In everyday English, to “backfill” means to refill. So if you want to plant a bush, bury a conduit, or repair a water main, you dig a hole, do what you have to do, and then put the dirt back in. Simple enough. The meaning is only slightly different in beekeeping, but different enough to be confusing.
In beekeeping, backfilling refers to filling an empty brood cell with honey or sometimes pollen. In other words, a cell that previously held one or more generations of developing bees, is now used to store food.
This may occur at different times of year. For example, in the fall when the colony has finished drone rearing, the drone cells may be filled with honey. There are several good reasons for this: drones won’t be raised again for many months, the cells are the wrong size for workers and, since they are close to the main nest, they are convenient storage areas. A well-stocked pantry right at your finger tips—er—tarsal claws!
But there is another time when bees start backfilling . . . just before swarm season. But first, let’s back up a few weeks.
In the early spring the brood nest expands in a process that is the opposite of backfilling. As the bees use up the nectar and pollen adjacent to the brood nest, they turn those empty cells into brood-rearing cells. This allows the colony to raise many more bees—enough bees to maintain the hive and prepare for winter, as well as enough bees to swarm.
Then, as swarming time gets close, the bees begin backfilling the outermost brood cells with honey. This shrinks the nest size, which in turn decreases egg laying by the queen. It means that, after the swarm leaves, the brood nest will be small enough that the remaining bees will be able to care for it. Backfilling provides a way of scaling down the entire operation so a greatly reduced workforce can still get the job done. Ingenious!
It is important to understand the concept of backfilling if you want to manage swarms. Checkerboarding—a popular but often misunderstood swarm management technique—is based on this backfilling behavior. Checkerboarding should be started at the peak of brood nest expansion, just when the nest stops getting larger and begins to shrink . . . but more on that later.
Just for the record, “backfill” can also be a noun, the name for the stuff you put in the hole, as in “They already capped the backfill.”