Locked Down! The 2020 Purple Emperor Season at Knepp

This is the bulk of the text of Matthew & Neil's report to Knepp Castle Estate on the 2020 Purple Emperor season there.

Like the human race, His Gloriousness suffered an annus horribilis in 2020. This was the worst Purple Emperor season since at least 1990, when similar weather events occurred. Everywhere, the Emperor was hit by a triple-whammy of misfortune, but Knepp suffered an extra, quadruple-whammy.  

Winter was mild, stormy and insufferably wet, culminating in the wettest February on record. The mildness meant that tit predation on hibernating Emperor larvae was alarmingly high. Ben Greenaway recorded an 85% predation rate in a large sample of larvae in Southwater Woods, near Knepp, the highest predation rate recorded to date. Matthew recorded a 50% predation rate in Savernake Forest (Wilts) and near Lambourn (Oxon, just), but from a smaller sample. 

This was the first whammy, and it was a biggie. No way was the Emperor season going to be good after 85% winter loss.

However, the fine April and May were ideal for the surviving larvae, which fed up speedily.  An early, and perhaps reasonable Emperor season was on the cards. Then, at the start of June when the more advanced pupae were wanting to hatch, the fine weather ended. Ten days of cool gloom ensued. The longer Purple Emperors spend in the pupal stage the fewer of them fly (we don’t know why). 

This was the second whammy, though perhaps a relatively minor one.

Nationally, the first Emperor of 2020 appeared on June 13^th. It was seen at Knepp, half-way down Green Lane. This meant that 2020 had failed to set a new official record for the UK’s earliest Purple Emperor, by two days. 

Matthew’s first sighting of the year foretells what the Emperor season will be like. Great Emperor seasons kick off spectacularly, but this one opened with a fragmentary sighting (confirmed only because Paul Fosterjohn then saw it too, more clearly). The writing was on the wall. 

Stops, Starts and Stutters

Purple Emperors tend to come out with a bang at Knepp, because of the size of the population.  Numbers almost double daily, for a week or so. This mid-June, though, the weather stuttered, and the Emperors stuttered too. Day counts rose from two on the 15^th, four on the 16^th (when the very first Emperors of the year were seen elsewhere [Southwater Woods and Epping Forest]); then, ten on the 17^th. The latter included six recorded on the Knepp Green Lane transect – a poor tally for Knepp’s Week 1 count. Then the emergence stalled: only seven were seen on the 18^th and seven on the 19^th. Oddly, sallow searching (the male’s habit of searching for females over the sallow stands) didn’t start before the 19^th. Something was wrong.... 

It was ominous that on the 18^th, after arriving in sunshine at high noon, sober, Neil managed to walk all the way down Green Lane, from Countryman Lane, to the pond, and bear left into Brookhouse 9 before encountering his first Emperor of the year – which promptly spiralled down, flew around him, and settled to feast on his flesh. For forty minutes Neil stood there, statuesque. 

Things picked up a bit on the 20^th, with a day count of 19, including the first female of the year (being courted and mated, in the Lady Chatterley’s Lover territory). The males were worked up that day: quite a bit of clashing and chasing was seen. The following day, the 21^st, the first Purple Emperors of 2020 were seen in Alice Holt Forest, Hants, and in Bernwood Forest, Oxon – two well-recorded sites; suggesting it was still very early days nationally. Knepp’s day count rose to 27. 

The weather improved on the 22^nd, and Matthew’s day tally reached 36. This included a courting pair in the south-east corner of Bentons Gorse, and a mated female rejecting a male unprepared to take “No!” for an answer, in a ‘tumbledown dance’. 

Anticyclone!

Midsummer saw a five-day anticyclone. This generated the bulk of the Purple Emperor emergence elsewhere, though was clear that the butterfly was already well out at Knepp. A ten-hour marathon produced a count of 72 Emperors at Knepp on the 23^rd – half of what Matthew had anticipated. The Green Lane transect that day totalled 24, six below par for Week 2. 

Midsummer Day, then, was Judgement Day for the 2020 Purple Emperor season, nationally. Another huge effort by Matthew totalled 84 Emperors, including four females. This was way below Knepp’s mighty standards, but proved to be the highest count for the year anywhere (not least because the population crashed in Fermyn Woods, Northants, the No 2 site, where many of the favoured ride-side sallows had been felled under a Back from the Brink conservation project). Everywhere else, Purple Emperors were deemed to be ‘scarce’, except for one part of Alice Holt Forest, Hants, where a day count of 17 was made – the highest day-count away from Knepp all season.   

June 25^th was hotter (33·4C at Heathrow). This was the day of the infamous Bournemouth Beach invasion, when the people of Lockdown Britain broke out, flocked to Bournemouth, and generated 558 parking tickets. Knepp’s day-total was only 70 Purple Emperors, but their activity was suppressed by the heat. Incredibly, sallow searching had all but ceased – indicating that the season was past peak, and on the wane.  

On the 26^th, a thundery breakdown began. A few Emperor males descended to feed, belatedly, on the moistened Knepp paths. Then the weather collapsed, horrifically.

Riders on the Storm

Purple Emperor butterflies are extremely vulnerable to nocturnal gales (see His Imperial Majesty, Ch. 4, final para). We’d been lucky, no Emperor season had been affected by gales since 2010, when numbers were reduced by around 50% overnight. We were perhaps overdue. Sure enough, an autumnal gale developed on Saturday 27^th and raged long into Monday 29^th. 

Neil managed to see 11 females (the highest tally at Knepp in 2020) in very strong and increasing winds on Sunday 28^th, proving that the main emergence of Herself coincided perfectly with the gales destined to ruin this year’s egg-lay. Bizarrely, he also witnessed two male groundings that day (both were immediately overturned by gusts), highlighting the complete lack of common sense evident in this species. The butterfly should not have been airborne that day, but it carried on regardless, perhaps because the air was heavy with pheromones. 

You would have thought that a large arboreal butterfly would have the sense to roost through gales on the leeward side of trunks and sturdy branches, but not for nothing is this butterfly’s motto Arrogans et Temerarius (Arrogant & Foolhardy). The vast majority of the 2020 Emperors, like those of the gale-strewn seasons of 2005, 2009 and 2010, chose to roost in the treetops – and got shredded. Maimed males were found crawling around in Green Lane, incapable of flight. One ‘sensible’ male was found roosting on a sap bleed on the leeward side of an oak on the east side of Bentons Gorse, but he was probably inebriated. 

Worse, the females were also massacred, before they had managed to lay more than a small percentage of their eggs. And a second batch of gales hit us in early July – after both sexes had stopped emerging.   

This was the third whammy, and it was total – a similar fate to that suffered by the British fleet under Admiral Sir Cloudesley Shovell off the Scillies in October 1707. 

 

Battered Survivors

The 2020 Emperor season had been shot to pieces, and by July 9^th – the 50^th anniversary of Matthew’s first sighting of Apatura iris, just a mile from Knepp - the landscape colours told of summer’s ending. Males became afternoon creatures of the leeward (east-facing) side of Bentons Gorse and Wagstaffs Wood, and the more sheltered oak territories along Green Lane. 

The most (of both sexes) counted in a day at Knepp during July was a meagre 20, on the 3^rd. The only other double-figure counts were 17 on the 5^th, 13 on the 7^th and 11 on the 12^th. The females were particularly scarce, only showing up on or near oak sap bleeds. 

Matthew’s final sighting of the year was in the glade at the north end of Bentons Gorse (The Consistorial Court of Discipline territory) on July 22^nd, though apparently a female was seen down Green Lane the following day. The butterfly had flown at Knepp for 41 days, just under six weeks. Our final message to the 2020 Purple Emperor season is, don’t come back.  

The Purple Emperor was not the only butterfly adversely affected by the gales: the Purple Hairstreak suffered similarly, and the White-letter Hairstreak even more so – hardly any were seen at Knepp. 

 

Transect Data

Poor weather, plus filming and safari commitments, meant that Matthew struggled to fit in the weekly Green Lane Purple Emperor transect, particularly during the peak season period (Weeks 3 and 4). 

Here’s the transect data -

2015 =   112

2016 =     95 (estimate)

2017 =   117

2018 =   201

2019 =   101

2020 =     61

This means that 2020 was by far the poorest season in six years of careful recording (we lapsed rather in 2016, when we did too many safaris and recording was hampered by wind).  However, the figure is by no means disastrous. The butterfly fared far worse elsewhere. Generally, Emperors were so scarce that only a singleton was seen all season in the best county site in Bucks.

The Fourth Whammy

A major heatwave developed almost as soon as the Purple Emperor season ended. It was severe over Sussex between August 6^th and the 13^th, when it broke. The sallow fields at Knepp responded to this new stress my shedding leaves, especially the sub-canopy foliage selected by egg-laying females and favoured by first instar larvae. Something similar had happened during the hot summer 2018, when no rain fell at Knepp between May 31^st and July 28^th. But this time the damage was worse, perhaps because the sallows (and other trees, including veteran oaks) had already been stressed by the gales. 

The canopies of the densely-packed, drawn-up sallows in the ex-arable fields suffered most, especially of the narrower-leaved sallows. Also, a lot of damage occurred along southern and western edges, probably due to gale stress. The pollard-type sallows, free-standing standards, coppiced trees and sallows along east-facing and north-facing edges were largely ok. As in 2018, the broad-leaved sallows fared better.

The good news is that some of the Emperor’s main breeding grounds were only lightly affected, notably the extensive sallow fields in Brookhouse 8, 9 & 11. However, sallow stands in the north-east sector of the Wildland were severely damaged: 27 Acres, Rainbow, Honeypools House and Oak Field, which are all major breeding grounds.    

Our guess is that in the region of 25-40% of eggs and young larvae perished because of the August drought. That wouldn’t matter much in normal year, let alone after the profuse egg lay of 2018; but after 2020’s very poor egg lay, it matters greatly. The sallows are likely to recover, as they did in 2019 (when they over-flowered, and leafed late). 

 

Prospects for 2021

As things stand, the prospects for next year are fairly dire, but much depends on the coming winter. The last thing the Knepp Wildland Purple Emperor population needs is another mild winter, as tit populations are booming - ‘when tits are up, Emperors go down’. Send for The Beast from the East and, better still, the bitter December of 2010 (to drive the tit flocks out of the Wildland)!

Hope is provided by the tale of 2012 and 2013. We suffered our wettest summer on record in 2012, and the Purple Emperor egg lay was consequently poor. However, that meant that numbers of wintering larvae were so low that the titmice didn’t home in on them, and winter actually showed up. 2013 was one of the best Purple Emperor seasons on record. 

 

Filming

TV cameras were rolling at Knepp in 2020, big time. Passion Pictures were filming a documentary of Isabella Tree's book Wilding, and Silverback Films were filming for the long-overdue BBC blockbuster on British wildlife. Both strongly wanted to feature the Purple Emperor. 

Neil guided Passion Pictures on 25^th, 26^th and 28^th June, with the first two days providing some stunning footage, as a dozen Emperors, all in pristine condition, swarmed at low level around the favoured ‘feeder tree’ mid-way along the west side of Bentons Gorse (Rosemary’s Tree). He and cameraman Simon de Glanville managed to manoeuvre a probe lens to within a couple of centimetres of a newly hatched Empress’s eyeball and, in the evening, filmed Emperors beating up Purple Hairstreaks in the canopy. The 28^th was too windy to film the Emperor action, but they secured some excellent footage of Turtle Dove.

Matthew helped Silverback, who intend to include a 3-4 minutes-long piece on the Emperor in the Woodland programme. It’s vitally important that they get everything they want as their series also features our arch rival, the Large Blue. However, the females gave them a hard time, and they narrowly missed out on filming a mating pair. They will be back filming at Knepp in 2021. Look out for the swallow-chasing sequence, it may be the silliest thing ever shown on TV.

 

Safaris & Purple Visitors

The safari programme was truncated and started late, due to Covid restrictions, and commenced after the weather had deteriorated. We ran morning and afternoon groups on Sat July 4^th & Sun July 5^th and Sat July 11^th & Sun July. The weather was dire on the first weekend but we managed to maintain our 100% record of showing Emperors to each group. The saving grace was Matthew spotting an inebriated male conked out by an oak sap run in heavy drizzle on the morning of July 4^th. 

In addition, Matthew set up the self-guided Purple Emperor safari trail again. It was very well used and appreciated, though several visitors commented that they found parking at Dial Post too much and would have preferred to be able to use the solar panel field parking area, as in 2019. 

Others, from Sussex in the main, parked along Countryman Lane and used the Green Lane corridor. Their cars were generally outnumbered by dogwalkers’ cars. Very few Purple Emperor visitors were found off-piste. 

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Please note that Knepp is not open access land, though it is crossed by some statutory rights of way and permissive paths. The best ways to see Emperors there are to come on one of the half-day safaris, under expert guidance, or use the self-guided Emperor trail mentioned above (details on Knepp Safaris website). Please do not wander about, you will be asked to leave.