Continuing the discussion on whether wild bees could provide pollination services, this is from almost 100 years ago:

Many careful experiments have shown that the honeybee is the most numerous and the most effective of all pollinating insects in California and is essential to the production of full crops of certain deciduous fruits and nuts. Many orchardists keep a few colonies for pollinating their trees, especially where the bees can reach alfalfa fields later in the season. 

Others find it more advantageous to rent the desired number of colonies from a professional beekeeper, because many orchard districts are so highly specialized that bees find it difficult to collect enough nectar to ward off starvation during the greater part of the year. 

Zierer, C. M. (1932). Migratory beekeepers of southern California. Geographical Review, 260-269.

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