Writing -- Ireland's Wildlife: Slugs

The following article is one of a weekly series on Ireland's wildlife commissioned for the popular national general interest magazine, Ireland's Own. It appeared in the 21 November 2003 issue.

Slugs have been present in the British Isles since the end of the last ice age. They are members of the mollusc family and, along with snails, belong to a large group of molluscs known as gastropods. The name literally means "belly-footed", and refers to the broad tapered foot on which these creatures glide.

Essentially a slug is a land-snail without a shell, although many species retain shell remnants internally. Gastropods originated in the sea, and those that have moved onto land remain very susceptible to loss of moisture. Without a shell, slugs have no built in shelter to help them conserve water, so they tend to find a cool, damp retreat during the day, emerging only at night to feed.

Contrary to popular belief not all slugs feed on healthy young plants: in fact it's only relatively few species that give the group a bad name as garden pests. Some species are carnivores, actively hunting invertebrates, including other slugs, but most feed on dead or decaying plant matter. These slugs play an important role in a healthy ecosystem, helping to break down dead plant material making the nutrients available to other plants and to their many predators.

One of the worst culprits in terms of attacking garden plants is the grey field slug (Deroceras reticulatum), which is active above and below ground whenever the weather is wet enough, even on milder winter days.

These small grey slugs can decimate young seedlings. Recent research shows that they actively select leaves with a high nitrogen content, so excessive feeding of young plants will only encourage further slug attack. They have also been shown to exibit a "memory" effect, which will bring them back to the same patch night after night until an entire crop of seedlings is destroyed.

Some of the larger slugs, like the the European black slug (Arion ater), and the introduced Spanish slug (Arion lusitanicus) also feed on young plants. These slugs, however, roam over much greater distances than their smaller relatives, and are difficult to erradicate from the garden. Slugs feed by means of a toothed tongue known as a radula which they use to rasp food into the mouth. It's estimated that the European black slug can have up to 27,000 teeth on its radula.

One of the things that makes slugs such a nuisance is their sheer numbers. If conditions are favourable populations levels can get very high, and the damage they cause can then be a real problem. It has been estimated that an acre of farmland may support over 250,000 slugs.

Slugs are hermaphrodite, which means that each individual possesses both male and female reproductive organs. Copulation can therefore result in two sets of fertilised eggs rather than one, and some species lay large numbers.

Controlling slugs is an age old problem to which there is no quick solution. Use of chemical pesticides, while widespread in the past, is becoming less common as people realise the long-term damage that these can cause to the environment and to the health of a garden as a whole. Gardeners have come up with some pretty inventive solutions over the years, but the simplest are usually the best.

A night-time forage in the garden with a torch and a bucket of salt water is a particularly effective way of putting a dent in the local slug population, and encouraging their many predators into the garden will help enourmously. Hedgehogs, certain birds and frogs all have voracious appetites for slugs. Another alternative is to introduce your own predators in the shape of a few chickens or ducks. Planting slug-resistant plant varieties is another strategy, and rearing robust young seedlings indoors before transplanting to beds also helps.

The war against slugs is one that gardeners will never win, but by working with natural solutions, planting strategically and encouraging wildlife we can at least minimise the impact of the slug menace.