I should have guessed as I was right 😆 but I have never had this problem so don't know for sure. If you use lemon or orange oil with soap you can melt all the caterpillars very easily. Alternatively make an imo capture with frass the fungus beauveria bassiana can make chitinase to do it for you, turning those caterpillars into fertiliser.

Been battling the buxus caterpillars with this at work!