Prelude: Death from above
Today, you are a honeybee and today you are going to die.
You enjoyed a summer full of industry, dance and frenetic activity collecting nectar for the hive along with your 10,000 sisters. But all of that is about to change.
They came from the East.
One arrived this morning, unnoticed, a vanguard. After spotting your hive it flew back to its nest to recruit others. Now, a storm is gathering above you. Shadows skate overhead, silent portents of impending calamity.
This darkness is cast by Asian hornets, Vespa velutina. They are specialised honeybee hunters. Each one patrols its own small territory above the hive. Their ferocious mandibles and instincts designed to cleave through the carapaces of you and your sisters. Together the hornets create a tightening net through which you cannot escape.
If you were an Asian honeybee the arrival of the first hornet would have triggered one of the most extraordinary defensive maneuvers in the natural world. You and your fellow workers would surround the hornet. Rapidly twitching your wing muscles to generate heat, you would have smothered it in a pulsating mass, cooking the hornet alive in a blaze of cooperative fury before it could call for reinforcements. Together, you might just have survived.
But you are not an Asian honeybee.
You’re a European honeybee and neither you nor your ancestors ever faced such a threat, until now. As such your genetic and behavioural make-up is bereft of the tools or tricks with which you might hope to defend yourselves. What little resistance you can put up is futile.
After their grizzly work is done, the hornets take no honey. That is not what brought them here. It is the sinews of you and your sisters that they feast upon.
This morning the chambers of your hive were alive with the buzz of activity. This evening things are different. Drips of honey echo in silence.
25 Genomes: The Asian Hornet
I’m Alex Cagan, a post-doctoral researcher in genetics at the Wellcome Sanger Institute. The Institute turns 25 this year and to celebrate we are sequencing the genomes of 25 species found in the UK that have never had their genomes sequenced before. Over the course of the year I’ll be going ‘behind-the-scenes’ to chronicle this ambitious project in various ways. In doing so I hope to throw some light on the scientists, institutes and species involved in doing this kind of large-scale science and the reasons why it’s being done in the first place.
The Asian hornet is one of these 25 species being sequenced by the Wellcome Sanger Institute (https://www.sanger.ac.uk/science/collaboration/25-genomes-25-years). It is one of five species that was decided upon by a public vote, in a head to head competition with other species. The case for sequencing the Asian hornet was championed by the Sumner lab, a eusocial insect research group based at University College London (http://www.sumnerlab.co.uk/im-a-scientist-get-me-out-of-here/). They tell us that the Asian hornet is ‘a dangerous invasive species that poses a huge threat to bee populations in the UK and elsewhere in Europe’. Of all the 25 species being sequenced it’s also the newest arrival in the UK. The first confirmed sighting of a nest was in Gloucestershire in September 2016, with a second found in Devon in 2017.
For many, the mere mention of the word ‘hornet’ is enough to set the pulse racing. Even if we’ve never encountered one, the image of a swarm of angry wasps on steroids looms large in the imagination. But what of the reality? Are they truly winged nightmares or is this reputation undeserved? We already have hornets in the UK, so what makes Asian hornets so special? In search of answers I headed to London to meet with Dr Gavin Broad, Principal Curator in charge of insects at the Natural History Museum.
Behind the scenes
Visiting the NHM is always a joy. Encounters with wonders there as a child inspired me, along with countless others I’m sure, to become a biologist.
I’ve always had a yearning to go behind the scenes and glimpse the vast collections that have never been on display. I have read stories of the museum’s legendary collections, particularly in the excellent book ‘Dry store room No1’, by Richard Fortey (for a review see: https://www.theguardian.com/books/2008/jan/26/history). I imagine endless corridors stacked with shelves containing specimens from all branches of the tree of life, gathered by naturalists across the globe. The biological equivalent to Borge’s fictitious Library of Babel, with specimens that display the near infinite variety of forms produced in nature. A library of life.
It turns out my imagination is not far off. After signing in as a guest I meet Dr Gavin Broad at his desk. It is covered with a combination of books and wasp specimens of different sizes, encased in tubes or glass-windowed wooden boxes. I’ve come to the right place, I think to myself.
We begin by heading straight to the museum’s collection of Asian hornets. Dr Broad leads me to a corridor lined with tall grey metal cabinets as far as the eye can see. Each one has a number and a brief description of the contents within. After a few seconds I’m already disorientated. We stop at one case that looks to me like all the others. This one is labelled 58 - Vespidae / Vespinae?. He spins a wheel attached to the cabinets and the stacks begin to move, making a central row of cabinets accessible. Opening one of the top doors reveals a stack of wooden shelves that immediately fills the air with the rich smell of timber.
Dr Broad retrieves the top two shelves and lays them on a table. Framed in glass are rows upon rows of hornets. They are beautifully laid out. A single pin connects each hornet to a paper record written with impossibly small, impossibly perfect handwriting. It is evocative of a golden age of natural history. An age, it dawns on me as I stand surrounded by cabinets of carefully documented insects, that may never really have ended.
We take the shelves to a nearby room to take a closer look. Peering at them through the glass the first thing I notice is that they’re smaller than I expected. I’d imagined something at least thumb sized. They’re still larger than common wasps, but not by much. They’re darker too, a deep velvety black covers most of their body, in brilliant contrast to the yellow and orange hues that highlight their face, abdomen and the tips of their legs.
The population of Asian hornet that has begun to colonise Europe, Vespa velutina nigrithorax, has a particularly dark appearance. Up close it becomes clear that they are also covered in a fine layer of fuzzy hair. I think it would be wrong of me say this makes them look cute, but it certainly adds... character. As we spoke Dr. Broad took a high-resolution photo using the museum’s special insect imaging setup. Take a look and decide for yourself:
A brief history of hornets
As I peer at hornets Dr Broad provides me with the broad brush strokes of their evolutionary history. Hornets, it turns out, are the result of a series of key evolutionary innovations. Hymenoptera, the order of insects containing hornets (as well as wasps, bees and ants), emerged in the Triassic, over 200 million years ago. One of the keys to their success was the evolution of the ovipositor, a specialised organ for laying eggs. This appendage really opened doors for the hymenopterans, as they could now lay their eggs in new places. These places could be hard to access crevices, where eggs would be safer from predators. Or, perhaps most ingeniously, on and even inside food sources such as fruits, or, in the case of the devious parasitic wasps, other insects.
Fast forward millions of years and we arrive at the emergence of the subclade Aceuleata. In these hymenoptera the ovipositor underwent a startling transformation. Instead of delivering eggs it became dedicated to delivering venom. The dreaded stinger was born. An organ used to create life metamorphosed into one that takes it. This sent the Aceuleate down a very different evolutionary road. While their ancestors laid their eggs on the food and then effectively abandoned them, the Aceuleata started to hunt and provision food for their offspring instead.
The next key development in the path leading to hornets was the emergence of eusocial societies. Eusociality is considered the highest level of organization among animal societies, defined by cooperative breeding, with breeding and non-breeding members dividing labour and working together. There are many startling parallels with our own societies here, yet they remain distinctly other and fascinatingly so. Eusociality appears to have arisen independently multiple times in the Aceuleata, in the social wasps, bees and ants. It may be that eusociality was made possible by the previous innovation, the stinger, which provided these fledgling societies with a viable way to defend their nests from intruders.
And what nests they are.
Not only does the NHM collection include specimens of the hornets themselves, they also have rows of boxes containing nests of different species. Dr Broad opens one box, at least an arm span wide, that is filled with a near complete example of an Asian hornet nest. It’s truly an architectural wonder. Created by wood pulp chewed and mixed with hornet saliva, it is both incredibly light and unbelievably sophisticated. The nest would have originally hung from a tree, and indeed a few remnant branches are still embedded in it. In this nest large swathes of the rippled paper shell are missing, revealing the interior structure. Horizontal discs, inlaid with hexagons that housed the larvae, are suspended by vertical columns that connect the levels. All the more remarkably, each nest is created and then abandoned within a single year.
It’s amazing to think that insects are capable of building such structures. One wonders what lessons there might be here for our own architects, given what these hornets can achieve with only a little wood and a little spit. Indeed, the field of biodesign continues to draw inspiration from these invertebrate constructs. Though for all their beauty the fact that they are usually packed with thousands of hornets would make me hesitant to admire one in the wild, where they are more akin to a devil’s piñata.
Having seen the rather beautiful hornets and their astonishing nests I’m starting to wonder what the big deal is about the threat of Asian hornets colonising the UK. Afterall, we have native hornets in Europe already, what harm could another species possibly do?
Apparently quite a lot. It turns out that unlike our native hornet species, which tend to be generalist hunters of many different insects, the Asian hornet is more like a precision honeybee-seeking missile. Asian hornets will search for honeybee hives and once they find one they will engage in ‘hawking’ behavior. This involves hovering near the entrance to the beehive, killing bees as they fly to and from the nest or try to defend themselves. The hornet, covered in a tough carapace, is practically impervious to the attack, while it’s own powerful mandibles are quite capable of tearing the poor bees apart.
In Asia, the local honeybees have coevolved with these hornets for millenia. This has resulted in the honeybees developing more effective defence mechanisms. The most famous being the ‘bee ball’ formed by Japanese honeybees. While European honeybees have been observed forming these bee balls, they do not appear to be of a sufficient intensity to effectively kill attacking hornets.
So for now the Asian hornets in Europe have the upper hand, mounting raids on honeybees that do not yet have a way to defend themselves. European honeybees have been in a perilous state for many years, with colonies collapsing at alarming rates due to a variety of factors, such as heavy pesticide use, that remain poorly understood(?). They really deserve a break. The arrival of a new and voracious specialised predator is the last thing that they need. Scientists and farmers alike are concerned that if Asian hornets become well established throughout Europe this will have a devastating impact on agricultural productivity. Or, in the worst case scenario, it could be the last nail in the coffin for the European honeybee.
How did they get here?
While there have only been two confirmed nests in the UK, Asian hornets are already established in France and Spain. How did they get to Europe in the first place? The most likely scenario, Dr Broad tells me, is that a hibernating queen arrived as a stowaway in a crate. It turns out that, like us, social insects such as the Asian hornet are particularly well-suited to becoming international colonisers. Though the reasons for their success are perhaps quite different from our own. The queen hornet mates with males only once in her life, during an initial mating flight. For the rest of her life the queen will carry the sperm she received in a specialised organ. She will use this to fertilise all future eggs she lays (apparently this remarkable process of internal fertilisation on demand is so precise that on average less than two sperm are released per fertilisation). Therefore, it only takes a single queen to establish a viable population. A lone voyager who contains within her the seeds of an entire society, waiting to bloom.
Vespa velutina have a particularly wide geographic range in Asia, this ability to tolerate a variety of climates likely contributes to their success as invaders. However, where exactly within Asia these invertebrate pilgrims came from remains a mystery. Here is where DNA could provide the answers. The DNA sequence of an individual is rich source of information but to understand it we first need a road map, a ‘reference genome’ from the same species that tells us the general shape and structure of the genome so that we can make sense of the pieces from any given individual. We hope that sequencing and assembling such a reference genome for the Asian hornet can shed light on this mystery, as well as many of their other secrets. What else can we hope to learn from sequencing the Asian hornet genome?
A genomic perspective
As well as helping us identify where exactly in Asia the hornets are likely to have come from the genome will enable us to design markers to help monitor their spread across Europe. This kind of information could be crucial to monitor which populations are spreading the fastest and which management strategies are proving most effective.
The Asian hornet genome can help us to understand the biology that underpins the amazing adaptations of this species. While this knowledge is fascinating in its own right, it could one day be used to help control their spread. For example, the Sumner lab proposes that if we can identify the chemical odorants and compounds that this species is sensitive to that information could be used to try and manipulate their behavior to limit their production.
Their website has more ideas on how the genome might be used, including the potential for biocontrol by gene-editing - and a list of great references if you want to get into the details.
So far, at least in the UK, vigilance has paid off. Both Asian hornet nests were quickly identified by the public and removed before they had a chance to spread. If you’d like to get involved there is an ‘Asian Hornet Watch’ app you can download (what a world!) to report sightings of Asian hornets (https://itunes.apple.com/gb/app/asian-hornet-watch/id1161238813) and a handy identification poster (https://www.pestmagazine.co.uk/en/library/posts/2015/asian-hornet-alert-poster).
I leave the Natural History Museum filled with a sense of awe, both for the Asian hornet and the museum’s collections which make such intimate encounters with the natural world possible. The carefully preserved and recorded specimens are a priceless trove of information. Sequencing the genome will be the latest chapter in our efforts to catalogue and understand the hornet. A new piece in an old game.