Friday 27 June 2014

Fighting the slug and snail takeover

I don't know about your garden but the molluscs are taking over in mine. We had a wet winter in which it never really got properly cold and so hibernating slugs, snails and their eggs didn't get killed off in their usual numbers. Now after a reasonably fine spring, we had a warm wet start to the summer. Lots of lush green leafy growth means lots for slugs and snails to sink their teeth (oh, yes, teeth)  into. One single slug or snail can reduce a courgette or lettuce seedling to a bare stalk in one session: in addition to a rough tongue called a radula, they also have thousands of tiny denticles, or tooth-like protusions. 

I must have tried every single slug/snail barrier or killing method known to mankind over the years. The problem is that there is no one ideal solution for all situations. Not everyone likes the idea of killing the pests in large numbers. One of the easiest ways to get rid of slugs and snails is to scatter slug pellets around your precious plants, yet it's well-documented that slug pellets will harm other wildlife as well. I only use them when the slug numbers are overwhelming and only under netting or in the greenhouse to try to minimise the danger of anything else ingesting them. I wish I didn't use them at all and I dislike dealing with the messy slimy death the pellets cause.

You don't, however, have to kill your slugs and snails: there are various ways to deter them, whether through physical barriers, or what I think of as diversionary tactics. I've found that repelling them without killing them tends to work well in years when the slugs/snails are not around in huge numbers; in very sluggy years like this one, they will just slope off to eat something else. I don't, for example, usually bother with slug protection for my chilli plants: slugs and snails don't tend to find them very appetising. This year, I've lost several chilli seedlings to slugs and snails: there are so many more of the pests and if they can't get at the salad leaves, or the young courgettes, they'll chomp down on whatever they do find. It's tempting just to pick them up and throw them over the fence, but both slugs and snails apparently have a homing instinct and they will just come back, even if somewhat slowly.
Caught in the act: young courgette and squash seedlings are a
favourite target, also salad leaves, brassicas, and my wasabi plants,
which I have to keep almost hermetically sealed away from
both slugs and snails.

One of the most attractive options (to me anyway) is to biologically control them with nematodes: near-microscopic organisms which live in the soil, and will eat into slugs and kill it. The nematodes do not interact like this with anything else, so are harmless to other wildlife, and the slugs are eliminated (good), yet the killing happens remotely and invisibly to me (even better). But even this is no cure-all: it's expensive, it needs to be done every six weeks or so throughout summer, and is really only effective against slugs, because the nematodes work in the soil, and snails tend to live above ground. 

The best solution of all, if available to you, is to create a slug/snail-hostile environment as naturally as possible. A pond with frogs, a resident hedgehog family and a population of thrushes and other slug-eating birds will help to keep numbers to a minimum. And one thing I've learned recently is that I shouldn't really be aiming to eliminate the molluscs altogether: only some of the 30 different slug species that live in the UK eat your tender vegetables, they and the rest will also feed copiously on decaying vegetable matter and so are an important part of your overall garden ecosystem.
In a dry spell, the slugs and snails seem to disappear. But if you overturn
pots, peel back black plastic or look underneath decaying wood, the
chances are you'll find them hiding in the dark and damp.

The trick, I think, is to use methods in combination. I've resorted to using copper tape to protect pots and individual plants such as brassicas and squash on the allotment, plus a good watering with anti-slug nematodes when the conditions are right and I can afford it, plus using organic pellets in very specific closed environments where there is no danger of other wildlife ingesting them, plus morning and evening inspections around the garden and allotment when the weather is damp and after rain (when I will despatch the slugs as quickly as possible by snipping them in half with scissors). It's time-consuming, but slightly less stressful than losing all your squash or salad seedlings to a fat, but still hungry slug.

I've collated the methods I've used (some more successfully than others) into the table below. I'd love to know about any other effective ways to remove, deter or eliminate slugs and snails and make this as comprehensive as possible.

The Method
Kills slugs and snails
Harmful to other wildlife
Easy to apply
Requires reapplication
Messy corpses

This is what happens when a slug ingests a slug pellet: the methiocarb
in the pellet causes the slug to over-produce mucus and it dies of dehydration, 
leaving you with a nasty slimy mess. Any predator that tries to
eat the dying slug may well be poisoned in turn by the methiocarb
in its system.

Beer traps
Kill slugs and snails
Messy corpses to dispose of
They die happy
Ineffective against large numbers
Requires maintenance
Waste of good beer
Other traps, eg, grapefruit halves
Does not kill, pests congregate in grapefruit halves
Live slugs and snails to dispose of

Copper tape
Does not kill; the pests will go elsewhere
Environmentally non-invasive
Relatively expensive
Risk of trapping pest INSIDE the pot
Taped pots can be reused

A simple copper collar made out of a section of plastic water bottle
lined with sticky copper tape

Coffee grounds
Does not kill; the pests will go elsewhere
A LOT of coffee grounds needed for more than one or two plants; you will need to befriend local Starbucks or similar
Environmentally non-invasive – grounds good as mulch
Ineffective against large numbers
Sharp grit, etc
Does not kill; the pests will go elsewhere
Do you really want your beds full of grit?
Risk of trapping pests inside the barrier

Torchlight patrols
Now you have to kill them
Environmentally non-invasive
Disposal of corpses
Effectiveness depends on your vigilance
Morale-draining in face of large-scale invasion

Encouraging or introducing predators: birds, frogs, hedgehogs, geese, etc
Easier to achieve in a rural environment
Potentially highly effective
Requires maintenance at least initially
Potentially expensive outlay (digging out a pond, acquiring geese)

Introducing predators: nematodes
Not nearly as effective against snails, only slugs
Requires reapplication

There are around 30 different types of slugs that live in the UK,
only around 4-5 of which will out of choice eat your vegetables.
These are two of the bad guys: the imaginatively-named
black slug, top, and the red slug, above.

Further reading - US-based article on natural methods of curbing or eliminating slugs and snails. - some more ideas for getting rid of slugs and snails and a photo which almost makes them look pretty. - how to buy and use nematodes as a biological control. - how to make your own nematode soup (warning: the method is somewhat icky) - know your slugs: an identification table to UK species.

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