Welcome to the 2026 Purple Emperor season, which opened officially this afternoon at Bookham Common, Surrey, with these two males cavorting about -
Congrats to Colin Kemp, and indeed to Bookham. This record equals the official all-time record for the earliest Emperor, set by boys from Marlborough College way back in 1893!
'In my beginning is my end...'
Knepp Wildland will be open for Purple Emperor visitors again this year, primarily from June 20th to July 12th (which should coincide with the peak season period). It involves a long walk and a full day - so come for the day.
Please park in the main visitor carpark, at the Swallow Barn visitor centre just off the A24 at Dial Post. This is now a pay & display carpark, but your parking fee will be reimbursed if you spend £20 or more in the shop and/or restaurant. Fees are £3 for 5 hrs (reimbursable) or £5 for up to 10 hrs (£3 reimbursable).
A Purple Emperor route map will be available from the shop (and also from the camping, glamping & safari centre at New Barn Farm).
Please follow this route. It runs past most of the best spots. Bring your lunch and plenty of water, especially if it's hot. It takes 10-15 minutes to get into the sallow areas, though the odd Emperor can be seen anywhere (including along the oaks along the lane near Swallows Barn, even from the queue for the loos).
The best male territories will be marked with Purple Posts, indicating where to stand to watch males up in the oak canopy gaps (afternoons only - the males are not on territory during the mornings, they're exploring). Other good viewing spots will be marked with Purple Bunting.
There will be a number of volunteers along the route, helping visitors to the hotspots. Once again, Purple Emperor head quarters will be in Lower Barn, along Green Lane.
Crucially, Emperors gather out of the wind: in a west wind, they will be on the east side of the oaks, and vice versa. Never look on windward edges.
During the first third of the flight season, Emperor males descend to feed along the Knepp tracks, especially on cowpats and fox scats - but they only do this during the first three of four days of their lives. This year, it is likely that males will not be descending to feed after July 1st.
fro from the camping and glamping centre
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:-
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...).
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 -
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 -
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 -
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...
