I'm new to this news group so if this topic has been beaten to death I'm glad of that fact but sorry to bore you. Been beekeeping for 25 years in Ontario, Sask, Alta, and now PEI. All these are in Canada.  Our big cash crop here is potatoes. Crop rotations are potatoes, grain (usually barley), red clover/timothy mix (sometimes a bit of sweet clover or alsike thrown in) for hay.  In 1996 potato farmers started using a new systemic insecticide called Admire (active chemical is Imidacloprid) on the potato crop. It is applied in the furrow as the potato sets are being planted so at first thought it should not cause any problems with bees since no spray drift and bees do not visit potato blossoms.  Apparently, from research done in France, the problem arrives on the clover crop.  The chemical stays in the soil a long time and comes up in the succeeding crops.  If they are pollen and necter producers then the bees can get the chemical.  Bees are very suseptable to imidacloprid poisoning at extremely low doses ( 3ppb ) That's 3 parts per Billion.  Problems start showing up as higher-than-normal winter losses.  My losses are normally 10 to 20% of hives wintered.  The past two winters have given me losses of 50% and 80%. I haven't done an inventory this year yet as we are still up to the hive tops (2 boxes) in snow. Most beekeepers, me included, on PEI experienced rather heavy losses last summer as well. I try to operate 300-400 hives but was down to 250 in the fall. My hives have NO MITES. 
Imidacloprid is in about a dozen brand name chemicals and are all made by Bayer( to my knowledge). Admire, Condifor, Gaucho, Premier, Premise, Provado, Marathon, Merit, and Impower are the names of which I am aware.  Imidacloprid is used on such crops as rice, cereals, corn, potatoes, sunflowers, tomatoes, various vegetables, canola, sugar beets, fruit, cotton, millet, hops and turf.  It has a half life fluctuating from about 9 months to 2 years depending on the soil and weather conditions. 
According to reports out of France, Imidacloprid has killed about 300,000 hives.  They had the problem working sunflowers. Bayer has been presenting very poor evidence (in my opinion) that Imid. does no harm to honeybees. The dead hives in France and here on PEI are still dead though. I again apologize for droning on and boring you but this chemical is or will be a problem for beekeepers world wide. By the way, France has suspended the use of Imidacloprid for one year and just recently extended that suspension for 2 more years.
I will monitor the group and if there is requests for more info I will try to comply. Please don't ignor this chemical.  I've had bees next to potatoes for many years with no problems but this is different.
Big Al