The Invasion of the Lusitanian Slug

 
Several slugs (Arion vulgaris) "assault" Roman snail (Helix
pomatia
). Picture: Monika Samland.

Two mild winters after each other demand their tribute. Some snail farms are suffering from large numbers of carabid beetles, such as the green beetle (Carabus auratus), hunting the snails cultivated there. In the south of Germany it is slugs invading the pens like locusts.

There is always a certain amount of slugs in the pens. The type of agriculture practised in the German Institute of Heliciculture in Nersingen is strictly biological and so no poisons may be used to exterminate slugs and other biological threats. Slugs found in the pens and on the pathways between them are picked up manually and done away with.

Snail Cultivation (Heliciculture).

Recently, however, Arion vulgaris, the Durham or Lusitanian slug, has mounted an offensive. Not only the large brown slugs are active very late in the year and can be found crawling around in December, but atypically they also become increasingly carnivorous. Usually Arion vulgaris is an omnivore, feeding on carrion and dogs' droppings, as likely as on weaker conspecifics. But only as an exception it actively hunts other snails.


Lusitanian slug first attacking a Roman snail from above in the
"neck" to then feed on it from inside. ( Picture on the right).
Picture: Monika Samland.
 

Slugs and the Variety of Food Sources.

In Nersingen in Southern Germany, slugs attack the cultivated Roman snails (Helix pomatia) in number and feed on it from inside. As Monika Samland, the proprietor of farm and institute, describes: The visceral sac is eaten first and the foot last.

Afterwards, the slugs lay their eggs inside the empty snail shells: 400 eggs one individual specimen of Arion vulgaris may lay in one batch! This is a part of the success formula allowing this species to spread over almost all of Europe and to largely displace the respective endemic Arion slug species, such as Arion rufus and Arion ater.

 
Pictures: Monika Samland.

In Nersingen the Lusitanian slugs will even follow their prey into the neighbouring pens, where the snails had been brought to uncertain safety.

There is no successful biocompatible remedy against those slugs as even Indian runner ducks will not be able to cope with such numbers of them.

Snails and Slugs Terror of the Gardener?.

As Monika Samland explains, the only way to get rid of the slugs is to collect them all with energetic workforce and to dispose of them in large buckets. Only, you never find all of them, and there are also the hidden batches of eggs waiting for the time to hatch.

It appears difficult to explain this behaviour of the slugs, taking into account that there was sufficient vegetable food available and even a bran mix fed additionally. It is also unclear, where such numbers of slugs come from, besides exceptionally many batches of eggs surviving the mild winters.

The prospects fort he next spring, when the new generation of young slugs will be hatched, are not exactly promising.

What is Arion lusitanicus?