The herbicide Glyhphosate kills plants by inhibiting 5-enolpyruvylshikimate-3-phosphate synthase. Protozoa and bacteria that utilize the shikimate pathway are also killed.
Patent US7771736 B2 discloses that
Generally a dosage of as little as about 1-2 milligram (mg) per kilogram (kg) of body weight is suitable, but preferably as little as 10 mg/kg and up to about 10,000 mg/kg of each of the glyphosate source and the dicarboxylic acid component may be used. Typically, a dosage from 15 mg/kg to about 5000 mg/kg of each is used. More typically, the dose is between 150 mg/kg to about 1000 mg/kg although any range of doses can be used.
Glyphosate is a ubiquitous systemic. It's use has greatly increased in the last decade and is now being used as a harvest aid (cantaloupe) and a ripening agent (sugarcane). It is absorbed by the tissues of plants and present in pollen and nectar gathered by bees in the field.
The USDA has confirmed its presence in water and air. Bees could accumulate glyphosate due to electrostatic attraction by simply flying through the air.
We have explored the theory that glyphosate kills beneficial bacteria in the gut and on beebread - leading to chronic malnutrition even in situations where food is present.