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It looks more like to be damaged by Pegomyia (Anthomyidae) : Sphenella just do not form such a foam capsulae. The easiest way is to collect some of the infested flower heads. Pegomyia pupates in soil leaving heads (pupae ochreous or orange), whereas Sphenella marginata pupates in the head (puparium bean-like, shinig dorsally & matt ventrally, BLACK).