A Sting in the Tale Page 14
Although they mimic their hosts in both colour and smell, the disguise of cuckoo bees is clearly not perfect, and they are often attacked. Sometimes I have seen them take refuge in the depths of the comb of the nest, where few workers ever venture. This might well allow them to improve their disguise by covering themselves in the oily hydrocarbons of their hosts, for usually after a day or two in hiding they venture out and assassinate the queen without her workers coming to her aid.
It is easy to think badly of cuckoo bees – I have met people who have been distressed to discover that a bumblebee nest in their garden has been taken over, and have even heard it suggested that we might somehow try to cull cuckoo bees in order to conserve their hosts. This attitude, although understandable, is as nonsensical as condemning a lion for eating a gazelle. Nature is red in tooth and claw, and is so much the richer for it. How sad would it be if we did not hear the sound of (avian) cuckoos in late spring? As we shall see in the next chapter, in addition to cuckoo bees, bumblebees are attacked by a huge range of predators and parasites, all part of a natural community which has existed and co-evolved over millennia. So long as there is enough natural habitat, bumblebees can support this rich diversity of life. Of course the flip side of this is that if we allow bumblebees to disappear, then we will also lose many other fascinating but less well-known creatures besides.
CHAPTER ELEVEN
Bee Enemies
A person who is too nice an observer of the business of the crowd, like one who is too curious in observing the labor of bees, will often be stung for his curiosity.
Alexander Pope
One of the more obvious features of bumblebees is that most are attractively coloured, with bright bands of yellow, and red or white bottoms. They are brightly coloured for a reason: they have a sting. Or at least the females do – it evolved from the egg-laying tube – and they use it to defend the nest against invaders, such as cuckoo bees, or predators. Bumblebees are generally good-natured creatures and almost never use their sting when away from the nest, preferring to fly away if disturbed when foraging. Even in their nest, many species are not very aggressive; I have dug up nests of the early bumblebee without needing any protective gear, the adult bees simply clustering around their brood and buzzing nervously rather than launching an attack. Buff-tails and tree bumblebees are a little more feisty, and their nests are best left alone – on rare occasions I have even been chased by particularly aggressive workers. When really riled, they will bite and jab with their sting at the same time, and are not easily deterred. I hate to admit this, but on more than one occasion I have accidentally pulled the head off a bee when trying to remove one which had fastened its jaws on to my clothing. It is a common misconception that insects die after they sting. This applies only to honeybees, which have strongly barbed stings which lodge in the flesh of their victim. The bee cannot then escape, and so continues to pump venom until it is swatted – even then the sting and contracting venom sac often remain in place. On the other hand the stings of bumblebees – and for that matter wasps – are not barbed, so they do not get stuck in their victim, and the stinging bee does not die. Hence a bumblebee can, in theory, sting you over and over again until she has run out of venom, so it is fortunate that they generally choose not to do so.
Stings are obviously a very effective defence, but it’s even better not to have to use them. That is where colour comes in; as an advert to warn potential predators that this particular prey is armed and dangerous. Many insects which have stings (such as wasps) or are poisonous (such as cinnabar moth caterpillars) have yellow and black stripes, a common signal aimed at predators such as birds that hunt primarily by sight rather than smell. The idea is simple – if birds can’t tell which insects are harmful and which are not, they will just attack indiscriminately. By the time they discover that the insect they are trying to eat has a sting or tastes awful, the insect itself may well have been badly damaged. Far better, then, for both bird and insect if the insect broadcasts the fact that it is not good to eat.
The more common the signal, the quicker predators will learn it. This is probably why lots of very different insects use similar signals – yellow and black stripes, or black and red spots (e.g. ladybirds, burnet moths). This leads to one of the great sources of frustration for anyone interested in bumblebees – they are rather hard to identify because many different species use exactly the same colour pattern and so appear very similar. Many species of bumblebee have black and yellow stripes with a whitish tail. Some are black with a red tail. Often, species with near-identical colouration may be quite distantly related, but natural selection has encouraged them to appear as similar as possible. Thus the red-tailed bumblebee and the red-shanked carder bumblebee both have the same colours; the only obvious difference is that the latter has reddish hairs on the pollen basket on its hind legs (from which it gets its common name), while the former has black hairs.
Harmful insects sending out a clear warning is all well and good, but it is a system which is open to cheating, for many insects that don’t have any such defences have copied the warning colours of genuinely noxious or dangerous insects, in the hope of fooling predators into avoiding them. Hoverflies provide a good example of a harmless mimic – many of them have yellow and black stripes, similar to wasps or bumblebees, yet they do not have a sting and are perfectly palatable to eat (I cannot confirm this from personal experience). This makes life pretty confusing for the predator. How does it know which signals are genuine and which ones are fakes? If the harmless mimics become too common relative to the models, predators will learn to ignore the warning signal.
Mimicry of this sort can even take place within species. Male bumblebees tend to be broadly similar in colour to their sisters, but they have no sting. Their only protection is presumably the fact that they look like their sisters, and that they appear after a spring and early summer in which the only bees on the wing have been females (with stings). If caught, male bumblebees jab their bottom at their attacker as if trying to sting, presumably in a last-ditch bluff. In fact the males of most species are fairly easy to distinguish to the human eye, particularly since many have tufts of yellow hair on their faces. Once you become experienced in bumblebee identification, you can amaze and amuse your friends by catching male bees with your bare hands. No need to tell them that male bees don’t have stings (at least not until later); instead, tell them that you have a natural way with bumblebees, that they trust you. Of course this takes a little confidence; best to practise without a crowd as you are likely to appear quite foolish if you get it wrong – not at all the heroic look you were going for. Bumblebee venom is similarly painful to that of a honeybee, i.e. an initial squeal and some hopping about is hard to suppress. It contains a whole range of chemical compounds, including histamines, which have been ‘designed’ by evolution to hurt as much as possible.
It has puzzled me for many years that a handful of bumblebee species do not bother to have warning colouration at all. The common carder bumblebee is, for instance, a scruffy brown with a few tufts of black, yet as well-defended as any other bumblebee. Many years ago I became convinced that they must have warning stripes in ultraviolet (which birds and bees can see but we cannot), but after several weeks spent working out how to take pictures in ultraviolet it emerged that they are just as dull in this spectrum as in the one we can see. Interestingly, the tails of white-tailed bumblebees are UV-reflective, presumably enhancing their signal to birds.
All of this said, it is rather rare to see predators eating bumblebees. In North America, robber flies and beewolves will take smaller bumblebees. Robber flies are splendidly ugly, powerful flies with humped backs, which grab their insect prey in mid-air. Beewolves are wasps which catch bees on flowers – they sting them to paralyse them, and then feed them to their grubs. In Europe, beewolves seem to mainly eat honeybees; I have watched them for hours returning with prey to their burrows on the sandy banks of the River Charente in France and have never once s
een one bring back a bumblebee. European robber flies are also mostly too small to tackle a bumblebee. There are some birds further south in Europe that are well known for eating bees – bee-eaters being the most obvious. The species of bee-eater found in southern Europe is an exotic, colourful bird, with a russet cap, blue wings and a yellow throat. They are very agile and fast fliers, snapping up bees on the wing with their sharp, down-curved beak, then deftly nipping off the sting. I have also seen a flock of them perched on fence wires next to honeybee hives in the Sinai Desert,19 picking off every bee as it tried to leave the hive. I’m sure they must take bumblebees too, but bumblebees tend to be rare in southern Europe because it is too warm for them.
Shrikes also eat bumblebees. These rather splendid birds have the gruesome habit of impaling their surplus prey on the spines of thorn bushes as a snack for later. They feed on a range of big insects such as bumblebees and grasshoppers, and also frogs and small mammals. Shrikes are moderately common in parts of southern Europe but are very rare in Britain, so here at least it seems unlikely that they pose much of a threat to bumblebees.
Not so long ago I was of the opinion that bumblebees had a pretty easy time in Britain, there seemingly being few predators able to attack them. Smaller workers sometimes get caught in spiderwebs, and are occasionally trapped by crab spiders,20 but otherwise they seem pretty safe. Or so I thought, until the spring of 2008, when I was sent a pile of dead queen bumblebees, enough to fill a large strawberry punnet, by a Mrs Barbara Baker who lives in the west of Scotland. She had picked the bees off her lawn, from beneath an overhanging sallow tree. There was a whole range of species, including many bilberry bumblebees which are rather rare in most of Britain. Each bee had been carefully dissected, with the top of its thorax removed and all of the flight muscles scooped out, leaving the thorax looking rather like a hard-boiled egg after one has spooned out the contents. Barbara hadn’t seen the culprit, but a bird seemed likely. Sallow catkins are a popular source of food for bumblebee queens in spring and clearly something was attacking the bees in the tree.
By coincidence, in July of the same year I received a letter from Anne-Marie Smout, a keen naturalist who is heavily involved in biological recording in central Scotland. Anne-Marie and her husband Chris had just come back from holiday in Denmark, where they had been staying with friends. The friends’ garden had some large lime trees. Buff-tailed and white-tailed bumblebees love the flowers of lime trees, although there is something in the nectar which seems to make them dopey and even sometimes to kill them. One morning when having breakfast on the lawn, Anne-Marie noticed many dead bumblebees under the trees. On closer inspection she found that each had been attacked in the same way – the back of the head had been opened up, and the brain scooped out. The abdomen had also been opened and hollowed out. Intrigued and slightly horrified, Anne-Marie and Chris set out to discover the culprit by hiding themselves out of sight and watching the tree with binoculars. They didn’t have to wait more than a few minutes. A whole family of great tits emerged, a pair of adults with their young, and they continued their banquet. Being apparently drunk on the lime nectar, the bees were easy prey to the birds, which had presumably learned how to avoid the stings and peck open the bees’ bodies.
I wrote a short article on this for Buzzword, the newsletter of the Bumblebee Conservation Trust, and this provoked further records of great tits eating bumblebees. In particular places, the birds seem to have developed different techniques. Some birds seem to open up the thorax, others nip off the tip of the abdomen, still others go for the head, or different combinations of the three. Tits copy one another, so presumably one of the adults in the family that Anne-Marie observed had discovered how to eat bumblebees, and the rest of the family copied the technique. It is reminiscent of the spread of milk-bottle-opening behaviour in blue tits in the 1960s. When I was a child, it was normal to have milk delivered to the doorstep. It arrived in glass bottles with foil lids. Every doorstep would have a tile or piece of wood to hand, which the milkman would balance on top of the bottles. If he did not, within minutes a blue tit would appear, peck open the foil, and drink the cream from the top of the milk. I must admit to feeling slightly sad that this rather cute behaviour has all but died out, as most people now buy their milk at the supermarket, or have the low-fat variety delivered, which has no cream on top.
Luckily for the bees, cases of great tits attacking them in this way seem to be very scattered and sporadic, and usually associated with a large flowering tree that is attracting lots of bees. However, this isn’t the end of this particular story, as we shall see.
Although foraging bumblebees may not have too many predators, bumblebee nests are a different matter. A large nest contains stocks of honey and pollen, and many tasty grubs and pupae, as well as the adult bees: a very valuable food resource to any creature able to overcome the defences of the bees. Steph O’Connor, my bumblebee sniffer-dog handler, has spent the last few years study-ing the predators and parasites that attack bumblebee nests. She has become remarkably good at finding bumblebee nests, considerably exceeding the abilities of poor Toby. Her project, funded by the Leverhulme Trust, was to find nests (with or without the aid of the dog), and then to follow their progress for the rest of the season, recording what attacked them, and whether they survived to produce new queens and males.
She and I investigated various camera systems, and in the end adopted one which had been developed by the RSPB for monitoring birds’ nests. The system has a small camera mounted on a post and trained on the bumblebee nest. The camera is connected to a large, buried Tupperware box in which resides the recording equipment and battery. Once installed the whole set-up is very inconspicuous, the camera being painted olive green for camouflage and nothing else being visible. The software records continuously, but immediately deletes the information if no movement is detected around the nest entrance. This greatly reduces the amount of data storage necessary, so that data needs downloading (and batteries renewing) only every few days. The camera is sensitive to infrared and has its own built-in infrared light so that it can detect movement through the night. In 2010 and 2011, Steph used this set-up to film thirty-six bumblebee nests from when she found them until they expired. The footage consisted of every ‘event’ during the remaining life of the nest – rather tedious to watch since most of the events were bees entering and leaving, or grass and leaves blowing in the wind. But every now and then the cameras revealed something exciting and new. No one has ever watched bumblebee nests in this way before.
It turned out that the most common visitors were mice, voles and shrews. Some nests had a steady traffic of small mammals going in and out, but we still don’t know exactly what they were doing. Charles Darwin was interested in the relationship between mice and bumblebee numbers. In his most famous publication, On the Origin of Species, he suggested that red-clover pollination (which is exclusively carried out by bumblebees) was better in the vicinity of villages, for villages have cats and cats eat field mice, which would otherwise eat the bumblebees:
‘The number of humble-bees in any district depends in a great degree on the number of field-mice, which destroy their combs and nests; and Mr H. Newman, who has long attended to the habits of humble-bees, believes that “more than two thirds of them are thus destroyed all over England.” Now the number of mice is largely dependent, as every one knows, on the number of cats; and Mr Newman says, “Near villages and small towns I have found the nests of humble-bees more numerous than elsewhere, which I attribute to the number of cats that destroy the mice.” Hence it is quite credible that the presence of a feline animal in large numbers in a district might determine, through the intervention first of mice and then of bees, the frequency of certain flowers in that district!’
Thomas Huxley, a great friend and supporter of Darwin, took this apparent logic even further by arguing that the power of the British Navy was attributable to spinsters, who keep cats, which eat mice, which therefore don’t eat bumblebee
s, enabling the bumblebees to pollinate clover, which is fed to cattle, from which salted beef was derived, which was the staple food of the British Navy. In fact there is little evidence to back any of this up, although it makes a lovely story. It is possible to turn the logic entirely on its head, because bumblebees regularly use the abandoned nests of small mammals such as mice to create their own nests. We don’t yet know the net effect of this interaction, although an educated guess might suggest that, for bumblebees, there is an optimum number of small rodents, sufficient to provide abandoned nests but not so many that most bumblebee colonies are then eaten. Despite the frequency of small mammal traffic in and out of Steph’s nests, she found no evidence that nests visited by many small mammals were more likely to die, suggesting that the mammals were not eating large numbers of brood. For all we know they may simply be sharing an entrance hole with the bees, for the tunnels to which such holes lead are usually variously branched and can go deep underground. Until we can find a way to get the cameras into the nest itself – which is a possibility using a flexible endoscope – we will not know for certain what is going on underground.