Sawfly larvae. They start in the centre of the bush so you don't notice them until they make their way to the edges, at which point they are quite large and can defoliate it quickly.

Thanks Don. I wonder if neem will help?

I don't know, but let us know your results. I grew gooseberries a long time ago and they got infested with those larvae just as the fruit was near ripe. Figured I couldn't use any harsh chemicals that near to harvest, so ended up pulling the plants in disgust. But I do love purple gooseberries, so planted 3 plants this spring and will have to find a way to deal with those worms. I believe Konrad said he had success wrapping the plants with some sort of transparent breathable fabric so that the sawflies couldn't get to the plants to lay eggs. I wonder if occasional treatments with neem or some other product could have the same effect.

From what I've read, you need to break the cycle of the fly if you want to control the damage. That means putting something down so that the fly cannot overwinter its larvae in the ground and then cover the plants with floating row cover or something similar to protect the fruit. I have also heard about a kaolin clay spray to put on the plants in the Spring but I don't know much about that.

The trouble with those worms is not so much establishing what their name is but that while you are reading these posts they are stripping your gooseberry bush bare, overnight, or in one or two nights.
A better question, I think is what to do about them. I believe that BTK takes care of them. I spray them when I spray my fruit trees and have not seen the worms in years, nor the ones that destroy black currant leaves. Fancying myself as a strictly organic gardener I do not use any chemical poisons but have used BTK with great satisfaction for over thirty years. It is a biological insecticide, rather than a chemical poison, and it only works on larvae. Fortunately for me, that is the majority of the problem pests I have to deal with. It also works marvellously on apple codling moth, and on cabbage and broccoli both of which might not even be worth growing if the worms that infest them could not be controlled.

I thought BTK didn't work on sawfly larvae because they're not caterpillars. I've been treating the bush twice a week with neem at the same time as the lilies and I can't see any damage yet. There are small gooseberries forming. I'll keep watching and report back.

Hmm, yes, from a BC Ministry of Environment publication on Integrated Pest Management....
"Currant Sawfly: Smooth, green worms with black spots found feeding on currants or gooseberries look a lot like caterpillars, but they are actually sawfly larvae (sawflies are insects related to wasps). Hand-pick them or spray with insecticidal soap or pyrethrins. Because they aren't true caterpillars, BTK sprays have no effect."

Too bad, it sounded like a good solution. BTW, the currant and gooseberry "worms" are one and the same.

I had planted a couple of Jostaberry, current crosses, it seemed that everything liked to feed on them. I dug them out after a couple of years, by then there wasn't much left of them but they were still trying to grow.

Another hitch could be that the bush is 8 feet away from a nest of robins... LOL. Robins are always stealing my fruit but I still love them. ;)