Box tree Caterpiller

I have recently arrived at my house in SWFrance in the Dordogne. I have a lovely formal terrace planted with topiary box. i have been in the UK for the past two weeks and during this period all the leaves on all the box have completely gone. From its appearance it looks like an attack from the box tree Caterpiller yet I can see none. There is evidence of webbing and piles of what I presume is poo. I don't know what to do, what to effectively treat this problem with and whether my box tree will now die? In short. Help.