A major study by the Government research agency FERA http://fera.co.uk/ finds a correlation between use of neonicotinoid pesticides and bee colony losses.   They combined large-scale pesticide usage and yield observations from oilseed rape with those detailing honey bee colony losses over an 11 year period, and reveal a correlation between honey bee colony losses and national-scale imidacloprid (a neonicotinoid) usage patterns across England and Wales.

http://www.nature.com/articles/srep12574

http://www.fginsight.com/news/major-study-finds-correlation-between-neonicotinoids-and-bee-colony-losses-5645