Bee populations are on decline, such phenomena is known as Colony Collapse Disorder. Goulson et al.2012 believes that neonicotinoid imidacloprid insecticides are linked to the decline as they occur at trace levels in the nectar and pollen of crop plants. 
The researchers exposed colonies of the bumble bee Bombus terrestris to neonicotinoid imidacloprid, observing reduced growth rate and an 85% reduction in production of new queens compared to control colonies. Imidacloprid insecticides are used worldwide to treat seeds such as rapeseed. During the growth of the plant, traces of the insecticide is carried to the flowers of the plant. Bees get there in contact with the poison.
 Whitehorn PR, O'Connor S, Wackers FL, Goulson D: Neonicotinoid Pesticide Reduces Bumble Bee Colony Growth and Queen Production. Science. 2012 Mar 29