Did you know there are millions of beneficial bugs called nematodes in your lawn that can help control pest insects and actually make your grass flourish?

Referred to as roundworms, beneficial lawn nematodes are microscopic insects that control pests such as weevils, white (lawn) grubs, sod webworms, cutworms, clearwing borers, and chinch bugs. They attack by boring into pest insects, injecting deadly bacteria, and then sucking out the cell contents. 

It’s literally amazing what’s going on in the soil just under the lawn in your garden. In the following article, we’ll outline why nematodes are so beneficial for the health of your grass, and answer many of the common questions about these lethal bugs. Thank goodness they’re on our side (well, some of them at least!).

What Are Beneficial Nematodes?

Spoiler alert: in this article we’ll be exploring deadly bacteria, feeding wounds, host cadavers, parasitizing, host liquification, and grub cavity-penetrating teeth.

Now you’ve been warned – let’s get started! Nema-what? Nematodes are in a phylum by themselves: the phylum Nematoda. Taxonomically, they are classified with insects. Experts say that there are over 25,000 nematode species.

Nematodes can be free-living, parasitic or predaceous. Some species attack animals and some feed on living plants. According to experts, 50% of the animal species on Earth has at least one parasitic nematode living with it. At LEAST one. 

Referred to as “roundworms”, nematodes are actually only worm cousins. Some nematodes are microscopic. Some, like the ones that live in sperm whales, can reach 30 feet long. Here’s a disturbing video of a nematode exiting an (almost dead) mosquito larva. 

A rotten apple may have tens of thousands of nematodes. Of all the lifeforms in ocean sediments, 90% are nematodes. There is even a species of nematode that only inhabits – German beer coasters. 

Nematodes in the soil are usually microscopic. Some are good for plants and some are bad.

Bad nematodes are about 1/50 of an inch long. They cause root knots or galls. Pest nematodes attack the root systems of potatoes, lettuce, carrots, peppers and cherry tomatoes. They can also cause stem injury to onions, rye, alfalfa and chrysanthemums.

Pest nematodes puncture cell walls with sharp mouths, inject saliva and then suck out the cell contents. The plant responds to this attack with distorted growth – root knots and galls. Plants attacked by pest nematodes may be yellowed or have excessive branches.

Nematodes do not exactly speed through your soil like a Ferrari. They don’t move far or fast, unless they hitch a ride in water. They can move quickly in moist soil. Nematodes can be spread by garden boots, tools and animals.

Beneficial nematodes attack pest grubs. These nematodes measure around 1/25 of an inch to several inches long. Some love to live in compost piles and are very helpful there, breaking down organic matter.

Good nematodes combat a long list of pests: weevils, white (lawn) grubs, sod web worms, cutworms, clearwing borers, and chinch bugs. Beneficial nematodes attack these pest insects by boring or biting into them, injecting deadly bacteria and then sucking out the cell contents. 

Some nematodes seek out grub hosts along watery pathways and some wait for them to pass by, like a praying mantis. Other nematodes follow grub poop trails. Once the grub host has been hollowed out, the nematodes lay eggs in the cadaver, and the cycle begins again.

Grossed out? Think nematodes are nasty? Nematodes are the most numerically abundant animals on Earth. Four out of every five animals on Earth are – nematodes.

How Beneficial Nematodes Control Lawn Grubs

Got grubs? Lawn (aka white, or curl) grubs feast on your grass roots until you have unsightly, spongy, yellow or brown patches. No amount of watering will revive these patches. You can pull grass up easily in the dead patches. If you explore underneath the dead grass patch with your garden spade, you can easily spot white lawn grubs in the dark soil. 

Curl or lawn grubs are pale white and characteristically “c” shaped. Here’s a photo. Lawn grubs are plump, and measure from 3/4 of an inch to 2 inches long.

Lawn grubs are the larval stage of certain insects, typically Japanese beetles, Masked chafer, European chafer, Rose chafer, fly larvae, Bill-bugs, Army worms, Strawberry root weevils, Banana Moths, Fungus gnats, June (or May) bug and Black Turfgrass Ataenius. The larvae feed on dead plant matter but also love chewing on live root systems.

A small amount of lawn grubs is natural. Check your grub level by digging 1 square foot of brown patch, about 3 inches deep. Overturn the soil. Experts say that is you have more than 5 grubs per 1 square foot of soil, then you may have a slight grub problem. Between 5 and 10 grubs, concerning but not alarming. More than 10 grubs – better get busy.

Not only will lawn grubs feed on your grass roots, there may be nightly visits of animals that feed on the grubs. Racoons and skunks will happily dig up your lawn looking for these tasty, high protein snacks.

What can you do to combat lawn grubs? If applying chemicals to your lawn is not your thing, you could try nematodes – the grub-killing ones.

There are a lot of beneficial nematodes (called entomopathogenic) that are used to kill over 200 pest types. For Japanese beetle grubs and white grubs, you’ll want the nematode named Heterohabditis bacteriophora (Hb). Hb nematodes have special “teeth” to help them chew through and enter those grubs. Once the Hb nematode enters the host, they inject bacteria into the grub. The grub dies within 24 to 48 hours.

Some experts recommend a combination of two grub-killing nematodes: Hb and Steinernema carpocapsae (Sc). Both are commercially available from dozens of companies. Sc is easier to produce but Hb is more effective for Japanese Beetles.

How To Apply Nematodes To Your Lawn

You won’t need to spray your entire lawn, only the brown patches. The objective is to overwhelm the pest grubs with an abundance of nematodes. You will apply the nematodes by spraying a liquid solution of them.

First, estimate the square footage of your brown patches and round up. Experts say that for 200 square feet of brown patches, you will need 5 million nematodes. Only infested areas need to be treated but add a few extra feet. For heavily infested areas, double the number of nematodes.

Plan your application out. Nematodes will die after the application’s expiration date. They will also die if you mix up a batch but don’t use it right away. 

First, water your affected areas well for a few days ahead of time. Commercial nematode applications can come as a liquid, gel, on sponges or in a powder. If you have a powdered application, mix it up with distilled water (under 86°F) and apply it the same day.

Nematodes die in bright sunlight, so start the application during low light, in the morning or evening. Apply the liquid with a watering can or hose attachment system. If you use a hose attachment system, make sure the nozzle opening is at least 0.02 inches wide and any screens are removed.

Spread the solution evenly over the brown patches, plus an extra foot or two. Wait 15 minutes, then gently water the brown patches. You should keep the brown patches moist for the next 2 weeks.

Some applications suggest another application two weeks after the first.

Usually, it will take 2 to 3 weeks from application to show results.

When To Apply Nematodes To Your Lawn

Some experts suggest two yearly applications, one in May and another in late August or September. In May, grubs are coming to the surface of your lawn in preparation for becoming beetles. May is the first recommended application.

Adult beetles lay eggs around July. One female will lay up to 60 eggs about 2 inches deep in the soil. Then, around a month later, the eggs hatch into grubs. September to mid-October is the second, and most important, time to apply nematodes.

If you only do one application, it should be in September. Make sure the soil temperature is above 60°F but below 93°F. You should see results the following Spring.

Climates vary – if you want to double check advice in your area, call your local Agricultural Extension office for advice.

Lawn Nematodes FAQ

How Long Do Beneficial Nematodes Last?

Most shelf lives of nematode applications is a month, but some are 2 to 3 weeks.

Do Nematodes Need To Be Refrigerated?

Yes! Please refrigerate at 37° to 50°F. 

Can You See Beneficial Nematodes?

No, they are too small, only a millimeter long. You can only check that they’ve worked by digging up a sample of soil and checking for grubs.

Will Nematodes Harm My Grass?

Nope. There are probably millions there already.

Will Fertilizer Kill Nematodes?

Manufacturers say that fertilizer should not be used for two weeks after a nematode application.

Will Nematodes Hurt Dogs, Pets, Worms or Butterflies?

Nope – only in science fiction films! Remember that cult classic ‘Tremors’ with Kevin Bacon? That’s what I’m picturing now 🙂

How Do I Get Rid of Nematodes? (At What Temperature Do They Die?)

When the soil temperature is below 60°F or above 93°F, nematodes will die. Nematodes will die in direct sunlight, so tilling soil will also kill the nematodes living there.

Conclusion

I hope you enjoyed our (slightly gory) foray into the under-lawn world of the beneficial nematode. In this case, they really are the good guys and can help us rid our grass of other more harmful insect pests.

Exactly how nematodes go about killing their prey – by boring into them, injecting them with deadly bacteria and then sucking out their insides – is very effective but totally the stuff of nightmares.

Just be grateful they’re not the size of those huge worms I mentioned earlier in ‘Tremors’, or worse still – we’re not as small as the children in ‘Honey, I Shrunk The Kids’.