It's a caterpillar. ...bug worm it is, from memory. They do the same sort of damage that gooseberry sawfly do to gooseberries. It's happening to one or two of,mine. It's hard to,see,the blighters but you can spray. A soap spray is effective as,is a systemic chemical,one.
I cut everything down......convince myself I'm doing the Chelsea chop (well. It does have the same effect).....and spray the remaining plant. In a few weeks it will flower again with fresh foliage. This year though I will anticipate a repeat attack and spray again just before the new flush of flowers.
These things are cyclical I,think. It's caterpillars and ants this year.