Snails and slugs were not included in the general pest control section, not because they are particularly difficult to control, but because of the wide range of strategies that have been adopted to control them. Also, their significance to the gardener appears to be greater than any other group of pests. The evidence for this is the amount of money that is spent on the baits generally used to control them. There are two sorts of baits, metaldehyde and methiocarb. Metaldehyde breaks down to water and carbon dioxide, and so is relatively safe to use. However, it is also the most common cause of pet and wildlife poisoning. Some animals seem to develop a taste for them, so it is best to place them under cover where slugs can get them, but not birds and dogs. In any event, they fall apart and decompose less rapidly when protected. (See Recipes for Alternative Pesticides for a superior recipe).
Methiocarb baits are not recommended on any account. The active ingredient was used until recently as a bird repellant on fruit, particularly cherries. However, approval for this use has been withdrawn, due to its toxic nature. Strangely, it is still available for vegetable gardeners to contaminate their vegetables. Methiocarb is also an insecticide, and so it poisons those insects that help to control slug and snail populations.
When using baits, due to their relative ineffectiveness (the control rate is a mere 10%), many gardeners apply more than the recommended rate. This is counterproductive, as the active ingredient is a repellant in concentration, and the kill rate decreases. In my errant past, when using methiocarb, I noticed that my slugs' trails showed they carefully avoided going within about 10 mm of any bait. A letter to the manufacturer brought the response of a free trial pack of methiocarb spray, even though the manufacturer knew that it was not approved for use on my crops!
Fortunately, there is a number of other strategies that will help to curb the predations of these slimy little creatures. One of the most effective is manual picking. The time to attack is dawn and dusk when conditions are moist and warm. Snails are readily crushed and are eagerly eaten by chooks if you are sensible enough to own them. Ducks are passionately fond of snails and in Indonesia, you can see duck-herds who hire themselves out to farmers for this purpose. Apart from their heavy feet, ducks do little damage in the garden, unlike chooks that scratch and eat the greens.
Slugs are not so easy to pick up. The best implement is a long hat pin to spike several for transfer to a container that traditionally holds salt. Some years ago, I was using this time honoured method during a particularly wet, slug infested summer, and very pleased I was with the result. There were noticeably fewer upon each visit to the garden. However, I used sugar instead of salt, as I wanted to add their corpses to the liquid manure tub, and I was reluctant to pollute it with salt. Some weeks later, I noticed that there were almost no slugs in my garden at all, and commented to a neighbour about this. He was most surprised, his slug problems being on the increase. I subsequently discovered that fermenting slugs for a few days in water and sprinkling the resultant mess about the garden, is a Biodynamic technique for slug control. With one exception, all gardeners I have passed this on to have commented very favourably on its efficacy. Capture rates can be dramatically increased by laying down boards or similar in the garden, and looking under these from time to time.
Some gardeners report excellent results from barriers placed around plants. Sharp sand, sawdust and lime being favourites. The idea is to dramatically increase the amount of mucus the creature has to secrete to reach its food supply. I believe that softwood sawdust is best, though many books state that hardwood is the stuff to use. Apparently this was an error in translation from a Swedish book, perpetuated by writers ever since. One elaborate contraption I saw was an electric fence made by laying down lengths of rigid PVC pipe and suspending an electrified wire 5 mm above the earth return, which was in contact with the pipe. Rather than using a fence energiser, the builder employed a 12 volt car battery.
Another approach in common use is the so-called beer trap. This consists of a shallow saucer of stale beer (don't waste the good stuff), set level with the ground. Apparently home brew is much preferred by the obviously epicurean slugs. They are said to crawl in and drown. A mixture of vegemite and water is also said to be effective, the attractant being the yeast. One disadvantage to this technique is that rain or irrigation rapidly dilutes the beer or substitute. One imagines the radius of effectiveness, and kill rate, would be about the same as one commercial bait, the attractant being bran.
Where large areas are concerned, a dilute solution of ammonia, copper sulphate or vinegar appears to be effective. It stimulates the production of excessive amounts of mucus, dehydrating them. It would seem wisest to use this approach on the morning of a day you know is likely to be clear and sunny, to enhance its effectiveness. Frequent use of these materials is probably unwise, particularly copper sulphate. Copper build up in the soil decimates earthworm populations.
No-dig gardens seem to have greater slug problems than those gardeners who clean-cultivated. Three passes of the rotary hoe are said to kill around 75% of the slug population. The no-dig gardeners' rejoinder is that cultivation also decimates the population of soil dwelling predators, carnivorous slugs, and centipedes. Centipedes appear to eat the eggs, rather than the slugs. Slugs prefer weedy, roughly cultivated soil with lots of places where they can avoid the sun's dehydrating effects, so creating a fine tilth and keeping weeds controlled helps minimise the problem.
It is worth bearing in mind that one grey field slug could have 90,000 grandchildren and 27,000,000 great grandchildren, so there is something keeping the population under control other than starvation. Once again, the general advice is to encourage as much ecological diversity as possible. The more ecological niches there are, the smaller the population occupying any one niche.
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© Jonathan Sturm 2003 - 2011