From Society to Genes with the Honey Bee
A combination of environmental, genetic, hormonal and neurobiological factors determine a bee's progression through a series of life stages
Although worker bees go through a rather consistent path of behavioral development, it is not rigidly determined. Bees can accelerate, retard or even reverse their behavioral development in response to changing environmental and colony conditions. For example, favorable environmental conditions in the late spring might cause a surge in worker birth rates, and that could result in a colony with a reduced percentage of foragers. Under these circumstances, young bees compress their period of hive work from three weeks to one week and become "precocious foragers." Conversely, a new colony founded by a swarm—a fragment of an old colony that leaves to establish a new colony—soon reaches a point at which it contains predominantly older individuals. In that case, some colony members retard their development and serve as overaged nurses. In those bees, hypopharyngeal glands that produce food for larvae continue with this function rather than producing other substances.
How does the behavior of thousands of individual bees generate a smoothly functioning colony? It seems unlikely that individual bees could monitor the state of their entire colony and then perform the tasks that are needed. Although some workers play special roles in organizing specific tasks, such as leading other bees to a new nest site during swarming, there is no evidence for real leaders or individuals—not even the queen—that perceive all or most of a colony's requirements and direct the activities of other colony members from one task to another. The challenge is understanding the mechanisms of integration that enable individual bees to respond to fragmentary information with actions that are appropriate to the state of the whole colony.
My colleagues and I and others have discovered that juvenile hormone—one of the most important hormones influencing insect development—helps to time the pace of behavioral maturation in honey bees. This hormone comes from the corpora allata, a gland that lies near a honey bee's brain. Indirect evidence for this hormone's role exists in the fact that young bees working in a hive have low levels of this hormone and older foragers have higher levels. Direct proof has also been obtained: Young bees given juvenile-hormone treatments become precocious foragers. Recently, my University of Illinois colleague Susan Fahrbach, graduate student Joseph Sullivan, undergraduate Omar Jassim and I found that removing the corpora allata does not prevent a bee from developing into a forager but does delay it for a few days on average. Juvenile-hormone treatments, however, eliminate that delay.
Manipulating hormone levels on a bee-by-bee basis is one thing, but demonstrating that bees alter hormone levels themselves in response to changing conditions is another. To show how the environment can modulate hormone levels, Robert Page of the University of California at Davis, Colette and Alain Strambi of the Centre National de la Recherche Scientifique in Marseille, France, and I induced precocious foraging by establishing colonies that consisted of only very young bees. Then we tested their blood levels of juvenile hormone and found that one-week-old precocious foragers had approximately 100 nanograms of juvenile hormone per milliliter of blood, which is about the same as that in three-week-old foragers and higher than the 5–20 nanograms usually found in one-week-old nurses. Two weeks later, we obtained overaged nurses from these colonies by preventing new adults from emerging, and these old nurses had levels of juvenile hormone that resembled young nurses, rather than foragers. From work with other experimental colonies, we found that bees that reverted from foraging to nursing were also "young" in terms of their levels of juvenile hormone.