Small Tortoiseshell (Aglais urticae)
Numbers have plummeted at an incredibly fast rate since then but this year seems to have seen something of a recovery. I am sure I have seen many more than in the last couple of years and I look forward to seeing the Dorset Butterfly Conservation statistics from the transect walks to see if this proves the point.
The sudden decline is still something of a mystery and Oxford University Zoology department are investigating as to what the reason(s) might be. One theory is that it is linked to the arrival from the continent of a small parasitic fly, Sturmia bella, in the late 1990's. The fly lays its eggs on nettle leaves and the caterpillars consume them. This is now the most frequently recorded parasite of Small Tortoiseshell caterpillars killing 60% of them where present.
Why have they done better this year? I don't know but may be last winter's colder weather was enough to reduce the Sturmia bella numbers and allowed more Small Tortoiseshell to flourish? I guess the research being done will tell us one day, hopefully soon.