Just got this from the beekeepers newsletter.
Guttation Drops Killing Honey Bees: Vincenzo Girolami, et. al., "Guttation Toxicity of Seedling From Coated Seeds, Journal of Economic Entomology, Volume 102, Number 5, October, 2009.
Guttation is a natural plant process where xylem fluid in a plant is excreted at the edge of the plants' leaves. In corn seedlings grown from seeds coated with neonicotinoids (systemic pesticides that are taken up by plants and expressed in nectar and pollen), the concentration of neonicotinoids in the guttation drops was found to be near those of the active ingredients in field sprays, or higher. Bees drinking these guttation drops died within minutes.