Take a walk around the Scottish Highlands in summer and you'll be wowed by the wildflowers that light up the landscape. Bees and hoverflies are also attracted by the flowers, to collect pollen. But these two groups of buzzing insects use different behaviours to get to this protein-rich food source. Bees often buzz the pollen out of the flower, by clamping their mouthparts onto the anther and speedily shaking it with their vibrations. Buzzing pollination has evolved 40 times in over half of the 20,000 bee species, yet just one species of hoverfly is known to buzz flowers; instead, the rest silently and slowly rub the pollen off with their legs and body.

Mario Vallejo-Marin and Gillian Vallejo are no strangers to Scotland. Working at the University of Stirling and at Natural Power, Stirling, UK, they are situated right at the gateway to the Highlands and are fascinated by pollinating insects. They were curious to know why bees and hoverflies go about it in different ways.

First, the duo wondered whether hoverflies are incapable of producing large enough vibrations to shake off the pollen, so they set out to measure the bodily accelerations of bees and hoverflies during their defensive buzzes. Greater acceleration means more vigorous buzzing and more pollen. But how to measure the vibrations of a tiny insect? Use an even tinier sensor. Vallejo-Marin and Vallejo used an accelerometer weighing just 0.2 g, to measure the vibrations. They collected bees and hoverflies from around Scotland and returned to the lab to identify the insects. They then held the insects gently against the exquisitely sensitive apparatus, just firmly enough to cause the insects to buzz with alarm and vibrate the sensor. Overall, the duo tested 299 insects, split evenly between bees and hoverflies, and measured over 4000 buzzes.

Surprisingly, the hoverflies were just as capable of producing flower-shaking vibrations as the bees. Vallejo-Marin and Vallejo found that larger insects produced larger and louder buzzes, but the buzzing produced by similarly sized bees and hoverflies were otherwise indistinguishable. They should both be able to buzz out pollen.

But buzzing a sensor is one thing; Vallejo-Marin and Vallejo wanted to see for themselves whether the hoverflies’ defensive buzzes were capable of releasing pollen from flowers. They repeated the previous experiment, but this time using a natural sensor – a flower – because buzzing against an anther should cause it to release pollen. While pressing the buzzing insects against the anthers of two plant species, the researchers collected the pollen that tumbled out, and in most cases the bees and hoverflies released over 2000 grains of pollen each time: a veritable feast.

Yet despite their apparent ability, hoverflies for the most part decline to use buzzing to collect pollen. The researchers have ideas of why this might be. A big clue lies in what bees and hoverflies actually do with the pollen: the hoverflies eat it themselves, sometimes even tucking in while they're still at the flower, but the bees are there for take-away, rushing the pollen home to their larvae. Buzzing quickly exfoliates large amounts of pollen, which is a lot more effective, but more energetically expensive, than the hoverflies’ relaxed and economical approach. With hungry mouths to feed, buzzing bees are certainly busier.

G. C.
Comparison of defence buzzes in hoverflies and buzz-pollinating bees
J. Zool.