nightmare with the buggers last year as they decimated my hedges. my hedges are only just recovering and were beginning to look a lot better. from a distance they looked fine but i decided to take a closer look. they are everywhere, big fat caterpillars. i think it is impossible to stop them. can't be healthy for me to every summer (yeah right...) have to keep spraying and it f**king rains as soon as i have finished spraying for all the good it does. i live in the SE (london). women down the road has a front garden full of fab' topiary bushes, they didn't appear to be touched. for some reason mine do.. 
i love those bushes. fighting a losing battle. dh thinks i should be done with it and put some plastic ones in 