The flowers on the red currants are just opening and I spotted this caterpillar in amongst them.
Resisting an initial urge to pick it off and squish it, I picked it off and tried to identify it. Conveniently (or it could be misleadingly) it adopted the classical “looper” pose, which pointed me towards the looper or geometer type moths.
My best guess is that it may be a magpie moth caterpillar – Abraxas grossulariata. That’s a beautiful moth and I don’t see them often, so I put the caterpillar back on the currant bush 🙂
A more troublesome caterpillar-like pest on currants and gooseberries are the larvae of the gooseberry sawfly (Nematus ribesii). These look very like caterpillars, but they have more of the chubby prolegs (the stumpy legs in the middle of the body) than butterfly or moth caterpillars do. They also tend to feed in groups and they are voracious: they can strip a bush of leaves more or less overnight.
UPDATE: 30th June 2013
The caterpillar has stayed on the same bush and I see it from time to time, getting plumper. It is eating the leaves a little, but only a little: it’s not a problem.
This picture clearly shows that it was indeed a geometer moth caterpillar: it just has the one pair of prolegs on segment 9, plus the claspers on its rear.
Happily, I don’t have any sawfly larvae to show, but if I did, they would (probably) have stubby prolegs all the way from segment 5 to segment 10. (I say probably, because there are lots of different sawflies and lots of different proleg configurations.)
Most caterpillars also have more pro-legs than this one, but most often they have four pairs, sometimes five, and they don’t usually have any on segment five.
If you are considering using chemical controls, it is important to know whether you’re dealing with “caterpillars” (the larvae of butterflies and moths) or sawfly larvae, as not all products work equally well on both. For example, Bacillus thuringiensis won’t work on sawfly larvae.