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‘Too much sugar in nectar slows down bees’

At a certain sugar concentration, the energy gain versus energy loss is optimised for nectar feeders, experts said Photo: Gareth Fuller/PA

When it comes to nectar, sweeter is not necessarily better.

Too much sugar slows down bees, taking up more of their energy, new research suggests.

Bumblebees drink nectar from flowers, then offload it in their nest – by vomiting – for use by other bees in the colony.

Sugar in the nectar makes it appealing, and the more sugar within the nectar, the more energy it contains.

However, nectar also gets thicker and stickier as the sugar content increases.

This makes it more difficult for bees to drink and regurgitate – taking more time and energy, scientists say.

The study published in the Journal of the Royal Society Interface, looked at the mechanics of both nectar drinking and regurgitation in one of the most common bumblebees in the UK – Bombus terrestris, or the buff-tailed bumblebee.

Bees find nectar that is low in sugar easier to drink, and very easy to vomit back up.

A bumblebee on a Dusky Cranesbill geranium plant Credit: Victoria Jones/PA

But as it gets more sugary, it gradually takes bees longer to drink, but swiftly becomes much more difficult to vomit.

Dr Jonathan Pattrick, formerly a PhD student based jointly at the University of Cambridge’s departments of plant sciences and zoology, is the first author of the study.

Now a post-doctoral researcher at the University of Oxford’s Department of Zoology, he said: “Bumblebees must strike a balance between choosing a nectar that is energy-rich, but isn’t too time-consuming to drink and offload.

“Nectar sugar concentration affects the speed of the bees’ foraging trips, so it influences their foraging decisions.”

The scientists say that while it has long been known that nectar with a higher sugar concentration takes bees longer to drink, its effect on nectar regurgitation has not previously received much attention.

They hope the new insights will help researchers make better predictions about which types of nectar bumblebees and other pollinators should like best, and consequently the kinds of flowers and plants they are most likely to visit.

This will inform crop breeders in producing the most appealing flowers for better crop pollination and higher yields.

For the research, bees were allowed to forage on sugar solutions of three different concentrations in the Department of Plant Science’s Bee Lab.

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While doing this, they were also timed and weighed.

When the flying insects returned to their “nest”, the researchers timed how long it took for them to vomit up the nectar they had collected.

“For low strength nectar, bees had a quick vomit that only lasted a few seconds, then were back out and foraging again,” said Dr Pattrick.

He added: “But for really thick nectar they took ages to vomit, sometimes straining for nearly a minute.”

For any given nectar concentration, bees regurgitate the nectar quicker than they initially drink it.

Dr Pattrick said: “It’s hard to drink a thick, sticky liquid, but imagine trying to spit it out again through a straw – that would be even harder.

“At a certain sugar concentration, the energy gain versus energy loss is optimised for nectar feeders.”

Scientists say the perfect nectar sugar concentration for the highest energy intake depends on the species drinking it, because different species feed in different ways.

The research was funded by the Biotechnology and Biological Sciences Research Council.