Our final instar larva in Savernake is just about full grown (Sat Oct 10th), photoed by co-workers Gary & Sarah -
It's done incredibly well to get this far, and will be wanting to pupate very soon. I hope to visit midweek to check it. I should be able to tell what sex it wants to be. Alternatively, it may try to hibernate.
Our foolhardy Emperor caterpillar in Savernake Forest is now midway through its final instar, photoed on Sun Oct 4th -
It's moved 3m, in-bush and then back out on another branch, with sparse, yellowing leaves infested with Willow Rust. Weather and seasonality permitting, it may try to pupate later this week.
Meanwhile, some other (less insane) larvae there are starting to colour up prior to hibernation - that's early, very.
It's a crazy year, and Emperors do Crazy big time...
Our kamikaze caterpillar in Savernake has made it into the 5th and final instar! Here s/he is today -
and this photo show what s/he's eating, coarse yellowing foliage with Willow Rust -
Also this weekend, a fair number of 3rd instar larvae, and a couple of late 2nd instars - so three instars in a day.
Butterflies push limits!
And whatever mistake you make in life, Never Underestimate a Caterpillar (or in Bad Latin, Eruca Nunquam Minoris Aestomate!)
The 4th instar larva we're following in Savernake is now changing to the 5th and final instar. Here it is, photoed today, Sept 20th, hunched up in classic skin changing pose -
And, as I pointed out in last week's post, if the Emperor is threatening to double-brood in northern Wiltshire, what is it doing in the warmer, sunnier Southeast???
Watch this space, and expect the unexpected... All things are possible with this the best of all possible butterflies...
Before I get too carried away, the other 25 known larvae we checked in Savernake Forest this weekend were in the 3rd instar, behaving normally (the trouble is, the Purple Emperor butterfly doesn't normally do normal, it pushes limits)...
Just when we thought that 2026 couldn't get any more amazing for butterflies, the Purple Emperor moves towards producing a second brood.
Last Sunday, Sept 13th, Team Savernake found an L4 larva!
Normally, Emperor larvae overwinter in L3 and change into L4 in mid-spring.
Here it is, on a distinctly autumnal leaf -
Photo Gary Knight
Now, if iris is moving towards producing some second brood adults in the frozen wastes of Wiltshire, what is it doing in the warm South East???Watch this space...
Purple Emperor larvae are developing steadily, and seem to be in 'average' numbers this early autumn.
They are bang on time, and are not showing any inclination towards second-brooding, despite the blazing hot August (we don't particularly want them to waste themselves by producing second brood individuals).
Yesterday, in Savernake Forest, half the larvae seen were in late L2 stage, whilst the other half had recently changed to L3 (in which they will remain until next spring). Here's a classic early 3rd instar larva (what we call EL3):-
Sallow foliage conditions are variable, but in general the sallows held up remarkably well through the heatwave summer. Sallow dieback has been noted on the clays, particularly on the Weald and Gault clays in the South East, with dying sallows reported in Alice Holt Forest. The sallow fields at Knepp are heavily droughted, though those along the water courses there seem OK. It looks as if we've just got away with it, but await more evidence...
Finally, here's a rather poor photo of my 2000th find:-
These are challenging times for Purple Emperor larvae in the wild, with heatwaves impacting on larval development and the ongoing drought affecting sallows and foliage quality.
In and around Savernake Forest, Wilts, larvae are developing steadily and the sallows are holding up relatively well, for now.
We have equalled the mid-August record for the earliest 3rd instar larva (set in 2018). All other larvae this last weekend (15th-16th) were in the 2nd instar, mainly in mid-L2 state.
As yet, there are no signs of larvae developing way ahead of schedule, and threatening to double-brood (and we don't want them to to that anyway, it's a waste of butterflies).
To date, larval losses have not been severe, or as high as they were this time last year. That may be because the females have laid in deeper shade this year, and/or because the heat may have reduced predator numbers significantly.
However, here's a dead 2nd instar (L2) larva found there yesterday -
Here's a nice late L2 larva from yesterday -
The problem here is that the Empress is forever moving the goalposts, adopting different laying strategies from year to year, and it takes us a while to crack what she's been doing. Her only consistency is her sublime disregard for our botanical taxonomy - she does not use Clive Stace's New Flora of the British Isles (2019, 4th edition) let alone Meikle, R. 1984. Willows & Poplars of Great Britain & Ireland. Note that the vast majority of sallows I find larvae on key out as hybrids (mostly as x Reichardtii hybrids).
Some districts have had no effective rain since June 11th (after a very wet start to the month). Wisley, Surrey, has just recorded 60 rainless days. Sallows in free-draining situations are dropping leaves (e.g. Cotswold Water Park [gravels] and on the Lower Greensand). Savernake's sallows are largely holding out well, but for how much longer...?
We need a Ministerial Statement on the State of the Nation's Sallows...
Big session looking for eggs and larvae in Savernake Forest yesterday, conducting an initial assessment of the 2026 'egg lay' (= number of eggs laid) and of larval survival in the heatwave. A gang of four searched one of the main breeding grounds (ground searching + shepherd's crooks, aiming to follow the larvae through).
Goodish news:-
1 The 'egg lay' looks good, though it is almost certainly lower than last year (an annus mirabilis in which the Purple Emperor won BC Wiltshire's Butterfly of the Year).
2 Larval survival seems to be OK-ish (judged by the number of finds of egg case bases + 1st instar silk pads and feeding, minus any live larva). Last year, we lost a lot of baby larvae to heat desiccation, and some eggs 'boiled in the bag'. This year, the females seem to have laid more in heavy shade, sensibly.
We found six 2nd instar larvae, 10 1st instar larvae; only one egg (about to hatch); one dead egg (predated by a sucking bug)) and five 'L1 Fails' (egg case base + pad & feeding - larva [egg case base is abbreviated to ECB, though that also stands for England & Wales Cricket Board; and about to hatch is abbreviated to ATH]). That's good.
Best, sallow foliage condition was Good to Very Good - heat and drought haven't adversely affected sallow foliage, though a few early-leafing sallows (males mainly) have gone through the phase of being suitable for young larvae.
Here's yesterday's best photo, of a late L1 larva (1st instar) next to an early L2 (2nd instar) -
Silver-washed Fritillary and Purple Hairstreak seemed to be over in the forest - imagine that, paphia and quercus gone by the end of July...
At present, it seems that the UK's final 2026 sighting of Apatura iris was of a female seen by Toby Ludlow in Lincs on Fri July 24th (perspective: July 26th used to be National Purple Emperor Day, usually coinciding with peak season...) - unless some second brood adults emerge in the autumn....
The 2026 Purple Emperor season (which was at best 'average') bumbles on, way past its sell-by date.
Today, Tues July 21st, 3 females in Chambers Farm Woods, Lincs, and a female on a sap run and, briefly, this old male on territory at Sapperton, Glos -
I dread to think how poorly larvae are faring in his desiccating heat, but will find out soon.
Only one thing can end this heatwave: Bristol Balloon Festival, due in early August...
The 2026 Purple Emperor season is stuttering towards its inevitable end.
Today, Fri 17th July, a female was seen in a wood near Basingstoke, Hants.
Yesterday, two males were seen on territory at Knepp, Sussex, and this dying male was found on the ground there -
In addition, yesterday, one was seen in Lincolnshire and this male was nicely active at Sapperton in Gloucestershire -
But watch this space, it may be that a small second brood appears this autumn, weather permitting: we already have 2nd instar larvae, by mid-July...
'In my end is my beginning'.
This photograph of an old male on territory in the 'Dead Beech Glade' in Savernake Forest, Wilts, is probably of my last Purple Emperor of the year. You can just make out a pair of antennae and the front of the forewings -
All we saw during an exhaustive and exhausting search of Savernake's best territories was this and another faded male, though there must be the odd female still on the wing.
Today brought very difficult weather conditions in which to work Emperors, especially late in the flight season: cloudless, calm and hot from early.
In such conditions, late on in the season, Emperors tend to stick to the vicinity of their feeder trees, which are usually sap bleeds. Sap runs are never easy to find, and can switch on and off from day to day. This year, they have been even scarcer than usual - Patrick's Tree, normally Knepp's No 1 feeder tree, has hardly functioned...
I was in a new part of Savernake today, and quickly learnt that weather conditions weren't right for surveying. The one sap run there, the best I've seen anywhere this year, attracted half a dozen male Red Ads, and briefly this old male Emperor (and it is male, it's completely lost its purple iridescence) -
and the next generation of larvae is advancing. Here's a late 1st instar larva, soon to change to the 2nd instar, and gain its horns - this early, rather begging the question of whether we'll see some 2nd brood Emperors on the wing this autumn???
Time was when the Purple Emperor season would normally start on or around July 7th. This year, the butterfly is now past peak and probably starting to go over at early sites in the south east. The length of the tail will be determined by the intensity of the current heatwave.
The woodland vegetation provides a better indication of real time than our calendar. Look at the oaks, and the sallows, and everything else. It's late July out there...
It appears that 2026 is at best a rather modest year for the Purple Enperor, numbers-wise - but populations have been hard to monitor due to extreme heat and unforecast winds, generating a pattern of days of relative activity punctuated by days of extreme lethargy.
Yesterday, I visited the central Malverns, straddling the Herefordshire - Worcestershire border, where Emperor males were discovered genuinely hilltopping last year, and were seen again last Wed and Thurs.
These are the first records I know of Emperors hilltopping in open terrain in this country, though odd individuals have been seen on the crest of the South Downs (seemingly wandering over, rather than setting up and defending territory).
It was too windy yesterday, with a Fresh westerly blowing on the Malvern tops. We saw a nice scatter of old sallows with suitable-looking foliage on the western (Herefords) slopes. Here's the habitat, where males have been perching on bracken tops -
Walked the Knepp Purple Emperor transect this afternoon, for Week 4. Counted 32.
That makes 2026 an average year for the PE here, even if they burn out fast in the coming heatwave (which they may well do).
They have been very hit and miss here this year - localised and episodic.
Today was my last day at Knepp. Off home early tomorrow, to work Savernake and elsewhere...
Here's my signing off photo from Knepp -
Struggled to see Emperors at Knepp today. It was too windy, yet again, but the two sap run trees which had been heavily favoured these last few days both drew blank, and most of the favoured male territories were empty too. Sap runs can suddenly switch off.
Conclusion: both sexes got so plastered during the football last night that they took the day off, hungover. I can't think of any other explanation...
The best I managed from today was this photo of a female sapping next to a Red Ad on the feeder tree next to the tree platform down Green Lane -
Watched the first half of England v DRC. Decided the cause was lost, so went out Emperoring on bike, listening to the match on the radio.
Both England's goals were set up by the Purple Emperor butterfly. Both goals resulted from magnificent 'tumbledowns' (mated female rejecting a wont-take-no-for-an-answer male). Here's Kane's winning goal -
Briefly, as I'm too tired. A pair joined at 4.05pm on the shady leeward side of a feeder oak, just 25' up, after meeting in flight. No courtship (he didn't even ask her name...).
They remained together, motionless, wings closed, for a marathon 4 hours and 2 minutes. The average duration prior to this observation was 3 hours 42 mins, from a sample of 19 pairings (now 20). Gentlemen of England, Ladies even, we are outdone, by a butterfly...
Onward!
The Purple Emperor is now at peak at Knepp.
Today, I walked the Knepp PE transect. This runs down 2km of oak-lined green lane and takes around 2 hrs 30 mins to walk. Weather conditions were Good, with Emperors nicely active (a wind sprung up towards the end - Knepp is a surprisingly windy place).
I totalled 38, including 5 females.
That's about par, suggesting that the butterfly is in average numbers at Knepp this year (though way down on last year's stupendous showing, which produced a record individual transect count of 80!).
However, if another extreme heatwave comes over, they'll shoot over early (as happened last year, and in 1976) - in which case this year will become a below average year...
At last, some of Knepp's oaks are producing sap runs. Both males and female cluster around these 'feeder trees', which means they tend to disappear elsewhere.
Here's some feeder tree action from 2025 -
Far too windy at Knepp today, again, after a quiet and productive day yesterday. Most of the male territories were unoccupied today, windswept. There was, though, some good activity out of the wind, along east-facing and north-facing edges.
Elsewhere, the Emperor seems to be having a good year in Lincolnshire. Down south, numbers seem at best moderate, but we can't get a reading when it's either stultifyingly hot or blowing a gale...
Here's the wrong Emperor, the Emperor dragonfly, eating a Marbled White -
