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Could anyone please identify these caterpillars, they are feeding on (totally stripping) mahonia plants in my garden, but only the mature leaves.New growth is unaffected.They are about 15mm long.
It is berberis sawfly, Arge berberidis
The species arrived in Britain in 2002 and continues to spread.
Watch out for it on Berberis as well; it can cause severe defoliation.
The first of the two links above gives advice on control.
If you like, you can report your sighting here