Disappearance of dimethoate, methamidophos and pirimicarb in lettuce

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Abstract
Foliar sprays of insecticides dimethoate at 150 or 300 g ai [active ingredient]/ha, methamidophos at 450 or 900 g ai/ha and pirimicarb at 140 or 280 g ai/ha were applied for control of the green peach aphid, Myzus persicae and the lettuce aphid, Nasonovia ribisnigri, .apprx. 2 wk before the lettuce started heading, and again .apprx. 1 wk from harvest. In lettuce, dimethoate partially oxidized to its oxon and pirimicarb converted to its methylamino- and/or formyl methylamino-analogues. Most residues were present in the outer leaves which were exposed directly to the sprays; only traces of residues were detected in samples of the inner head leaves. Total residues disappeared rapidly. Pirimicarb was the least persistent and only traces of residues (< 0.01 ppm) were detected in marketable heads. Concentrations of dimethoate, including the oxon and of methamidophos were well below their respective tolerances of 2 and 1 ppm, respectively.