Sounds like raspberry sawfly. The only control I know is to spray with an insecticide just before the flowers open. Look for one which mentions the more common gooseberry sawfly in the list of pests it deals with. I don't know of any organic controls, but others here may have ideas, if you don't want to use any chemical sprays.
Do we have Raspberry Sawfly in the UK? Thought they were an American problem.They are possibly another Sawfly: a Gooseberry Sawfly. Rasp Sawfly are green and Gooseberry a pale Green almost greyish with black heads.
I would avoid chemicals if you want to eat the fruit. A hose over the plant and esp under the leaves washes them off and or using an old sock soaked in water and manually removing them.
There are several species which could be called that, without necessarily being the american genus. Without a photo it's rather hard to say but if they cause so much damage that the plants don't crop, it's almost certainly a sawfly. See here:
Manually removing them with a jet of water etc will work ok, but sawflies can strip plants overnight, so vigilance is of the utmost essence, as it is with gooseberries!