The first Purple Emperor of 2019 will take to the air at lunchtime on Friday June 14th, somewhere in Sussex or Surrey. An earlier appearance was on the cards but larval progress has been slowed down by cool nights and evenings during May (at one staged a May emergence was a possibility).
This prediction is based on the progress of larvae being followed in the wild (last year my prediction was less than 24 hours out...).
Most years we get records of odd specimens being seen ridiculously early. These seem to be misidentifications (of White Admirals) or result from specimens bred in captivity, which often emerge early (and are undersized).
Numbers could be exceptionally high, though with some local exceptions. This is because of (1) a high egg lay last year, (2) normal levels of larval losses during the autumn, (3) relatively low losses of hibernating larvae, (4) relatively low losses of early spring larvae. Much, though, depends on flight season weather.
There will be exceptions, particularly in parts of the South East which were severely droughted last summer.
Numbers will almost certainly be down at Knepp Wildland, I fear significantly. No rain was recorded at Knepp between 31st May and 28th July 2018. As a result, leaves withered on many of the sub-canopy sprays on which most eggs are laid and on which young larvae feed. Also, many of Knepp's sallows have shown signs of severe drought stress this spring, and many more have been attacked by moth larvae (mainly Mottled Umber moth). Similarly stressed sallows have also been noted at Bookham Common, Surrey.
Also, numbers will be down in Fermyn Woods, Northants, where several hundred sallows have been felled during extensive Forestry Commission ride widening work, as part of a Back from the Brink project in liaison with Butterfly Conservation. Hopefully, this will only be a temporary blip. Much depends on what happens, and doesn't happen, next.
Conversely, prospects are extremely good elsewhere, including in Hampshire and Wiltshire. For example, sallows (and Purple Emperor larvae) in Alice Holt Forest were saved by 10mm of thundery rain on Fri July 13th 2018.
The season will be over by around July 23rd.
Cold nights are forecast for next week. This could induce temperature shock on freshly formed pupae - so watch out for aberrations...
Enjoy your Emperors! This is going to be a cracker...