Flight of the bumblebee
Zooming in like a mouse on wings, a bumblebee prepares to gorge on a favourite food source: tree lucerne. Cast in popular imagination as quaint, cumbersome and “of no use because they don’t make honey,” the bumblebee’s virtues as a tireless and effective pollinator are only now being appreciated and put to work. Far from being a droning bumpkin which somehow defies the laws of gravity, the bumblebee is being heralded as the gardener’s true friend.
Stroll with me through Auckland’s Parnell Rose Gardens on a glorious summer afternoon. Drink in the scent that wafts from sun-warmed roses, drenching the air with floral extravagance. Taste the tang of the sea breeze rising from the harbour below. This is a great place for bumblebees. All abuzz, these lumbering zeppelins zigzag from flower to flower. Follow one if you can. Here burrowing into a rose that hasn’t quite opened, six legs scrabbling furiously: there frolicking amongst the stamens in the centre, wriggling its abdomen and buzzing intermittently.
See how choosy they are. That bed of sagging lilac roses is popular, as are the red and yellow roses further down, but the rest may as well not exist. Even within a favoured bed, many blooms are ignored. Do the bees have some secret sense which informs this capriciousness? Indeed they do. Like university students choosing courses for a degree, each bee majors in a small number of flower species, learning the fastest way to extract the nectar or collect the pollen, and the best way to force an entry into the flower. Yet each will minor in a few other flowers by periodically sampling their rewards, just to cover its options.
When we think of flowers being pollinated, we generally think of honeybees, but in many ways bumblebees are superior pollinators. The larger, furrier bodies of bumblebees collect more pollen from the stamens (male flower parts) and make better contact with the pistils (female flower parts) than do honeybees or other insects. “It’s like comparing a powder puff with a cotton bud,” remarked one bumblebee enthusiast.
Honeybees and native bees are often too small or selective to effectively pollinate some flowers. Flowers that provide only pollen, such as tomatoes and kiwifruit, are generally not attractive to honeybees, but bumblebees will happily work these blooms if the hive is supplied with a sugar solution to compensate for the lack of nectar.
Tongue length is also important to the bumblebee’s success. Honeybees have short tongues compared with those of bumblebees, and are not interested in working tubular flowers (such as broadbeans and red clover) because they usually cannot reach the nectar at the bottom of the flower. In fact, the lack of adequate red clover pollination—with the resultant lack of seed set—was the reason bumblebees were brought to New Zealand in the first place. Their importation from England between 1876 and 1885 made history: it was the first time that insects had been introduced to a country specifically to pollinate a particular flower. New Zealand has no native bumblebees, and red clover is an important pastoral crop. Farmers could not afford to continually import seed from England to replenish their pastures.
Four species of bumblebee were imported before it was realised that bumblebee tongue length differs from species to species. The most widespread species in New Zealand, the large earth bumblebee (Bombus terrestris), was later found to have the shortest “tongue”—strictly a proboscis—rendering it less effective for pollinating red clover. The other three species have longer tongues, but have more restricted distributions (see fold-out following page 96). Compared with honeybees, though, even the large earth bumblebee has a long tongue.
Bumblebees do have one vice: they are inclined to cheat. If a large earth bumblebee cannot reach the nectar of a particular bloom, then it will often bite a hole in the base of the flower and rob the nectar that way. This bypasses all the pollination specialisations of the flower and prevents seed-set, because other insects will use the same hole to rob the flower on subsequent visits. The mandibles of honeybees are usually not strong enough for this sort of breaking and entering.
The length of the bumblebee tongue is easily appreciated if you have the patience to quietly sit and observe bumblebees working. Occasionally, when collecting from large vertical inflorescences such as foxgloves or delphiniums, the bumblebee will not retract its proboscis between flowers, and resembles a tiny, flying, striped elephant.
One situation where bumblebees have demonstrated their clear superiority over honeybees is in the glasshouse—especially tomato glasshouses. In Warkworth, Dave and Alison Taylor have been using bumblebees to pollinate their hothouse tomatoes for four years, and are thrilled with the results. The day I visited, Dave had just received a fresh shipment of bees and was about to set them to work. We stepped inside the glasshouse—rows and rows of plants were supported by strings from overhead supports, and thousands of little yellow flowers promised a feast of pollen for the newcomers.
Alison told me that bumblebee pollination sets more fruit and produces better-shaped and tastier fruit than artificial methods do (either a hormone spray or handheld vibrators). She says that they get about a third more fruit using bumblebees. Indeed, in their glasshouse, nearly every position of the ripening racemes was filled by a developing tomato.
But why use bumblebees? Why not honeybees? For the simple reason that honeybees refuse to work tomatoes. As well as having no nectar, tomato flowers have their pollen inside the anthers, rather than clumped on the tips like sherbet. Honeybees are not boisterous enough in their gathering to dislodge the pollen. Bumblebees, on the other hand, buzz the flowers vigorously, and are rewarded with a shower of pollen falling on to their bodies. To a honeybee, a tomato flower is about as interesting as a used bus ticket!
Another reason why bumblebees make good glasshouse workers is that they are available in small quantities. Whereas the average honeybee hive contains many thousands of workers, commercial bumblebee colonies contain fewer than 100. Honeybees would work a glasshouse in a matter of minutes. then be bashing their heads against the panes trying to get out. Honeybees naturally forage over a range of several kilometres around the hive, and are therefore ill-suited to the confines of an 800square-metre glasshouse. Bumblebees, on the other hand, prefer to forage close to the nest, and don’t seem to mind a closed environment.
When it comes to work, bumblebees are the draught horses of the bee world. In a tomato glasshouse, a single bumblebee is capable of pollinating 450 flowers per hour, taking an average of eight seconds to harvest pollen from one bloom before travelling to the next one.
Bumblebees fly and forage in misty, rainy weather, and at temperatures just a few degrees above freezing—conditions in which a honeybee won’t budge from home. Bumblebees can fly in adverse circumstances because, unlike most insects, they are able to maintain their flight muscles at a more or less constant 34-40°C—well above air temperature.
The muscles of most terrestrial animals don’t work properly if they’re cold. To overcome this problem, furry and feathered animals maintain a constant body temperature (we call this being “warm-blooded”), while most insects and reptiles heat their muscles up by sitting in the sun or vibrating their wings.
Bumblebees can decouple their wings from the flight muscles that drive them, and warm up these muscles simply by shivering. Using this approach, a bumblebee can raise the temperature of its thorax from 24°C to 37°C in one minute. Bumblebees are also clad with a fine fuzz, which plays a major role in conserving body heat. These peculiarities are a heritage of the bumblebee’s geographic heartlands: cool temperate latitudes of the northern hemisphere, with a few species even extending beyond the Arctic Circle.
Although airborne or foraging bumblebees maintain a nearly constant thoracic temperature, the temperature of the abdomen (which has no flight muscles) can be varied by the bee to suit its needs. On a cold day, the bumblebee uses a countercurrent heat exchange arrangement in its blood vessels to conserve heat in the thorax, allowing the abdomen to cool close to ambient temperature. On a hot summer’s day, when extensive flying threatens to cause overheating in the thorax (insects can’t sweat, although bumblebees can evaporate nectar off their tongues to cool down), the heat exchanger in the bee’s waist is bypassed, so heat can be dissipated from the abdomen. When they are not flying, bumblebees allow their bodies to cool down to ambient, to save energy.
Bumblebee bodies are partitioned into three parts, like those of most insects. Sensory and feeding structures are concentrated in the head. The thorax contains little but blocks of muscle to power legs and, especially, wings. Most else happens in the abdomen. Honey is stored there in the honey crop (a highly extensible bladder between the oesophagus and intestine), and air sacs also take up significant space in the anterior of the abdomen. Further back are excretory organs, reproductive paraphernalia and the sting. Yes, bumblebees do sting!
An oft-quoted myth states that a bumblebee can’t fly because its wings are too small, but nobody has told the bumblebee yet, so it keeps on flying. This fallacy may have arisen in the 1930s when, during a dinner conversation, a biologist asked an eminent Swiss aerodynamics professor to comment on the aerodynamic capabilities of the wings of bees and wasps. The aerodynamicist became intrigued by this question, and made some preliminary calculations on the basis that the wings were flat plates, and therefore prone to stalling. These theoretical calculations “proved” that the bumblebee could not fly.
In reality, the aerodynamics of many insect wings, including those of bumblebees, have more in common with helicopter blades than with the fixed wings of conventional aircraft. Two pairs of wings attach to the thorax of the bee, but the wings on each side are joined with hooks, so that they act as one. Even so, the surface area is quite small, and the wings must beat at about 200 beats per second to keep the bee aloft. This is 10 to 20 times faster than nerve impulses can travel from brain to muscle, so, unlike vertebrate muscles, where each contraction is the result of a separate nerve impulse, bumblebee muscles are asynchronous. This means that the contraction of one pair of muscles stretches the opposing set, and the stretching is sufficient stimulus to initiate contraction of the stretched muscles, and so forth. Occasional nerve impulses serve simply to indicate that flight is still required.
The lift produced by this system is sufficient to hoist at least twice the bumblebee’s body weight—an important consideration, since a full load of nectar and pollen can weigh nearly as much as the worker bee carrying it. Furthermore, a bumblebee can still fly when half the wings have worn away. Far from being an aerodynamic non sequitur, these creatures could teach us a thing or two about staying aloft.
The same wing muscles which power flight are also used to produce the characteristic bumblebee buzz. In Christchurch, I visited Industrial Research, where a small team of people investigates vibrations of economic significance—including, surprisingly, the buzzings of bees. Industrial Research invented the rubber shock absorbers which are now being used internationally to protect buildings and bridges from earthquake damage, and the company’s interests also include reducing vibrations in band saws to reduce wood wastage and monitoring vibrations in the Hamilton jet engine to make it more efficient. One of the group, mechanical engineer Marcus King, has been investigating the effect of buzzing on the anthers of various flowers.
Of particular interest has been kiwifruit, for which bumblebees are excellent pollinators. Kiwifruit anthers look rather like the rockets in early cartoon strips. Vibrations of the right frequency cause the “booster region” of the rocket to twist and warp, splitting the “fuselage” and hurling the pollen out into space. Kiwifruit anthers require vibrations of 6930 Hz before they spontaneously explode, but the bumblebee buzz ranges from 300-400 Hz (a frequency close to the G above middle C on a piano). How then do bumblebees get the pollen off the anther?
The secret, Marcus found, was in the loudness of the buzz and the way in which the bee collects the pollen. If you watch a bumblebee on a poppy (or some varieties of rose), you’ll quickly notice that it uses its legs to rake up a bunch of anthers and hold them against its body, then “buzzes” them before moving on to the next group of anthers. The buzz causes the hard plates that make up the exoskeleton of the bumblebee to vibrate, and the amount of energy that can be transferred from the buzzing bee to the anthers is so great that the pollen literally explodes outwards. It is not just the thorax, where the muscles are situated, which vibrates, but the whole body, head included. It is this powerful buzz that makes the bee such a good pollinator. If you hold the stem of the flower while the bee is buzzing, you can feel the strong vibrations through your hands.
As well as using the buzz to dislodge pollen from anthers, bumblebees produce it during nest building (to compact soil), when threatened by a predator (to scare it away) and when “angry,” such as on a windowpane.
By recording the bumblebee buzz and then slowing it down, it has been shown that the bee does not emit a constant sound, but a series of short (less than a tenth of a second) buzz pulses. Nearly everything that a bumblebee does is geared towards obtaining food for the hive (as pollen or nectar), and a series of short pulses requires less energy to produce than a continuous bombination.
Marcus King says that a bumblebee can’t fly and buzz at the same time because the same muscles are used in both actions. “The hoverfly, which looks rather like a bee, can fly with one wing, and buzz with the other,” he told me, “but a bumblebee buzzing while flying up a windowpane is actually alternating flying with buzzing. You can’t really tell with the unaided ear and eye, and the bumblebee doesn’t lose any height in the process because the pulses are so short.”
Exactly how bumblebees buzz remains something of a mystery. It is thought that the flight muscles are decoupled from the wings (as in shivering) and used to rattle the thorax at a frequency about double that of the wing beat in flight.
Horticulturally Speaking, the 1990s are shaping up to be the decade of the bumblebee. In New Zealand. the first person to try his hand at rearing bumblebees for use as commercial pollinators was Nelson Pomeroy, who is now one of the country’s largest bumblebee producers.
His enthusiasm for insects started when he was a youngster growing up in Taranaki. “I used to catch admiral butterflies,” he told me, “and then I became fascinated by bumblebees. I kept hives of them in my wardrobe. They got in and out through a copper pipe in the wall. Later, at Massey University, I was still able to keep bumblebees in the wardrobe of the hostel.”
After completing a PhD in Toronto on the finer points of how bumblebee colonies grow, Nelson returned to Massey as a lecturer. Although he had expected to abandon Bombus, it was not to be. The Development Finance Corporation, which was financing aspects of the kiwifruit industry, predicted that within a few years there would be insufficient honeybees to pollinate all the country’s kiwifruit orchards, so he was urged to mount a project on the mass rearing of bumblebees for kiwifruit pollination. European horticulturalists were becoming intrigued by the possibility of using bumblebees for pollinating crops in glasshouses, and a Dutch company started to sponsor his research. He left the university to carry out contract research full-time for his Dutch clients, and eventually set up a commercial bumblebee-rearing operation of his own—Zonda Resources—in Hawkes Bay.
“Bumblebees can pollinate virtually anything that isn’t well pollinated by honeybees, especially if the honeybees’ failure is due to cold,” Nelson told me. “Tomatoes, capsicums, melons, kiwifruit [the predicted shortage of honeybees never arrived] and overseas blueberries are well pollinated by bumblebees.”
Zonda has been selling bees in a worthwhile way only since 1992. Before then, the use of insecticides in glasshouses would have wiped out any bumblebee foolish enough to venture its proboscis inside the door.
The bugbear of tomato growers’ lives is whitefly, and in 1992 new methods became available to control it. One was a minute parasitic wasp that deals death to whitefly, and another a chemical that prevents whitefly moulting, and so stops its development. This chemical isn’t toxic to adult insects such as browsing bumblebees. Nonetheless, the chemical is mainly used as a back-up, and the wasp is the main controller of whitefly.
West Auckland tomato grower Tony Ivicevich is full of enthusiasm for bumblebees. He explains the advantages: “In winter, especially, much pollination used to be done with a hormone spray that produced small, pulpy, poorly-shaped fruit. Little pollen in pollination results in few seeds in the growing tomato, and few seeds result in low hormone levels and small, poor fruit. The hormone spray was like pollination with little pollen. Using bumblebees, there is maximum pollen in each pollination, and every flower sets fruit. In fact, we can now afford to thin out any lower quality fruit. We also get a lot less botrytis, because the bumblebees carry the pollen back to their hives. Using hormone spray, flowers full of pollen would fall on the lower leaves of the plant, and the pollen provided a wonderful substrate for fungal growth. So with bumblebees, our fungal spraying is way down, too. Using bumblebees as pollinators benefits the consumer in better fruit and far fewer sprays, and it saves me 12 hours’ manual pollination work each week.”
The only drawback to bumblebees is that they don’t come cheap, Tony confides. “There goes another $4 flying past,” is his wry comment upon spotting one of his charges. “I pay $180 per hive, but the price varies depending on how many hives you buy and how many bees there are in each.”
Nelson Pomeroy tells me that it takes ten weeks to raise a hive containing 50-100 bees, and it has a short “shelf life” once ready. He guarantees the viability of his hives for a month after purchase, but some survive two to three times as long.
Zonda was the first commercial bumblebee operation in New Zealand, but it is no longer the only one. A small but highly competitive industry has sprung up around bumblebees, including one firm that supplies bumblebee queens to European growers.
Bee Pacific, in Christchurch, supplies hives to both local growers and the Japanese market. Director Clive Washington explained that the road to successful bumblebee exporting has not been smooth. “New Zealand growers are much more adaptable than most, and are willing to spend a bit of time in setting up a hive and modifying it if necessary, whereas Japanese growers want a plug-in hive that is completely ready to go on arrival.”
Bee Pacific was the first to introduce bumblebees for pollination into Japan, but European producers quickly introduced more sophisticated “ready-to-go” hives. To compete, Bee Pacific has had to continually modify and improve its packaging and presentation. They are into their sixth design in just two years.
Looking back on the whole experience, Clive commented: “I don’t think I would have bothered with bumblebees if I had known in advance how much effort and capital would be required, but we’re in too deeply now to quit.” One of the biggest headaches initially was the transport of hives. The hives have a gravity-fed sugar feeder on top. and a number of shipments arrived in Japan upside down with the hives in a sticky mess. Cargo handlers in both countries are starting to learn how to treat a shipment of live bumblebees, Clive says, but problems still arise. Recently, most of a shipment of hives was lost because they were transported around Japan in sub-zero temperatures on the back of an open truck—the bees just froze to death. Likewise, leaving a container of hives outside in the noonday summer sun means that the bees will cook.
The bumblebee industry is cloaked in secrecy. Clive thinks that the Europeans are able to produce their own queens and colonies year-round, but they aren’t letting on. Here it is no different. Bee Pacific cultivates its bees in cargo containers, each outfitted with temperature and humidity control systems. I was only permitted to peer into a container where the hives were in the final stages, because the rest of the procedure was said to be too commercially sensitive. Everybody I spoke to was cagey about their methods, and wouldn’t let on just how many bees they produced or where they were distributed. This gave me quite a different perspective on that fuzzy little bee bumbling around in my garden—not only a hard worker, but a focus for commercial intrigue as well.
At dusk, a venerable green Subaru station wagon slips through the back streets of Blenheim. On the driver’s door a large round decal proclaims “Bee Wares: Bombus Collectors.” At the wheel is Don Perkins, en route to a suburban property to collect a few Bombus.
Don and Maria Perkins run a small bumblebee enterprise from their home in Grovetown, just north of Blenheim. In the past they have caught wild queens for Nelson Pomeroy and others, but are now raising and selling their own hives. To build up their stocks, they advertise to remove unwanted bumblebee colonies from people’s back yards, and it was on one of these expeditions that Don was embarked.
He arrived at a section that would have made Steptoe & Son’s premises look like a public library. Meditatively, he inspected debris littering the floor of a large garage, and then, motioning us into silence, pressed an ear to the concrete. Slowly he crawled about, listening here, hesitating there.
Dissatisfied with the sensitivity of the unassisted ear, he resorted to technology. An inverted plastic kitchen funnel attached to a foot of garden hose served as something between ear trumpet and stethoscope. Vibrations detected by the inverted funnel sprang from the hose into his attuned ear.
But not this evening. “I can’t pick up anything, but the hive has got to be here if that’s where you saw them going in.” The owners nodded assent. “Vacuum cleaner ready? Let’s see what we’ve got, then.” After another 20 minutes of delving and trowelling among the concrete slabs, we had nought but the corpses of a couple of workers (bumblebee workers, that is), and the vacuum cleaner remained silent and empty. “Looks like they are all dead now. I’m sure they won’t trouble you again,” said Don, gathering up his equipment. While round one hadn’t exactly gone to the bumblebees, neither had it gone to the Bombus collector.
By now, the night was upon us, and our slightly furtive appearance climbing over a fence at the end of a drowsy surburban street should have troubled even the most complacent Neighbourhood Watch group. We walked a hundred metres through an area of waste ground to a point where Don had earlier observed bee activity. He started burrowing under the sward. No more than 20 cm down, torchlight revealed the furry blackand-gold of bumblebees, scrabbling surprised, like miniature bears disturbed in hibernation. “This is where the vacuum cleaner would be really useful. You can just suck the bees right into the dust bag where they’re quite safe until we get home. But because there’s no power here I have to pick them out by hand.”
Don is anxious to recover brood from the nest, not just adults, so he carefully separates the plump clay-coloured brood cells from the dirt. Without the vacuum cleaner there is little alternative to fingers for recovering the bees themselves. A few sting him. “They get a solid grip with their feet and jaws, and can really force that stinger in,” Don grimaces, “even through gardening gloves.” The bumblebee sting lacks the honeybee’s barb, so the bee can withdraw the sting and use it repeatedly, like a wasp. Within 30 minutes we were back at the car, bearing somewhat meagre booty: perhaps 20 bees and a small number of brood cells secured in a box.
Maria has the task of sorting out the hive and reasssembling it in a nest box. Part of their garage is w-lled off to form a darkroom for bee handling. Subdued light and warm temperatures (provided by heaters) keep the bees placid and facilitate handling. Our hive proves to be on its last legs—no queen, few workers and little brood. Maria transfers them to a box where, nestled between layers of underfelt, they have some chance of survival if a new queen can be introduced.
Over the last few years, much of the Perkins’ operation has consisted of catching young queens with butterfly nets while they forage in the wild (Maria’s speciality). then selling them for between two and three dollars each to nest producers. Now the two have become so bitten by the bumblebee bug that they not only sell colonies to horticulturalists, but offer “pet” observation hives for sale to schools or budding home entomologists, and are building a tourist exhibit to be called Bumbleland.
Don sucks at his stung fingers. Is the pain worth the gain? “I’d much rather be doing this than pruning pine trees, which is what I did before,” Don assures me.
Bumblebees belong to the same family of insects as honey bees, and both groups make a living from servicing flowers, but their differences are surprising. Honeybees (see New Zealand Geographic, Issue 2) form vast, highly organised hives populated by tens of thousands of bees. If the honeybee hive is a metropolis, then the bumblebee colony is a small rural village, rarely holding more than a few hundred inhabitants.
Bumblebee colonies generally only last a summer, and the cycle is rekindled each spring by the emergence of young queens from burrows in which they have overwintered. You’ll see them wheeling low over the ground, occasionally landing to investigate a promising hole or mat of grass. Each is looking for a place to establish a nest. Any dry, dark hole containing plant fibres will do—abandoned mattresses, compost heaps and mouse or insect holes are favoured sites.
Once she has found a site, the queen will rearrange the fibres to create a cavity at least a few centimetres wide and high, and build a thimble-sized cup out of wax to store nectar she collects from flowers. Wax is produced by glands on her abdomen and is scraped off by her legs. She will also make a small ball from pollen and lay a batch of eight to ten eggs on it.
Suitable nest sites seem always to be in short supply, so she may well have to fight off other covetous young queens. When hives are dug up, as many as 30 dead queens can been found. It’s not easy holding the fort!
The queen marks the position of the eggs with a dab of pheromone (scent) so that she can find them again to brood. The bumblebee queen broods with her abdomen, which is relatively hairless underneath (just as the brood spot on a hen is featherless), by holding it tightly against the eggs, and purposely heating it to between 34.5 and 37.5°C. She will sit on the eggs for as long as she can, sipping nectar from the honeypot for energy, and will only leave to collect more food. Provided she has food, she can maintain the temperature of her brood at up to 25°C above ambient for long periods.
Under ideal conditions, the larvae will hatch in four to six days and grow rapidly, feeding on the protein in the pollen ball along with freshly collected pollen and nectar the queen gathers. At first, the larvae share the same cell, which the queen enlarges as they grow and moult, but later each spins a tough silken cocoon creating a chamber of its own.
The larvae develop into pupae after 10-20 days, and the queen lays a new batch of eggs between the closely spaced pupae. The adult bees emerge as worker bees after about 11 days of pupation (about a month after egg-laying), just in time to help rear the next batch of larvae. There can be a range of sizes in the newly-emerged workers, from hardly bigger than a thumbtack to only slightly smaller than the queen, but once the bee has emerged, it won’t grow any larger.
It is thought that lack of space in the shared cell and unequal feeding results in bees of varying sizes. Some advantage may well accrue to the hive from this multisizing of workers. Larger individuals have longer tongues that can extract nectar from flowers inaccessible to smaller bees, but, conversely, smaller bees tap smaller flowers faster. Workers will live for three to ten weeks.
Rimsky-Korsakov was perhaps a little optimistic when he penned “The Flight of the Bumblebee.” No bumblebee I’ve seen can match that music’s zinging pace. In fact, with a flight speed of just 1020 kilometres per hour, the time taken to reach a foraging area can be considerable. This fact possibly limits colony size, for workers from a big hive would need to cover too much ground to make nectar collection viable. Unlike commuters whose homes are hours from work but can travel by night, bumblebees need at least a vestige of daylight to operate by. Using sensory structures termed ocelli on their heads, they are able to navigate by polarised light patterns in the sky. Landmarks are also used, and novice workers appear to experience some difficulty in finding their way about in flat, featureless terrain. Mature workers seem to establish regular haunts, revisiting the same plants day after day, like hunters patrolling traplines.
The bumblebee queen can start taking things a bit easier now she has children to help. The larger workers will join her in foraging for nectar and pollen as soon as they’ve lost the wet, white tousled look of newly emerged bees, and their wings have hardened. Unlike honeybees, bumblebees do not dance in the hive to direct a stream of workers to a rich food source. Each bumblebee worker has to use her own initiative to bring home the honey. This behavioural difference may reflect the fact that honeybees evolved in tropical forests where localised food sources (such as a nectiferous tree in flower) are more common than in temperate areas.
Even for an insect as large as a bumblebee, a small amount of honey in the hive represents a lot of labour. In measurements made in North America, the collection of one pound of red clover honey represents about a month’s work for a dozen bees, and entails visiting some 10 million clover blooms.
Most collecting trips do not return a full load of nectar-0.1 ml would be average, with 20 mg of pollen per load as well. Accumulating even this tiny cargo may demand 0.5 to 1.5 hours’ effort, and the daily routine for a foraging bee entails perhaps eight or nine such trips, with only a 2-4 minute break for unloading in the hive between forays. In a 10-12 hour day (longer than a honeybee’s) each working bumblebee may contribute 0.3 ml honey to the colony and 0.2 g pollen.
In a strong colony with, say, 250 workers, the total food reserves of the hive could be 300 ml of honey and 6 g of pollen. Such small amounts make bumblebee hives unattractive for beekeepers, although the honey is reputed to surpass honeybee honey in taste. Analysis has indicated it contains less glucose and sucrose than regular honey, but more fructose. Unlike honeybees, bumblebees don’t deliberately evaporate moisture out of nectar to make honey. Presumably the warmth of the hive (usually maintained at 30°C by the bees) reduces moisture.
In a Jacob and Esau analogy, smaller bees often stay at home and help look after the brood and the nest. Once enough workers are foraging, the queen stays home, too, and concentrates on building new cells and laying eggs. She no longer has to fight off rival queens singlehandedly, for the workers, too, have a sting in their tail (the male drones don’t), and some guard the hive. To get past the guards, it only requires that you bear the scent of the hive. Occasionally, intruders such as wasps manage to get in, and once they take on the correct scent they will be treated like any other bumblebee resident.
Having reached this size, the colony will double in size almost every week for the next 3-8 weeks, and eventually contain up to 300 working bees, depending on the species. Colonies in florally rich areas, such as fields of lucerne, can develop as many as 1000 workers.
The Nests of Bumblebees and honeybees are quite different in structure. Unlike the honeybee’s regular hexagonal comb pattern that we all know, a bumblebee nest resembles a lot of egg-shaped thimbles loosely stacked in layers. The bottom layer contains the remnants of the first brood, upon which the second brood build their cocoons, and so on. Bumblebees are thrifty housekeepers: excess wax is scraped off the pupal cocoons and re-used, and empty pupal cocoons are cleaned out and used to store nectar and pollen. Bumblebees never store as much nectar and pollen as honeybees do, as they don’t plan on overwintering.
When a bumblebee colony is reaching maturity, with sufficient numbers of workers and adequate pollen and nectar on hand, it stops producing workers, and all brood develops into either drones or new queens. What exactly initiates the change from growing, working colony to reproductive factory is not known, but it is of commercial importance. If a colony can be kept growing and producing mostly workers, then it will be able to pollinate commercial crops for longer.
Drones are produced from unfertilised eggs. As with honeybees, the queen is able to lay such eggs by shutting off the flow of sperm from the sac in which they are stored. It is thought that some factor in the first four days of larval life determines whether a fertilised egg will develop into a worker or queen. It does not involve feeding royal jelly to the infant queens as in a honeybee hive. Nor does it seem to be simply the amount of food they are fed, although queen brood are certainly fed more, and spend a few extra days as larvae, meaning that they tend to end up larger than workers.
The newly emerged queens are probably the most timid bees in the colony, quite unlike the founding queen bee, but after mating and overwintering in their solitary burrows, they mature into warlike Amazons ready to do battle for a prime nest site and control surly workers. Sometimes young queens will help the mother queen for a while or forage for the hive before striking out on their own.
Drones may also help incubate the brood for a few days after emerging, but once they leave the nest they never return. They live for three to four weeks and, unlike honeybee drones, are able to survive independently in the field by foraging like workers.
The milder New Zealand winters and the presence of flowers throughout the year allow some bumblebee hives to continue or even be established over winter—unlike the situation in Britain. These overwintered hives will contain more workers earlier in spring and be able to take full advantage of spring flowering to eventually produce an unusually large numbers of queens, sometimes more than 500.
The pheromones which enable bumblebees to recognise their own “family” are more than just dabs of eau de bee lingering on bee bodies and around the hive. They are sophisticated chemical signatures which govern the life of the colony.
The most powerful of these is emitted by the queen. If the worker bees sense any weakening of the queen’s scent, then they will stage a peasant revolt which soon results in anarchy. They’ll down tools and start laying their own eggs (workers are all female, remember), killing and tossing out any larvae not their own, and fighting amongst themselves. But because workers are unfertilised, they can produce only drones, and the hive will die out.
This dominance pheromone is produced by mated queens only, and it prevents the development of ovaries in workers. Artificially introduced into a hive, it could conceivably maintain order for a few weeks in the absence of a strong queen.
There are many different types of pheromones, and their exact composition depends on the message they are designed to convey. The incubation pheromone seems often to be mistakenly released, causing bees to incubate the floor, a piece of wood or the nest entrance. Some sort of footprint substance is used to mark and follow trails, as can be demonstrated by letting a bumblebee walk over filter paper, then shifting the paper. The next bee will follow the exact trail.
While in Christchurch, I visited Barry Donovan, an entomologist who has been researching bumblebees for 25 years. On one occasion, bees were nearly the death of him. In the course of opening a hive, two bumblebees sank their stings into a vein in his hand. The injection of venom directly into his bloodstream caused a violent anaphylactic reaction, and he collapsed. Fortunately, prompt medical attention, including an injection of adrenalin, was on hand, assisting his recovery.
Despite this incident, his enthusiasm for the insects has remained unquenched. He showed me around his hives at the Lincoln Agriculture and Science Centre. The lidded, wooden boxes contain a fold of brown underfelt which takes on a lumpy look when the colony is young, and looks decidedly pregnant when it covers a mature colony.
He showed me a young colony that had died off, leaving behind a few pupae in a teased-out hollow of carpet felt. In another box a very large colony was on its last legs. When we pulled the felt aside, very few workers rushed out to fend off the intruders. However, there were still a number of young, beautifully fresh and fluffy-looking debutante queens present.
Last year, Barry found one hive which had produced 575 young queens, but he explained that fewer than one per cent of young queens ever establish a nest, and only a small minority of those nests will successfully produce new queens. Life is hazardous for a bumblebee queen. She must find a hole in which to hibernate, or perish—as she will in an encounter with a hungry bird or a car windscreen. Even if she succeeds in establishing a nest, she might still have to fight off rival queens, slaters, spiders, earwigs and mice.
I had heard suggestions that queen collection in the wild may be reducing the total bumblebee population, so I asked Barry Donovan for his opinion. “Unlikely,” he said. “On average, each hive produces around 100 queens, and generally the number of hives that can be sustained in an area is limited by the availability of nesting sites and food. Each hive only has to produce one reproducing queen each year for the number of colonies to remain constant. So I don’t think there is any danger of bumblebees becoming a threatened species.” Indeed, in areas rich in the flowers they relish, it has been calculated that there can be as many as 8000 bumblebees per hectare!
So, next time you are sitting out on the patio, savouring a glasshouse-grown-tomato sandwich, spare a thought for that hardworking, black-and-yellow striped bumblebee that is probably at this very moment pollinating your beans. Without her help, your garden wouldn’t be anywhere near as successful.