They are evil, pure evil, look up evil in the allotmenteer's dictionary (someone should really write one of those) and you would see their photo. They start off at the very bottom of your beloved plant and leave little tiny pin holes in the leaves. Turn that leaf over and you'll see teeny tiny almost see through catepillars, probably lots of them. grrrr!
If left unchecked (by that I mean, if left alive) they will quickly and I mean quickly, eat the leaves of your plant down to little skeletons in no time. They get fat - thus easier to see I suppose - quickly too and that my friends is when WAR is declared. I not the biggest fan of gooseberries (I don't dream of them or anything...) and I'm generally a pacifist but when something is eating MY food - well, all niceties go out the window.
Andrew squishes them, I'm more girly (I hate things popping in my hands and leaving innerds and green slime everywhere, but that may just be me) and plop them into a container with water in it. I think salt would be good here too, but I haven't got any down at the plot and anyway our Blue Tit chicks are growing big and strong (and noisy) and they probably like the unsalted variety of sawfly. You can of course go down the pestiside route too but then I don't really like that.
Be careful about the desire to just shake the plant and watch them all drop off, they just climb back up again or if they're old enough, they will thank you for the help - they go to ground in order to pupate (I hate that word!)
Here are some of my pictures of the new enemy in our plot - I guess now we are becoming a more established allotment garden we're going to get more of these pests....