Any species that is totally dependent on another is vulnerable should its host encounter difficulties and so it was with the white-letter hairstreak whose dependance on elm has seen its population levels fall dramatically since dutch elm disease wiped out most of the elms from the English countryside in the 1970s. Fortunately, diseased elms can produce new growth from their roots and survive until they reach a reasonable height and this has enabled the white-letter hairstreak to survive as a species and it is thought that in recent years its numbers may even be increasing, possibly due to climate change.
The white-letter hairstreak has a short flight period from the beginning of July until mid-August. The reports we have in the Nature of Dorset records database for 2017 and 2018 however show emergence in week 25 in late June and last just four weeks until week 28 in mid-July with no records for August at all. This variance from the textbooks may be due to Dorset's southerly location and perhaps further north they emerge later and fly until three or four weeks after they have stopped here?
There are reports from four sites in Dorset so far but the bulk of reports come from Butterfly Conservations exceptional reserve at Alners Gorse. It is quite possible there are small populations at other sites yet to be identified and recorded of course.