>>  Lowbush blueberries are there to start with so the fields are not actually planted but rather managed to promote the blueberry plants at the expense of other native plants.
>
>Right. So this is the part I don't get. If there already are millions of acres of blueberries, then there almost must be pollinators in equally large numbers. Why are folks convince that there is a pollinator deficit that must be remedied with non-native bees?

The natural stands are partly limited in density by the natural pollination available, and are interspaced with other plants, possibly providing habitat and alternate forage for native pollinators.

Cultivation greatly increases the blueberry density and reduces the nearby alternate forage as well as possibly disrupting habitat for pollinators.  The added density also requires larger numbers of pollinators at a distance from refuge and alternate food.  Unlike honey bees, some of these pollinators only range a few hundred feet in their entire life.  They would never reach the centre of the crop. Moreover, some might get lost on a large featureless expanse.  (That can be a problem with any pollinator, including honey bees).

Additionally, in nature, there are years with no or few pollinators, just as there are years when pollinators may show up in abundance.

The nature of agriculture is to try increase the concentration of producing plants and to ensure reliable, adequate and cost-effective supply of all inputs, the shortage of any of which could potentially affect production adversely, including pollinators.

Reducing risk is a huge part of successful farming and is the part of agriculture that is entirely lost on many non-farmers as well as many idealists and academics.  Maybe that is why they are not farmers.  Anyone who misses that class is not a farmer for long.  (The other thing a farmer needs to know for survival is that money spent is money spent, but money earned is only a maybe).

Managing pollinators is part of that risk management, and assigning pollination to experienced professionals is usually part of that risk management.  I know of many large companies which require pollination, but only a handful which do not hire it out to professionals, and those that do it well hire professionals with a long practical (commercial) beekeeping background.

What is not commonly realised is that some years, even careful observers cannot figure out how the pollination was accomplished on a given field, and how much the bees actually did of the job, since flies and other pollinators contribute.  However, since bees are so cheap to put on a crop compared to the increase in production each hive adds (up to a point), and because bees guarantee a set, smart operators will try to put an excess of bees on each acre and worry if they are short bees.

Of course, it is hard on the bees, being oversupplied on a crop, and that is why the beekeeper are paid the big bucks.   They have to figure out how to repair the damage to the bees after each set and build them back up for the next set.   

Make no mistake.  Providing optimal pollination is hard on bee colonies and the assumption that many make that having a few hives around and that those hives or other pollinators will thrive is a big mistake once the fields get to a certain size.  The expenditures to get into production and the value of the crop far overshadows the expense of hiring bees.

There are some pollination sets which actually build the bees (often some years and not others), but many do not, and if there is a decision whether to save the crop or the bees, ask any beekeeper, in the grower's mind the crop comes first.  

In an isolated area, should spraying be necessary, where would th bees go and who would move them?  A professional pollinator is set up to work around the grower's needs, whereas a grower may be spread to thin trying to manage the crop and bees at the same time.

At any rate, native pollinators can work well in limited plantings, and where there is not a huge expense or dependence on the crop, but where there is a large investment and huge expanses of one crop, prudence calls for a pollinator which is manageable and predictable.  Where crop insurance is available, the bees supply is a condition of coverage and insurance companies may require evidence of hive condition and assurance that the bee supplier is qualified.

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