The 2026 Purple Emperor Season... Prospects and Timings

Much has happened in the Purple world these last few weeks: a cold start to May, with cool nights; then a mid-month spell of damaging localised squalls and downpours; then a heatwave with record temperatures (day and night) at the month's end. Crucially, larval development stalled during the first half of May.

Many larvae are now full grown and there are a few early pupae around (there's a BUT coming up, several in fact).  

At this range, it looks as though the first Purple Emperor of 2026 will appear on Sat June 13th, behaving appallingly badly, BUT this is not an easy season to predict (it may be as early as June 10th...).

Sallow foliage quality is very variable from district to district. This is important. In some regions, sallows responded to the 2025 heat-shocks by over-flowering copiously this spring: they produced flowers rather than foliage. The same thing happened after the 2018 heatwaves.  

Also, there have been localised infestations of Geometer moth larvae (Mottled Umber etc.), and infestations of tree hopper nymphs (+ cuckoo spit). This means that Emperor larvae have been feeding on morsels rather than whole leaves, which slows their development down (and may contribute to mortality).

Here's a recent view of sallows in one of the main breeding grounds at Knepp (May 21st), note the sparse foliage, with some dead or dying sallows:-

And here's a view of lakeside sallows at Knepp, note the white seed on the lefthand sallow and the paucity of leafage (the white stuff in the water is seed), May 21st:-

However, most of Knepp's broad-leaved sallows are fine - it's the narrow-leaved types that have suffered most. And Knepp hosts 60ha of sallow jungle. Don't worry, Knepp can cope with this...

Over-flowering and leaf laceration have been noted elsewhere, e.g. in Alice Holt Forest, Hants, and Cotgrave, Notts.  

However, sallow foliage quality in and around Savernake Forest, Wilts, is remarkably high (Sav could produce an annus mirabilis...).  

All this suggests that adult numbers could be very patchy - very good in some districts, more modest in others - BUT much depends on weather during the pupation and pupal stages, and especially during the flight season period. 

Knepp Wildland, by the way, will be open for Purple Emperor visitors again, primarily from June 20th to July 12th, with parking at the main Dial Post centre. I'll post details later. Knepp's famous Purple Emperor Safaris are also running during this period (but are fully booked - though some more may be arranged, if there's demand...).