Helix aspersa, commonly known as the garden snail, is served as escargot in French cuisine. The large numbers in the complex are beginning to pose a threat to crops in nearby farmland, they said.

The 3-centimeter-long snails were first spotted last October by Tetsuhisa Kanda, a resident of the complex and member of the Nature Conservation Society of Japan.

Katsuki Nakai, chief curator of the Lake Biwa Museum in Shiga Prefecture who examined the snails, said it is not yet known why the snails have been spreading in the complex, while pointing to the possibility that snails imported into Japan for consumption have escaped.

"They are resistant to dry conditions and can be active during winter, so it is feared they could spread further to other areas," Nakai said.

The conservation society will hold a lecture for local residents Sunday on how to differentiate between garden snails and domestic species, which can be difficult for ordinary people, and how to eradicate the alien species.