A Sting in the Tale (11 page)

Of course flowers refill with nectar and using smelly footprints to detect empty flowers would only work if the footprint wore off. This seemed to be exactly what happens. We recorded how long a comfrey flower remains repellent to passing bees after it has been visited, and found that the effect seems to wear off after about forty minutes. We then carefully measured how quickly the flowers refilled with nectar, and found that they took between forty minutes and an hour to refill. In other words there was a pretty good match between when bees would visit a flower and when it was likely to be full, or nearly so.

With our feeble sense of smell, this all seems terribly impressive. However, there is more. It turns out that different flower species refill with nectar at different rates. For example, borage produces nectar very fast, whereas comfrey is middling, and bird's-foot trefoil is very slow. When feeding on borage, bees start revisiting a flower just two minutes after the previous visit, and again this roughly corresponds with the time it takes to refill. On comfrey, as I mentioned, bees revisit flowers after about forty minutes, while on bird's-foot trefoil a flower seems to remain repellent for at least 24 hours.
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Yet the footprints are the same.

How does this work? It seems that bumblebees are able to tell how old a footprint is, perhaps by the strength of the smell, and that they learn an appropriate threshold for any particular flower. This is helped by the fact that individual bees tend to specialise for several days at a time, and sometimes for their entire life, in visiting just one flower species over and over again, gaining a lot of experience. So a bee visiting borage quickly learns to ignore all but the freshest smelly footprint, while a bee visiting bird's-foot trefoil learns that a flower that smells even faintly of another bee's feet is best avoided.

We later looked at other insects, and found that this system seems to work across species. Both honeybees and all of the bumblebee species that we studied seem to use footprint smells to judge which flowers to visit, and are able to recognise the footprints left by other species just as well as their own. This of course makes sense, for it does not matter who has emptied a flower – it is still empty. Bees even seem able to tell when a flower has been visited by a hoverfly.

It is probable that without their ability to detect and avoid empty flowers, bumblebees would struggle to survive. In our comfrey patches in the Itchen Valley Country Park there were hundreds of bumblebees foraging together, and most of the flowers were empty or nearly so at any one time. Landing, pushing her tongue into the flower and then taking off again all takes time and energy for a hungry bee. Even saving a fraction of a second can, cumulatively, pay huge dividends, for each bee must find tens of thousands of full flowers per day if she is going to fuel all of her flight and bring back a net return of nectar for the nest.

This aspect of bee behaviour was tremendous fun to explore over a number of years, but it turned out that we got one part of it wrong. Jane and I stopped working on bees' smelly footprints in about 2000, conceitedly thinking that we had pretty much wrapped up most of the interesting angles on this. I started working more on the ecology of rare bees, and Jane got herself a lectureship at Trinity College Dublin, where she has since made a name for herself studying the pollination of invasive weeds such as rhododendron. In 2005 I found myself at a conference in St Petersburg chatting to Thomas Eltz from Dusseldorf University. He had been analysing the speed at which the chemicals in bee footprints evaporated from flowers, and had found that it was far too slow to fit with our explanation. The compounds are very large and not very volatile at all, so that they remain on the flower more or less indefinitely. In fact he found that flowers accumulate chemicals from successive bee visits so that, with careful analysis, the flower can provide a record of all the insects that have visited it during its life.

This then begs the question: how can bees tell how old a scent mark is? If it doesn't evaporate, then it will not fade over time, and it should be impossible to know whether the flower has had time to refill with nectar, yet clearly the bees somehow do know. We still don't have a definitive answer. My guess is that perhaps the chemicals deposited by bees' feet slowly sink into the waxy coating of the flower – for just as bees are waterproofed with oils, so flowers are waterproofed with oils and waxes – and by sinking into the flower's surface they become less detectable, and effectively fade even though they are still present.

Interestingly, the same smelly footprints, when encountered in a different context, have a different meaning. Bee footprints around the entrance to the nest, rather than having a repellent effect, help returning foragers to find their way inside. It seems that bumblebees learn to interpret a particular smell in an appropriate way, depending on the circumstances in which they encounter it.

It is humbling to reflect that though a bumblebee has a brain smaller than a grain of rice, it has powers of perception and learning that often put us mammals to shame. Next time you are sitting in your garden while the bees are visiting your bee-friendly plants (if you haven't got any, I hope you'll plant some next spring), take the time to watch what they are doing. You will quickly notice them dismissing some flowers after a quick sniff. But I'm sure that there is still much more to learn. Certain flowers seem to be visited by particular species of bee, and often we have no idea why. Individual bees may collect pollen or nectar or both, and seem particularly disposed to collect one or other depending on the flower, but again we often have no explanation for their choices. On some days, even in high summer when the weather is fine, foraging bees may suddenly become scarce, as if they have all decided to go on strike for a few hours; we do not know why. Bumblebees are one of the most familiar and intensively studied of all the insects on earth, but there is still an enormous amount that we do not understand about their lives.

CHAPTER SEVEN

Tasmanian Devils

Burly, dozing humblebee,

Where thou art is clime for me.

Let them sail for Porto Rique,

Far-off heats through seas to seek.

I will follow thee alone,

Thou animated torrid-zone!

Ralph Waldo Emerson (American poet)

In Tasmania the first bumblebee was recorded in 1992. There was no mistaking her, for the local bee species are tiny – small enough to hide under a grain of rice, mostly drab and not very furry. Nor do bumblebees naturally occur anywhere near Tasmania, for they are mainly creatures of the northern hemisphere. So these new furry giants would not have escaped notice for long, especially since they first appeared in the gardens of Hobart, the most densely populated area of the island.

These new arrivals were quickly identified as buff-tails. Now buff-tailed bumblebees only actually have buff tails in the UK. In the rest of Europe, buff-tailed bumblebees have white tails, which, as you might imagine, makes them awfully hard to distinguish from white-tailed bumblebees. Anyway, these were buff-tailed buff-tails, which meant that they had to have originated from the UK, 10,500 or so miles from Tasmania. If your geography is a little rusty, Tasmania is the southernmost state of Australia, a roughly triangular island floating 150 miles off the south coast of the mainland, with New Zealand lying about 1,500 miles to the east. So how did they get there? The answer is not quite as mysterious as all that, for as I have already mentioned, English buff-tailed bumblebees have been living quite happily in the wild in New Zealand since 1885 or thereabouts. The Tasmanian bumblebees presumably came from there, but this is still a 1,500-mile journey against the prevailing wind, across a stormy and cold Tasman Sea. With the best will in the world they could not have flown.

We may never know how they made the journey. They may have been accidentally transported on a ship; a young queen may have hibernated in a plant pot and been brought over with some nursery plants. But I find this unlikely. It is probably no coincidence that in about 1988 tomato growers the world over turned to using bumblebees to pollinate their crops. Researchers in Belgium and Holland had discovered that bumblebees are fantastically efficient at pollinating greenhouse tomatoes, and they had also worked out how to breed buff-tailed bumblebees in large numbers. As a result, bumblebee-rearing factories quickly sprang up in Europe and then in North America and Asia to cater for the demand. The only tomato growers left out of the bumblebee bonanza were those in Australia, where there are no native bumblebees, and where importing foreign species is strictly forbidden. Tomato growers on mainland Australia still have to hand-pollinate their plants; teams of workers are employed, each of whom is equipped with a slender vibrating wand. Every flower has to be touched with the tip of the wand if it is to set fruit. As might be imagined, this is tedious work in a large glasshouse – some commercial operations cover hundreds of acres and contain literally millions of tomato flowers – and the labour costs are substantial. Bumblebees are not only much, much cheaper, but the tomatoes produced by bumblebee pollination are also larger and apparently taste better than those pollinated by humans, so in the late 1980s, Australian tomato growers suddenly found themselves at a distinct disadvantage. That bumblebees mysteriously arrived in Hobart in 1992 may just be coincidence. Bumblebees had failed to cross from New Zealand to Tasmania for 100 or so years, but suddenly they managed to do so just after it was discovered that they had huge commercial value. Draw your own conclusions.

Yet why shouldn't bumblebees be imported to Tasmania and, for that matter, to mainland Australia? After all, they are cute, furry and beneficial insects which pollinate crops and wild flowers. Who wouldn't want them? The answer is that man has a rather poor record with regard to introducing non-native species, and nowhere more so than in the Antipodes.

Australia is a remote island, with a remarkable and unique fauna, of which the bizarre marsupial mammals are the best known. Only distantly related to the mammals found in most of the rest of the world, they evolved into many wonderful forms: kangaroos, bandicoots, koalas and numbats, among numerous others (and top marks to the Aboriginal people for giving them such memorable names). Australia also has thousands of indigenous bees, butterflies, flowers and so on. New Zealand is the same, only more so as it is even more isolated than Australia, so no mammals got there apart from bats (which, of course, can fly). There, giant flightless birds took the roles of large mammals, and enormous crickets (known as weta) evolved to fill the role of mice. When Captain Cook first arrived, New Zealand was clothed in verdant forests teeming with birds and insects, almost every one of which he had never seen before; they were all unique to New Zealand.

Tragically, the early European settlers in Australia and New Zealand soon became homesick, pining for such familiar creatures as foxes, rabbits and hedgehogs. They formed Acclimatisation Societies, dedicated to introducing as many non-native plants and animals as possible, and awarding medals to those who were most successful or dedicated in their efforts. With spectacular naivety, in New Zealand they even experimented with introducing zebra and giraffe. Aside from these, the introductions were remarkably successful. When I first flew to Christchurch (with Mick Hanley in 2003) it all looked tremendously familiar. We had left behind a cold and drizzly winter's day in London and arrived to glorious summer sunshine, but there the dissimilarity ended. Perhaps not surprisingly, the buildings in Christchurch look decidedly British. After all, all the older ones were built by British immigrants. The trees lining the streets are limes and planes, just like those in London. The birds chirping from their branches were blackbirds, sparrows, thrushes, starlings and greenfinches. We hired a car and drove out of the city across the Canterbury Plain, the main agricultural region of New Zealand. So far as I could tell as we rocketed along with Mick at the wheel, the blurred trees, roadside flowers and farm animals that we shot past appeared to be much the same as those at home. When we stopped – in my case, saying a silent prayer of thanks that I had survived the journey so far – a pair of goldfinches flew by, and skylarks trilled overhead. Had it not been for the snowy mountain peaks in the distance, we could have been in Cambridgeshire or any other flat, agricultural part of lowland England.

It was several hours and 300 miles before we saw something new; a small friendly bird which my bird guidebook suggested was a fantail. New Zealand is famous for its beautiful scenery (think
Lord of the Rings
), but it is an ecological holocaust. In 200 or so years we have wiped almost all the native animals from vast tracts of the country, clearing native forests to make way for farming, and especially for sheep ranching. In the remote mountains there are still forests of native trees, but the birds that once inhabited them have mostly gone, eaten by the introduced foxes and stoats, against which the native species seem to have no defence. Famously the kakapo, a chubby moss-coloured flightless parrot, attempts to escape from predators by shinning slowly up the nearest shrub and then jumping from the top, only to thud down to the ground a yard or so from where it started. Hardly enough to throw a wily fox off its trail. Hence this once widespread New Zealand bird was down to about sixty individuals at the last count.

The situation is almost as bad in Australia, where the native marsupials have struggled to cope with introduced pigs, rabbits, foxes, cats, camels, dogs and goats. Vast tracts of forest have also been cleared to make pasture for cattle and sheep, and have subsequently become overrun with European weeds such as the appropriately named Patterson's curse,
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while the tropical north-east is swamped with the South American shrub
Lantana
. Perhaps most famously, cane toads (also from South America) have bred in their countless millions in Queensland, from where they are spreading remorselessly southwards, consuming everything that they can fit into their capacious mouths.