The very short version is that there are a bunch of factors, and wild bees aren't in danger.
I don't know if anyone knows where the bees are going, exactly, but it looks to me like they're leaving because the living conditions in commercial hives aren't tolerable.
Bees normally live on honey and bee bread (fermented pollen). They aren't happy being fed sugar, fructose, and/or honey. Feeding them honey might not be such a bad idea, except that a lot of the honey available for sale is fake honey, and in the interest of making honey untraceable, the pollen (which bees need) gets filtered out.
The fungicides which are needed to control fungus in the hives also interfere with the fermentation for the bee bread.
There's no pesticide that can kill the mites which are deadly to bees are also at least someone damaging to bees-- they're both arthropods.
Things to do: plant flowers. Oppose habitat destruction. Buy honey at farmer's markets. Be careful about imposing laws-- in Europe, neonicotinoids are forbidden, and farmer's are using the more dangerous pyrethroids.
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