Q and A about nematodes

Nematodes are widely used to control harmful larvae or slugs. That's why I thought it was time to take a closer look at the most frequently asked questions about nematodes. With this blog, you will literally get all the answers to your questions about nematodes.

Can you see nematodes?

Nematodes are small, thin roundworms ranging from 0.1 to 2.5 millimeters in length, making them not visible to the naked eye. When you buy a packet of nematodes, you won't see them inside. They are also invisible in water or potting soil. However, you can see nematodes under a microscope once they are dissolved in water.

When nematodes are applied against fungus gnats, people often think they see the nematodes in the potting soil. However, it is the larvae of the fungus gnats that sometimes become visible after applying nematodes. It may seem as though they are trying to escape from the nematodes attacking them in the soil.

Where do nematodes come from?

Nematodes naturally occur all over the world. There are many different species that have adapted to various habitats.

Most nematodes live in the soil, both in natural soils and in gardens and parks. They can also survive in various types of water (freshwater and seawater). Think of lakes, streams, rivers, and oceans.

Some nematode species can survive in extreme conditions such as deep under the sea, in hot springs, deserts, and polar regions.

Some specific nematodes live in or on plants. They feed on the roots, stems, and leaves of specific plants, which can be either cultivated or wild plants.

There are nematodes that live as parasites in insects, mammals, birds, and reptiles. This group includes the commercialized nematodes that parasitize slugs or insect larvae. Commercialized nematodes are species-specific and do not harm other animals, birds, or humans.

Some specific nematodes can infect humans, such as roundworms.

What do nematodes do in the soil?

Nematodes are naturally present in the soil and are part of the soil food web, just like all other organisms. In the soil, it's also a matter of eating and being eaten. Nematodes play an important role in this.

• Organic material that we apply to our garden soil is broken down by earthworms, millipedes, centipedes, and woodlice. Bacteria and fungi then process the organic residues with the help of enzymes, making them available to plant roots. Nutrients (nitrogen and phosphorus) can be stored in the bacteria and fungi. Many nematodes feed on bacteria and fungi that have stored nitrogen (and phosphorus) in their structure. Nematodes do not need this nitrogen and excrete it back into the soil, making it available to plants. In this way, nematodes contribute to the slow release of stored nutrients.
• Nematodes, like earthworms, improve soil structure. They wriggle through the soil, creating small tunnels that aerate the soil.
• Predatory nematodes are insect-parasitic nematodes that feed on harmful organisms in the soil. There are different groups of naturally occurring predatory nematodes. These parasitize bacteria, fungi, and other nematodes. The commercialized predatory nematodes also naturally occur in the soil. Examples include Steinernema feltiae (Stenema or Ferti-care), Steinernema carpocapsae (Carpo-care), Heterorhabditis bacteriophora (B-green), and Phasmarhabditis hermaphrodita (NO-slug).
• There are also plant-parasitic nematodes that feed on plant roots. These can sometimes cause problems in commercial cultivation, but not in private gardens. It is often a result of monoculture.

Can nematodes withstand fertilizer?

In general, nematodes thrive best in soil rich in organic material. Worm castings, compost, or composted manure (not fresh manure) belong to that category. The breakdown of organic material boosts soil life. More bacteria and fungi mean more food for the nematodes.

Chemical and organic fertilizers (granular or liquid) containing chemically bound nitrogen (urea or ammonium) are harmful to nematodes. High salt and nitrogen levels must be avoided. Fertilizers containing only phosphorus or potassium have less negative effects on nematodes. However, phosphorus is present in high amounts in most gardens, so this fertilizer does not need to be added. Soil life can make stored nutrients in the soil available to plant roots.

Adding external fertilizers changes soil biology, which negatively affects soil life, including the nematode population. Conversely, adding organic material boosts soil life, which also enhances the nematode population.

How do nematodes come to life?

Like many organisms, nematodes go through different stages from egg to adult nematode. After the egg stage, there are four larval stages. When you buy nematodes, they are in the third larval stage. So, you are buying a living product. In this stage, they can temporarily survive without a host but will actively seek prey to parasitize as soon as they enter the soil.

After the fourth larval stage, adult nematodes emerge. There are both male and female nematodes. They mate so that the female can lay eggs again. In the case of insect-eating nematodes, mating occurs inside the host (slug, beetle larva, fungus gnat larva) where they live. Free-living and plant-eating nematodes mate in the soil.

Can nematodes swim?

Insect-eating nematodes cannot swim like fish but move through the moisture around soil particles. They do this by bending and stretching like a snake. As they move, insect-parasitic nematodes search for prey to invade. They find their prey through chemical signals, called kairomones, emitted by insects. This is similar to pheromones in moths and butterflies.

Purchased nematodes are first dissolved in water and then used to spray the soil. Nematodes can survive in the water solution for a maximum of a few hours. Therefore, they should be distributed over the soil as quickly as possible in the case of soil treatment and over plant leaves in the case of leaf treatment.

Can nematodes be harmful to humans?

Purchased nematodes are harmless to humans, animals, and the environment.

How often should nematodes be applied?

Nematodes can be used to kill various harmful insects such as slugs, grubs, fungus gnats, vine weevil larvae, and leatherjackets.

How often you need to apply nematodes depends on the number of harmful insects in the soil and the life cycle of those insects. It varies depending on the insect you want to control.

Fungus gnat larvae can usually be controlled with one treatment, unless you have a large number of fungus gnats, in which case you may need a second treatment.

Slugs are controlled once or twice a year. May and autumn are the best periods because the soil temperature is ideal then.

Grubs are controlled once a year, but this control should be continued for two to four consecutive years.

Vine weevil larvae are ideally controlled in March and September. Leatherjackets are best controlled from September to mid-October.

Which month should nematodes be used?

• First of all, the prey you want to control must be present. Using Kraunema when vine weevil larvae are not present in the soil is pointless.
• B-green nematodes against grubs work best on young beetle larvae, so it's important to use the nematodes when the grubs have just hatched. Generally, this is for all grub species between mid-August and the end of September. For more specific information on this, click here.
• In the case of a leaf treatment, you should apply nematodes when young larvae (Colorado potato beetle) or caterpillars are present. This obviously varies from species to species.
• It is important that the soil is moist and the soil temperature is ideal. Even though slugs, for example, are present in the soil all year round, you should not apply nematodes during heatwaves, extremely dry periods, or in winter.

How long do nematodes survive in the soil?

How long nematodes survive in the soil depends on your soil. In general, nematodes can survive in open soil for a few weeks to a few months. For the best results, ensure ideal soil conditions.

1. Nematodes need moist soil. They die off more quickly under dry conditions.
2. The soil temperature should ideally be between 15°C and 30°C. Too high or too low soil temperatures are counterproductive. Check the ideal soil temperature for each commercialized nematode.
3. There must be food available for predatory nematodes. Without harmful insects, bacteria, and fungi, they cannot feed and reproduce.
4. A soil with good structure and sufficient organic material provides better survival chances for nematodes. Such soils contain many bacteria and fungi that contribute to their survival.
5. Nematodes are in turn eaten by protozoa (single-celled organisms and other nematodes), which can affect their survival.

Are there nematodes in potting soil?

Nematodes can indeed be present in potting soil. These are mainly free-living nematodes that naturally occur in potting soil. They feed on bacteria, fungi, and algae present in the potting soil. They are harmless to the potting soil and the plants.