Britain’s snail invasion

Why the population explosion?
Normally a dry, hot summer kills off huge numbers of gastropods (the mollusc family that includes slugs and snails) and dries out their eggs. But two humid summers have provided ideal conditions not only for survival (snails live up to 12 years) but also for some species to fit in extra breeding cycles. Slugs can lay as many as 500 eggs a year, so each one could have hundreds of thousands of grandchildren. The Royal Horticultural Society has named slugs and snails as Britain's leading pest for the second year in a row, and a wet spring this year could cause another infestation. "As soon as temperatures start approaching 5C or more at night, we can expect quite a lot of damage," says Dr Andrew Salisbury from the Society's entomology department.

Why lump snails and slugs together?
Aside from the shell, the two creatures are very similar. There are 29 species of slug and snail in the UK but all share essentially the same biology and all are 'unsegmented gastropods'. (The word gastropod – 'stomach foot' in Greek – describes the way they move about on their bellies.) Their skins are moist and sensitive, they are most active in the rain and at night, and they are hermaphrodites with very exotic copulating techniques.

And do we know how big the population is?
In 2007, scientists at Bayer CropScience reported that between the spring of 2006 and July 2007, slug counts had risen from 35 slugs to 61 slugs per square metre, suggesting a UK total of 15bn slugs. There is no formal estimate of the snail population, which becomes dormant in late autumn and winter, but gardeners across the country have reported infestations. Richard Jones, gardening expert for the BBC's Gardener's World, has noted in a recent blog that he could "barely walk to the front gate without the familiar sound of snails crunching underfoot".

Are they really such a menace?
Only a few species eat healthy plants and have the potential to become pests. Most feed on dead and rotting vegetation, helping to decompose waste and recycle nutrients for the rest of the ecosystem. They are also important prey. Fragments of snail shell are often found around 'thrush anvils', where they have been bashed open by birds. Frogs, toads, slow worms and hedgehogs all feed on slugs. So do humans: 'heliculture' (snail farming) has been catching on, even in Britain. (But be careful about foraging: the snail best for eating – the Roman Snail, Helix pomatia – is protected in the UK, and some wild-caught snails can harbour a parasite that may cause a rare kind of meningitis.) Snails have also had more spiritual uses: traditionally they've been used in divination. Psychiatrist Carl Jung claimed the snail represented the self in dreams: the soft insides analogous to the subconscious, and the shell to the conscious.

Then why all the fuss about them?
Because some species eat healthy plants, and since slugs and snails can eat twice their body-weight daily, this poses a big problem for gardeners - particularly in spring, when they eat seedlings and attack vegetables and ornamental species, like hostas and tulips. More serious is their impact on farmers, who have to spend a lot more on pesticides as a result. Slugs (particularly the keel slug, which lives underground) have the potential to destroy entire fields of crops by eating them before they germinate. Some slug attacks this autumn have been so severe that farmers have had to re-sow their fields.

Do British farmers suffer most?
No, things are far worse in parts of Scandinavia where a plague of Arion lusitanicus, a species from Spain which can grow to 15cm (and is known as the 'killer slug' because of its unsavoury habit of eating weaker brethren), is such a pest, it has become a political issue. Swedish politicians have called for the unemployed to be mobilised as a 'fifth column' to exterm-inate the creatures, while Norway's government has offered 10,000 krone ($1,500) for the best method to eradicate them.

So how can snails and slugs be controlled?
Pellets are the most common chemical control, but they often contain metaldehyde, a substance toxic to game, wild birds and animals (including cats, dogs and hedgehogs) and harmful to fish. In recent months there have been reports that the chemical might be banned after traces were found in drinking water. Farmers have warned this could lead to much-reduced oilseed rape harvests, and a 30 per cent fall in wheat production. But less toxic chemicals are available, including anti-slug powders based on aluminium sulphate and pellets made from iron phosphate. Iron phosphate is especially promising as it is a naturally occurring compound - it is even used as a human nutrition supplement - that interferes with the gastropod digestive system, causing death within six days.

What of non-chemical methods?
Gardeners swear by various techniques, some more bloodthirsty than others. These range from going out at dusk with a sharp spike, spearing slugs as they emerge to feed and stamping on snails, to mowing your lawn at night, ensuring a damp massacre but annoying your neigh-bours. Traps and protective devices also work – eg jars of beer sunken into the earth (not quite flush with the ground, to avoid killing beetles), and plastic fences around the most vulnerable plants. Gardeners in Schleswig-Holstein, in Germany, rent slug-eating Indian runner ducks to clear their borders. Scientists at the University of the West of England have even built the SlugBot, a robot that picks up slugs and puts them in a fermentation chamber, where they become biogas and charge up its batteries. ·