This depends on the species of bee. The honeybee has been subject to several pests, including varroa mite, hive beetle and a plethora of viral diseases which all affect the health and productivity of the hive. There have also been many media articles concerning colony collapse disorder (CCD), which is likely to be due to a combination of factors including bee stress and the use of insecticides.
Several species of bumblebee are also in decline and the principle factor causing this is habitat loss. The species most affected are those which inhabit field margins, chalk downland or other specialist habitats. We don’t know enough about their specific habitat requirements to effectively conserve many of these.