My lilac bush's newly-emerged leaves are crushed together into little balls. Inside each of the little balls is a caterpillar about 15mm long. They are white with black heads and short legs, so I think they're caterpillars not maggots. Since they're living in the leaves and not under the bark I don't think they're lilac borers, although they look similar. Can anyone help identify this pest? And do you know how to get rid of them and/or stop this happening again next year? I'm attaching a picture of the leaf damage. I have a picture of the caterpillar too but it seems I can only attach one image.