Though the UK Fly mines website says the leaf mines of the little orange fly Acidia cognata are made in October-November, I’m sure the mines will still be visible this month, and possibly further into the winter, so this is one to look out for now when you’re out walking.
One of the plants this fly mines – this is the one I’ve found these mines on – is Winter heliotrope (Petasites fragrans), which is particularly noticeable now, due to the pretty pink flowers that appear from November to February. Other favoured larval plants are Butterbur (Petasites hybridus), a plant very similar to Winter heliotrope, and Colt’s-foot (Tussilago farfara), the leaves of which don’t appear till after the flowers, so approximately from April onwards.
After hatching, the Acidia cognata larvae create a corridor on the upper surface of the leaf. As the larvae grow, the mine widens and eventually becomes more of a blotch, especially if there are several larvae on a single leaf and their mines meet. I’ve found these mines in two local patches of Winter heliotrope but not found any sign of them in other locations, so distribution does seem a little random. If you spot any, please do record your sightings as, like many leaf-miners where the adult flies are not often seen, this species is probably under-recorded.