Slug population booms in damp, cool conditions

Published on Saturday 25 August 2007 01:13

WHILE the cool, wet summer has been a disappointment for most Scots, it has been a gift to the nation's slugs. The weather conditions have lead to a massive build-up in slug populations which are now likely to cause major headaches for Scotland's farmers and gardeners.

Slugs do not have an enviable reputation with the general public. Many find their physical appearance repulsive, and anybody who has had the misfortune to tread on one with bare feet will know how disgusting they feel. In general, they receive little attention except when they are causing trouble. With rain in July being 141 per cent of the norm, the slimy pests are likely to wreak havoc.

All animals need water to survive but whereas most, including humans, have a watertight skin or cuticle surrounding their bodies, slugs are entirely surrounded by living moist tissue, similar to the inside of our mouths.

This makes them particularly susceptible to desiccation during dry conditions, as water can evaporate from their entire body surface.

To stop this happening during dry periods, slugs become inactive and rest in the soil or under stones. In typical years the ground becomes progressively drier throughout the summer, limiting slugs' ability to feed and breed. Slugs can survive without feeding for several weeks so it is only in exceptionally dry years that slugs will be killed by drought. However, if conditions are moist throughout the summer the capacity of slugs to feed and reproduce remains unchecked.

Their propensity for population explosions under favourable conditions is helped by the way they have sex.

Slugs are unusual in that they are hermaphrodites, each animal having both male and female reproductive organs. However, it still takes two to tango. When they mate, slugs mutually exchange sperm, then shortly after both slugs lay fertilised eggs.

Biologically speaking, this is one of the most efficient reproductive systems there is. In most sexually reproducing animals, sexual reproduction brings the benefits of increased genetic diversity and vigour to offspring, but the system is inefficient because males use the same food resources as females but do not in themselves produce offspring.

Conversely, hermaphrodite species that can self-fertilise tend to suffer from harmful genetic diseases (for the same reasons that humans should not marry close relatives).

Slugs have developed a system representing the best of both worlds: all animals reproduce and genetic diversity is maintained.

Most people realise that slugs are a serious pest in the garden, but few know how devastating they can be in agriculture.

Dr Andy Evans, a slug expert at the Scottish Agricultural College in Edinburgh, said: "Scottish potato growers have seen an increase in slug problems within their fields this year. Even a small amount of slug damage can cost potato farmers a lot of money."

In late summer and autumn, many farmers sow wheat and oilseed rape crops - both favourites of slugs. "If the weather does not change we are set to see serious slug problems in both these crops," added Evans.

Ultimately, problems for farmers will become problems for consumers. "The impact of slug damage in Scotland could lead to higher prices for many vegetables including cabbage, swede and potatoes," warns Dr Evans.

There is only one sure method to control slugs, and that is to use lots of different methods.

• Hand collecting. Go out at dusk with a torch. This is an easy way to collect lots of the large black slugs that can do so much damage (assuming you are not too squeamish to pick them up). Slugs can be drowned overnight in a bucket of water.

• Beer traps. Plastic cups half full of beer sunk in the ground will catch lots of slugs. Make sure to raise the rim of the cup about half an inch above the ground surface. This stops beneficial insects such as predatory beetles falling in.

• Copper ring barriers. These can be placed around the base of individual susceptible plants. Slugs are repelled by copper and will not climb over these barriers. Copper rings are effective at protecting newly planted seedlings, but are expensive.

• Biological control. Tiny parasitic worms can be bought (under the trade name Nemaslug) from many internet suppliers. The worms are emptied from the pack into a watering can and then applied to the soil, where they infect and kill slugs. They are particularly good at killing the smaller slugs that are difficult to collect by hand. Many professional lettuce growers use these worms.

• Slug pellets. Garden centres typically carry several brands of slug pellet containing one of two poisons: metaldehyde or iron phosphate. Metaldehyde can be quite toxic to pets and wildlife. If you use metaldehyde slug pellets you MUST follow the instructions on the pack. Failure to do so not only puts wildlife and pets at risk, but is also against the law.

• Dr Michael Wilson is a lecturer in the School of Biological Sciences at the University of Aberdeen. He is working at The Scotsman as a Media Fellow, funded by the British Association for the Advancement of Science.