What are they?
With over 20,000 classified species, and a possible million unclassified, nematodes are the most numerous multicellular animals on earth. A handful of soil will contain thousands of these microscopic worms, many of them acting as parasites on insects, plants or animals.
Nematodes can be beneficial or harmful. Gardeners use them as a biological control on many plant pests, such as slugs, chafer grubs, vine weevil and sawfly. But some nematodes, such as the potato cyst, also cause huge damage to crops. See Further Reading, below.
How do they work?
If you apply nematodes to the pest - either in the soil or on the plant - the nematode ejects bacteria inside the pest's body. These bacteria multiply and cause blood poisoning, leading to death. The host tissue is then fed on by the nematodes, and as they feed they multiply, and immediately start searching for a new host.
For the organic grower, this is a biological - not chemical - pest control. It also has the advantage of being reasonably specific to the target pest, so that other wildlife is not affected. You can buy nematodes online. However, they aren't necessarily cheap and you do need to follow the precise instructions.
How to use nematodes
It is important to choose the correct nematode for the right type of pest. And to use them in the right conditions ie when the soil is above 5C (and will remain so) and when pests or their larvae are active. Nematodes are also light sensitive, so use them early morning or dusk, when light levels are low.
They arrive in a sachet suspended in a paste. It is best to use them straight away, however you can store them in the fridge for up to a few weeks (do not freeze). Mix the paste with a small amount of water to make a slurry, before adding the rest of the water. Stir the solution once again before applying. If you use a watering can, use a coarse rose, to make sure that the nematodes don’t get stuck in the holes.
Keep the soil moist and warm for at least a few weeks, to make sure that the nematodes remain active. You may have to make repeat applications - especially when treating slugs, for instance.
How Long Does It Take To See Results?
Usually 3-7 days, with maximum effect occurring over 2-4 weeks. Nematodes disintegrate the pests from the inside out, so you will not see dead insect bodies as you would with a chemical knockdown.
How Often Should Nematodes Be Applied?
Nematodes are recommended for use whenever larvae or grubs are present. Generally, this is during the spring and autumn. Because larvae feed on plant roots, beneath the soil surface, severe damage can be done before realizing there is a problem. Look for signs of an adult insect, such as leaf-notching. If adult insects are present, their eggs will be hatching soon. Again, you may have to make repeat applications.
Where can I get nematodes?
You can buy them online. We recommend the range from the Organic Gardening Catalogue.
The following is a list of pests, and the best nematodes to deal with them:
Ants Nematode Steinernema will ambush the ants when passing. As the bacteria poisons them, it will weaken the colony, eventually killing off the queen who has too few workers to feed her. It is best used between April to September by drenching the nests.
Chafer grubs If your lawn has yellow patches and looks like it has been dug up, chances are you have chafer grubs. The damage comes from the birds, foxes and badgers who dig for these tasty morsels. The grubs can be controlled by the nematode Heterorhabitis bacteriophora, one of the oldest known and best of the insect parasitic nematodes. It is best to deal with the young grubs in August and early September.
Fruit flies, carrot root fly, onion fly, gooseberry sawfly and codling moth. All these pests can be treated with a generic mix called Nemasys Natural Fruit and Veg Protection Pest Control. You can use it as a general treatment after planting out and when the soil has warmed up, or to target specific pests when you see them, such as gooseberry sawfly caterpillars. These (and other caterpillars) need to have direct contact with the spray while they are on the leaves.
Leather jackets Leatherjackets, the larvae of the crane fly or daddy longlegs, attack the roots of your grass lawn. They can be controlled by the nematodes Steinernema feltiae. The best time to put them to work is in the autumn, when the adult daddy-long-legs are laying and the soil is warm.
Slugs The grandly named Phasmarhabditis hermaphrodita is another weapon in the arsenal against slugs. The nematodes can modify the slug's behaviour so that it remains below the ground surface before death. After eating the cadaver, they produce another generation, which move off through the soil in search of new slug hosts.
Vine weevils Steinernema kraussei seeks out the Vine Weevil larvae. Unusually, this nematode is active at soil temperatures as low as 5C, which means that applications can start in March in many areas. This will give much greater control of larvae when they are present - either March to May, or from July to October.
Nematodes can also destroy a crop. The worst examples are the potato cyst. To prevent infestation, use a crop rotation of at least 3 years and buy resistant varieties where possible.
Here also is some research on the nematodes which devastate legume crops, particularly clover http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1088311/
And some interesting information on how the American fruit fly has evolved to fight back against the nematode. https://www.theguardian.com/science/2010/jul/08/bacteria-fruit-fly-nematode-parasite