Thanks for the reply, the sight of so many beasties did unnerve me a bit.
I went back to look at them and realised that they had done no damage to any of the shrubs roundabout.The seal is under a Yew tree.Over the years i had noticed that they had been reduced to a skeletal form but had not been unduly concerned as it happened after they finished flowering.I also had never seen the culprits before.
I will take your advice and feed the plants.The destroying of the larvae, is not going to be easy.There are so many, and i am a bit squeamish, but i will try.thanks again Anne