Rare Gall found on Tormentil at Stoer
On 22nd July 2021, we carried out our annual survey of the plant-life on five sites within the 1 km square which straddles the start of the Stoer Peat Track (NC0429). This is a local contribution to the National Plant Monitoring Scheme, which we have been doing since 2016. It requires close scrutiny of small, previously-defined, strips or squares of habitat, to estimate the current abundance of all species found.
Site 2 is a 10 x 10m square of grassland on an old shieling at the foot of a crag at NC047293 (photo 1), which we survey on our hands and knees. Some 40 species have so far been listed from this site. One of the more conspicuous is tormentil Potentilla erecta, with its four-petalled yellow flowers. However, a particular example of this familiar plant caught the eye, since it had a row of neat swellings along its prostrate stem, each about 2mm across (photo 2).
Closer inspection suggested that these might be galls, possibly ones caused by the gall wasp Xestophanes brevitarsis, which is specific to this host. Dr Margaret Redfern of the British Plant Gall Society was able to confirm our provisional identification from the photograph. Only 20 records of this species are mapped on NBN (National Biodiversity Network) for the whole of the British Isles. Just two of these are from Highland Scotland, neither in the north-west, so a good find.
Ian Evans and Gwen Richards