When Honeybees Become Foragers

Many species of insect, including the familiar honeybee, exhibit considerable flexibility in their behavioral development. Honeybee colonies usually contain tens of thousands of sterile female worker bees laboring on behalf of a single queen, who lays most or all of the eggs in her hive. At first, the young workers feed the larvae that hatch from the queen's eggs (most of which will become new workers), but after about three weeks of nurse duty, they graduate to the job of collecting pollen and nectar outside the hive.

Research indicates that the age-related transition between nursing and foraging (collecting food) is regulated by hormonal changes. Young nurse workers have very low concentrations of juvenile hormone in their blood, whereas older foragers have much higher concentrations of this hormone. If young bees are treated with juvenile hormone, they become early foragers. In contrast, the onset of foraging behavior is delayed by removal of the corpora allata, the glands that produce juvenile hormone; furthermore, bees without corpora allata that receive hormone treatment regain normal timing of the switch to foraging. Bees that collect pollen and nectar also have bigger mushroom bodies, an anatomical feature of the honeybee brain. Neither juvenile hormone nor foraging experience appears to be necessary for this change in brain structure to occur, although both factors influence the rate at which the mushroom bodies grow. The expansion of the mushroom bodies takes place in anticipation of the needs of foragers, as they must be able to recognize spatial landmarks so they can travel back and forth between the hive and distant patches of flowers.

As it turns out, the changes in juvenile hormone concentrations that take place within the bodies of honeybee workers are not absolutely fixed with worker age. This conclusion is based on experiments with colonies that have been manipulated so that all the workers are the same, relatively young, age. Under these conditions, a division of labor still manifests itself, with some individuals remaining nurses much longer than usual while others start foraging as much as two weeks sooner than average. As a result, the larvae are cared for continuously while the colony also receives food supplies.

What enables bees to make these developmental adjustments? One hypothesis is that a deficit in social encounters with older foragers may stimulate the developmental transition from nurse to forager behavior. This possibility has been tested by studies in which groups of older foragers were added to experimental colonies made up of only young workers. he higher the proportion of older bees, the lower the proportion of young nurse bees that undergo an early transformation into foragers. The behavioral interactions between the young residents and the older transplants must inhibit the development of foraging behavior. Transplants of young workers have no such effect on young workers already at the site. Thus, the social environment of young honeybee workers influences their behavioral development by regulating the release of a key hormone. When released, the hormone becomes part of a bee's cellular environment and modifies the properties of the brain's nerve cells. After these hormonally induced changes in brain structure have occurred, workers change their behavior to match the special needs of their hive.

The interaction between genetic information and the environmental causes of task-switching in the honeybee has been highlighted by a study of how genetic variation affects the rate of behavioral development in this insect. If one keeps genetically different lineages of honeybees in identical conditions, some genotypes (genetic makeups) respond to exactly the same hive environment differently by making the transition from nurse to forager more quickly than others. If the physiological cause of task-switching is the increase in juvenile-hormone concentrations caused by changes in the social environment, then the genes of fast-developing lineages can be predicted to influence things like the production schedule for juvenile hormone or the sensitivity of individuals to the age composition of their colony. As it turns out, some genotypes do tend to make more juvenile hormone sooner than usual, while other genotypes are especially sensitive to increases in the numbers of older workers in their hives. Thus, several different physiological routes can lead to fast (or slow) behavioral development in the honeybee, each dependent on a particular mix of genetic and environmental factors.