Something is going wrong with our bees. Across North America and Europe, large numbers of bees are routinely wiped out by colony collapse disorder, putting many agricultural crops at risk. ‘There is no single cause of colony collapse. It looks like it may be a mixture of pests and diseases; a decline in bee habitats and pesticides may be involved too. There will also be a nutritional component,’ says Sue Nicolson from the University of Pretoria, South Africa. Knowing that bees have to regulate their pollen and nectar consumption from a variety of sources to remain healthy on a balanced diet, Nicolson and her colleagues, Solomon Altaye, Christian Pirk and Robin Crewe, decided to find out whether young nurse bees can maintain a balanced diet when offered a choice between imbalanced diets made from different protein sources (p. 3311).
Designing the diets that the bees would dine on, Nicolson selected three protein sources: casein, a pure protein derived from milk; royal jelly, produced by bees as a food source for larvae and the queen; and Feed-Bee®, a commercially available bee food supplement derived from plant sources. Calculating the protein and carbohydrate content of the royal jelly and Feed-Bee® supplement, Nicolson and her colleagues added sucrose to each protein source to produce four diets with protein-to-carbohydrate ratios of 1:50, 1:25, 1:10 and 1:1. Then the team collected groups of 100 newly emerged adults from local African bee populations – which seem to be immune from colony collapse – isolated each group in a cage in a warm (35°C) incubator (to simulate hive conditions), and provided them with water and a choice of diets. Some of the caged groups were offered the choice between 1:50 and 1:10 diets, while others were given 1:50 and 1:1 diets. Other colonies had access to 1:25 and 1:10 diets, while the last group were offered 1:25 and 1:1 diets.
Monitoring the bees' consumption and mortality rates over 14 days, Nicolson and her colleagues could see that each colony adjusted how much it consumed from each food source to ensure that it got a balanced diet. While the colonies provided with casein-based diets balanced their intake so that they consumed 1 mg of protein for every 12 mg of sucrose, the colonies on royal jelly diets consumed slightly more carbohydrate (14 mg) to every milligram of royal jelly protein, and the Feed-Bee® protein bees consumed the least amount of carbohydrate, at only 11 mg of carbohydrate per milligram of Feed-Bee® protein. The team also noticed that the bees on casein-based diets had lower mortality rates than bees fed on the other protein sources, and the bees fed on royal jelly had the highest ovary activation rates, suggesting that a bee's diet can dramatically affect its life history, and that high dietary protein levels may not be as essential as many beekeepers think.
Nicolson admits that she was surprised that the bees required so much carbohydrate in their diet, and suspects that the workers have much higher metabolic rates than other social insects as they all contribute to regulating the environment in the colony. She is also keen to find out how adding larvae to the equation could alter the bees' diets. According to Nicolson, young adults produce royal jelly, which has a 1:1 protein-to-carbohydrate ratio to feed young, and this could shift the bees' nutritional requirements, forcing them to increase their protein consumption to meet the larvae's needs. Ultimately Nicolson hopes that this information will help us to understand one of the many causes of colony collapse and reverse its devastating effects.