Box bother.

I've used xentari this year with good results, it targets the caterpillar and almost immediately they stop eating the box leaves and die. Again it's not cheap and supposedly not for sale in the UK but is available of Amazon I bought it on the co.uk site and it comes from Holland, I know a few others on here use it as that's where I got the idea from. I'd tried everything up till then and almost given up my hedges as a write off, still not sure if parts of them will recover as the caterpillars stripped all the leaves off some sections.

If you give the box plants a good shake then return a few minutes later lots of caterpillars dangle down on a silk thread, you can pick them off by hand but it's only a small percentage each time. They wrap themselves up in the webbing to avoid predators. The first couple of years bluetits were eating them but gave up as they must taste awful.

 

That's what is in xentari.

A friend has replaced all his box hedging with ilex crenata, looks very similar to box but is slower growing, it's not affected by the caterpillars.