Who, What, Why: Are British slugs under threat?

The Spanish stealth slug can grow to 10cm

A "super breed" of slugs has arrived from Spain, but are they a threat to the UK's native slugs?

The wet weather has resulted in myriad problems for people around the country and now there is another - it has helped a "super breed" of slug from Spain flourish in the UK.

Numbers of the creature commonly known as the Spanish stealth slug, Arion flagellu, are growing fast in the UK - helped by the wet weather - and threatening native molluscs, say experts in the field.

Another Spanish slug, Arion vulgaris, has already become a huge problem in Norway, attacking farms and gardens. Researchers fear the same could eventually happen in the UK.

Why the Spanish stealth slug poses such a problem is its aggressive breeding cycle. It produces hundreds more eggs than native slugs and so numbers multiply quickly. It also brings with it new diseases and parasites which can kill British slugs.

"It is a real concern," says Dr Les Noble, a slug expert from Aberdeen University. "I have been collecting and analysing data on slugs for 30 years and it is clear to me that this incoming species is getting a foothold in the UK and spreading."

Their aggressive breeding cycle mean they lay around 400 eggs in a year, compared to the 100 or so laid by native species, says Dr Noble. Another problem is the diseases and parasites the slugs - which can grow to 10cm - carry.

"The Spanish stealth slug has evolved to deal with these diseases and parasites," says Dr Noble.

"Our slugs self-fertilise and are so highly inbred they can't deal with these new diseases and parasites and as a result several slugs and snails may become extinct. The result would be a drop in biodiversity. Several species being replaced by one is never good."

Vegetation types could also start to change as the slugs attack certain plants, which again puts the native ecosystem out of balance, he adds.

This year's mild winter and wet spring are said to have provided ideal breeding conditions for molluscs. The native slug and snail population is said to have trebled in some parts of the countryside, but it has also resulted in an increase in the Spanish stealth slug.

The increasing number of all slugs poses a real threat to agriculture, say experts. They also pose a threat to drivers as cars can skid on the slicks left where slugs have been run over and squashed.

"Slugs are also cannibals so when they smell dead slugs that have been crushed by cars they gather on the roads to feed on them," says Dr Noble. "This can leave slicks on roads which can cause accidents."

The most common way foreign slugs and other pests arrive in the UK is in imported produce, like fruit and vegetables, says slug expert Howard Drury.

"Fresh produce from countries such a Spain is packed and arrives in the UK within 36 hours," he says. "Pests which pose all sorts of threat can come in this way."

Figures for the number of native slugs vary, but estimates go up to 32 different types. While they are hated by gardeners they do have their uses like eating dead plants, say experts.