How bees communicate by smell

What a pheromone is, and what it is not

A pheromone is a chemical produced by one individual that changes the behaviour or physiology of another of the same species. In honey bees they fall into two loose groups: releaser pheromones, which produce an immediate behavioural response, and primer pheromones, which change an animal's physiology over days.

Neither group is a message in the sense of a sentence. A pheromone shifts a probability. Alarm pheromone does not order an attack; it lowers the threshold at which nearby bees will attack, and whether any individual does depends on her age, her genetics, the weather and what else is going on.

The queen's chemistry

Queen mandibular pheromone is a blend rather than a single compound, and no single component reproduces its full effect. It suppresses worker ovary development, inhibits queen rearing, attracts a retinue that grooms and feeds the queen and then distributes the pheromone onwards by trophallaxis, stabilises a flying swarm around her, and attracts drones during a mating flight.

The distribution mechanism is what makes it a colony-level signal at all. A queen cannot touch twenty thousand bees. The retinue picks the blend up and passes it on, so its concentration falls with distance from her and with the size of the colony. A colony that is crowded enough for the signal to become dilute is a colony that starts building queen cells, which is one of the mechanisms behind swarming.

Alarm, and why a beekeeper avoids bananas

When a worker stings, the sting apparatus releases a blend whose most abundant component is isopentyl acetate — the compound that gives bananas much of their smell. It marks the target and lowers the attack threshold in nearby bees, which is why a single sting is often followed by more in the same place. A second alarm pheromone is released from the mandibles and acts at shorter range.

Orientation and marking

• Nasonov pheromone is fanned from an abdominal gland at a nest entrance, a water source or a settling swarm. Bees adopt an unmistakable head-down, abdomen-raised posture to broadcast it.
• Footprint pheromone is deposited passively as bees walk, and marks recently visited flowers so that foragers avoid revisiting an emptied one. It is also deposited at the hive entrance and contributes to orientation.
• Comb wax and stored food carry a colony-specific odour signature that guards use to decide who belongs.

Brood pheromone

Larvae release an ester blend that shifts nurse behaviour, influences how much pollen is foraged, and modulates the age at which workers begin to forage. It is a good corrective to the picture of a colony as adults managing a passive brood: the brood is signalling its own requirements and the adults are responding.

Why the system is redundant

Functions overlap. Several pheromones influence foraging effort; several influence swarming; queen and brood signals interact. That redundancy is characteristic of a system that has to work in a dark, crowded, noisy environment with no addressing and no acknowledgement, and it is why a single compound is rarely a clean explanation for a colony-level behaviour.