Sympathy for the Devil's Butterfly?

The report (http://www.telegraph.co.uk/earth/main.jhtml?xml=/earth/2008/03/29/eabfly129.xml) from Butterfly Conservation that the UK population of Small tortoiseshell (Aglais urticae), formerly known in Scotland as 'The Devil's Butterfly', has declined by around 80% is a concern. The agency point one finger at an 'alien parasitic' fly Sturmia bella that utilises the butterfly's eggs that are laid in large batches on nettles. This fly is normally found on the continent but was recorded in the UK for the first time in 1998 (and, you guessed it, its appearance has been linked to our recent milder winters). As far as I am aware, the parasite is currently restricted to Southern England so it seems likely that the current shortage of the butterfly are at least partially down to our cold, excessively wet, summers (the adult butterfly normally spends a lot of time basking to raise its body temperature). It would be interesting to do a systematic survey of the nettle food plant for eggs and larvae throughout the UK and to relate that to the parasite's distribution.