can lay waste to sod webworms.
These living organisms are rather delicate, so be sure to purchase a package that has not passed its expiration date.
Use the product right away, and be sure to follow package instructions carefully.
#10 – Natural Pyrethrins and Pyrethrum
Natural pyrethrins and pyrethrum, as well as synthetic pyrethroids, are relatively mild products that have been proven effective in fighting sod webworms.
However, they will also kill good bugs and beneficial fauna. That’s why it is best to use them only as a last resort, and then very sparingly.
You may also like: What Is A Pyrethrum Spray And How To Use It On Plants?
#11 – Use Conventional Pesticides
Here are some of the conventional pesticides that are commonly used against sod webworms:
- Spinosad Organic Spray
When using chemical deterrents, be sure to water your grass thoroughly a couple of days before applying the product.
After treating with a chemical pesticide, refrain from watering for a minimum of three days to give the product the best chance for effectiveness.
Should You Really Worry About Sod Webworms?
Luckily, severe sod webworm infestation is not common.
If you practice good lawn care and encourage good bugs along with friendly fauna to take up residence, you shouldn’t have much trouble with them.
If you do have to fight a severe infestation, remember to use pesticides sparingly and strive to establish sound, consistent lawn care management techniques to naturally discourage these pests from taking up residence in your lawn.
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