How bees smell and taste

Smelling with antennae

Odour is detected by pore plates and hairs along the antennae, which carry receptor neurons in the tens of thousands. Their signals converge on the antennal lobes, where each odour produces a characteristic spatial pattern of activity across roughly a hundred and sixty glomeruli. A scent is, in effect, a shape drawn across that map, and learning a scent means learning to recognise the shape.

Bees learn odours fast. A single pairing of a scent with sugar is often enough to produce a durable association, which is why olfactory conditioning is the standard tool for studying insect memory. They discriminate between molecules differing by a single carbon in a chain, and between mixtures differing in the ratio of components.

The small sense of taste

The honey bee genome carries a large family of odorant receptors and a strikingly small family of gustatory ones — around a tenth as many taste receptor genes as a fruit fly. A bee is built to smell, not to taste.

What survives is what a nectar feeder needs. Bees respond strongly to sugars, and an individual's sucrose response threshold is a stable, measurable trait that predicts what job she takes on in the colony and how she behaves as a forager. They detect salt, and water. Bitter detection is limited compared with other insects, which has a practical consequence: bees are poor at rejecting some contaminated nectar, and a syrup a bee will happily drink is not evidence that it is good for her.

Taste is not only in the mouth

• The antennae carry contact chemoreceptors, so a bee tastes what it touches before it drinks.
• The mouthparts and tarsi — the feet — also carry gustatory sensilla, which means a bee standing on a nectary is already sampling it.
• Sucrose responsiveness measured by touching an antenna is the standard assay, and it varies consistently between individuals and with age.

Why the colony smells like itself

Chemistry is also how a colony holds together. Comb wax, stored food and the cuticular hydrocarbons of the bees themselves combine into a colony odour, and guards use it to decide who belongs. The threshold shifts with conditions — guards are more permissive during a strong nectar flow and stricter during a dearth — which means the errors are systematic rather than random, and drifting bees from neighbouring hives are routinely accepted.

Learning a scent, and forgetting one

Olfactory learning in bees is fast, durable and specific enough to be the standard tool for studying insect memory. A single pairing of a scent with sugar produces an association detectable minutes later; several pairings produce one that survives for days and requires new protein synthesis to form, which is the same molecular signature long-term memory has in vertebrates.

It is also revisable. A scent that stops paying is extinguished, and a bee that has learned one flower will switch when a better one appears. Flower constancy — sticking to one species during a foraging trip — is a strategy rather than a fixation, and it comes apart when the reward does.