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I noticed that the ripe capitula of the Senecio jacobae (maybe now called Jacobaea vulgaris) in my garden all have a topping of white foam, which is now dry and solidified.
I opened one capitulum and found a dipteran larva as expected, presumably a tephritid. Several tephritidae are known from this plant, Sphenella marginata perhaps being the commonest.

My question: is the foam indeed produced by the larva, is it species-specific behaviour and may it help to identify the larva? is this common for tephritids in flower heads, or could it be related to the toxicity of this plant?

thanks in advance, Louis