Posted by ChrisR on 08-03-2012 19:25
I just had a look at that link - it's the first time I have read it. I quote:
The primary reason for this decline is due to the expanding range of a tachinid parasitoid fly called Sturmia bella which was first recorded in Britain in 1999.
Crikey, I hardly know where to start :D Sadly, the page just repeats the same rumours that were put about by the media years ago when the story first broke. For a start Sturmia
was first recorded in the UK in 1998 (many years after Small Tortoishell populations started their decline) and was reared from a Peacock butterfly in Hampshire. If you look at Owen Lewis's research it shows that populations of Small Tortoishell have actually risen in some places where Sturmia
is found, while areas where Sturmia
has not been found (in the far north) have experienced terrible declines.
It is without doubt that any parasitoid will have *some* effect on their hosts but it is absolute fantasy to say that the decline in Small Tortoishell is primarily due to Sturmia bella
. Parasitoids that wipe out their hosts are very rare indeed! ;)
My own personal theory is that the Small Tortoishell has been in decline for quite some time (at least the early 1990s) and we can probably blame this on man's unsympathetic use of the countryside and to the effects of climate change, although it is going to be very hard to point the finger at anything specific. Sturmia
is quite common where it occurs but it is unlikely to a major factor in the decline because it occurs across the entire palearctic region and doesn't seem to cause much of a problem there.