Nematodes: Allies or Enemies?

By Monica Mansfield
Published: May 9, 2019 | Last updated: May 4, 2021 11:15:20
Key Takeaways

Nematodes are among the oldest organisms on the planet and can either destroy a garden or help it thrive. Monica Mansfield gives us some inside information on these good and bad parasites that are lurking in our dirt and water.

Nematodes can either be a gardener’s best friend or worst enemy. Plant parasitic nematodes invade our gardens and cause yellowing leaves, stunted growth, and poor harvests. Beneficial nematodes are powerful allies that feed on harmful bacteria, fungi, insects, and other nematodes that want our crops for themselves. Beneficial nematodes also break down organic matter and recycle nutrients in the soil.


Nematodes are microscopic roundworms that can range in size from one-fiftieth of an inch up to several inches long. They were some of the first organisms to exist on the planet, having been around for an estimated one billion years. They live in water and soil and, depending on the type, feed on bacteria, fungi, protozoans, other nematodes, insects, plants, animals, and even humans. They are a vital part of the soil food web, serving as predators, prey, and nutrient recyclers.

There are over 15,000 known species of nematodes on the planet, with an estimated 15 percent of the species being parasitic towards plants. They are so abundant that if you were to remove everything on earth except nematodes, we would still be able to see an outline of everything on Earth. In fact, just one handful of soil can contain millions of them.


Harmful Nematodes

Plant parasitic nematodes are only about one-fiftieth of an inch long, with smooth, unsegmented bodies. Most are long and slender, while some species are more pear-shaped. They have sharp, pointed mouths, called stylets, which puncture cell walls and allow them to feed on tissues.

There are four classifications of plant parasitic nematodes. Migratory ectoparasites feed on the outsides of the roots. Sedentary ectoparasites burrow their heads into the roots to feed and stay there. Migratory endoparasites tunnel into the roots and then search for another host when they are done feeding. Sedentary endoparasites burrow into the roots and stay there permanently to feed. Nematodes will also eat parts of the plants above ground at different times in their life cycle.

They can cause some serious harm, to the tune of an estimated $77 billion in damages to crops worldwide. The most common plant parasitic nematodes are root-knot nematodes. They get their names from the damage they cause to root systems. When they puncture the root and move in, they actually expand the size of the root to make more room for themselves and their offspring. When you dig your sick plants up at the end of the season, you’ll find the roots have knots in them.


Symptoms may include yellow wilted leaves, signs of infection, stunted growth, and poor yield. Nematodes also present as patches of poor growth in an otherwise healthy field, which may spread if left untreated. If you suspect nematodes are the cause of the problem, you can gently lift the plant out of the ground and examine the roots. You may see root rot, injured root tips, small lesions and knots, or excessive root branching.

The symptoms may be mistaken for other issues until you dig up the plant, and sometimes there is no damage to the plant at all. Healthy plants can actually tolerate small infestations without suffering a loss in production.


How to Get Rid of Harmful Nematodes

The first parasitic plant nematodes were noted in wheat seeds in 1743, followed by root knot nematodes on cucumbers in 1855, and cyst nematodes on sugar beets in 1859. By the early 1900s, the field of agricultural nematology had taken root. Nematodes were first treated by soil fumigation in the 1940s, however, most nematicides are now strictly regulated or banned due to their harmful effects on the environment.

Nematodes may not move more than a meter in their lifetime, but they can travel long distances in a variety of ways. They can be carried on shoes, farm tools and equipment, in dirt that is moved, in water during floods, and on plants and seeds. Dried nematodes in their dormant state can even be carried in the wind. These modes of travel make it challenging to quarantine and kill nematodes, so the best option is to manage their populations as best we can in our gardens.

Follow Proper Sanitation Practices

Because of the way nematodes can travel, it is important to follow proper sanitation practices. Be sure to clean farm equipment, tools, shoes, and clothing when moving from field to field, or garden to garden.

When bringing in new plants, be sure to quarantine them for a short time to make sure they are healthy before introducing them to your garden. You can also check their roots for knots and lesions before transplanting. Bringing sick or infested plants into your garden is one of the most common ways to spread pests and disease. This raises a great argument for starting your own plants from seed.

Crop rotation is an effective way to manage nematodes. By planting non-host plants in alternating years, you can decrease their populations. Cauliflower, broccoli, and millet are ideal. French dwarf marigolds and common vetch are effective when used as a cover crop and then turned into the soil. Growing non-host plants for two years in a row will significantly lower nematode populations.

You can also plant nematode-resistant varieties. For example, many tomatoes are nematode-resistant, such as Best Boy, Big Beef, French Rose, Lemon Boy, OG 50, Sugar Snack, Supertasty, and Winter Red. Your seed catalogs should be able to point you in the right direction.

Neem oil kills parasitic nematodes without harming beneficial nematodes. Neem oil works by disrupting their growth cycle, which will prevent them from laying eggs before they die off. Neem won’t harm most beneficial insects, such as bees, and is also an effective fungicide and insecticide ideal for preventative use.

Bring in Some Carnivorous Fungi

Nematophagous fungi are carnivorous and feed on nematodes. These fungi actually set traps to snare nasty nemotodes — either sticky traps or circular rings that capture and kill their prey. What’s remarkable is these fungi will only set the traps when they detect the nematode’s ascarosides, which are the chemical cues nematodes use to communicate with one another.

Nematophagous fungi are found in abundance where there is rotting organic matter, such as the compost pile, leaf mold, and decomposing bark. Adding compost, leaf mulch, or layering your garden with wood chips will encourage the fungi that protect your garden from parasitic nematodes.

Healthy plants resist well and perform better than plants suffering nutrient deficiencies, even in the presence of harmful nematodes, so regularly adding compost and organic matter to your garden serves a double function.

The most effective method of managing harmful nematodes is to use a combination of these methods, as just one will probably not be effective on its own.

Beneficial Nematodes

Beneficial nematodes are a gardener’s best friend. Instead of attacking our plants, they attack a wide variety of garden pests. The most helpful strains in the garden are endoparasites of insects, which introduce Xenorhabdus sp. bacteria into the insects they eat. This bacteria kills them within 24-48 hours and breaks down their tissues so the nematodes can make their home inside of the insect, lay their eggs and feed on the decomposing tissue.

The most commonly used beneficial nematodes are Steinernematidae carpocapsae, S. feltiae, S. glaseri, Heterorhabditisheliothidis, and H. bacteriophora. They are effective against many pests including weevils, cutworms, chinch bugs, white grubs, clearwing borers, fungus gnats and sod webworms.

These beneficials are sold commercially as biological insecticides. They can stay viable for months as long as they are kept at the correct temperature, and mix well with fertilizers and pesticides. They are considered environmentally friendly by the EPA since they occur naturally, are not genetically modified, and do not harm vertebrates. There is no evidence insects develop resistance to the bacteria these nematodes produce.

How to Use Beneficial Nematodes in Your Garden

Beneficial nematodes can be purchased from most garden centers, dormant in a powder. To use, add them to water and spray them on your plants and soil. Be sure to remove the screen in your sprayer so they can get through.

Because they must be stored at the correct temperature to remain viable, you can guarantee their viability before use by adding them to water and observing them under a microscope.

It is important to apply beneficial nematodes in the correct conditions. They require warm, moist soil to be effective, so it is a good idea to irrigate the garden before and after application. Because they travel in water, watering the garden after application helps to move them around and find hosts. The ideal conditions also include high humidity, moderate temperatures, and indirect sunlight. Applying them in the morning or evening is best.

As gardeners, we need to understand and prevent potential threats to our gardens. By applying beneficial nematodes and using best practices to reduce plant parasitic nematodes, we can take care of these threats in a way that’s effective and won’t harm the environment.


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Written by Monica Mansfield | Homesteader, Owner & Writer of The Nature Life Project

Profile Picture of Monica Mansfield

Monica Mansfield is passionate about gardening, sustainable living, and holistic health. After owning an indoor garden store for 5 1/2 years, Monica sold the business and started a 6.5-acre homestead with her husband, Owen. She writes about gardening and health, as well as her homestead adventures on her blog at

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