The descriptive text, below the map, is from the Cornish Red Data Book (2009). The map on this web page depicts the organisms distribution and shows the records made pre-2000 and those made since.
Range & Status
This is a rare species and is at the north-western extreme limit of its distribution in England. Its main strongholds are in the New Forest and Pembrokeshire. The nearest population to Cornwall is a small colony on north-east Dartmoor in Devon.
Records from the early 1900s suggest the species had a stable population in West Penwith. The last confirmed Cornish record came from a ' Menyanthes trifoliata bog' at Tregorvans, St. Buryan on 1 July 1957 when J. Cowley collected 10 male and 12 female specimens.
Habitat & Ecology
This is predominantly a species of slow flowing, base-rich streams on acid heathland. This habitat has the attribute of maintaining a constant water temperature which at source seldom drops below 10\'b0C which is key to larval survival in winter.
The main threat to the species is the removal of grazing animals required to maintain the open aspect around the stream required for successful breeding.
Since the last RDB in 1997, there have been no records in Cornwall, despite searching suitable habitat in West Penwith. Given the success of the small colony on Dartmoor, surveys of suitable similar sites on Bodmin Moor may prove productive. The species is subject to Biodiversity Action Plan work and retained its globally threatened status in the new BAP species listing 2007.
I.J. Bennallick, S. Board, C.N. French, P.A. Gainey, C. Neil, R. Parslow, A. Spalding and P.E. Tompsett. eds. 2009. Red Data Book for Cornwall and the Isles of Scilly. 2nd Edition.Croceago Press.
The Cornish Red Data Book Project was led by the Cornwall and Isles of Scilly Federation for Biological Recorders (CISFBR). The full text and species accounts (minus the maps) are available on the CISFBR website.