A butterfly species that was thought to be extinct in the Bath and Bristol areas has been discovered on the grounds of Wessex Water’s Tucking Mill treatment works.

Caterpillars of the rare Marsh Fritillary butterfly have been found in a flower-rich meadow near the works thanks to surveys by ecologists from the water company’s estates team.

Found mainly in open damp grassland, the Marsh Fritillary was once widespread but was spotted at fewer and fewer sites until it disappeared from the local landscape in 1993.In 1997 there was an attempted re-introduction scheme for the butterfly at former colony site Folly Farm, an Avon Wildlife Trust reserve near Chew Valley Lake. This sadly failed, and the last previous recorded sighting in the Bath and Bristol area was back in 2005 on Bathampton Down.

Patrick Hancock, assistant ecologist at Wessex Water, said:

"Marsh Fritillary is a favourite for enthusiasts for breeding in captivity, and recent records for the species have often been attributed to 'unofficial' releases into the wild.

"Numbers of Marsh Fritillary butterflies fluctuate from year to year for reasons that are not fully understood, so it will be interesting to see how many of these caterpillars survive the winter to become adult butterflies."

Marsh Fritillary caterpillars live communally in webs spun on or near their main foodplant, Devil's-bit Scabious.  A recent survey of the Tucking Mill site recorded 18 larval webs.