The Large Blue Butterfly in Bath

Posted by Michael Williams - MCIEEM BSc (Hons) on 28/10/2014

The Large Blue butterfly reintroduction is one of the conservation success stories of recent times. Declared extinct in 1979, it has been successfully reintroduced to a number of sites in the south-west of England.

The long history of the butterfly in Great Britain begins very close to home for us. The exact location of the very first record is vague, but at least good enough to narrow it down to a handful of sites - in 1795, William Lewin recorded the first specimen from “hills near Bath”, and gave the species its English common name (incidentally, the same William Lewin was responsible for naming the Bath White butterfly, after a piece of needlework from the city, forever after misleading budding Lepidopterists that the butterfly could be found in the locality). With this rough location, narrowing down possible sites becomes easier when the ecology of the butterfly is taken into consideration.

The caterpillar feeds on the flowers of wild thyme, and until after it went extinct in Britain, this was thought to be the only thing it ate. However, once it reaches the fourth instar, the butterfly then assumes a hunched position and begins to produce chemicals that attract red ants. The caterpillar, now being mistaken for an ant grub, is taken back to the nest where it feeds on their eggs and larvae. Two ant species in particular are attracted to the caterpillar, Myrmica scabrinodis and M. sabuleti. For many years, M. sabuleti was thought to be a subspecies of M. scabrinodis, and the major difference between the two is a microscopic ridge on the scape of the antennae. Although known to occasionally survive in the nest of M. scabrinodis, it stands a much better chance in the nest of M. sabuleti. These two ant species have very specific habitat requirements – M. sabuleti requires grasssward height to be no higher than 2-3 centimetres. Any higher than this, and they are out-competed by M. scabrinodis. Added to this is a third ant species that is often important – the yellow meadow ant, which makes the large ant-hills in meadows. The structure of the ant-hill, which structurally more closely resembles a sand dune than the surrounding meadow, creates ideal conditions for wild thyme. M. sabuleti  is also known to partially, and sometimes completely, take over yellow meadow ant nests.

So in order to have a good guess at the location of the original British Large Blue site near Bath, we need to find somewhere suitable for the red ants, which require south-facing hill slopes with a short sward, and preferably an abundance of ant-hills with wild thyme growing on them. Both can still be found in the Bath landscape, sometimes in abundance. Many of the ant-hills at Solsbury Hill and Prior Park are over a hundred years old, and some are even older. However, in other parts of “hills near Bath”, agricultural improvement has led to the destruction of traditional meadows and grazing pasture. Going back to 1795, it was likely that the optimal habitat for Large Blues was much more abundant in the local area. However, as has been shown at Collard Hill near Street in Somerset and other reintroduction sites, habitats can be restored, and given time and appropriate management, the butterflies can spread to neighbouring habitats. Often habitats managed for Large Blues benefit other species, such as Pearl-bordered Fritillary, which feeds on violets (Myrmica ants are often tricked into depositing violet seeds in the highly nutritious waste from their nests).

My personal opinion (informed by much walking and cycling around the area seeking out the location!) is that Lewin found the first Large Blue within the vicinity of Lansdown, on the south-facing slopes behind the racecourse and around Kelston Roundhill, although I have found several other suitable candidates. With news of further reintroduction sites around the country, is it possible that the Large Blue will once again be found in the “hills near Bath”?