As harvest season begins, agriculture officials warn that even one pesticide-treated seed in a binful of corn or soybeans could spell financial doom for the farmer who sold the grain to the elevator.

Seeds treated with insecticides and fungicides are commonly used by farmers to protect seedlings at planting. These seeds are dyed bright colors to differentiate them from untreated seeds. If even a few grains of the seed show up in a truckload or storage bin, the entire lot is considered contaminated and cannot be used in food for humans or animals.

“Federal law has zero tolerance for trea...