The butterfly books give the impression that Purple Emperors fly for at least a six week period, from late June through to late August. That is very misleading, not least because of the clear trend for butterfly ‘flight seasons’ (when adults are on the wing) to occur earlier and earlier in the year, seemingly due to climate change. We hold no recent record for a Purple Emperor in England later than 7th August and live in expectation of the butterfly appearing in mid June or earlier. We are also anticipating a partial second brood.
Although the timing of the Emperor season varies somewhat from year to year, according to spring and summer weather, it seems to be steadily becoming earlier and earlier, and it may be becoming shorter too. We have not had an old fashioned ‘late’ Purple Emperor season for years (e.g. in 1972 and 1977 the butterfly did not appear before ca 20th July), and gone seem to be the days when the Emperor season begins in mid July. No, the books have become nonsense, burn them. Visit this website instead.
With most butterfly species, there are ‘early’ sites, ‘mainstream’ sites and ‘late’ sites, so that a species may be well out in one place and either finished or not yet begun elsewhere. This is particularly true with the Chalkhill Blue. Also, the flight season for a species tends to last longer at sites supporting very large populations.
With the Purple Emperor, though, there seems to be far greater flight season synchrony, with only a gap of a four or five days between flight season commencement and peak in the warm south and in the cloudier Midlands. The season seems to finish everywhere in late July, with only the odd individual lingering into August. Also, there are no huge populations with longer than average flight seasons.
Down south, the Emperor season tends to commence today between 24th and 28th June, when a few early males emerge. Male numbers build up well within three or four days, and the first females then appear. What we call ‘peak season’ – the time when the maximum number of Emperors (and Empresses) is flying – tends to be about a ten day period commencing 4-5 days after the start. This tends to equate with the last day or so of June and first 10 days of July. Males tend to emerge over a period of 10-12 days, from Day 1. Female emergence is harder to determine as we see so little of Herself, but probably lasts for a little more than a week. Crucially, males are most active when there are virgin females emerging (understandably…).
We do not know how long males and females live – as we have only managed to recapture a single marked specimen of each sex, in both cases after 7 days! Much depends on the weather. Purple Emperors are vulnerable to burning out in very hot weather, and are also decimated by gales (as they roost in tree tops).
After mid July, the males become increasingly scarce and we are left largely with egg-laying females in what tends to be a two week ‘tail end’ to the season. If you are restricted to weekends, there are two weekends for you. Do not miss them!