Last July we added White-letter hairstreak to the campus species list. It’s impossible to say whether it was a stray individual (revisiting my photograph it was a pretty beaten up specimen!) or part of a breeding colony on campus. It can persist in quite small colonies, often centred around a single tree as for the Sheffield colony mentioned in this recent Guardian article. The larvae usually start out feeding on flower buds, so white-letter hairstreak prefers more mature and therefore flowering elms. However, it has been recorded subsisting solely on younger growth.
There’s plenty of young elm scattered through the Wilderness, especially towards the Beech Lane gate, so I think it’s worthwhile at least keeping an eye open for white-letter hairstreak in any of its life stages throughout the year. The surest and perhaps easiest way to confirm a breeding colony would be to locate eggs. They’re laid at any height, typically on the scar between new and old growth or at the base of new buds. Best of all they look like miniature flying saucers (see below)! Actually finding one is a very long-shot, but I’d be very grateful if anyone walking through the Wilderness and passing some elm took a moment to look for hairstreak eggs. Close to a flower bud – if we do have any flowering elms here – would be the best starting place, and I imagine a hand-lens would be useful. They’ll probably be easiest to find in the next couple of months whilst the twigs are bare, by late March the eggs will begin hatching.
Elms have tiny dark brown or black buds that alternate along the stem – for more on identifying trees in winter see here or ask one of our friendly resident botanists!