I'm no expert here, Anniel, but on a recent "Gardeners' World" programme, Monty Don was showing little blue beetles which lived on mint.......believe it or not, they were called blue mint beetles!

Just done a quick bit of research and its an odds on bet they're Dock Leaf Green Beetles - they eat all broadleaved vegetation, and particularly like rhubarb leaves. Check the underside of the leaves, where you may see clusters of small brown eggs - rub these off. These beetles usually arrive in hordes, munch the leaves and lay eggs and depart reasonably quickly. I'd destroy any eggs present, but wouldn't worry too much about their eating the leaves - the rhubarb is not being picked now its so late in the season, and will recover anyway.