The sweetest nectar of all the flowers may not be best for bees, slowing them down instead of giving them a boost in energy.

Once bees slurp as much delicious nectar as they can, they buzz away to share some of this sweet bounty with their nestmates – in the form of regurgitation, or vomit. The more sweet, the more sugar, and the more energy the nectar contains. At first, this sounds like a win-win scenario. However, too much sweetness can make the nectar thick and sticky – a challenging and time-consuming substance for the bees to regurgitate into "honeypots", receptacles in the nest to store collected nectar.

"For low strength nectar, bees had a quick vomit that only lasted a few seconds, then were back out and foraging again, but for really thick nectar they took ages to vomit, sometimes straining for nearly a minute," said lead author Dr Jonathan Pattrick, a post-doctoral researcher in the University of Oxford’s Department of Zoology. 

Nectar "offloading," as the researchers refer to regurgitation, "is poorly explored." To address this, the team studied solutions of 35 percent sugar all the way up to a sweet high of 65 percent. The researchers let loose Bombus terrestris – a common bumblebee in the UK – to feed on the solutions, weighed them, timed their feeding, and finally watched as they returned to their plastic nest box. At the extreme end of sweetness, the bees took longer to lap it up and longer to barf it back out. 

"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," said Pattrick.

Previous work suggests the best concentration for energy rates when drinking is around 50-60 percent for bumblebees. For offloading, a 65 percent solution took the bees up to half a minute, while at 35 percent it took them only 4-5 seconds. With this taken into account, bees may prefer nectar with 3-4 percent less sugar than previously thought, in order to account for the energy expenditure of retching it back up.

Now this study isn’t just about bee barf for kicks. The researchers wanted to know what flowers are most enticing to the bees to help pick the best plants to boost pollination, a key component of the world’s food supply. There are more people to feed on Earth than ever before and yet pollinator populations are dwindling. The study will help researchers understand which flowers and plants the bees are most attracted to in order to elevate the chance of crop breeding success.

Different bee species feed in different ways, including the sweetness level they prefer. Nectar is a solution of sucrose, glucose and fructose, though a variety of other compounds may be present as well. Most bees feed faster than they vomit, dipping their feathery glossa (tongue) to sip nectar and then forcing the substance back up through a tube-like structure to regurgitate. Some, such as orchid bees, have to suck the nectar up, making high concentrations of sugar even harder to consume.

"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,” said Pattrick. “At a certain sugar concentration, the energy gain versus energy loss is optimized for nectar feeders."

The research was published in the Journal of the Royal Society Interface.