For the purposes of this series, the term solitary bees include the relevant species native to the farm in the Shenandoah Valley of Virginia: carpenter bees, mason/orchard bees and bumble bees. Relevance to other regions still exists as the botanical family that includes these bees has native species to much of the northern hemisphere.
Declining populations of solitary bees is due to one single factor: Petroleum.
Not long ago, animals were the engines that drove farms. As a result, most of the fuel for these animals was grown right on the farm in the form of forage crops like clover. Vast pastures of clover provided nourishment to the solitary bees. The arrival of petroleum and cheap fuel yielded a shift to mechanized equipment becoming the engine that drives farms. Accordingly, the desired pasture composition shifted toward nectar-less grasses to fatten livestock bound for the dinner plate. Access to nourishment for solitary bees was greatly diminished.
Exacerbating the problem was petroleum fertilizers to grow the grasses tall. Previously, little to no fertilization beyond the biological waste of farm animals was used on the flower-rich pastures. Fertilizers applied to grasses helped them shade out wildflowers completing the shift in pasture composition away from bee-sustaining flowers. Unlike honeybees that will travel miles to forage, most solitary bees prefer to feed within 300 feet of their nest. Thus the massive, unbroken fields of monoculture crops have vast areas that cannot be reached by native pollinators. The resulting trend is that yields of insect pollinated crops are getting increasingly unpredictable. (Source, page 41)
Mechanization of agriculture also yields a significant shift in land use. Habitat providing forests and nourishment providing meadows are both removed to plant rows of monoculture crops or grass-dominant pastures. These highly specialized land uses are only possible due to machinery powered by cheap fuels.
In fact in Britain, most wildflowers that bumblebees depended on are now absent from the landscape. Unsurprisingly, the last native short haired bumble bee in Britain was observed in 1988. I believe efforts are being made to re-introduce bees previously exported to New Zealand where no native pollinators existed to produce seed from the red clover crop.
The arrival of the various petroleum-based insect-, pest-, herb-, fungi-, etc. –cides was the figurative nail in the coffin for these poor bees. Applications of pesticides have a collateral damage effect to the tune of 85% reduction in queen production and 55% population reduction in solitary bees. (source) Some parts of China has used so many chemicals that both native bees and honeybees cannot survive resulting in the need to hand pollinate entire apple orchards (source). Unsurprisingly, bee-pollinated orchards have extremely higher production rates than hand-pollinated orchards.
In Britain, out of 25 bumble bee species, only 6 remain in significant numbers with 3 gone from the region and 8 more following suit. Ireland has 2 species on the brink of extinction with 4 more considered endangered. The Irish are currently in danger of losing pollination ability that would destroy the agriculture economy. Here in North America, we have 3 species in danger of extinction with one additional species thought to already be extinct.
Cultivated honeybees can only pollinate open faced flowers. All other plants require solitary bees –usually bumblebees- to provide adequate pollination. Without these solitary bees, plants like tomatoes, eggplants, wildflowers and clovers to name a few could cease to exist, or at least be economically unviable to produce as food crops. Therefore native pollinators are more deserving of the fame and more in need of the conservation given to honey bees.