The emperor strikes back
Once thought extinct in many areas, purple emperors are now turning up everywhere, even in supermarkets.
COVER STORY Having once eluded even the keenest lepidopterists, purple emperors are now widening their range
Though most of our UK butterflies have suffered horrific declines in recent decades, a brave few are bucking the trend. None more so than the purple emperor, Apatura iris, long regarded as a rare denizen of southern oak woods – even though its essential requirement is the humble sallow bush, the caterpillar’s foodplant.
The State of the UK’s Butterflies 2015, a review produced by Butterfly Conservation and the Centre for Ecology and Hydrology, shows that during the 10 years leading up to 2014, the purple emperor increased its distribution by a staggering 135 per cent. Its closest rival was the silver-washed fritillary, another showy butterfly, which expanded its range by 55 per cent during that time. The next review, likely to be published late next year, should show the continuation of that positive trend.
In recent years, individuals of this distinctive butterfly have turned up in an impressive range of unlikely situations, which is not surprising, as it has eccentric tendencies and pushes limits – all limits.
Purple emperors have been seen in several supermarkets (mainly Tesco, but also Sainsbury’s and Waitrose), two nursing homes, two public schools, a hospital, a borstal, a prison, a trout farm, an ammunition dump, a crematorium, the national film archive at Berkhampsted, outside the Royal Courts of Justice in central London, and, best of all, the departures lounge at Gatwick Airport. Most of these were dispersing males.
These intriguing records suggest that this is a surprisingly mobile butterfly, capable of traversing large tracts of landscape; both rural and suburban. It may also be an occasional short-haul migrant, as suggested by the report of a male flying around a fishing boat two miles off the Brighton coast in 1919. It had come from France. This butterfly has to move about to track down new habitat patches – it’s just that it now seems to be doing so on a scale previously unknown.
Secret beauty
Our difficulty with this large and magnificent butterfly was that we did not know how to look for it, and what we thought was knowledge was mostly myth and assumption. This is a canopy-dwelling species, which we normally only see through narrow fissures in its treetop world. Also, it doesn’t feed on flowers, favouring various oozings from deciduous trees and, less frequently, revolting substances on woodland rides, such as fresh fox scat. In fact, it looks
Victorian butterfly collectors sought it with unrestrained ardour, to form the centrepiece of their precious collections.
and acts quite like a tropical forest butterfly, living up top and descending only to imbibe.
The purple emperor has long been the centre of naturalists’ attentions. The Victorian butterfly collectors sought it with unrestrained ardour, to form the centrepiece of their precious collections. They had two techniques for precuring specimens: a net on the end of a very long pole (6m or longer) and catching males as they descended to feed on something nasty. The latter necessitated hanging around gamekeepers’ gibbets and middens, or carting some stinking excrement into the woods.
The early Victorians discovered that male purple emperors favoured particular trees, branches even, which every year were utilised by territorial males in the afternoons. These trees became known as master trees, or master oaks. At one point, it was believed that whole populations would vanish if the master tree was felled.
Moreover, people tended only to visit a few well-known localities. The insect undoubtedly became seriously under-recorded as a result. This bad habit is only just changing now, as more and more localities are discovered as the result of concentrated survey efforts.
Just before the start of this century, lepidopterists at last began to get to grips with the purple emperor. Surveying and monitoring came to the fore in an effort to develop science-based conservation.
We began to look for so-called master trees and, within a short while, we had developed a survey technique that works in most situations. The results were impressive and are reflected in The State of the UK’s Butterflies 2015.
Butterfly hunt
Andrew Middleton and Liz Goodyear, of Butterfly Conservation’s Hertfordshire and Middlesex Branch, started searching for the purple emperor in Hertfordshire first, one of many counties where the butterfly was deemed extinct. Their initial surveys, from 1999 to 2002, located small populations at six sites. The butterfly has since been found to be fairly widespread in the county, though populations are low.
Inspired, and enabled, Andrew, Liz, and their co-workers, went on to rediscover purple emperors in Middlesex, Essex, Suffolk and Cambridgeshire – other counties where the butterfly was considered long extinct.
The butterfly was also found in Norfolk, reappearing in woodland between Cromer and Holt in early August 2016.
Declaring the purple emperor extinct in a county almost inevitably leads to rediscovery. In recent years, the butterfly has made a comeback in no fewer than 15 counties from where it was deemed lost during the middle period of the 20th century. Some of these were almost certainly recolonisations; others may have resulted from the resurgence of lingering populations; whilst a reappearance of the butterfly in Warwickshire was kick-started by a couple of introductions from which natural spread has then occurred.
Andrew, Liz and other members of the purple emperor fan club had worked out how to find male territories. This is relatively easy on sloping or undulating terrain, though most populations are very small – ones and twos hither and thither, mostly thither. Males gather on prominent trees – both broadleaved and conifer – on wooded and sheltered high points, away from air turbulence. There they await the arrival of females in need of male services. These territories are upslope of sallow bushes. The secret is to study maps and aerial photos during the winter, locate sallows when they flower in spring, and then home in on likely male territories during the midsummer flight season. Many territories are along the leeward side of wood edges.
This technique works brilliantly, except in woodland of an even height on level ground, where males seldom establish territories. Here, the butterfly has another strategy for ensuring that boy meets girl: males search areas of sallow-rich scrub during the mornings in the first half of the flight season, when the females are emerging. However, once the female emergence is complete, the males stop doing this and take the mornings off, behaving rather like ageing rock stars.
Aerial combat
The afternoon shenanigans of male purple emperors have to be seen to be believed. These guys, for all their iridescent beauty, really do know how to behave badly. They are utterly belligerent and fearless, especially for an insect lacking a sting or bite. Males vigorously dispute the possession of gaps between tall trees. When an incomer invades an occupied canopy gap, the resident male will launch himself into the air and
The afternoon shenanigans of male purple emperors have to be seen to be believed. They know how to behave badly.
then the two will circle around each other two or three times, spitting hellfire, before one will chase the other up and away, at speed, often out of sight. Regularly, the two return for a rematch. Eventually, one will be driven away and will steam off unruffled to the next territory, and the next punch up.
They will attack anything above the size of a bee that enters their airspace. The range of birds on the modern hit list includes all three species of woodpecker, crossbills, siskins, cuckoos, turtle dove, hobbies (a mistake…), goshawks and white storks. The birds tend to ignore the disgruntled butterflies, of course. Willow wonders The purple emperor was wrongly considered to be an oak woodland species. Oak is useful because its dense foliage provides shelter up top and because both sexes feed regularly on oak sap bleeds. However, the butterfly is essentially a species of sallow-rich landscapes, preferring goat willow, Salix caprea, though true willows, such as white willow, S. alba, are also being utilised.
Herein lies the problem, for sallow bushes were regarded by landowners and foresters as invidious weeds, especially after World War II when the ‘dig for victory’ ethos was still prevalent. It is this prejudice that rendered the butterfly a rarity, forcing it to subsist at population levels that were so low we could scarcely detect it.
Today, it occurs in a diversity of habitats, and may well be moving into new ones, including sallow scrub on both urban brownfield sites and colliery wasteland. There is a colony on Hampstead Heath, and populations are also being discovered in river valleys and around disused gravel pits.
The prospects for this butterfly look rosy or, even better, purple. However, its success means that it will be demoted down the conservation prioritisation list, especially by foresters and nature reserve managers wishing to clear sallow scrub.
The purple emperor may be moving north, outside its historic range – but no one has looked. The extent to which it is benefiting from climate change is debatable, as mild winters impede larval hibernation and lead to high levels of predation by flocks of titmice. Also, early springs followed by poor early summer weather render sallow foliage too coarse and thick for young larvae in July and August – the insect becomes out of sync with leaf development.
Critically, this butterfly simply needs large areas of sallow scrub, especially of the broader-leaved varieties. The purple emperor is not a rare species, but one that has been severely suppressed by prejudice against sallows, and one we didn’t know how to look for. We do now.