Beneficial Nematodes

When it comes to nematodes, it's the destructive ones that get all the press. And they deserve it--plant parasitic nematodes cause an estimated $78 billion in damage to crops worldwide.

It's only been recently that beneficial nematodes have stolen the spotlight. After decades of trying, researchers now know how to efficiently mass-produce these insect-parasitic nematodes for use in the farming industry, and that's good news for gardeners. These tiny critters help control many common garden pests, including armyworms, rootworms, fleas, fungus gnats, stem borers, root weevils, cutworms, and billbugs. In field research, they've been shown to be just as effective as traditional insecticides on these pests.

Today, there are dozens of American companies selling beneficial nematodes to farmers and home gardeners. The worms can be used as a pest control for lawns and golf courses, gardens and greenhouses.

Nematodes for Gardeners

For gardeners, beneficial nematodes are attractive as biological pesticides because of their effectiveness and environmental safety. They contain no toxins and are harmless to humans and all other warm-blooded animals. They won't harm fish or plants and are ideal for insect-infested areas around drinking wells or other environmentally sensitive locations that preclude the use of chemical pesticides. As a testament to their safety, the U.S. Environmental Protection Agency waived registration requirements for beneficial nematodes.

"We see a lot of gardeners using beneficial nematodes on white grubs," says Jim Cate, president of Integrated Biocontrol Systems in Lawrenceburg, Indiana. "They're good on a number of beetles that live in the soil. For example, carrot weevil, asparagus weevil, black vine weevil."

However, effective use of beneficial nematodes requires knowledge of the nematodes and the insect you want to control. Simply applying them like a traditional pesticide won't work.

What is a Nematode?

In an average backyard, there are billions of naturally occurring nematodes (also known as roundworms) in the soil. They are 0.6 to 2 millimeters long and often hardly visible. Some feed on plants, some feed on animals, some feed on decaying organic material or bacteria, and some (the beneficial nematodes) feed on insects.

Beneficial nematodes belong to one of two genera: Steinernema and Heterorhabditis. Four species of Steinernema and one species of Heterorhabditis are commercially available in the United States. Steinernema is the most widely studied beneficial nematode because it is easy to produce. Heterorhabditis is more difficult to produce but can be more effective against certain insects, such as the white grubs of Japanese beetle.

"There are hundreds of nematode species parasitic on insects, but only a few that show commercial potential. Currently, only five species are produced commerically in the U.S.," says Harry Kaya, professor of nematology at the University of California-Davis. "We expect to see more beneficial nematodes available in the future because some nematodes are more effective on certain insect pests."

How Nematodes Work

The life cycle of beneficial nematodes consists of six distinct stages: an egg stage, four juvenile stages, and the adult stage. The adult spends its life inside the host insect. The third juvenile stage, called a dauer, enters the bodies of insects (usually the soil-dwelling larval form). Some nematodes seek out their hosts, while others wait for the insect to come to them. Host-seeking dauers travel through the soil on the thin film of water that coats soil particles. They search for insect larvae using built-in homing mechanisms that respond to changes in carbon dioxide levels and temperature. They can also follow trails of insect excrement. Other species have a "sit-and-wait" strategy, like a praying mantis. When the mobile insect tunnels by them, they attack.

After a single dauer nematode finds and enters an insect (its skin or natural openings), the nematode releases a toxic bacteria that kills its host, usually within a day or two. In less than two weeks, the nematodes pass through several generations of adults, which literally fill the insect cadaver. (Steinernema reproduction requires at least two dauer nematodes to enter an insect, but a single Heterorhabditis can generate offspring on its own.) Nematode adults feed until they exhaust their food supply (the insect carcass), and as that time nears, the life cycle is halted.

As if they know time is running out, the entire host population of nematodes--as many as 200,000--become dauer nematodes, each with the ability to kill another insect. Eventually, they leave the insect carcass and begin to search for another host. Since it is nonfeeding, the dauer nematode can last for weeks in the soil.

How to Use Nematodes

For the home gardener, localized spraying is probably the quickest and easiest way to get beneficial nematodes into the soil. Although there's no need to worry about wearing rubber gloves or protecting your clothes, take reasonable precaution against splashing them on you. Beneficial nematodes, unlike many of their cousins, are harmless to mammals.

Producers ship beneficial nematodes (dauers) in the form of gels, dry granules, clay, and water-filled sponges. All of these dissolve in water and release the millions of nematodes. A typical spraying will introduce hundreds of millions of nematodes--each ready to start seaching for an insect--into your garden.

Nematodes should be sprayed on infested areas at a time when the targeted pest is in the soil. Timing is important, or else you'll have to repeat the application.

Northern gardeners should apply nematodes in the spring and fall, when the soil contains insect larvae. "Most of the beneficial nematodes are adaptive to cold weather," says Cate, at Integrated Biocontrol Systems. "In fact, the very best time to control white grubs is in the fall."

If you're in a warmer climate, beneficial nematodes are most effective in the summer. In any case, if you're unsure of when to apply beneficial nematodes, call your Extension service. Find out when and for how long the soil-dwelling stage of the target insect will be present and plan your nematode application for that time.

Moisture

Beneficial nematodes move in water-filled spaces between particles of soil. If the soil is too dry or too tight, they are not able to move and search out host insects. Commercial suppliers recommend watering the insect-infested area before applying nematodes. An additional sprinkling after releasing the nematodes will help rinse them off plants and grass into the soil.

Ultraviolet Light

Exposure to ultraviolet light or extreme temperatures can kill nematodes. During the warmer summer months, apply nematodes in the early evening, when the intensity of light is reduced and soil temperature is lower. Remember, watering your lawn before application will lower the soil temperature. Nematodes are either killed or less effective when soil temperatures are below 60oF or higher than 93° F.

For the typical garden, nematodes will take about two to three weeks to show results. You should see fewer of the targeted adult insects in your yard or garden. If you applied the nematodes in the fall, you should see results the following spring.

How can you make sure the nematodes have worked? If you want hard evidence, you'll have to do some digging.

"Look for insect activity," says Kaya. "If you dig up the soil you can usually find a dead insect. If the nematodes killed the insect, it should be brown or reddish orange, depending on what type of nematode was used. Secondly, it shouldn't have a putrid smell." Insects killed by Steinernema turn cream to dark brown, while Heterorhabditis turns insects orange-red. (They may even glow in the dark!) For most gardeners, nematodes must be reapplied each year.

Though be nematodes will overwinter in the soil, there will not be enough of them to effectively control pests for more than one season. Effective use of nematodes depends on a high enough number of the organisms to overwhelm your garden pests. Surprisingly, nematodes will work even when combined with some powerful pesticides. Farmers use nematodes in combination with diazinon, but home gardeners should check the package instructions before combining the two. On the other hand, the pesticide carbaryl will kill beneficial nematodes.

Because of its high nitrogen content, fertilizer can reduce nematode effectiveness. Manufacturers recommend that fertilizers not be used two weeks prior to and after nematode application. Again, check the package instructions.

Choosing a Supplier

How will you know if the nematodes have arrived at your home alive and well? Unless you test the product on water-covered grubs in a bucket, you won't. If you want viable nematodes delivered to your door, you should pick a reputable supplier. Choose a company that sells several different nematode species. Ask how long they've been selling them and which target pests each of the nematode species controls. You should get a pretty good sense of the supplier's expertise. After you get the nematodes, it's up to you.

Beneficial nematodes attack many soil-dwelling insects, but not all of them. Control of some insects is questionable. For example, researchers say the beneficial nematodes commercially available may or may not be effective in controlling Japanese beetles (white grubs), however nematode suppliers claim a combination of Heterorhabditis and Steinernema will do the trick. Also, nematodes shipped in clay or granule form may not be certified organic. Ask your supplier if they ship nematodes on live hosts. An application of 100 million nematodes will typically cover 2,000 square feet and cost between $15 and $22. If you've got a severe insect problem, however, you may want to double the recommended density to increase their effectiveness.

Dan Hickey is a former editor at National Gardening.

Photography by Keith Weller/USDA-ARS

Choosing a Supplier

How will you know if the nematodes have arrived at your home alive and well? Unless you test the product on water-covered grubs in a bucket, you won't. If you want viable nematodes delivered to your door, you should pick a reputable supplier. Choose a company that sells several different nematode species. Ask how long they've been selling them and which target pests each of the nematode species controls. You should get a pretty good sense of the supplier's expertise. After you get the nematodes, it's up to you.

Beneficial nematodes attack many soil-dwelling insects, but not all of them. Control of some insects is questionable. For example, researchers say the beneficial nematodes commercially available may or may not be effective in controlling Japanese beetles (white grubs), however nematode suppliers claim a combination of Heterorhabditis and Steinernema will do the trick. Also, nematodes shipped in clay or granule form may not be certified organic. Ask your supplier if they ship nematodes on live hosts. An application of 100 million nematodes will typically cover 2,000 square feet and cost between $15 and $22. If you've got a severe insect problem, however, you may want to double the recommended density to increase their effectiveness.

Dan Hickey is a former editor at the National Gardening Association.

Photography by Keith Weller/USDA-ARS

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Articles → General → Garden Care → Pests and Problems
Articles → General → Garden Care → Soil, Water, and Fertilizer
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