Recent studies described in Iowa State and Purdue literature explore some ways that honeybees are affected by insecticide treatment, especially neonicotinoids.
"...neonicotinoid exposure is likely a combination of direct contact; indirect contact with dosed weeds/crops, talc or soil; and through ingestion from pollen in dosed plants. "
Neonicotinoids will be used on about 250 million acres of cropland this year, says the ISU paper. It is extremely lethal to honeybees. Poncho,Cruiser and Gaucho are some products with this insecticide.
No one is saying definitively that neonicotinoids are the root cause of the honeybee decline, but it is pretty hard to avoid the conclusion that they may be detrimental to honeybees, and we need bees.
These papers have some recommendations that all of us can use to minimize the effect of neonicitonoids on honeybees.
http://www.extension.iastate.edu/CropNews/2012/0406hodgson.htm
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0029268
