Natural enemies of slugs & snails | The Good Slug Guide

Natural enemies of slugs & snails

The idea that the predators or diseases of pests can be used in their control has been around for a very long time, and today about 300 are available commercially around the world. There is, though, just one biocontrol of slugs, a microscopic nematode worm with the awkward moniker Phasmarhabditis hermaphrodita. This common inhabitant of soils lives as a parasite in a wide range of both slugs and snails, living and multiplying inside its hosts, but death is brought about by bacteria associated with the nematode rather than the nematode itself. Both are now cultured together in an industrial plant that looks very much like a brewery, and the nematode’s tough larval stage (called the  ‘dauer’  larva, from the German for durable) and the bacteria are harvested together, packaged and stored for distribution. The most popular formulation, Nemaslug, is very expensive but only partly effective, for some molluscs can encapsulate and neutralize invading nematodes. For example, the grey field slug is susceptible, but the green cellar slug is both susceptible when immature and resistant as an adult. The Spanish slug doesn’t bother to fight them off because it is tolerant of high nematode loads.

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Garden tip:

Nematodes thrive in damp conditions. Nemaslug is best applied in the evening using a watering can, and if it looks like rain so much the better. A new formulation, Nemaknights can be applied dry direct from packet to soil, but it still has to be watered in.

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As for natural controls, we are always advised to ‘encourage hedgehogs, frogs and toads’ in the garden, and sometimes that list is extended to include birds, beetles and even the slow-worm, but it is asking us to take a huge leap of faith.

Although we can see the evidence of damaged shells left behind by song thrushes, for example, slug predation leaves no direct evidence, so how can we know if slug predation is even a thing in the garden? Here, we owe a debt of gratitude to a single organic gardener in Yorkshire, Jillian MacDonald, who has amassed convincing indirect evidence for significant levels of mollusc predation in gardens.

The UK’s native terrestrial flatworms, which are not flat but resemble tiny black or brown slugs without feelers, rank among our least-studied creatures. However, Jill noticed some in her garden in the tiny village of Kirby Wiske near Thirsk in Yorkshire. She was curious enough to have their identity confirmed by flatworm expert Hugh Jones, and she went on to study them for four years. Jill found thousands – many, many more than ever found anywhere else – and even discovered a species new to science.

Importantly, Jill often saw these little creatures feeding, which meant she could collect information on their diet. Microplana terrestris had a relatively broad diet, of which molluscs formed the major part. On the occasions when M. scharffi was seen at food, 88% were on earthworms and the remainder on slugs. A very few M. kwiskea (the new species, named after Jillian’s village) were seen feeding, and these observations suggested that it prefers earthworms. All very exciting, but no matter how Jill tried she could not induce M. terrestris or M. scharffi to attack live earthworms, slugs or snails in captivity; damaged prey had to be provided. It seems, therefore, that our native flatworms are scavengers rather than predators. Something attacks the molluscs in Jill’s garden, thereby turning them into flatworm food, but we still do not know what that something is. Jill extended her study by looking for native flatworms in friends’ gardens. She found them everywhere, and suspects now that these creatures are very common indeed.

In the garden: Whodunit?

It’s long been known that different predators attack snails in different ways and thefore the way in which a shell has been broken often provides forensic evidence about the fate of its occupant.

In 2018, I made a point of collecting all the empty snail shells I found while gardening. Of 91 empty shells of the common garden snail, just eight showed no external signs of predation and two of those contained pupal cases of a humpbacked fly. All this suggests that relatively few snails die of natural causes, and that predators (and to a lesser extent, parasitoids) exert a useful degree of control – in my garden at least. The same might be happening in your garden.

• Broken shells with long, relatively straight fracture lines are the result of song thrushes bashing them on anvils.
• Short and irregular fracture lines are typical of toothy attacks by small mammals.
• Neat circular holes are typical of drilid attacks (see page 55).
• Other types of damage are more mysterious. We  don’t know what causes large irregular holes in some shells, for example, or tiny round holes in others.
• Although the remains of insect pupae can sometimes be found in undamaged shells, if you break them open, they don’t tell us for sure what caused the snail’s death. For that, live snails have to be collected and kept in order to see what emerges over time.

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Why not do your own experiment and collect damaged snail shells in your garden and see if you can identify the predators? (By the way it’s best to wear disposable gloves when handling snail shells). Make a note of your findings and dates, and  over  time you can build up a picture of your garden and its inhabitants.