What Is a Beneficial Nematode? How These Worms Control Garden Insects
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What is a beneficial nematode? A beneficial nematode is a microscopic roundworm that attacks insects. The term usually refers to entomopathogenic nematodes, often shortened to EPNs, which kill insect pests by carrying bacteria into the insect’s body. This explainer answers the question directly, describes the mechanism of infection, lists the pests involved, and explains the conditions that decide whether these worms work in a garden. The answer draws on university extension material and avoids product recommendations.
Quick answer
- A beneficial nematode is a roundworm that parasitizes insects. Extension material describes EPNs, sometimes called beneficial nematodes, as worms that only attack insects.
- The nematode enters an insect and releases bacteria that cause the insect to die. The host dies from blood poisoning within a few days.
- Moisture is the main condition for success, because the nematodes need free water to move.
- University sources describe these nematodes as harmless to people, pets and plants.
What a beneficial nematode is
Nematodes are microscopic, worm-like organisms that are found in almost every natural landscape. Plant-parasitic nematodes attack plant roots and can be a major crop pest. Entomopathogenic nematodes are a different group. According to the University of New Hampshire Extension, EPNs are the type that is sometimes referred to as beneficial nematodes, and these only attack insects.
The Colorado State University Extension describes the same group in plain terms. Insect parasitic nematodes are small round worms that kill insects but are harmless to other organisms. The same page notes that they are also known as predator nematodes, beneficial nematodes, or entomogenous nematodes.
The University of Florida IFAS Extension places these nematodes in a wider context. Entomopathogenic nematodes are described as fitting into integrated pest management programs.
How the nematodes kill insects
The infection process has two stages: the worm gets inside the insect, and the worm’s bacteria finish the job. The UNH Extension explains that beneficial nematodes parasitize insects by entering their bodies and infecting them with bacteria from their gut, which causes death of the insect host.
The CSU Extension describes the same sequence in more detail. Upon entering the insect, the nematodes kill their host by releasing bacteria into it. The bacteria develop within the body, and the host dies from blood poisoning within a few days.
The nematodes themselves then continue to develop inside the dead insect. The CSU page states that thousands of nematodes can be produced following a single infection, and that the body wall of the dead host eventually ruptures and releases them.
Finding the host is part of the process. The CSU page notes that nematodes use carbon dioxide and perhaps other chemicals produced in waste products of insects as cues to find their hosts. The CSU page also states that insect parasitic nematodes are mobile and move short distances in search of host insects.
Which pests they target
Beneficial nematodes are not a general-purpose insecticide. The UNH Extension lists shore flies, fungus gnats, caterpillar larvae, beetle larvae, leaf miners and thrips among the crop pests that the parasitic relationship can be used to control.
The CSU Extension gives a more detailed picture by species. Among the pests effectively controlled are most turf insects and several other insects that live in soil. Steinernema carpocapsae tends to be most effective against caterpillar larvae. The same species has controlled sod webworms, cutworms and certain borers in trials. Steinernema feltiae is effective against certain fly larvae, and it is sometimes used against fungus gnats.
White grubs are a special case. The CSU page states that Steinernema are less effective against white grubs, root maggots, rootworms and black vine weevil. The genus Heterorhabditis is treated differently. Field trials consistently show that this genus outperforms Steinernema for control of white grubs.
Not every insect is a good target. The CSU Extension notes that maggots, crickets, grasshoppers, and termites appear less susceptible to the commonly available strains of these nematodes. The same page adds that other nematode strains may hold future promise against these pests, which means the list of suitable targets can change as new strains become available.
What conditions decide whether they work
Environment is the largest single factor in field performance. The CSU Extension states that one of the greatest factors to determine susceptibility under field conditions is the environment where the pest insect occurs. The CSU page states that the nematodes are most effective against insects that occur in moist, dark locations, such as various soil insects or borers within woody plants.
Moisture
Moisture is the most important factor. The CSU page explains that insect parasitic nematodes require free water to move and are susceptible to death by drying. High humidity and free water are the primary conditions for effective use. The nematodes are also susceptible to drying and ultraviolet light, which is why they perform best in moist, dark locations.
Irrigation can help the process. The CSU Extension notes that adequate moisture is essential and that pretreatment irrigation is recommended in soil applications. A post-treatment irrigation may help push nematodes through a thatch layer. Irrigation can also move some soil insects, such as white grubs, higher in the soil, where they may be more easily found by the nematodes.
Soil texture
Soil structure affects how far the nematodes can travel. The CSU page states that nematode movement is more extensive in sandy soils with large pore size than in clay soils. Nematode penetration may be expected to be poor in tight soils common to some regions. The CSU page also states that extensive thatch accumulation may prevent nematode penetration into the turf root zone where white grubs feed.
Temperature
Temperature is the second limit. The CSU Extension reports that short exposure to high temperatures (above 90 to 95 degrees F) can be lethal to commonly available strains. Low temperatures are also a limit. The page states that low temperatures (below about 55 degrees) inactivate most Heterorhabditis species. Strain and species differ in temperature sensitivity, so the label conditions matter.
Cold, dry climates create additional problems. The CSU page notes that nematodes are sensitive to freezing temperatures which can damage their cells and limit their mobility. Most nematodes rely on moist environments to survive and move through soil, so dry conditions reduce their populations and activity, especially near the surface.
Application and storage
Nematodes are sold in a very small stage of life. The CSU Extension states that insect parasitic nematodes are sold in the infective juvenile dauerlarva stage, which are barely visible to the unaided eye. The CSU page states that the active stage of the nematode that invades an insect is the juvenile (dauerlarva) stage.
The application method is simple. According to the CSU page, to apply, dilute the nematodes in water and drench the soil or inject them into plants. Regular spray equipment can be used, because nematodes are quite tolerant of pressures found in many sprayers. The UNH Extension adds that nematodes are easy to apply through existing irrigation systems, with a simple handheld sprayer, or even by the placement of infected cadavers into the soil.
Compatibility with other inputs is a common question. The CSU Extension states that insect parasitic nematodes appear compatible with many chemicals used in lawn and garden care. Storage is the other practical concern. Most packaged products have a shelf life of several weeks or even months if refrigerated, according to the same page.
Why they fit integrated pest management
The UF IFAS Extension describes entomopathogenic nematodes as fitting nicely into integrated pest management programs, because they are considered nontoxic to humans, relatively specific to their target pests, and can be applied with standard pesticide equipment. The UNH Extension makes a related point about pesticide use, stating that EPNs help keep pest populations low enough to prevent significant crop damage, while also reducing the need for pesticides.
The safety point is widely repeated. The UNH Extension states that EPNs are harmless to humans and are safe to use around pets, livestock, and plants. The CSU Extension adds that they cannot attack or cause disease in birds, mammals, or fish.
How we chose these sources
The sources were limited to university extension pages from New Hampshire, Colorado and Florida. Each page was fetched directly and checked for a successful response before any statement was used. Two other extension pages returned HTTP 403 when fetched, so their content was not used. Retail and product pages were excluded.
What we could not verify
The three pages used here agree on the mechanism, the host range and the main limits of moisture and temperature. Some questions could not be answered from them, and the explanation does not fill those gaps with general assumptions.
What the sources do not say
The sources do not give a single application rate for a home garden. The CSU page gives rates per acre and per square foot in its discussion of commercial use, and those figures should not be translated into garden dosages without checking a current label. The HTTP 403 responses from two additional extension pages mean their statements on timing and moisture were not checked here.
Frequently asked questions
Are beneficial nematodes harmful to people?
The University of Florida IFAS Extension describes entomopathogenic nematodes as considered nontoxic to humans.
Do beneficial nematodes harm plants?
The UNH Extension states that PPNs attack plant roots, but EPNs only attack insects. The CSU Extension states that insect parasitic nematodes kill insects but are harmless to other organisms.
Which pests do beneficial nematodes control?
The UNH Extension lists shore flies, fungus gnats, caterpillar larvae, beetle larvae, leaf miners and thrips. The CSU Extension adds that most turf insects and several other insects that live in soil are among the pests effectively controlled.
Why does soil moisture matter for beneficial nematodes?
The CSU Extension states that adequate moisture is the most important factor. It also states that insect parasitic nematodes require free water to move and are susceptible to death by drying.
How should beneficial nematodes be stored?
The CSU Extension states that most insect parasitic nematodes packaged for sale have a shelf life of several weeks or even months if refrigerated. It also states that they can be killed or inactivated by extreme temperatures, so they should never be allowed to freeze.
Internal links
For related reading on soil conditions, see how to keep garden soil healthy. For a look at soil improvement in general, see how to make garden soil better.
Sources
- University of New Hampshire Extension, EPNs 101: An Introduction to Beneficial Nematodes, https://extension.unh.edu/blog/2021/04/epns-101-introduction-beneficial-nematodes
- Colorado State University Extension, Insect Parasitic Nematodes, https://extension.colostate.edu/resource/insect-parasitic-nematodes/
- University of Florida IFAS Extension, Entomopathogenic Nematodes (EENY-530 / IN944), https://edis.ifas.ufl.edu/publication/IN944
