Echinacea. Caterpillars in the flower heads

Question from Isabel Wakefield

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My white swan Echinacea are currently plagued with small greenish caterpillars that are living in the flower heads. They spin threads and pull the petals in to cover themselves whilst they eat little furrows across the flower head. When they are exposed and irritated they wriggle furiously and drop off to the ground. Any idea what they are and how to combat them?