GARDENING / Of salt, real ale and nematodes: Michael Leapman on a new and natural way to kill slugs - by infesting the slimy foe with roundworm

NOW look, you are not going to enjoy this. You will have to read about nasty, slimy hermaphrodites with gross appetites being put to death by parasites that wriggle beneath their skin and perform unspeakable acts on their flesh. I shall weave tales of watery mucus, pus-filled swellings and putrid decay. Anyone of a sensitive nature should turn right away to a more wholesome page - food and drink, perhaps, or motoring or property.

End of health warning. We stouter-hearted gardeners know that slugs and snails are facts of life that you cannot just . . . well, slide over. They nibble away at young seedlings and root crops, especially potatoes, and they leave holes and slime on the leaves of lettuces and other green vegetables. They are, in short, an enemy to good cultivation; and to destroy your enemies you have to know them.

Until now, though, even knowing slugs and snails has not helped a lot. The best available remedy, poisoned pellets, kill only about half the molluscs in an average garden, and many people are wary of them because they could harm pets or birds. Other common deterrents smack of desperation - putting ashes or broken eggshells in their path, luring them to drown in real ale or just going out with a torch at night (when they are at their most active) and treading on them; make sure to leave your boots outside when you go back indoors. One recent book suggests a high-tech but messy execution: mowing the lawn at night, to emulate the effect of a kitchen blender.

The good news is that relief may soon be at hand, and environmentally sound relief at that. Scientists at Long Ashton Research Station, near Bristol, have isolated a nematode, or small roundworm, that is a natural, merciless enemy of the slug. Its larvae get under the skin of the beast's mantle - just behind its head, near the centre of its back - and breed there. This causes a swelling that finally bursts, spreading pus and nematodes all over the victim's body. The worms themselves carry on breeding, while the slug loses its appetite (wouldn't you?) and dies.

'We've had very good results, significantly better than chemical control,' says Dr David Glen, head of Long Ashton's Crop and Environmental Services Department, who has been involved with the research since it began in 1987. 'The nematodes could be on the market for gardeners in two years.'

There are some 30 varieties of slugs and snails in British gardens. Not all do damage, but most do. A rough rule is that they cause harm in inverse relation to their size. The great grey slug and the large black slug, for instance, can grow up to eight inches long, but though they may give you a nasty turn they will not usually eat your crops. On the other hand the common snail, the strawberry snail, the garden slug and the grey field slug, all about an inch long, can decimate a row of seedlings before you notice anything is amiss.

Pellets are the only way of poisoning them because they have a natural defence against sprays or anything applied externally. They are covered with a layer of watery mucus which repels liquid, so a spray would roll off into the soil and become harmless. 'Slugs are very good at producing additional mucus when they're irritated by something like a poison,' Dr Glen explains. 'Then they leave both the mucus and the chemical behind in the soil.'

There are two problems with pellets, even if you are not ethically opposed to using chemicals. The first is that they are a danger to pets and birds, though only a slight one. Pets would have to be undiscriminating eaters to swallow enough pellets to harm them. As for birds, slug pellets are all dyed blue because birds do not associate that colour with food.

But what if the birds eat slugs that have consumed poisoned pellets? 'When you calculate the amount of molluscicide that's likely to be in a slug,' says Dr Glen, 'a bird couldn't possibly eat enough of them to be affected.' (There is no evidence that the nematodes harm birds, or any creatures except molluscs).

The practical drawback of pellets is that they are applied to the surface, although at any time half the slugs in your garden will be underground, feeding on the roots of your favourite plants and vegetables. When they eventually come to the surface, sometimes

after several days, they often scorn the pellets: for a slug, a week-old pellet is as unappealing as a stale baguette. That is why, with pellets, you seldom get more than 50 per cent control.

But isn't getting rid of half the slug population better than nothing? Not much, for slugs breed prodigiously. As hermaphrodites, every one of them is capable of laying eggs - and they can produce up to 500 if the conditions are right. What they like is lots of rain: if they dry out they cannot produce the mucus that allows them to slide over the earth, so they become immobile and eventually starve.

In field trials, the nematode - full name Phasmarhabditis hermaphrodita - has proved much more effective than pellets in keeping slug damage down. One test involved simultaneous sowings of Chinese cabbage in three separate locations. In the first, where no control was used, virtually all the cabbages were damaged. In the second, treated with pellets, half the crop was lost. In the third, using the nematodes, there was a loss of only 15 per cent.

The man who did most to discover the worm's lethal qualities is Mike Wilson, an enthusiastic young Liverpudlian who began work on the project shortly after graduating from Bristol University. He showed me a small plastic bag, about 4in x 3in, roughly the size of those bags of toffees you buy at petrol stations. He assured me (and I took his word for it) that it contained 27 million tiny nematodes, bred by the Agricultural Genetics Company (AGC) at Littlehampton, Sussex, which will eventually be marketing them. One or two bags should be enough for a small garden.

The worms are packed in a damp clay which needs to be refrigerated, and even then has a shelf life of only about three months. To use it, you just add water and apply it to the soil with a spray or watering can.

Mr Wilson unpacked a bit of the clay, mixed it with water, put a speck on the tray of his microscope and invited me to peek. I saw little white squiggles squirming this way and that. Mr Wilson took a look. 'They're wriggling quite nicely,' he judged, 'although you can get them to wriggle faster than that.'

Then he produced a saucer of four slugs, in various stages of infection from the nematodes. I was not sure whether he planned to stage a demonstration attack for my benefit, like throwing Christians to the lions, but before he had time to suggest it I and my stomach decided I had seen enough.

In two years' time, when the worms are on the market, we shall know for sure whether they mean a miraculous new quality of life for persecuted gardeners. In the meantime, it's back to the broken eggshells.-