I have a lot of lilies in pots and last year I decided I would not grow them again if they were damaged as much this year by the lily beetle. When I saw the first pair (always in pairs on top of one another..ahem) I killed them and then sprayed all of them with a general purpose insect spray I bought from Focus in their sale when they were taken over. Not seen any since. I thought I was going to have to buy expensive Provado!
We've managed to stop the little blighters just by hand picking and squashing, no sprays needed!
Unfortunately they are not always in pairs. I have about 30 pots of lilies and find it best to look at them whenever I walk past them and squash the beetles as I see them. Never used an insecticide yet. As far as I know there is no effective insecticide. The trouble is they can fly so can easily spread from neighbours lilies.
I always squash any I see when passing. Only the eggs and larva are susceptible to treatment with Provado (same as for vine weevils). I treat my pots with a drench in September/October,