Pests and diseases


Nematodes are used as a biological control to help manage garden pests such as leatherjackets, slugs and vine weevil – without using harmful toxic chemicals.

What are nematodes?

Nematodes are microscopic creatures that act as parasites on other insects. They release bacteria into the host’s body to kill them, then eat the host.

For the organic grower, this is a biological - not chemical - pest control. Unlike a chemical spray, which may drift off target, nematodes are specific to the host pest, so that other wildlife is not affected.

How to use nematodes

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.

They can be used on leatherjackets, ants, slugs, chafergrubs, fruit flies, carrot root fly, onion fly, gooseberry sawfly, codling moth and vine weevils.

Nematodes are really effective but can be expensive so it's important to choose the right nematode for the specific pest, and to use them in the right conditions.

You will find detailed instructions on the packet, but in general...

  1. The soil needs to be above 5C (and remain so, even at night).
  2. The pests or their larvae need to be active (generally, this is during spring and autumn).
  3. Nematodes are light-sensitive so should be applied when light levels are low in the very early morning or at dusk.

Where to buy nematodes

You can buy nematodes online. We recommend the range from the Organic Gardening Catalogue, where Garden Organic members get a 10% discount.

How long does it take to see results?

Nematodes usually work in three to seven days, with the maximum effect occurring over two to four 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?

One or two doses, when larvae and grubs are present, should be enough to treat the problem, but in cases of severe infestation space applications ten to 14 days apart to ensure complete coverage.

However, it depends what you are treating so always read the instructions. Slug nematodes for example only work once, for around six to eight weeks.

Common pests and the best nematodes to deal with them:

Leather jackets

These are the larvae of the crane fly or daddy longlegs, and they 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. It's best to apply nematodes for leatherjackets in the autumn, when leatherjackets are young, and the soil is still above 5 degrees.


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's 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


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, the nematodes produce another generation, which move off through the soil in search of new slug hosts. Slug nematodes are killed off in soil after six weeks by a predatory fungus, so repeated applications don't work, and unfortunately, they don't live long in the soil, which is why you need to be very time accurate in applying - just when shoots are emerging or potato tubers forming for example.

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 southern areas. This will give much greater control of larvae when they are present, which is either March to May, or from July to October.

Further instructions on using nematodes

Nematodes 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 it.
  • 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.
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