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...).  

Good News!

Purple Emperor larvae have slowed right down, so the prospects of a late May start to the flight season are receding fast - though we are still on for an early June kick off, with everything depending on the weather over the next few weeks.

Cool nights have slowed them right down, making them miss out on their evening feed. Also, the East and North-East winds of late April stopped them feeding - PE larvae hate wind - and made several larvae change position. Many larvae spent the best part of a week changing skins to the 4th instar (L4).

Here's a mid-L4 larva - 

There are a lot of Willow Sawfly Euura viduata larvae feeding on the sallows at present. They can mimic PE feeding damage. Beware. Here's one -

Onward! We are still heading towards an early and potentially very good PE season (depending on the weather...).

 

Held up!

Emperor larvae have only made slow progress this last week, despite the fine weather - they've been held up by cool nights (and cool evenings, missing out on their evening feed).

Yesterday, of 19 larvae seen in Savernake Forest, only 8 were in the 4th instar (L4); the rest (bar one retard) were skin changing to L4. The most advanced were halfway through the 4th instar (I was hoping to find one skin changing to L5).  

It seems that they've got held up skin changing to L4, as has my captive larva (who spent 5 days changing). 

This means that the prospect of a May Emperor have receded, but everything depends upon the weather... We are still on for an early flight season...

Here's a nice mid-L4 larva from yesterday -

Also, I was pleased to find the leaf beetle Gonioctena viminalis yesterday. The first time I've found this distinctive beetle. It's called the Willow Leaf Beetle, but certainly isn't at all common on the Emperor's woodland sallows - 

 

Spring of 2026

Purple Emperor larvae are continuing to develop ahead of any accepted norms... 

They got held up a bit by a cool and showery week in mid-April, but are set to romp ahead this week, under a strong anticyclone.

Over the weekend of April 18th-19th, 49 larvae were checked in Savernake Forest. Of these, two had recently changed to the 4th instar (L4), but the majority were changing skin. That's early, but not unprecedentedly so. 

Crucially, the sallow foliage is shooting ahead, and caterpillars tend to be in sync with their foliage (White-letter Hairstreak larvae are developing apace).

PE Larvae can race through the 4th instar, in fair weather, though they then take much longer in the 5th and final instar - and they can get stuck at red traffic lights (or a full motorway closure) at any stage, in wet weather. 

Here's an Early L4 larva -

Here's an L3 larva about to shed the skin its worn since last September, and has wintered in. Note the hunched position - 

In late April, Purple Emperor larval feeding 'damage' is fairly distinctive. They feed only at leaf edges, and do not make holes (that's moth feeding). The only thing you can confuse PE feeding with at this time of year is the feeding of the common Willow Nematine sawfly Euura viduata -

The 'record' for the earliest wild 5th instar PE larvae is May 1st (2011). That record is under serious threat this year. 

I repeat that PE larvae have come through the winter remarkably well, and that 2026 has the makings of an astoundingly good Purple Emperor year - unless the weather misbehaves.

Also, again and yet again, this butterfly could be on the wing exceptionally early this year.

Watch this space...  We are living through very exciting times...

     

Spring of 1976...

Fifty years ago, the great butterfly season of 1976 was brewing. I was there, big time: having had the previous eight summers wrecked by the examination system*, I took a self-funded sabbatical, and butterflyed on over a hundred days between mid-April and early September – playing cricket on most of the others. This was low budget stuff, my main form of transport was a three-speed bicycle – and the tarmac melted. I was living near Selborne in East Hampshire, a top butterfly hotspot, but thought nothing of cycling to Chiddingfold Forest. Crucially, I kept detailed diaries.

The previous summer had been hot and dry, and 1974 had been reasonable too. Butterfly populations were on a high. Truly great butterfly summers are the second or, better, third in a sequence of hot summers. ’76 was the third. Incidentally, we have not had a hot summer sequence this century...

The 1976 butterfly season did not begin unduly early, like modern seasons. Here’s some first and last dates, from the Selborne & Alice Holt Forest district:-

Orange Tip                     April 16^th        June 12^th

Speckled Wood              April 19^th            -

Green-veined White       April 20^th       June 18^th

Dingy Skipper                 May 6^th         June 22^nd

Duke of Burgundy          May 7^th          June 10^th

Pearl-bordered Frit         May 6^th          June 6^th  

Small Pearl-bordered      May 23^rd       June 24^th

 

Those are typical start and end dates for central southern England from that era. Butterflies, that summer, didn’t start emerging early until the start of June. Then, Large Skipper commenced on June 1^st and Meadow Brown on the 2^nd (my earliest Meadow Brown of the 20^th century).

The Holly Blue was staging one of its periodic revivals, having been scarce since 1970. The Pearl-bordered Fritillary had an astounding year, with colonies in most young conifer plantations (there were at least ten colonies in Alice Holt Forest that summer, and the butterfly truly abounded in Chiddingfold Forest). 

But Butterfly of the Spring / early Summer 1976 was the Wall Brown, which was almost omni-present – not just on the downs and grassy heaths, but in open woodland rides, along road verges and on garden banks. My diaries record some 250 Wall Browns, between May 10^th and June 18^th, but I didn’t diarise roadside and garden sightings. I saw over 50 in Lodge Inclosure of Alice Holt Forest on May 23^rd. This abundance occurred after a massive 3^rd brood during the sublime autumn of 1975, blowing a hole in the theory that a 3^rd brood depletes populations and is responsible for the current decline. Common Blue and Small Copper were comparably numerous.

The spring weather wasn’t too extreme. April was dry and merely pleasant, and May started and ended rather poorly, though it included a couple of strong anticyclones. The heat and the drought were to come later. This meant that spring butterflies were not unduly stressed, laid a lot of eggs, and had lengthy flight seasons. It was also a great time for larval development. All the while, the Purple Emperor season was brewing...

Next Time: June & July '76.

*Rant: why oh why didn’t we reset the academic year during the Covid pandemic, so that exams take place in the autumn, and we allow our young folk to experience spring and early summer? Rant over.

Ministerial Statement

Here is a Ministerial Statement on the State of the Nation's Caterpillars (Rhopalocera):-

'Led, as ever, by His Imperial Majesty, the Emperor of the Woods,  the High Spirit of the Midsummer Trees, The One of Whom the Nightingale Sings - to cite a few of His numerous epithets - the larvae of some of our butterflies are unusually advanced.'

'Purple Emperor larvae had commenced feeding in Savernake Forest, Wilts, by March 18th, equalling or narrowly beating the record set in 2019 and 2020.' 

'Moreover, the sallow foliage is now unprecedently advanced, following a mild winter (our last cold spell was at the start of January) and the mid-March anticyclone.  Sallow blossom is currently at peak.'   

'Also in the woods, some White Admiral larvae are already feeding, some Pearl-bordered Fritillary larvae have reached their final instar, and if you want something utterly barking, look at the advanced state of White-letter Hairstreak larvae - they're likely to be on the wing in early May.'

'At this range, unless the weather deteriorates, the Purple Emperor is likely to be out by early June, breaking the long-established record set in 1893. Numbers could be astoundingly high, as larvae have come through the winter well.'  

Some Emperor larvae have been feeding by 'bud biting', like this (Savernake 18th March 2026):- 

Most, though, are lined up alongside sallow buds, waiting for leaf buds to loosen, or sepals to darken and open, so they can commence feeding. They are well greened up, like this:-

Watch this space, and get very worked up...

 

The New £20 Note...

We are pleased to announce the new Royal Mint £20 note, with an Emperor as Monarch...

Meanwhile, Purple Emperor larvae are getting ready to start feeding. Many are greening up next to flower or leaf buds. Their first meals will be on loosening buds, by 'bud-biting', or on darkening sepals (they wont feed on very pale, 'anaemic' sepals). 

In general, Emperor larvae are decidedly advanced - as is the spring of 2026. It is quite possible that a few larvae have already had their first post-hibernation meals in the warm South East.  

That's remarkable, as it was only a couple of years ago that the first-ever March meals were detected, at the very end of the month. Now we're heading for a mid-March feeding start. 

We may be heading for a Very Early Flight Season... Watch this space.