Why bumble bees?
In comparison with other pollinating insects like honeybees, bumble bees are very effective pollinators. They are first of all fast workers (for instance, they visit twice as many flowers per minute as honeybees), and because of their size, they can carry relatively heavy loads, which enables them to make long foraging trips. Also due to their relatively large size they often achieve better contact with stamens and pistils than smaller insects.
Furthermore, bumble bees make relatively few demands on the circumstances under which they work. They feel more at ease in greenhouses/tunnels than honeybees for instance, particularly where restricted areas are concerned. Bumble bees are still active at relatively low temperatures (around 10°C) and low light intensity levels. Even strong wind and drizzle will not keep them from doing their job.
One important advantage of bumble bees over honey bees is the absence of a communication system. Honeybees inform each other by means of the so-called bees' dance of the presence of an attractive food source outside the crop in which their pollination activities are required, as a result of which the bees may leave collectively. Bumble bees do not have such a communication system. Should an individual bumble bee find an attractive food source elsewhere, it cannot inform its companions, so that the other bumble bees will continue to work in the crop in which their services are required.
Another advantage of bumble bees over honeybees, which manifests itself particularly in fruit crops, is the fact that they are not so much tied to a specific area in the crop. They change trees more often and more easily than honeybees. This benefits the cross-pollination which is often required in fruit (especially when it depends on pollen of special "pollinator trees").