Omethoate

Omethoate is a systemic insecticide used for the control of (mostly) sucking insects and mites in a wide variety of crops.

Contact dermatitis from omethoate-dimethoxon is rare.

Omethoate is the P=O analogue (oxon) of dimethoate. Dimethoate is metabolised via oxidative desulfuration to omethoate which is the active anti-acetylcholinesterase metabolite. This biotransformation occurs in all media: hence the metabolic pathways of both compounds have much in common. However, degradative pathways acting directly on dimethoate such as O-demethylation, N-demethylation and hydrolysis of the amide function mean that the balance between activation and degradative metabolism will influence their respective selective toxicities. In mammals there are two main routes of degradation: (i) glutathione transferase mediated O-demethylation (ii) hydrolysis to N-methylthioglycolamide which is S-methylated by S-adenosylmethionine and subsequently thiooxidised. In soil and plants the N-methylthioglycolamide moiety formed from the hydrolysis of omethoate undergoes a complex series of C₁, C₂ and C₃ biotransformations ultimately leading to oxalate and citrate respectively and total degradation to C0₂. The information reported below is derived largely from an evaluation prepared by the UK MAFF Pesticide Safety Directorate (PSD, 1993).

Orally administered omethoate to rats is rapidly metabolized and excreted in the urine; the main metabolites are O-demethylomethoate and N-methyl-2- methylsulfinylacetamide. O-Demethylation and hydrolysis of the P?S bond are main degradation routes both in mammals and plants. Omethoate is rapidly degraded in soils with DT₅₀ of a few days.