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It is very unusual to see Rhagoletis alternata anywhere other than on roses, where the larva develops in the hips. With picture-wings, it is always a good idea to say which plant you found it on - many are very host-specific. Adults do occur on non-hosts, but knowing the plant is a good start.
recording wildlife with The Recorder's Year on www.hbrg.org.uk/TRY.html.
Syrphus, thank you for your comments.
Does the fact that this specimen was photographed on a cotyledon of a runner bean indicate that this is more likely to be Trypeta zoe? And, might I expect to the work of leaf miner in the near future?
No - this is obviously the rather inconvenient exception! T. zoe mines a variety of Asteraceae, including Artemisia and Senecio. The site at http://www.nhm.ac.uk/research-curation/research/projects/british-insect-... is very useful for info and links.
Your beast certainly looks like T. zoe, and is definitely not Rhagoletis.
Thanks again! I shall certainly take your advise.
Lat/Lng: 50.410049125766, -4.1007328033447
OS grid ref: SX508587