As plantings have grown larger (monoculture), the need for concentrated pollinators at bloom time has grown. At the same time populations of many pollinators has been delining, and this decline has become a major environmental issue today.
Feral honeybee populations in the US have dropped about 90% in the past 50 years, except for the southwest where they have been replaced by africanized bees. At the same time managed honeybee colonies have dropped by about two thirds.
Some think that other pollinators will replace the lost honeybees, but general pollinator decline was already happening before we had tracheal and varroa mites in honeybees. Only in a few areas, wild populations are building up; in most areas they are declining as badly as honeybees.
Furthermore pollinators cannot be exchanged on a 1 for 1 basis. They are not all equal. Some are generalists, some are specialists. Some have long tongues; some short. Bees may deliberately collect pollen, but have different collection techniques.
Monoculture needs very high populations at bloom, but can make the area quite barren, or even toxic when the bloom is done.
The decline of pollinators is compensated to some extent by beekeepers becoming migratory, following the northward in the spring from southern wintering locations.
Here are some of the reasons for pollinator decline:
Pesticide misuse
It is a label violation to apply most insectides on crops during bloom, or to allow the pesticide to drift to blooming crops that bees are visiting. Yet it is frequently done, with little enforcement of the bee protection directions. Pesticide misuse has driven beekeepers out of business, but can affect native wild bees even worse, because they have no human to move or defend them.
Bumblebee populations are in very bad shape in cotton country. Bumbles LOVE cotton, and they are hit over and over when pesticide applicators (illegally) apply insecticides on this blooming crop while bees are foraging.
Widespread aerial applications for mosquitoes, med-flies, grasshoppers, gypsy moths and other insects leaves no islands of safety where wild bees can reproduce and repopulate. One such program can knock down pollinator populations for several years.
The chemlawn philosophy has convinced people that dandelions and clover are weeds, that lawns should only be grass, and that highly treated with pesticides. This makes a hostile environment for bees, butterflies and other pollinators.
Rapid transfer of parasites and diseases around the world.
Increased international commerce within modern times has moved diseases such as American foulbrood and chalkbrood, and parasites such as varroa mites, acarine mites, and the small African hive beetle to new areas of the world, causing much loss of bees in the areas where they do not have much resistance to these pests. Imported fire ants have decimated ground nesting bees in wide areas of the southern US.
Loss of habitat and forage
The push to remove hedgerows and other "unproductive" land in some farm areas removes habitat and homes for wild bees. Large tractor mounted rotary mowers may make farms and roadsides look neater, but they remove bee habitat at the same time. Old crops such as sweet clover and buckwheat, which were very good for bees have been disappearing. Urban and suburban development pave or build over former areas of pollinator habitat. Migratory pollinators, such as monarch butterflies and some hummingbirds depend on nectar corridors for their annual migration, and development or agricultural practices have disrupted some of these vital corridors.
Bee paranoia
If one runs a seach for "carpenter bees" the Internet, one will mostly find information about how to kill these valuable pollinators, not how to encourage and use them. This attitude ("Get the bug spray, ma, I just saw a bee!") is one of the worst problems our pollinators have. The "killer bee" hype has increased this paranoia. Beekeepers find increased vandalism of their hives, more difficulty in finding locations for bee yards, and more people inclined to sue the local beekeeper if they are stung, even if it is by a yellow jacket.
'Resources on Pollinator Decline, Protection, Enhancement and Management:'
Forgotten Pollinators by Drs. Stephen L. Buchmann and Gary Paul Nabhan is a classic work describing the pollinator crisis. Their opening chapter, "Silent Spring and Fruitless Falls" describes the risk in a nutshell. They go on to illustrate the problem and propose some solutions.
Many people, even folks in agriculture who should be highly aware of this issue are not. Dr. Malcolm Sanford of the University of Florida has written an important position paper: Pollination, the Forgotten Agricultural Input, which is online at: [1]
The United Nations Food and Agruculture Organization has also issued a position paper on the subject of pollinator decline: Biological Diversity: Pollinators [2]
The International Initiative for the Conservation and Sustainable use of Pollinators: A proposal for a plan of action [3]
Xerces Society Pollinator Conservation Program (North America) [4]
POLLINATOR BIODIVERSITY A CO-ORDINATED CLOBAL APPROACH (FAO) [5]
Article in Conservation Ecology: The Economic Impacts of Pollinator Declines: An Approach to Assessing the Consequences [6]
Brazillian Pollinators Initiative [7]
African Pollinator Initiative [8]
THE SAO PAULO DECLARATION ON POLLINATORS [rgm.fmrp.usp.br/beescience/arquivospdf/workshop.pdf]
The Pollination Home Page [9]
The North America Pollinator Protection Campaign [10]
Pollinator Conservation Handbook [11]