Characteristics of Residues of Methyl Parathion Applied to Field Cotton1

Abstract

Methyl parathion applied to field cotton by a high-clearance spray machine at the rate of 0.5 lb. in 6 gal. of water per acre was found to persist on and in the foliage up to 12 days. The principal site of the residues of the insecticide was in and under the cuticle of the leaf and not on the surface. Bioassays indicated the residues of methyl parathion found 1, 3, 7. and 12 days after treatment of the cotton were toxic to the boll weevil ( Anthonomus grandis Boheman). Analyses using p32-labeled methyl parathion indicated the residual half-life of the insecticide applied to cotton leaves was approximately 24 hours. Maximum penetration of the leaf by the insecticide occurred within the first 2 hours after application. The p32-labeled methyl parathion was not translocated from its site of application on the cotton leaf. Locale analyses using radioactive methyl parathion indicated that most of the residual deposit was located within the leaf tissues, with some remaining in the cuticular layer of the leaf. Four compounds containing p32 were found in the residue. Two were identified as methyl parathion and methyl para-oxon (dimethyl p -nitrophenyl phosphate) and the others were not identified. Under conditions of high temperatures (72° to 112° F.) more of the residual methyl parathion in the cotton leaf was converted to methyl para-oxon than under conditions of lower temperatures (70° to 90° F.). The toxicity of the residues of methyl parathion over extended periods was due in part to the presence of methyl para-oxon.