Box caterpillars

I don't have box in my garden but my nephew is loosing his hedge to them. He has asked for advice. What should he do?

Hello I'd say try a Permethrin (pyrethrum )based insecticide mixed at the absolute correct dose and water it around the roots,make sure the soil is damp as the liquid would run off dry soil ,not environmentally friendly,please dont take old gardeners advise to just slip  the liquid into a watering can and add water    jezza