4.1.1. Pollen trapping
The number of foragers from a honey bee colony that is collecting pollen can be estimated using pollen traps (Goodwin, 1997; see Human et al., 2013 for more details). This is particularly relevant if the crop of interest only produces pollen (kiwi) or if pollen foragers are more efficient pollinators than nectar foragers (apple, almond) because they have a greater likelihood of contacting the stigma. Pollen traps are devices with grids (Fig. 8) that fit across the entrance of a hive. With some designs, the hive entrance is blocked and the trap forms a new entrance. Returning foragers must walk through the grid to enter their hive. Bees prefer not to walk through pollen traps if they can avoid it and will use any other gaps in a hive body as an entrance once a pollen trap is fitted. These holes need to be blocked to ensure that all bees are using the pollen trap. It is worth checking the hives several days after the trap is fitted to make sure all bees are entering and leaving the hive through the pollen trap.
As returning bees carry pollen through the trap the
grid scrapes some of the pollen pellets from their corbicula. The pellets then
fall into a tray where they can be collected. The proportion of pollen pellets
removed depends on the size and shape of the holes in the grid and the size of
the pollen pellets the bees are carrying. Pollen pellet size, and consequently
the efficiency of a pollen trap may vary with both the plant species, the time
of day the pollen is collected, and meteorological conditions (Synge, 1947).
The inside of a pollen trap can become blocked over time. Depending on the
design of the trap it may be difficult for hive cleaning bees to carry dead
bees through the trap and for drones to move through it. When these accumulate
on the inside of the trap it can reduce the ease with which foragers move
through the trap and hence the foraging ability of the colony. If traps are to
be used for extended periods of time they should be checked regularly for
Fig. 8. Grid on a pollen trap