Gardening: Slugs ordered off for fowl play: Ducks and worms are the latest biological weapons which could win the war against slimy pests, says Anna Pavord

Slugs are like icebergs: you see only a tenth of the problem. The bulk of the population is lurking underground - 200 slugs to every square metre of soil. It is an outrageous number and the shortage of hedgehogs in full-time work means that more survive than is healthy for hostas and delphiniums.

Zeneca, a division of ICI, has just come up with a novel way of attacking slugs in the garden, by means of nematode worms and associated bacteria. You mix the worms with water, apply them with a watering can, and they swim about in the moisture between particles of soil until they find a slug. One dose lasts six weeks.

They attack through the open pore at the back of the slug's mantle - the hump just behind the head that works as a primitive kind of lung. The bacteria infect the slug and, when it is dead, the nematode swims on to find another victim.

The slug nematode is the latest in a series of biological pest controls which include treatments for aphids, caterpillars, red spider mite, vine weevil and whitefly. The conditions in which you use them have an important bearing on the success rate. The dampness of the soil is critical if the slug nematodes are to do their job. Without moisture, they cannot move around. Warmth also matters. They are most effective when the soil is around 15C (60F), get more sluggish as the temperature drops, and give up altogether if it is less than 5C (40F). Mine is 8C (46F) at the moment.

'The nematodes work best in open, friable soils,' advises Zeneca. 'Poor results may occur in very heavy soils.'

But slugs adore heavy, clay soils, which should improve their odds in the war against the nematodes. We shall see. I need a break and am willing to risk pounds 12.99 on a packet of nematodes in the hope that this might provide it.

There is a slug haven on the bank, where the things lurk submerged like baleful hippopotamuses watching what I am up to. In the usual hiatus between my meaning to go and fetch slug pellets to protect some newly planted lush bit of greenery, and my actually doing so, the signal goes out and the slugs charge into action. End of lush greenery. Why is 'slug' so commonly used as a synonym for teenage torpor? Slugs are staggeringly energetic. And hermaphroditic to boot, which means all the energy they do not dissipate on sex is available for eating.

Sally Fallon of Wimbledon, south- west London, has a different way of dealing with slugs in the garden: a small fleet of Khaki Campbell ducks which rollick about the lawn and dibble diligently in the borders. She says they make a brilliant job of de- slugging the entire place.

'What about the plants?' you may ask. Mrs Fallon attributes slightly better behaviour to the ducks than they show. Even as she was assuring me that the only things they ever attacked were the pinks, I was watching them making short work of all the new shoots on a juicy-looking phlox. An aberration, I hope.

The present trio have been with her only since September, but she and her husband have had fowl of various sorts - guinea-fowl, geese, bantams, ducks - in the garden for almost as long as they have been in the house. Ducks, she says, are the best: tractable, stay-at-home, reliable producers of eggs. She gets one each a day from her three.

The garden is quite large, 150ft long by about 50ft wide, with trains to Waterloo running along the bottom boundary. It has a brick wall on one side and a wooden fence on the other. Lawn takes up most of the space and the trees, of which there are plenty, are productive.

At the bottom is a big walnut, then a little orchard of apples and pears, plus loganberries, tayberries and apricots. A flower border runs down in front of the wall, turning into a vegetable border about two-thirds of the way down.

The ducks' base is a portable A- frame coop on the terrace. They range free during the day, but have to be shut up at night to keep them safe from the foxes that use the railway embankment. 'The birds are entirely a part of the garden,' says Mrs Fallon. 'They add so much to it. Even if our garden were half the size, I would still keep ducks in it. Our children loved having them, too.'

'What about the neighbours?' I asked, thinking of Corky the cockerel and the row over his dawn crowing. Mrs Fallon has been luckier with her neighbours than Corky's owner. The only trouble, she said, occurred with the guinea-fowl, which used to bounce over the walls into other people's gardens and peer in at their windows. When, finally, only one guinea-fowl was left, it got very stroppy and terrorised the neighbour's dog. The solution was a dinner party at which the Fallons and their neighbours feasted on a fricassee of the ill-tempered fowl.

Water is the next thing you think of. Surely ducks need a pond to splash about in and to keep their feathers in good nick? Apparently not. In Wimbledon they have an old stone sink, which they use enthusiastically, and a plastic bowl of fresh water every day.

The present little troop is not yet quite as well trained as Mrs Fallon's last lot, which used to follow her down the garden like formation dancers, gobbling up the slugs as she lifted each stone in turn along the edge of the border. Even so, she says, she has only once noticed slug damage in the garden, and that was at the top of the runner-bean poles. Those slugs, she thought, had sneaked up the boundary wall and climbed directly on to the poles without ever having had to run the gauntlet of the ducks.

As well as their pickings, the ducks get a daily feast of chicken layer pellets and are bedded on a mixture of sawdust, leaves and mown grass. Khaki Campbells are not quite as winsome as the white Aylesburys that you see in children's picture books; they are buffish brown, smaller-bodied than the white, but are better layers, says Mrs Fallon. The Aylesburys, once bred intensively around that town for the London market, were always more of a meat bird.

The geese were the least successful of the Fallons' birds. They came to help with the lawnmowing and were supposed to keep the grass looking neat, but it happened to be 1976, year of the big drought. The grass did not grow and the geese covered the lawn in evil-smelling, slimy droppings. But the ducks, said Mrs Fallon, as they waddled flat-footed over her crocus, are paragons.

Zeneca'a slug control, sold under the brand name Nature's Friend, will be test-marketed this year in 50 garden centres around the country; pounds 12.99 will buy you an empty box and a postcard which you send off to the suppliers. They get the nematodes from breeders in the Netherlands and post them to you, alive and kicking, within the week. The sachet contains about six million nematodes, sufficient to treat 20 square metres of ground.

Ducks can be bought through Exchange and Mart or through the livestock advertisements in regional papers such as the Western Gazette.

(Photographs omitted)