AUSTRALIA: Bees warm up with a hot drink

Humans are not the only ones to enjoy a hot drink on a cold morning. Bees have the same idea when the weather is chilly and the discovery could be highly significant with the advent of climate change. And not just to bee keepers.

If the atmosphere heats up and bees switch from the plants they currently pollinate to others, this may have serious implications for agricultural and ecological systems around the world.

Monash University researchers in Melbourne, Dr Melanie Norgate and Dr Adrian Dyer, set out to investigate whether Australian native stingless bees had a preference for nectar at different temperatures. In a world-first discovery, they showed that air temperature had a definite impact on bees' choice of warmer or cooler nectar.

"To get precise measurements, we needed a controlled environment and we could do that with the little native bee," Norgate says. "Their hive is only 30 centimetres high and the bees leave it at about the right density to count them so you don't get overwhelemed with huge numbers."

The research team used a controlled temperature laboratory and, when not being subject to experiments, the bees were maintained at a constant temperature of 23oC. Their 'flight arena' was a steel-framed box with white wooden side panels and a floor painted foliage green to resemble the ground the bees would normally fly over.

The lid of the box was transparent flexiglass and two postgraduate student observers monitored what the bees were doing for an hour a time. A colony of some 4,000 adults and 800 foraging native bees was propagated and established in a pine nesting box connected to the foraging arena by a plexiglass tube.

Two artificial feeders to represent flowers were located on each side of the flight arena with heat blocks beneath them so the temperature of the sugar solution 'nectar' could be raised.

"We wanted to see if bees could associate the location of the flower with the temperature of the nectar it contained and we found they could," Norgate says. "We gave the bees a couple of hours in the arena to learn which feeder they preferred but between each trial, we switched the feeders at random to ensure they were not just learning to go to one side or the other for some reason.

"The hive and whole flight arena was in the constant temperature room so we knew what the temperature was where they were flying," Norgate says. "We couldn't cool the nectar to below ambient temperature so we just had one feeder slightly above and one at room temperature."

The researchers used a thermal camera to measure the bees' body temperature and when the air temperature was below 34oC they flew to the warmer nectar.

"Bees need to be warm to fly because their flight muscles have to be at a particular temperature for the biochemical energy processes to work," Norgate says. "In flight they heat up so they are warmer than the ambient air temperature but when they land on a flower, it takes them 20 seconds to imbibe the nectar and while they are sitting there, their body is cooling down.

The researchers found that if the feeder was at ambient temperature they cooled down and had to use more energy to warm themselves up before they are able to take flight. When they went to the warmer feeder, they did not cool down and this kept them at the same flying temperature - which was an energy reward.

The experiments showed that when the air temperature was raised and the nectar in the feeder was above the bees' flight temperature, they switched to the cooler feeder. That is, the bees have a novel behaviour mechanism to modify their body temperature at just the right level for flight, Norgate explains.

She says it would be necessary to determine what the different pollinating insects are doing if the earth is warming. Should a temperature threshold for bees be crossed that might change their preferences and those of other pollinators, thereby affecting the reproductive success of plants.

A report on the research is available in PLoS One