A Sting in the Tale (17 page)

So what were these bees doing? Remember that, in her own nest, it is not in a worker's interest to lay (male) eggs so long as her mother is laying female eggs, since she is more closely related to her sisters than to her sons. This argument assumes, reasonably, that the nest has finite resources (i.e. food) and cannot rear unlimited numbers of both. But if a worker can get into a nest of entirely unrelated bees, she should not care that any eggs she lays might be reared in place of the queen's offspring. Any reproduction she can get away with in this context is a bonus, increasing the genes she passes on to the next generation. We do not yet know how common this is in more natural situations, but a recent study of the Japanese bumblebee,
Bombus deuteronymus
, found unrelated workers in three of eleven wild nests studied, with these workers producing 19 per cent of the males from these three colonies, so it is clearly not just confined to buff-tailed bumblebees.

A related tactic is also exhibited by bumblebee queens. Some queens emerge from hibernation later than others, even within the same species. Perhaps they choose a particularly shady spot to hibernate, or manage to burrow deeper into the earth where it is cooler. Or perhaps they are just naturally late risers. Whatever the reason, by the time they emerge from hibernation many of the best nest sites have already been taken. One can imagine one of these late queens, repeatedly exploring promising-looking holes in the ground, and each time finding another queen already in residence. As each day passes without her starting her own nest, the season slips away. It takes time to build up a large nest and produce lots of daughter queens and sons, so if she starts her nest too late it is unlikely to be very successful.

In these circumstances, it is common for the queen to attack. If she can kill the resident queen, then she can claim control of the nest site and also take over the resident queen's brood. You may wonder why she should wish to look after another queen's offspring. In most organisms this would be a very silly strategy, but not so in social insects. The brood are destined to become workers, and they will work just as hard for their adoptive parent as they would for their mother, not being able to tell the difference. A similar strategy is used by Australian choughs (birds of the crow family), which live in family groups with just one breeding pair and lots of younger helpers. These groups will readily kidnap half-grown birds from other groups, incorporating them into their team of helpers and thus improving their chances of rearing more offspring.

In the case of bumblebees, by killing the resident queen the intruder is saving herself all the hassle of looking after a colony in its very early stages, albeit at the cost of having to engage in a fight to the death against an individual of similar size and strength. We do not know how common such nest usurpation is, but there are accounts of bumblebee nests being excavated to find as many as twenty dead queens inside, and one live one. Whether all of these dead queens were failed usurpers, or whether the nest had been successively taken over by twenty different queens in succession is impossible to say (at least without DNA fingerprinting the queens and workers, which has not yet been done).

Once a nest has adult workers they should help their queen to repel or kill a usurper, since if their mother dies they are doomed to slavery. For this reason it seems likely that usurping is harder the larger the nest, but on the other hand the prize to be won also becomes greater.

Such usurpation can take place between different bumblebee species, but generally only between closely related species. Buff-tailed bumblebees, for instance, will often try to invade nests of white-tailed bumblebees, but rarely the other way round since the white-tailed bumblebee queens tend to emerge a little earlier than buff-tails. Similarly, in Arctic North America,
Bombus hyperboreus
frequently usurps
Bombus polaris
– and because the Arctic season is so short, the usurping queen does not rear any workers of her own, but instead only new queens and males.

One group of bumblebees, known as cuckoo bumblebees, have become specialists in this tactic, entirely giving up their social lifestyle in favour of life as specialist assassins. There are six species within the UK, all belonging to the same genus as the ‘true' bumblebees,
Bombus
. This means that they all have a common ancestor, and would once have all had a similar life cycle, probably similar to most ‘true' bumblebees today. But at some point in their evolutionary past, the ancestor of the cuckoo bumblebees evolved down a different route. It is easy to imagine how it happened, and it presumably began as opportunistic usurping, with one late-emerging species often trying to usurp queens of a related, earlier-emerging species. If the likelihood of success in founding a nest by the conventional route was significantly lower than the odds of successfully usurping a queen from an existing nest, then over time the usurping species may have specialised. So doing, they opened up the possibility of evolving physical characteristics to make usurping easier. As a result, these cuckoo bumblebees tend to be larger and have a thicker external skeleton than the ‘true' bumblebees, which presumably makes it harder for the resident queen or her workers to sting them to death. It is certainly harder to push a pin through the thorax of a cuckoo bumblebee compared to a ‘true' bumblebee.
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Cuckoos can also be recognised by the lack of pollen baskets in the females; they don't need them since they don't do any foraging for the nest.

We don't know for sure how cuckoo bees find bumblebee nests to attack, but it must surely be by smell. When keeping artificially reared nests in boxes outdoors it is common to see cuckoo bees flying and walking around searching for an entrance, so they can clearly tell that there is a nest close by. Having found the entrance, the cuckoo bee barges past any workers that get in her way and attacks the resident queen. The queen will usually fight to the death, and with the aid of her workers she may sometimes succeed in killing the intruder. Should she be killed, however, the cuckoo bee will take her place. Occasionally the resident queen will even acquiesce rather than fighting to the death, become subservient to the cuckoo bee and behave like one of the workers. In either case the cuckoo bee will lay eggs, and the bumblebee workers care for them as they would their own. The cuckoo may also eat any eggs or young larvae in the nest, but tends to leave older larvae to develop into workers which will help care for her own offspring. Moreover, cuckoo bees do not produce their own workers, so the female is not, strictly speaking, a queen. All of her eggs develop into fertile offspring, either males or females. Having taken over a nest, the cuckoo bee queen will continue to lay eggs until the workforce she has coerced into her service begins to die off. There is no supply of further workers, so once taken over by a cuckoo bee, the nest will not last for long. But it will usually survive long enough to produce more cuckoo bees, so continuing the cycle.

The inherited workers continue to work for their new mistress presumably because they have few other options. It is said that they often try to lay eggs, but the cuckoo will chase and bite workers that she finds attempting to do so (just as their mother would have done if she were alive), in a largely successful attempt to keep order. It would be interesting to see whether these enslaved workers are more prone to drifting off to other nests of their own species to try laying eggs there. Presumably this would be in their interests since otherwise they will spend the rest of their days rearing offspring of an entirely different species.

Although cuckoo bumblebees all have a common ancestor, there are now thirty or so species in the world, each specialised to some degree on a particular host. The commonest cuckoo bee in England is usually the southern cuckoo, which targets buff-tailed bumblebees (it presumably being no coincidence that buff-tails are the commonest ‘true' bumblebee species). In most respects the life cycle of cuckoo bumblebees is rather similar to that of their hosts. Mating occurs in mid- to late summer, and only the females will hibernate. Males tend to be much more common than females, and can be very abundant – sometimes the commonest bumblebees to be seen – when feeding sluggishly on flower heads of thistles, knapweeds and bramble. Interestingly, the cuckoo species often have a very similar colouration to their hosts – the hill cuckoo, for example, is black with a red tail, and is superficially very similar to its host species the red-tailed bumblebee. Some years ago, it occurred to me that cuckoo females might also mimic the smell of their host; if they did, it would be less likely that the queen and her workers would sound the alarm and mount their defences. All bees are coated in an oily mix of hydrocarbons, the same compounds which make up the smelly footprints on flowers. The precise mix differs between bee species and also probably differs a little between members of different nests within a species, enabling workers to distinguish nest mates from non-nest mates. I started collecting any female cuckoo bees that I could find and storing them in sealed vials in a deep freeze, with a view to analysing them when I had examples of each species. Unfortunately for me, some cuckoos are rather rare and this progressed slowly. Before I could collect enough to do anything useful, I noticed a new paper by Steve Martin at the University of Sheffield in which he had investigated this in some detail, and shown quite convincingly that cuckoo females do indeed have a smell which closely matches that of their host. My samples are still in the freezer.

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.