Pearl-bordered fritillaries in Cumbria

PEARL-BORDERED
FRITILLARIES
IN CUMBRIA

The Pearl-bordered and Small Pearl-bordered Fritillaries have contrasting fortunes in Cumbria, despite sharing the same foodplants of Common Dog Violet and Marsh Violet, plants that are found in almost every tetrad within the county.
The Pearl-bordered is the first Fritillary to appear in the year and is usually on the wing in Cumbria from about the end of the second week of May. The Small Pearl-bordered emerges about a fortnight later and its flight period covers the whole of June. There is therefore an overlap between the two species that can make identification difficult.
The Pearl-bordered has declined both locally and nationally; it is now almost entirely restricted to the limestone areas at the head of Morecambe Bay, for example Arnside Knott and the Whitbarrow area. There are probably now no more than 15 or so colonies in Cumbria but that comprises around 10% of all the colonies remaining in the UK.
The Small Pearl-bordered is widespread throughout the county with the number of colonies probably numbering over 200. As well as grassland sites, where it can be found alongside the Pearl-bordered, it is more likely to be seen on its own as it can make use of much wetter areas (where Marsh Violets are probably the main foodplant), for example along the wet edges of woodland and in marsh areas besides streams and rivers where trees don't grow.
The Small Pearl-bordered does look smaller (just!) but has a generally duskier appearance. On the upperside of the wings the orange is deeper (when fresh) and the black more pronounced, the black being noticeably dense around the outside edges of the wings (see right photo above). The underside of the wings (see left) seem more contrasty as there are several silver areas on the forewing whereas the Pearl-borded looks more overall yellowish and has only two silver patches (one large and one small).
Both species overwinter as the partially developed caterpillar. As the Pearl-bordered needs to grow fast in the spring for its earlier pupation and emergence in May, it needs a very warm micro-climate. This is provided by dead bracken and dead leaves which absorb the warmth of the spring sun. However, the layer must not become too thick otherwise violets will not grow. The species also rapidly declines if tree cover builds up and light is excluded. Limestone pavements provide ideal conditions for both the Common Dog Violet and the butterfly, as pockets of dead material are trapped between the rocks (see photo right).
The Small Pearl-bordered has less demanding requirements and longer to develop. There are plenty of open, but sheltered, marshy areas in Cumbria receiving good sunlight and which also have light coverings of dead material.
Curiously, the neighbouring county of Durham has only four remaining colonies of Small Pearl-bordered Fritillary.