A small tortoiseshell in the garden yesterday caused me to reflect that this was once such a common butterfly. It was some years ago now but I can clearly remember counting fifteen on ice plants in our garden at one go, all jostling for space and a chance to get at the nectar.
Numbers have plummeted at an incredibly fast rate since but this year (2022) seems to have seen something of a recovery. I am sure I have seen many more than in the last few years and I look forward to seeing the Butterfly Conservation statistics from transect surveys 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 as small tortoiseshell number fall so there are less caterpillars for the be Sturmia bella to paratise and so their population reduces thus allowed more small tortoiseshell to flourish? I guess the research being done will tell us one day, hopefully soon.
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