Scurrying between nectar-laden blossoms to transfer pollen for seed and fruit production ready for the next harvest, honeybees must be the ultimate pollinator poster child. However, Karin Nordström from Flinders University, Australia, is a fan of another industrious insect. ‘I love watching hoverflies on warm sunny days’, says Nordström, who has been investigating why hoverflies are attracted to certain flowers with her colleague Shannon Olsson, from the National Centre for Biological Sciences, India. Describing how the insects, which are often mistaken for honeybees, are happy to share large blooms with other pollinators, Nordström adds that they prefer to remain in solitude when feasting on smaller florets and chuckles, ‘They seem to fight over yummy flowers’. Scientists have investigated how the hoverfly nervous system controls flight for almost 40 years, but little was known about how the insects respond to confrontations and the factors that trigger a hoverfly to desert a delicious dining opportunity. So graduate student Malin Thyselius settled down for a Swedish summer of hoverfly watching.
Working with Paloma Gonzalez-Bellido and Trevor Wardill from the University of Cambridge, UK, Thyselius developed the techniques that allowed her to film the insects with a pair of high-speed cameras as they visited flowers. Then, Thyselius painstakingly reconstructed the insects’ movements in 3D, which she says was difficult and time consuming because it was hard to distinguish the flitting hoverflies against the cluttered background of vegetation.
Monitoring the females as they loitered on poppies, bristly hawkbits, yarrow flowers and two species of daisy, it was clear to Nordström and Thyselius that the hoverflies were not keen to share their flowers, departing even before approaching interlopers landed. And when the duo analysed the invaders’ approach, they found that the looming intruders did not accelerate as they approached the hoverfly's flower, suggesting that the incomer was not staging an attack. It was also evident that the departing hoverflies were keen to make a quick getaway, taking off 3.5 times faster than usual when approached by other hoverflies and 5 times faster when the newcomer was a predatory wasp that could attack. And when the duo analysed the direction taken by the departing females, it was clear that they were fleeing the intruder, as they consistently flew in the opposite direction from the inbound visitor. However, Nordström and Thyselius were surprised that the hoverflies did not time their departure to coincide with the point at which the image of the invader reached a specific size on their retina; instead, the fleeing hoverflies seemed to respond to how fast the image of the visitor expanded on their retina.
‘The take-off was not hard wired, but a lot more loosely triggered’, says Nordström, who also speculates that the females may be able to distinguish between the approaches of a threatening wasp and a benign hoverfly based on differences between the incomers’ speed and their appearance. Either way, once disturbed foraging hoverflies have departed, they often delay their return, but eventually do so to make sure that they don't miss out on a feast just because they choose not to share.