Deposition, mobility and persistence of sprinkler-irrigation-applied chlorpyrifos on corn foliage and in soil

The insecticide chlorpyrifos, (O,O-diethyl-3,5,6-trichloro-2-pyridyl phosphorothioate) was applied to sweet corn (Zea mays L.) by chemigation, injecting either an emulsifiable formulation or technical chlorpyrifos dissolved in soybean oil into 0.25 cm or 1.27 cm sprinkler-applied irrigation water. Half of the plots treated with chlorpyrifos in 0.25 cm water were immediately irrigated further with 1.27 cm water without chlorpyrifos. Half of the plots treated with chlorpyrifos in 1.27 cm water were irrigated with another 1.27 cm water without chlorpyrifos 10 days later. Neither the volume of water used to apply the insecticide nor subsequent irrigation affected chlorpyrifos resides on the corn foliage or in the soil. Chemigation of the soybean oil solution resulted in three times more chlorpyrifos on the foliage than chemigation of the emulsion formulation, apparently because of greater adhesion of the oil droplets to the foliar surface. Chlorpyrifos residues in foliage declined with an initial half-life of one day. Chlorpyrifos on or near the soil surface declined with an initial half-life of approximately four days.