Are predatory insects plaguing your garden?
While there are many types of beneficial nematodes, insecticidal ones are naturally occurring microscopic organisms that live and feed on insects in the soil.
There are different types of nematodes that kill insects. However, many of them are difficult to mass produce or have very narrow host ranges.
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The beneficial nematodes that we will talk about here are entomopathogenic. This term derives from the Greek word entomon, or insect, and pathogenic, or disease-causing.
What makes these insect-killing nematodes special is that they kill their hosts quickly, due to special bacteria that live in their systems.
In contrast to the nematodes you are probably used to thinking of that parasitize plants and animals, these beneficial types can be a great help to your garden.
And in a wonderful gift from nature, insecticidal nematodes will kill pests that feed on plants while typically ignoring beneficial insects.
These fascinating organisms can help control up to 200 different species of insects from 100 different families. That’s pretty wide-ranging coverage.
Here’s what’s to come in this article:
What You’ll Learn
- Different Types and Behavior
- Identification, Biology, Distribution, and Life Cycle
- Nematode Prey
- Choosing the Right Nematode for Your Climate
- Application and Use
- Personal Protective Equipment (PPE) Is Not Required
- How Do They Look When They Arrive?
- Compatibility with Garden Chemicals
- Irrigation: Keep the Soil Moist
- How to Physically Apply
- Where to Buy
- Get Growing with Nematodes
Let’s talk more about what they are before we get into how you can use them, and where to buy them.
Beneficial nematodes offer a safe form of organic pest control for use in the garden. So safe, in fact, that the Environmental Protection Agency (EPA) even waived the need for them to be registered for pest management.
Not only are they safe for use around pets and people, they are ideal for treatment of soils like those around drinking wells that are sensitive to traditional pesticides.
And you can use your existing equipment – hoses and sprayers are no problem for them.
Different Types and Behavior
There are two genera of nematodes that are popular as marketable products to protect your garden.
The first genus is Steinernema – the most commonly sold genus for insect protection.
The second genus, Heterorhabditis, is more difficult to produce for commercial sale. However, these species are highly effective for eradicating insects that live deeper in the soil, particularly grub worms.
They are fascinating little organisms that go through six stages of life.
First is the egg stage. Next, they go through four juvenile stages before finally reaching the adult stage.
Those in the third juvenile stage, known as “infective juveniles” (IJ), or sometimes dauers, are the most resistant to environmental stress.
This is the only stage in their life cycle in which they will seek out insects to kill, and boy are they active!
Some sit and wait for prey to pass by. These types are known as “ambushers,” and are particularly effective against insects that move around in the soil. An example is S. carpocapsae.
Some species of Steinernema jump to attack their prey – sometimes up to nine times the length of their bodies!
The other types are known as “cruisers” because they are highly active and can move long distances to find their prey underground.
H. bacteriophora is a cruiser. This behavior makes it highly effective against pests like white grubs that don’t move around much.
Some species, such as S. feltiae and S. riobrave, combine the two types of behaviors.
Nematodes travel via water between particles of soil and identify their prey by monitoring changes in the carbon dioxide levels, temperature variation, and sometimes by excrement trails.
Once the prey is identified, it enters the chosen insect through a natural opening in its body like the mouth, anus, or spiracles (openings that are used to exhale carbon dioxide).
Species in the genus Heterorhabditis can actually penetrate the body of an insect through the cuticle if there are no other options to enter their prey.
You’ve probably noticed that these guys are pretty aggressive. Once inside, the different species behave in basically the same manner.
They release a strain of bacteria that lives inside their digestive system. These bacteria will rapidly multiply and kill the insect within 24 to 48 hours.
The different types each carry their own special bacteria – Xenorhabdus for Steinernema and Photorhabdus for Heterorhabditis.
The interactions between the nematodes and their bacteria are symbiotic, meaning each organism helps the other out.
The nematodes locate and enter the host, providing a home for the bacteria. In exchange, the bacteria weaken the insect’s immune system. And they produce chemicals that kill the insect as well.
Another thing that the bacteria do is release chemicals that are toxic to other microbes. This ensures that the nematodes can stay in their new home full of nutrient-rich food without having to fight off foreign invaders.
Steinernema species need both male and female nematodes to reproduce, while those of Heterorhabditis are hermaphroditic and produce offspring solely through females who have both male and female reproductive organs.
Once the nematode is inside its insect host’s body, it begins to reproduce. The babies thrive and grow by eating the nourishing tissue of the host insect and the dead bacteria.
When the insides of the host insect have been consumed and fully utilized, hundreds of thousands of nematodes in their juvenile stage emerge and begin hunting for their next meal.
If you have introduced beneficial nematodes in your garden, you can see if they are being effective by digging through the soil to look for grubs. You want to find grubs that are discolored.
Insects attacked by Heterorhabditis are usually orange-red, while those parasitized by Steinernema are typically cream to dark brown.
The reason these insects produce such odd colors is due to remnants of the pigments from the bacteria that grew inside them.
This may seem weird, but you should also smell the dead insects. They should not have a putrid smell. If they do, that means they were killed by other types of nematodes in the soil, and not the ones you introduced.
Identification, Biology, Distribution, and Life Cycle
Nematodes are usually between 0.6 and two millimeters long, with a cylindrical shape and an unsegmented roundworm body.
Steinernema includes the more shallow-dwelling, less mobile types, while Heterorhabditis species are more mobile, moving and hunting up to seven inches deep in the soil.
It’s important to remember that they move by traveling in the water in the spaces between particles of soil. They often move more effectively in sandy soils compared to clay because the pores in the soil are larger.
Free water is critical to the efficacy of your beneficial nematode attack plan! Irrigating before you apply will help them to control pests for a number of reasons:
- The pest grubs will move higher in the soil when it is wet – so it will be easier for the nematodes to attack them.
- Watering will lower the soil temperature.
- If the nematodes are trapped in tight thatch, watering may help them move down through the thatch.
Next, let’s take a look at the insect pests that these organisms target.
The two species of Steinernema most commonly used in pest treatment include S. feltiae and S. carpocapsae. These types attack insects that are closer to the surface of the soil, hunting to a depth of about three inches.
S. feltiae is highly effective at controlling pests like fungus gnats and fly larvae, and it will also parasitize some types of caterpillar larvae. This species acts most effectively in clay or silt soils.
S. carpocapsae attacks mobile insects, so it acts as an excellent tool for controlling caterpillar larvae. These prey include such voracious pests as cutworms, armyworms, and webworms.
S. carpocapsae is particularly useful against insects on the surface of the soil, which is where the Lepidopteran pests tend to be. It is most effective in sandy soils.
S. riobrave is another commercially available species in this genus. It has an unusually wide host range that spans different orders of insects and is usually sold to control weevils and mole crickets. This species combines the characteristics of an ambusher and a cruiser.
Two species of Heterorhabditis are also commercially available, and neither of these species is fussy about soil type. Since these nematodes pursue pests so deeply in the ground, they are used to control grubs like those of the Japanese beetle that live deeper in the soil.
H. bacteriophora is very versatile in its choice of host. It can attack both caterpillar and beetle larvae. It is also highly effective against root weevils.
The other Heterorhabditis species that is commonly sold is H. indica. This species will also attack a range of insects, including caterpillars and beetle larvae, weevils, fungus gnats, and thrips.
Choosing the Right Nematode for Your Climate
While you want to choose the right species to control the particular pest you are dealing with, you also need to consider the temperature of your soil during the growing season.
Some prefer hot weather, while others are effective at lower temperatures.
Those in the genus Heterorhabditis are fans of hot weather. H. indica is particularly heat tolerant and can parasitize insects when the soil temperature is 86°F or even higher.
H. bacteriophora is also a warm-temperature species and is much less effective at soil temperatures less than 68°F.
Species of the genus Steinernema vary in their temperature preferences. S. riobrave is active in a wide range of temperatures, from 59 to 95°F.
Originally isolated in the Rio Grande Valley in Texas, this species can even tolerate semi-arid conditions.
S. feltiae is unique among the insecticidal nematodes that are commonly used because it is active at temperatures as low as 50°F.
In contrast, S. carpocapsae is the most effective at soil temps between 72 and 82°F.
One reality of selecting nematodes for pest control is that their action is not immediate, although it is quite effective over time. The first host insects will begin to die after 24 to 48 hours of application.
We have to be realistic about the behavior and timeline of these beneficial creatures. Now that they have infiltrated their insect hosts, they are feeding and reproducing, preparing the next generation.
They will continue to hunt and stay active unless they run out of food, or the temperature gets too hot or cold for them. As with any organic management practice, you should scout and pay close attention to detail.
Don’t be afraid to get your hands dirty.
Spend some time digging in your garden, looking for infected grubs and larvae. Note what you see. Monitor before you apply your beneficial nematodes, 48 to 72 hours after you’ve applied them, and then a week or two later.
This is a holistic approach that will take time to create change in your garden. Note other factors in the garden like soil temperature on a daily basis, and how that correlates with your findings.
Application and Use
Now you may be wondering how to use these useful little organisms in your garden! They can be applied to your vegetable garden, ornamental beds, containers, lawn, or pasture.
While we are focusing on soilborne insects, insecticidal nematodes are sometimes applied to plants above the ground as well – to eradicate leafminers in plant tissue, for example.
Identify the pest or pests you’d like to kill, and evaluate based on the behavior of each species we’ve talked about.
For example, is your target pest a shallow-dweller or a deep diver? What temperature is the soil where your target pest is living? Is the soil compact or sandy?
The size of the area that you will be treating affects the number of nematodes you will need to purchase. For example, you would buy 5 million of them to treat 1,600 square feet of land, and 50 million to treat an acre.
If your plants are growing in containers or a greenhouse, you may need to apply higher numbers.
Nematodes are very sensitive to UV light, so you should apply them in the morning or evening to avoid direct sunlight.
Applying in the evening is particularly important in the summer when the surface of the soil could be too hot for them to survive. Soil temperatures will be lower at dusk.
A list of many of the insects controlled by commonly sold species follows, but this is by no means a complete compilation.
After answering these questions, you will have a more clear idea of which nematode species to select.
Personal Protective Equipment (PPE) Is Not Required
Applying these beneficial organisms to your garden is simple. No major protective gear is required.
If you prefer a little distance, you can wear gloves, but it’s not essential. These organisms are even safer to use than Bt and pose no risk of toxicity.
How Do They Look When They Arrive?
You will usually receive your nematodes in the form of a gel, clay, dry granules, or water-filled sponges. All of these forms will dissolve in water. Just follow the package directions.
If you require organic certification, you should be aware that nematodes shipped in granules or clay may not be certified organic.
And don’t be alarmed if you receive a bag of dead insects! Beneficial nematodes are frequently sold while they are inside their insect hosts. Wax mealworms are a common source of commercially available insecticidal nematodes.
In this case, you just scatter them on the ground where you want them to act, and the juveniles will emerge.
Apply when the time is right. Make sure to do your homework and find the target pest so that those beneficial nematodes have something to eat.
The best time to apply is in the spring or fall, when most of the insects you’ll be targeting are in their larval stage. You want to focus on the larvae because they are still in the soil and therefore vulnerable to attack.
For example, fall is the best time to control white grubs like weevil larvae.
If you’re in doubt about the life cycle of your target pest, you can always reach out to your local extension agency and ask for more information on the specific pest in question.
Compatibility with Garden Chemicals
Surprisingly, juvenile insecticidal nematodes are compatible with many types of garden chemicals. They can even be tank-mixed with most fungicides and herbicides.
Fertilizers are typically not a problem, unless you are using fresh manure or high concentrations of chemical fertilizers like urea.
These organisms can even tolerate many insecticides, like organophosphates such as diazinon, although the common insecticide carbaryl (Sevin) is moderately toxic.
You can wait one to two weeks to apply your insecticidal nematodes if you are not sure if your selected chemical insecticide is compatible with these organisms.
Package labels will typically provide the information you need to know regarding which chemicals your chosen species is compatible with.
Irrigation: Keep the Soil Moist
Pay attention to the moisture content of your soil. It is a good idea to thoroughly water it before you apply.
After application, go ahead and water again to rinse any nematodes off the leaves of plants in the area, and down into the soil closer to their targets.
This is particularly important if you are treating turf with a white grub problem. The thick thatch can make it difficult for the nematodes to penetrate the soil.
Watering after application will help the organisms travel through the thatch to get down to the soil where the grubs are.
Once you’ve applied them, you want to keep your soil moist for the next couple of weeks to make sure they can stay active and keep killing those pesky insect larvae! If it doesn’t rain, you should water every three to four days.
How to Physically Apply
To apply the nematodes, you can use a watering can if you are targeting a small area. For larger areas, you can use a pump sprayer, end-hose sprayer, irrigation system, backpack sprayer, or motorized sprayer.
The easiest way to apply them is by using a hose end sprayer that controls the dose applied.
Make sure to agitate the mix of beneficial nematodes and water so that they are evenly dispersed from whatever sprayer you are using. The nematodes have a tendency to settle if they are not mixed properly.
One note of caution: be careful if you use a hydraulic pump with high internal and shear forces. They can shred your nematodes!
You can expect to see results within two weeks.
After the initial application, you should apply again in seven to 10 days to make sure there are enough insecticidal nematodes to assault the populations of your target insects.
If you still have pests, keep applying the organisms every seven to 10 days until the pest populations dwindle.
Remember that if you have a population of insects that has built up over the course of years, it is likely to take more than one application to contain them.
The likelihood of your nematodes surviving the winter depends on your climate. They can typically survive one hard freeze. However, prolonged hard freezes can kill them.
Even if you live in a mild climate, you should reapply a newly purchased batch of nematodes in the spring. You will need a high population of them present to effectively attack your pests.
The amount of time that the nematodes can be stored varies according to the species. S. carpocapsae survives well in a dessicated state and can be stored for several months at room temperature. However, other types such as H. indica have a shorter shelf life.
Your nematodes should be shipped to you in an insulated container. Typically, they can be stored in the refrigerator for several weeks or even months. Since they are sensitive to extreme temperatures, make sure they don’t freeze.
And keep them away from high temperatures! Temperatures over 90°F will kill most insecticidal nematodes, so be sure not to leave them in a hot vehicle or sunny spot.
Where to Buy
You can often find packages of beneficial nematodes at your local garden center or plant nursery.
Alternatively, there are a variety of online sources as well. Many of these will be listed under a brand name, so you’ll need to check the specific species before purchase.
S. carpocapsae is available as NemAttack™ Sc or BioLogic Sc.
You can find NemAttack™ Sc in packages of 5 million to 500 million available at Arbico Organics.
Biologic Sc is available in packages of 5 million to 25 million via Amazon.
Steinernema feltiae is available as NemAttack Sf or Biologic Sf.
You can find Biologic Sf in packages of 5 to 25 million available from Biologic via Amazon.
Arbico Organics carries packages of NemAttack™ Sf containing 5 to 500 million nematodes.
Steinernema riobrave is sold as NemAttack™ Sr.
Find packages containing 5 million S. riobrave available at Arbico Organics.
Heterorhabditis bacteriophora is sold under the brand name NemaSeek™ Hb.
Pick up packages of 5 million to 500 million from Arbico Organics.
NemaSeek™ Hi contains the species Heterorhabditis indica.
Find packages containing 5 million available from Arbico Organics.
Get Growing with Nematodes
Beneficial nematodes can offer your home garden immense perks.
You have a wealth of choices available, which will let you target the pests you want to control, while accounting for your type of soil and the ambient soil temperature.
These wonderful little organisms offer great protection for your gardening spaces and are totally safe for use in proximity to humans, other animals, and even beneficial insects.
Now that you know how they operate, add them to your garden, and watch your pests dissipate!
Have you used insecticidal nematodes in your garden? If so, tell us about your experience in the comments.
And for more information on controlling pests in your garden, check out these guides next:
About Helga George, PhD
One of Helga George’s greatest childhood joys was reading about rare and greenhouse plants that would not grow in Delaware. Now that she lives near Santa Barbara, California, she is delighted that many of these grow right outside! Fascinated by the childhood discovery that plants make chemicals to defend themselves, Helga embarked on further academic study and obtained two degrees, studying plant diseases as a plant pathology major. She holds a BS in agriculture from Cornell University, and an MS from the University of Massachusetts Amherst. Helga then returned to Cornell to obtain a PhD, studying one of the model systems of plant defense. She transitioned to full-time writing in 2009.