Must be larvae of something, but a photo would help - unlikely to be causing plant death, whatever they are, unless they've demolished every single leaf on the bush, so what makes you think they're dying other than the presence of these larvae?

Havent got camera at moment leaves are curling and dropping off

Drought would cause that too. Why don't you buy a Provado ultimate bug killer spray and hit it with that as well as giving the plant a jolly good soaking at the roots.