What are temporal castes in honey bees?

Yes, she’s a worker, but what kind?

Don’t let the phrase “temporal caste” scare you away. Temporal means “related to time” and castes are social groups with physical differences designed for special jobs. All beekeepers already know about temporal castes in honey bees, but few know the name.

Queens and workers

Female honey bees are born into one of two physical castes: they are either workers or queens. These bees have some obvious physical differences that we can see. Queens are bigger than workers with long abdomens and legs that splay away from their bodies. Their wings are about the same size as worker wings, but they appear short compared to their abdomens. They also have a large bare spot on their thorax and lack corbiculae on their hind legs.

Queens also have differences we can’t easily see just by looking. They have well-developed reproductive systems and pheromone-secreting glands, for example. Mated queens have an overflowing spermatheca for storing sperm for years and their stinger is smooth and knife-like, suitable for reuse.

The caste honey bees are born into doesn’t change. Queens don’t shrivel into workers, and workers don’t grow into fully functioning queens.

Temporal castes are temporary

Temporal castes are based on temporary periods of highly specialized work. The work is so specialized, in fact, that it requires changes in the physiology or anatomy of the individual. This phenomenon is called temporal polyethism, a word derived from “many gods.” Worker bees go from one job assignment to another as they age. In times of colony stress, older bees can sometimes revert to a job they held as a youngster.

The key to defining a temporal caste is a physical change in an individual. For example, nurse bees and comb-building bees are both workers. Structurally, they look the same. But nurses have highly developed hypopharyngeal glands that allow them to feed the developing larvae. Comb-building bees have shrunken hypopharyngeal glands and lose the ability to provide brood food but produce large amounts of wax from glands in the abdomen.

We don’t fully understand how they change from one temporal caste to another. On the surface, a worker bee appears to go from one job to another in a more or less orderly fashion, with each temporal caste lasting about a week. But the roadmap is unclear—not all bees do all the jobs.

What causes change during colony stress?

Some researchers say that if a bee can go “willy nilly” from one job to another, there is no temporal caste system. Others point out that stimuli from within the colony cause the changes, not individual choice. And because the change requires physical re-tooling of the bee’s body, it meets the definition of temporal polyethism.

As in the previous example, workers with developed hypopharyngeal glands don’t have developed wax glands—time is necessary to make the change. Hence, the stages are temporal. Other temporal assignments, some with fewer physical changes, include cleaning, guarding, undertaking, scouting, and foraging.

Even laying workers can change back to regular workers if enough open worker-brood pheromone is present to suppress their ovaries. It is a temporary, although often terminal, situation.

Are winter bees their own caste?

To make things more complicated, the physiology of winter bees differs from spring and summer bees, but they don’t behave like a temporal caste. Winter bees are born as winter bees. They are not summer bees that change into winter bees. Pollen shortages in late autumn appear to stimulate the production of winter bees in the colony, a natural part of winter preparations.

Winter bees have obvious physical differences that allow them to store large amounts of brood food within their bodies. They also have unseen differences that extend their life expectancies from weeks into months. And even when their fat bodies become depleted, they do not become regular workers.

We know this because, unlike summer bees, winter bees do not have a division of labor in their ranks. Winter bees are generalists, doing what’s needed in the hive with no physical changes required. Instead of shape-shifting to do a different job, they simply do what’s needed. What one can do, they all can do. 

Taken together, all these factors suggest winter bees are not a temporal caste, but perhaps a third female caste, giving us queens, workers, and winter bees.

Other insects with temporal castes

Insects with both physical castes and temporal castes include honey bees, stingless bees, and ants. Others have similar social structures that are less defined and obvious, such as termites, bumble bees, sweat bees, and social wasps.

Rusty
Honey Bee Suite