Honey bees are famous for their highly structured societies in which each individual plays a well-defined role. But how do they reorganize their workforce in response to changing needs? A new study shows that workers regulate the number of foragers using pheromones that arrest development in young bees.
Worker honey bees begin their lives as cell-cleaners, then advance up the ranks to become nurses and then food storers, finally graduating to foraging and colony defense at 2 to 3 weeks. The transition to foraging is associated with hormonal and structural changes in the brain and increased expression of foraging genes (ScienceNOW, 10 October 2003 ), but what triggers these changes is not known. Scientists suspected that old bees hold back the development of young bees through some chemical means, as past experiments have shown that this happens only when they can touch each other.
Isabelle Leoncini, a graduate student at the University of Avignon, France, and Gene Robinson, an entomologist at the University of Illinois, Urbana-Champaign, set out to find the mystery substance. They and their colleagues found that levels of a fatty acid compound called ethyl oleate, a component of a brood pheromone that inhibits behavioral development, were about 30 times higher in the crops (honey stomachs) of foragers than in nurses. When young worker bees ate a mixture of ethyl oleate and sugar candy, they started foraging 2 or 3 days later than did controls, the researchers report online this week in the Proceedings of the National Academy of Sciences. That delay is substantial, says Robinson, given that the bees only live 5 to 6 weeks. He and his colleagues think the foragers likely pass the pheromone to younger workers while exchanging food.
The discovery suggests a nifty mechanism for negative feedback, says Francis Ratnieks, a behavioral ecologist at the University of Sheffield, U.K. The bees most likely to receive the pheromone from the foragers are the nectar receivers, who are the next in line to become foragers, Ratnieks explains. If foragers abound in the colony--when it's raining, for example--ethyl oleate levels in the hive will be high and the development of receivers into foragers will be delayed. With fewer foragers present, levels will drop and receivers will mature more quickly. In this way, the colony can regulate the division of labor to suit changing needs and environmental conditions without any centralized control, adds Robinson.