Widespread and common where foodplant is abundant
Easily confused with the Common Blue (Polyommatus icarus). In the male the presence of the patch of hairy androconial cells in the basal 2/5th up the upper forewing is diagnostic, but this can only be seen if the wings are open and it can be hard to see unless the light and angle of view is right.
The absence of the basal spot on the underside of the forewing is an indication that the specimen might be thersites, but is not diagnostic, as often Common Blues are also lacking this basal spot (form icarinus), which is much more frequent in eastern Europe than in the UK. There is often a difference in the ground colour of the underside of the wings in males, which tend to have a pale milk-chocolate ground colour contrasting with the pale grey of the underside of the forewings. However this is a subtle difference, but with experience I have found it possible to pick out thersities from icarus when they are puddling together on damp mud.
Dry grassland often in rocky or bushy places.
Foodplant: Species of sanfoin (Onobrychis spp.)
Recorded in 38 (2%) of the 1900 10km squares of Bulgaria.
Last recorded in June 2019
Recorded date range (Adult): 05 May to 08 August