Friday, 19 April 2013

Slugging it out - using nematodes as pest control

As the soil warms up and new spring growth appears, so the slugs and snails emerge again, looking for soft new leaves to devour. Sometimes gardening feels like a constant battle against the march of the munching molluscs. 

As we're getting to plant our potatoes ready for the Secret Garden Club workshop on heritage potatoes this Sunday, we've been looking for effective methods of slug control - they can cause major damage to growing potato tubers, especially in damp conditions.

There is no one-size-fits-all method to controlling slug and snail numbers. Certainly encouraging slug predators to visit, if not live in, your garden is a good start. Frogs, hedgehogs, beetles, and many birds including ducks, thrushes, robins and starlings, all consider slugs and snails to be a good meal.

Methods like traps baited with beer can work well in small areas like a patio or courtyard garden. For a bigger area or an allotment you could not hope to trap all your slugs and snails with a few beer traps.

Deterring them with coffee grounds or crushed eggshells can work for single plants - although I bet a slug would crawl over broken glass to get to a juicy hosta - and for anything bigger than that you would need a very large number of eggs or coffee grounds.

Pellets can be effective but the blue ones, containing metaldehyde, are also toxic to birds, frogs and hedgehogs, not to mention small children and pets - if they inadvertently ingest slug pellets, or a slug in the throes of pellet poisoning then they too be harmed, possibly fatally in the case of smaller animals. The white pellets, which are marked as being safe for wildlife, contain ferric phosphate. In practice, I've found these much less effective at killing the slugs and snails. 

Until now the Secret Garden has relied on deterrence, using copper rings around individual plants. Copper rings work as a barrier - slugs and snails won't cross copper as the metal reacts with the mucus coating their bodies. However, it's only a deterrent. It won't reduce their numbers, so while they won't munch your copper-protected brassicas, they will slink off elsewhere and eat something else instead. And the Secret Garden is very sluggy, and the wet conditions last summer were ideal for slugs and snails, making it likely that the population will rocket again this year with a bumper crop of eggs surviving the winter deep in the soil, now ready to hatch.

So the Secret Garden Club is going to war. We're fighting back with nematodes. Phasmarhabditis hermaphrodita, to be precise. And we have 24 million of them.

Nematodes are microscopic organisms, barely visible to the naked eye. Our particular nematode, P. hermaphrodita, is a slug killer. It seeks out slugs, penetrates their bodies, and poisons them. It will then reproduce, generating hundreds more nematodes hungry for more slugs.

These nematodes will also kill snails but because they live in soil they tend to encounter far fewer of them, since snails tend to live on the surface or climb walls and fences.

They have no effect on any other creature, so are safe to use around pets, children, and other animals.

1. What you need - a water supply, clean watering can and a packet of slug-killing nematodes. 2. Each 8-litre watering can will take
a quarter of a pack of nematodes. Add the nematodes to the can first.
3. Fill the can with cold water and mix well. It will look like a murky soup.
Water the nematodes into your soil evenly. An 8-litre watering can will cover
around 10m2.

4. Now refill the watering can, this time with clean cold water. 5. Cover the same area with plain water so that the nematodes are
thoroughly soaked into the soil.
One application of nematodes will last around six weeks, then the operation can be repeated. We'll be monitoring our population of slugs and snails and will report back on progress.

We bought our slug killing nematodes from Gardening Naturally. They're also available from Crocus, Unwins, and the Organic Gardening Catalogue.


  1. I used nematodes on my plot for the first time a few weeks ago. Will let you know how I get on!

  2. Please do - that means you're a couple of weeks or so ahead of us, so any news you have from the Slug War front will be eagerly received.

  3. Since we applied the slug killer, the weather has been dry, so it's difficult to judge how effective the nematodes have been. But some early morning rain gave us ideally sluggy conditions and sure enough I crunched across the lawn inadvertently squishing snails at every footstep. Yet I only saw two (tiny) slugs. Hardly a scientific study, but encouraging all the same.

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