you could spray with a systemic insecticide but the best way is to creep up on your plants and look for the beetles, squishing them when you find them. look out for their larva as they look like a splash of bird poo. often underneath the leaf. when they drop off they land on their backs and are black undermeath so hard to spot.

I think that being vigilent and squashing them is the only real control.

Put one hand or a pot under the beetle and flick or pick it off with the other. They do drop off when they hear you coming, and land upside down so you can't find them against the soil - crafty little wotsits. :-(

I carefully place some light coloured polythene around the base of my lillies. then i just go to the beetle. It drops off the stem turns upside down and lands on it's back, with its soil coloured tum showing. I also remove the birdlike poo as sunburngirl says.

Thanks for your advice. I already collect and squash the little blighters but didn't realise that the gooey substance underneath the leaves contained their lava. Removal and dealing with both beetle and their poo is a messy job which I will have to undertake!