Culprit

This extraordinary creature - which would look like an alien monster from a science fiction film, I think, if wasn't so small - is a sawfly. There are several black sawfly species, which can be difficult to differentiate from photos, but if you view this full-screen you should be able to make out a small orange protuberance at the end of her abdomen, just above her sawsheath, and according to the chaps at the Sawfly Recording Scheme this feature seems to be exclusive to Arge berberidis, the Berberis Sawfly. I generally take a "live and let live" approach to the insects proper gardeners think of as pests, but if I'd identified her on the spot the two of us might have had words, because my purple berberis was stripped so bare of leaves last summer by sawfly larvae that at one point I thought they'd killed it altogether. Luckily it survived, and is leafing up again now.... just in time to feed her offspring.

The Berberis Sawfly was first reported in the UK in 2002, but the affected Essex garden was already so infested that entomologists concluded it must have been there for a couple of seasons already. Since then it has spread rapidly northwards and westwards, has now colonised most of England, and seems to be moving into Wales as well. The secret to this success is probably that its favourite larval foodplants - berberis and mahonia - are very popular garden shrubs.