The Munch Bunch

I spotted these caterpillars munching away on the leaves of my silver birch tree.  Quite a colourful lot but research shows that they won't be turning into attractive butterflies or moths.  Seems they are the caterpillar of the Sawfly (Nematus Septentrionalis) and whilst in large numbers they can do a lot of damage, they are otherwise harmless.  Other than spraying them with insecticide (which I'm not prepared to do) they can only be controlled by individually picking them off and squishing them (which I'm also reluctant to do!).  They've stripped a few leaves from one of the spindly lower branches of the tree but that is one which will wither and fall off in a couple of years anyway, so I'm not too bothered.  I'll keep an eye on them though; if they suddenly increase in numbers and take over the tree I may have to rethink!