Several excellent photos of two Glanville Fritillaries – a species not expected to be found in Dorset – were sent in to us by Mark Pike on 30 May. This led to a confusing exchange of emails with Mark, as he saw them at “Compton Down”. Glanvilles are generally restricted to the Isle of Wight, where one of their key sites is Compton Down, but Mark confirmed that the photos are of ones seen on Dorset’s Compton Down, which is near Shaftesbury in north Dorset. He tells us that this is the same place he saw this species in 2019. Mark also tells us that three Glanvilles were reported at Whitesheet Hill in Wiltshire, which is only about twenty miles away as the crow flies.
The likely origin of the small numbers being seen over several years in this area is a regular release by somebody, who is hoping they will breed. Unfortunately, unless global warming has an effect, it is not very likely; our previous Records Officer, Bill Shreeves says:
The foodplant for the caterpillars is Plantain, which is common enough, but the requirements of this species are more complex than just providing the right food. Research on the Isle of Wight shows that the females will only lay on very young Plantains and these only survive in the large numbers required on the unstable cliffs which continually supply the bare ground needed. If none of this erosion occurs for a few years, a tall dense sward grows up, which is of no use. Superficially, the chalk quarry between Clubmens Down and Compton Down doesn’t look a bad choice, but there is not enough erosion or Plantains to sustain a colony, and the surrounding downs would need a lot of work to create the hot eroded hollows which are needed.
To know the species is breeding, we need to see eggs, caterpillars or chrysalises. The caterpillars are probably the easiest to spot, which would be in February and March, when (judging by the colony on the Isle of Wight) the black caterpillars huddle together in warm hollows, and singletons venture to crawl out to feed on young plantains before hurrying back to rebuild their body temperature.
We all know of the very successful re-introduction of the Large Blue into Somerset, but this was preceded by years of research to determine the exact habitat needed, and a lot of work to ensure it was created. Whether it is kind to keep releasing butterflies who are very unlikely to be able to successfully breed is something we will leave with the reader to form their own opinion.