Nematodes: Control Or Use As A Beneficial Insect?

Last updated on October 23rd, 2023 at 08:30 pm

Nematodes are nematodes that can be found in the garden on plants or in the soil. They are sometimes good and sometimes bad, because depending on the species, they eliminate beetle larvae as beneficial insects, but they can also be annoying plant pests that need to be controlled.

What are nematodes?

They are small. Tiny to be exact. Maybe 0.5 millimeters, two at the most. What’s more, the worm-shaped animals are whitish to colorless. Nematodes are therefore almost invisible to the naked eye in the garden and on plants.

There are more than 20,000 species worldwide, which can live almost anywhere, whether in water, as predators free in the soil or as parasites in plants or even animals.

Nematodes are among the most species-rich and also the most individual-rich animals of all – hundreds of thousands of nematodes can live in one square meter of soil. Nematodes move actively with sinuous movements, but are always dependent on a film of water. The animals ingest their food with a mouth spine, which they use like a hypodermic needle to suck in their food.

Nematodes living freely in the soil eat fungi, algae, bacteria or carrion, but also hunt and prey on snails or insect larvae. Prey gives the false impression that the animals penetrate the prey and release a special bacterium that eventually kills the pest.

Are nematodes harmful to humans?

The hunting behavior of the nematodes can make you feel a little queasy. After all, the scriptwriters for horror films could not have thought up a better way to deal with the prey. In fact, there are also nematodes that are harmful to humans, which are larger and thus more disgusting than the species relevant to plant protection. These, in turn, are completely harmless to humans, pets, fish, and even earthworms. In the garden, one is neither attacked by nematodes nor infected in any way. You don’t notice anything from the animals, which have a very narrow host range and only go after harmful insects. Otherwise, nematodes would not be allowed to be used for pest control, nor would they be allowed to be bred for this purpose, and even nematodes that are harmful to some plants would remain on them and not scurry around wildly in the garden.

Beneficial vs. harmful nematodes

There are nematodes that are harmful to plants as well as species that can be used as beneficial insects to attack beetles such as the large leaf weevil or the garden leaf beetle or their larvae – completely biologically without poison. The nematodes actively search for their hosts in the soil.

Some species, on the other hand, you just want to fight, because nematodes that are harmful to plants suck on the roots or infest the plants through the soil and migrate to the leaves and are safe in the plants from pest control. The plant pests among the nematodes suck plant cells or root sections with their mouth sting and drool salivary secretion into the plant, which leads to deformations.

Nematodes as beneficial insects to control pests.

Beneficial nematodes are used in the garden as a biological pesticide, but also in composting. This is because many types of nematodes are among the microorganisms that form the soil and are therefore also important for the garden. However, the use against pests in the soil is by far the most important in the garden. Nematodes arrive in an inactive state as a powder conveniently by mail, and all you have to do is dissolve them in water and spread them as quickly as possible.

Per square meter of garden area you pour out a good half million nematodes – you adjust the amount by diluting the concentrate with water accordingly. When doing so, follow the manufacturer’s instructions, because the use of nematodes is only effective at certain times, when the respective pests are in susceptible development stages as larvae or pupae. Outside of this time, nematodes do nothing. Adult beetles are left alone. The application also only works when the soil is moist and above 12 degrees Celsius. In summer heat, nematodes become rotten or simply dry up.

What pests can be controlled with nematodes?

Since nematodes only help against certain pests, you need to know exactly what they are. Nematodes against thick-mouthed weevils would simply leave codling moth larvae to the left. In the garden, these species in particular excel in biological pest control: Steinernema feltiae (SF), Steinernema carpocapsae (SC), Heterorhabditis bacteriophora (HM), Steinernema kraussei (SK), Heterorhabditis bacteriophora (HB) and Phasmarhabditis hermaphrodita (PH).

What else helps against fungus gnats?

To control fungus gnats works well in a biological way with nematodes. However, yellow boards against the adult fungus gnats are also suitable for control. In addition, you can sprinkle a layer of sand a good one centimeter thick on the potting soil, then the larvae can not come out of the soil. A piece of fly screen cut to size has the same effect. Likewise, you can control fungus gnats with special predatory mites and the larvae with Bacillus thuringiensis.

How long does it take for nematodes to work against the pests?

Nematodes, of course, do not act as quickly as plant protectants. Nevertheless, the beneficial insects do their job quickly. The first successes in the form of dead larvae can be seen after just 24 hours. After a total of 3 weeks, the nematodes have done their job. This is how long you should keep the treated soil constantly moist.

How long do nematodes survive in water?

That is quickly said: not long. After preparing the broth for watering, you should also apply the nematodes quickly. After some time, the animals settle at the bottom of the watering can. Then stir again.

Harmful nematodes in the garden

Whether on perennials, fruit or vegetables – the pests of different genera such as Ditylenchus, Pratylenchus or Meloidogyne suck from the outside as so-called root or root gall lice on young roots, penetrate them or migrate in the plant to the leaves, where they provide as leaf or stem lice for glazed spots or deformed leaves and stems. The pests do not always cause uniform symptoms. However, brown or black spots, deformities, growth retardation, yield loss, few flowers or partial plant death are always involved somehow. To be sure, cut up an infested leaf on a mirror with a razor blade and add some water. The nematodes will leave the leaf and float around in the water. The pests may be hard to see, but on the mirror, the pests visually double in size, making it easier for you to spot the nematodes.

Nematodes are also responsible for soil fatigue, which makes it difficult or impossible to plant roses in places where they have been before. The host plant spectrum of nematodes is wide: in addition to fruits and vegetables such as apple, tomato, strawberry or cucumber, the pests also attack houseplants such as hydrangea, hyacinth and begonia.

Tips: How to control and prevent harmful nematodes.

Direct control is not possible. Cut off infested plant parts and completely dispose of severely damaged plants. Actually, the soil should also be removed, but this is usually not possible. Therefore, change the crop rotation regularly – this reduces the risk of infestation. Strengthen beds and plants by liming and organic fertilization. This is because healthy soil contains plenty of helpful fungi and bacteria that inhibit or even kill nematodes.

There are also plants against nematodes, marigolds or mustard for example. The plants stimulate nematode eggs in the soil to hatch with certain root excretions, but then provide no food for the larvae and the pests die. Nematodes of the genera Meloidogyne and Pratylenchus can be starved out by not planting anything in the area for months and also by rigorously destroying weeds. When planting tomatoes and potatoes, use the special varieties ‘Dolcevita’ and ‘Alexandra’, respectively, which are resistant to nematode infestation.