Bees versus flies – which flex their muscles most?

New research by experts at the University of Stirling has debunked existing assumptions about why hoverflies use different techniques for pollination than those used by bees.

Led by Dr Mario Vallejo-Marin, of the Faculty of Natural Sciences, the research - Comparison of defence buzzes in hoverflies and buzz-pollinating bees – has been published in the renowned Journal of Zoology.

More than 11,000 species of bees have evolved the capacity to ‘buzz pollinate’ flowers – where bees use their thorax muscles to create powerful vibrations, responsible for the loud buzz, to eject pollen locked inside flowers. In contrast, of more than 6,000 species of hoverflies, only one – the cactus hoverfly – uses the technique.

An existing hypothesis of why hoverflies do not buzz pollinate was that they are incapable of producing vibrations of the required magnitude or amplitude to dislodge the pollen. The Stirling team conducted experiments to measure the amplitude of vibrations produced by hoverflies, and compared them to those generated by buzz-pollinating bees.

The findings showed that bees and hoverflies produce vibrations of similar amplitude – providing no support for the previous ‘weak vibrations’ hypothesis. Unexpectedly, the research also revealed that male bees produce weaker vibrations than females of the species.

Dr Mario Vallejo-Marin, of the Faculty of Natural Sciences

Dr Mario Vallejo-Marin said: “For the first time, we tested one of the hypotheses put forward to explain why hoverflies do not buzz pollinate, and the results are surprising. Our study suggests that the lack of buzz-pollinating flies may be best explained by their life history, and the fact their predatory larvae depend much less on pollen in their diet, than bee larvae.

“These research findings can have benefits for society and industry as understanding why some insects, but not others, are capable of buzz pollination will support preparations to face the consequences of rapid changes in insect populations around the world.”

The research team sampled bees and hoverflies across three months during summer 2020, in central Scotland, the Outer Hebrides, and the Orkney Isles to ensure a cross-section of the diversity of insect pollinators in Scotland. Vibrations were measured and recorded using a portable miniature accelerometer and custom-written software.

The project was funded by a research grant from the Leverhulme Trust.