I found this in the lower paddock yesterday evening, and it had me very confused. It looks very much like a caterpillar doesn't it, and that's exactly what I thought it was (they were, there were two) at first.
I was pretty sure I'd never seen a caterpillar like this before though, and my field-guide didn't help, so I posted a photo on the WAB site.
Always very helpful, I was quickly told it was a Sawfly larva, NOT a caterpillar, although the WAB mamber who told me that couldn't tell me what species of sawfly it belonged to.
I have taken photos of mating sawflies in the garden, click HERE, but I'm not convinced it's this species I'm looking for.
I think my sawfly larva is a TURNIP SAWFLY LARVA (or Coleseed sawfly larva), Athalia rosae. You can identify it by its black colour (when it nears adulthood), with a grey/olive green underside. Mine were feeding on my Field Mustard plant, but I'm sure you'll see them on Oilseed Rape, should you care to look...
This was considered to be a massive pest a hundred years ago, especially for turnip and brassicae growers. It was thought to have been pretty well eradicated until the 1940s, but recently it seems they are once again on the increase, sometimes swarming and mating in vast numbers.
One doesn't need turnips in the garden to have turnip sawfly larvae. They are voracious feeders on Oil Seed Rape also, (getting to be a real problem) and if conditions are right, they'll turn up anywhere.
My larvae seemed distinctly keen on my Field Mustard Plant in the lower paddock!
NB. (Later). After a little research on the web about Turnip Sawfy (larvae) it seems after we eradicated them, it was in France that they reappeared first. They then flew over the channel and have now become a problem again in the southeast of England especially.
There you go.
Blame the French. As always...!