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Important is the life cycle of a swarm. Unlike wasps, bumble-bees and lots of other insects a bee swarm survives the winter as a whole and can start the pollination cycle earlier than other insects. Furthermore honeybees have a structured way of flying "on: plants, meaning that they do commonly not change pollination rate.

Bees are actively used for guided pollination, which is not possible with most other insects (not withstanding some experiments with bumblebees in greenhouses). Interesting fact is that you will often find the phrase "crop xxx requires yyy hives" while describing the role of bees in pollination.

hth.