Social insects such as honey bees live in complex societies, and it is a fascination to many scientists how bees co-ordinate their behaviour according to the needs of the colony. Worker bees must perform a number of tasks in order to keep the hive functioning correctly. Younger bees tend the larvae and the hive and, from the age of three weeks, can abandon their hive tasks and switch to foraging for pollen and nectar. The performance of these two types of task within the colony relies on the bees altering their behaviour as the colony's needs change. Brood pheromone is secreted by the larvae and stimulates younger bees to switch from hive tasks to foraging to feed the youngsters. On the other hand, the presence of older bees, which do most of the foraging, inhibits the onset of foraging in younger bees. The sensitivity of the bees to either brood pheromone or older bees alters according to the needs of the colony, but can also be manipulated experimentally.
Octopamine is a neurochemical that can modulate the activity of neurons and alter the behaviour of an insect. Andrew Barron and his colleagues at the University of Illinois used two types of bee colony that have controlled population structures to investigate how octopamine affects the responsiveness of honeybees to the presence of older bees in the hive and brood pheromone released by larvae.
First, the team tested how octopamine alters the effect that older bees have on the foraging activity of younger bees. They took single cohort colonies, which consisted of a queen and young worker bees that are all the same age. Two colonies were treated with octopamine mixed with sucrose that was provided as a food source, and one of the colonies received a transplant of 100 older forager bees. Then the colonies were sealed for 3 days. Upon opening the hives again, all the transplant bees were removed and the time of onset of foraging as well as the number of foragers was noted. They found that octopamine makes the younger bees less responsive to the inhibitory effect of the older bees that usually prevents the youngsters from foraging. After treatment with octopamine, more younger bees foraged even when the older bees should have inhibited them!
Then the team tested how octopamine affects the number of bees foraging in a colony when they are exposed to brood pheromone. This time they used triple cohort colonies in which the worker bees belong to one of three different age groups. Dishes containing brood pheromone, which activates foraging, were placed in the comb of two colonies, and one of these colonies also received octopamine. It was discovered that the bees were more likely to forage when octopamine is available in their food because it makes the bees more sensitive to brood pheromone.
This study shows that octopamine plays an important role in the complex organisation of bee behaviour, reducing the foraging workload when there are already enough bees on the job while ensuring that enough pollen and nectar are collected when there are hungry mouths to feed.
