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Dancing in the Spring: Purple emperor butterfly

Updated Thursday 1st April 2010

From an ancient woodland in Wiltshire, conservation advisor for the National Trust, Matthew Oates, talks to the Saving Species team in an extended interview, and opens the door to the hidden world of the purple emperor butterfly.

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Copyright The Open University


Respondent: Well welcome to the magical world of the Purple Emperor. Now he is not actually a butterfly at the moment because it is early February and this insect spends the winter as a hibernating caterpillar. So what we’re here to do is to look for the hibernating caterpillars or larvae of this very elusive difficult to see butterfly. One of the problems with Purple Emperors is that they occur at very low population density. So you never see loads of them. The nearest of our common butterflies we’ve got to that is the Comma, which you only ever see in ones and twos. Well the Emperor’s similar to that but he’s worse because he’s an arboreal, canopy dwelling butterfly. So actually seeing them is incredibly hard.

Anyway one technique which I’ve been developing is actually that of trying to survey for them by looking for their larvae; tiny little caterpillars as they are at that time of the year in late summer and early autumn, and we’ve actually had some success on all that. But our knowledge of this butterfly is, always has been terribly inadequate. There’s a lot of mythology, which of course is part of our relationship with it, but it’s very difficult to get involved in the conservation of something about which you know very, very little.

So what I’d been doing, last summer I got on a bit of a roll. I actually made a breakthrough, it just happened, you know, serendipitously by happy chance, as things do, and I started finding the eggs. And then I started finding the young tiny little larvae, little baby caterpillars. And then I started looking at the type of leaf situation in which I was finding these eggs and larvae, and started writing everything down, meticulously described everything, tried to categorise everything, in particular the leaf texture and colour. And I made a bit of a breakthrough there and in very, very crude terms I found that the butterfly was preferentially selecting leaves of a certain colour and texture in four words: mid green, soft matt. Mid green, soft matt - now as things stand that’s probably the nearest we’ve got as yet to cracking the essence of the Purple Emperor.

Now it breeds on sallow trees, oh yes everybody knows sallow trees, pussy willows and so on. Well actually they’re a taxonomic nightmare and some of them are extremely difficult to actually separate. There are species, subspecies and hybrids. But what’s really important is the texture of the leaf which we really view in terms of colour.

Interviewer: And is that the texture of the leaf at the egg stage or at the larval stage?

Respondent: It seems very much that the females select leaves of a certain texture on which to lay their eggs, and they’re clever, these empresses, they know what they’re doing; they’re investing in their children. They seem to be laying their eggs primarily where they know that the caterpillars are going to be able to successfully feed. Because this tree here, which very much fitted the mid green, soft matt category revealed no less than eight Purple Emperor larvae.

Interviewer: And one of the things we were seeing, as we were coming up, you have wonderfully named all these larvae after English poets. So who do we have on this tree at the moment?

Respondent: Yeah. As a scientist one is obliged to give them all thoroughly boring numbers. And indeed these guys have thoroughly boring numbers. But I’m afraid I’m not just a scientist, I also suffer from parrotry, as many other people do, and we have a tree full of English language Welsh poets here. We have several Thomases; RS Thomas, Dylan Thomas, Edward Thomas and so on. And the trouble is I’m not entirely sure who everybody is here because they’re so close together. This was one of the most amazing moments of my life when I suddenly discovered a branch with six Purple Emperor larvae on it.

Interviewer: And that is very unusual in terms of Purple Emperor ecology?

Respondent: Absolutely. Most of the trees I searched had nothing on them. And how on earth did I stay sane? Well that’s because I did most of my searching during the Ashes series last summer and I was listening to test match special. But anyway you’re looking at one here.

Interviewer: I just can’t see anything, where is it?

Respondent: He’s next to a bud, he’s under a centimetre long, and he has very cleverly camouflaged himself to match in with the bit of twig next to the bud. And you may just be able to see it. There’s a tiny bit of silk around him. He’s actually sitting on a silk pad. He’s very, very clever. So he’s lined up on a sallow twig, near the tip and he’s next to a bud, and he’s in tune with that bud. He’s listening to it, he’s waiting for it, waiting for it to slowly, slowly, slowly swell and grow, and as that bud swells and grows it changes colour, and as the bud changes colour you will find, I know this, I’ve been breeding them in captivity, that the caterpillar will change colour to match the bud.

Interviewer: So they’ve got this real harlequin lifecycle in this larval phase?

Respondent: Absolutely. Now as larvae lasts, when I was finding them last July and August they were matching the colour of the leaf they were on, and they sit in a very particular place on the leaf, on the upper side of the sallow leaf at the leaf tip, where the tip curls down and where the raindrop gathers, and they sit in the raindrop.

Interviewer: And why not?

Respondent: It’s probably a very good way of avoiding predators. But that’s where they always sit. But they match, in July and August and into September they match the colour of the leaf. Now as the leaf changes colour, so the caterpillar changes colour too. And then by late October they’ve gone a very strange colour, they are not green at all; they’re sort of a grey brown, you think oh that’s dead. And then, as the leaves start to fall off the trees, the caterpillars very wisely crawl off the leaves, and they’ve got to do that before the first wicked autumn gale of the year of course. And so they are seeking to hibernate on the bare stems of the sallow trees.

Now I followed, I think I lost track of my sense of direction, I think I followed 46 into hibernation, and it’s never been done before. I mean prior to this I personally have found I think three in hibernation and lost the lot around about Christmas, New Year period so I’d never, never had them this late into the winter. So this is very much terra nova. Anyway they come off the leaves wisely before the leaves come off the trees and crawl around seeking somewhere to hibernate.

Now some of these 46 I’ve followed have just gone less than 10 centimetres and just conked out on the nearest bud; others have moved an awful long way. John Milton no less has gone 3.5 metres and he’s less than a centimetre in length.

Interviewer: He’s a long distance mover?

Respondent: He’s done incredibly and I did incredibly well to find him. You know, it took me hours to find Milton. I mean the efforts one has to put in on behalf of Milton in this life are just incredible. But anyway most of them have gone into hibernation next to the buds. A few haven’t, and you must look at this fellow here because he hasn’t. He’s gone into a fork, now can you see that - it’s extremely difficult.

Interviewer: Oh! Yes, yeah, right in the sort of the ‘v’ of the fork.

Respondent: He’s absolutely in the ‘v’ of the fork, his head rolled over, turn the thing around you see its backside, it’s a bit better there.

Interviewer: And he’s a different colour to the other one we’ve just been looking at?

Respondent: He’s, well it doesn’t look alive does it? He looks dead, he’s a horrible, grey, dirty, oh there’s no life in that! And incredibly enough the actual fork, the actual sallow twig he’s got on is actually covered in almost a little mucus of algae. Grey green algae has developed during the winter in the damp weather, and it’s developed over this caterpillar as well, and he’s matching it. But you’ll find that that guy is alive and well, and these little caterpillars, you know, are now fast asleep in hibernation absolutely conked out. It’s probably in some sort of Buddhist state in fact.

In a few months’ time they’re going to be beating each other up and beating other things up. They’re going to rule this forest. So we’re waiting for spring and we’re waiting for those sallow buds to swell and then burst, and then of course we’re going to start feeding. And these tiny little caterpillars are then going to feed up on warm spring evenings late April and throughout May, and they grow incredibly fast, and he’ll grow almost to the size of my middle finger, especially if it’s a female because a female’s a bigger butterfly and bigger caterpillars. And then they will pupate. We don't rightly know mechanics of that in the wild. From breeding them in captivity we know that they pupate on the underside of the sallow leaf and that the pupa matches in colour the colour of the underside of the sallow leaf perfectly, and they are horribly difficult to find.

I’ve found one, two Purple Emperor pupae in the wild, and in both cases I have to confess by accident rather than by deliberate design. And then they’ll spend about three weeks as a pupae, and then around about Midsummer’s Day they start emerging. It used to be early or mid-July but flight seasons of most of our butterflies have got earlier and earlier really these last 20 years and that may be an attribute of climate change. And so they then start emerging around about Midsummer’s Day. They don’t all emerge on the same day, they emerge over about a two-week period; first mostly males and then the latter part of the emergence is mostly female. And we don't know how long an individual Purple Emperor lives for because we have been unable to do effective mark and recapture on them.

It’s not easy doing mark and recapture on an arboreal, canopy dwelling butterfly that doesn’t visit flowers and which occurs at very low population density. I’m on to it. I’ve got some data but nothing like enough. Anyway I can tell you they can live for at least two weeks. We know that much. Late July they start to die off and really by something like 7th or 8th of August these days they’re gone. We’re left with eggs which then hatch after about a couple of weeks depending on the weather and then these little green larvae which feed incredibly slowly at the leaf tips, sitting on a leaf tip day on day and day and end, and they feed very intermittently.

The study I’ve done here found that they merely feed two or three times a week. It’s completely stochastic. I can’t predict when they feed and when they’re not going to feed. It is in dry conditions rather than during the rain. But they feed very slowly, grow very slowly and then as I said change colour, go into hibernation mostly on the bud. Some of them like this grey fellow here are at forks and you occasionally get them on leaf scars as well. There’s three main colour forms. Sort of yellow green, which is on the twig next to the buds, and these grey ones in the forks, and there’s a rather rare colour form which is almost a chestnut brown. It’s rather pretty which you get on sort of the browner coloured, cleaner stems. But I’ve only found three of those. We must go see them because one of them is no less a person than Lord Byron.

So that’s basically the year of the Purple Emperor in a nutshell but the big thing of course is first we sleep the winter, then we dance the spring and that’s what we’re waiting for.

A purple emperor butterfly Copyrighted image Icon Copyright: Wiki CreativeCommons Kristian Peters
Purple emperor butterfly





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