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The species richness of wild bees and other pollinators has declined over the past 50 years, with some species undergoing major declines and a few going extinct. Evidence of the causes of these losses is patchy and incomplete, owing to inadequate monitoring systems. Managed honey bee stocks have also declined in North America and many European countries, although they have increased substantially in China. During this same period, the demand for insect pollination of crops has approximately tripled, and the importance of wild pollinators in providing such services has become increasingly apparent, leading to concern that we may be nearing a “pollination crisis” in which crop yields begin to fall. This has stimulated much-needed research into the causes of bee declines. Habitat loss, which has reduced the abundance and diversity of floral resources and nesting opportunities, has …