It's a sickening statistic: 50% of the UK's bee species have been lost since the 1950s and the number of honeybee hives in the US has plummeted from 6 million to 2.5 million. No matter where you are, bees and the agriculture that they support are at risk, so maintaining their health is of paramount importance. Sara Leonhardt from the University of Würzburg, Germany, explains that diet can significantly affect the insects’ health – high protein diets can shorten the life expectancy of adult honeybees while benefiting larvae – so bees would clearly benefit from being able to assess the quality and nutrient content of their food. However, Leonhardt says, ‘It remains unclear whether bees are actually able to perceive and assess the quality of pollen’. And even if they could, which senses might they use? Explaining that bumblebee foragers have to be able to assess the quality of the pollen and nectar that they deliver to the nest, because their nest mates do not provide feedback on the quality of the provisions, Leonhardt and Johannes Spaethe decided to ask bumblebees more about their ability to distinguish between the proteins in their diet.
Snugly sitting the bees in a tube with their heads protruding from the end, Fabian Ruedenauer was able to take advantage of the bees’ tendency to stick out their tongues when they detect sugar to learn more about their preferences. Knowing that bumblebees can be trained to transfer their tongue extension behaviour to show when they recognise odours and flavours, Ruedenauer embarked on a painstaking set of experiments to test the insects’ ability to recognise proteins from different sources. Training the bees to recognise apple and almond pollen and the protein casein from milk, the team were pleased to see that the bees were capable of learning to recognise all three forms of protein. Then, Ruedenauer tested whether the bees could distinguish between the two pollens – which have different odours – by gently blowing a puff of pollen-flavoured air over the bee's head. Sure enough, the bumble bees could clearly distinguish between the two, sticking their tongues out whenever they encountered the pollen flavour that they had been trained to recall.
But could the bees distinguish between pollen mixtures of different concentrations and if so, which senses would the insects use? Mixing pollen and cellulose in different proportions and blowing the mixture's odour over the faces of restrained bees, Ruedenauer could see that the bees were indifferent to the strength of the mixture, responding equally happily to both the strong and weak mixtures. However, when Rudenauer offered the bees the opportunity of tasting the mixture by dipping their antennae into the pollen/cellulose and casein/cellulose mixtures, the insects distinguished between the dilute and concentrated mixtures with ease. ‘Bumblebees are able to learn the odour of pollen and casein using olfactory cues, but can only differentiate between concentrations of these substances when they can touch the food,’ says Leonhardt.
Having shown that the insects are capable of differentiating food with different protein contents, Leonhardt and her colleagues are now testing whether bumblebees are able to translate this into foraging practice by selecting better quality food when offered a choice. They also want to discover whether the foragers’ customers – the larvae that they supply – have an impact on the bees’ sensitivity to the quality of their foraged food.