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Jan: You are absolutely right, Scaptomyza flava is an option. I cannot prove, of course, that all my different flowers are mined by the same fly species, and adults are only seen on the sunflowers. However, the mine shape of Scaptomyza is a bit different (https://bladminee...yza-flava/): though the mines in my Tropaeolum are not fresh, none of them has reached the blotch stage yet that is typical for Scaptomyza.

John: I totally agree. I shall label the pictures "Chromatomyia cf. horticola", which cannot be too wrong. It is a pity that Milos isn't around at the moment, he'd probably have supplied us with more detailed information.

Many thanks to both of you!