Gardening: How do you fight slime-trailing pests?

Scientists are on the trail of more environmentally-friendly ways of killing slugs. Helen Lewis reports

An essential gardener's weapon is threatened with extinction. For years the dedicated gardener has scattered slug pellets in blissful ignorance, concerned only to protect plants against slime-trailing pests. However, a vocal chorus of "hazardous to wildlife" is causing the slug pellet to come under intense scrutiny.

It appears that the pellet's active ingredients - metaldehyde and methiocarb - are not broken down inside the slug or snail but remain active and harmful, not just to domestic pets but also to wildlife such as mice, hedgehogs and birds. The slug pellet is being accused of causing decline in Britain's song thrush populations, and may also have an impact on mammals and birds of prey further up the food chain.

At present there is a so-called wildlife-friendly slug pellet available, based on aluminium sulphate. However, it is easily washed away by rain. Not environmentally sound, and a big minus considering that slugs cause most devastation in wet conditions at night. Nevertheless, scientists are changing their tactics away from chemical pest control, and have come up with some intriguing solutions.

Allowing certain weeds to grow could protect agricultural crops and well- tended gardens alike from slug damage. Research at Kingston University has shown that slugs eat cultivated plants because there is generally nothing else on the menu; if the pests are offered a diversion in the form of a more appetising meal, they do less harm.

It was found in trials with 12 different weed species that slugs showed a preference for dandelions, with groundsel the most unappetising. Tests using wheat seedlings found damage was almost halved when dandelions were present, compared to wheat only.

Some plants, though, produce their own defence mechanisms against slugs. One research team found that many species of the carrot family triggered nervous activity in slugs, and that the poisonous weed hemlock contains a chemical that curtails their desire to eat. Tests are now being carried out on a synthetic form of that chemical extract, applied as a spray on various crops.

In the meantime, rather than showering the vegetable patch ad lib, try to catch a few slugs to test if enough are present to justify using pellets. The University of Newcastle, in conjunction with the pharmaceutical giant Rhone Poulene, has carried out experiments to find the best slug trap.

They tried ceramic tiles, plastic saucers, carpet, black plastic sheet, hardboard, dustbin lids, bricks, egg trays and linoleum. Easily obtained baits were used, such as wheat, potatoes, bran, layers' mash, cabbage, beer, and cat food.

The winning combination was a hardboard square baited with the hen food, layers' mash. This madean ideal way to determine whether enough pests were present to justify pellets being applied.

Perhaps the most revolutionary technique is to water the garden with a solution containing a tiny parasite. Discovered by Long Ashton Research Station, the nematode parasite preys exclusively on molluscs and poses no threat to other wildlife. It is killed at temperatures above 25C, therefore will not survive if accidentally eaten by a warm-blooded predator. One application lasts a season; the parasite dies out during cold weather.

For gardens, high-value horticultural crops, and organic farmers forbidden by Soil Association ethics to use chemical methods of control, this is an effective technique. However, it is too costly for general agricultural use.

Lastly, a tried and tested method of slug control that costs nothing: Place one boot, very heavily, straight down on to the pest and twist hard. Leave for the count of 10 seconds and remove.