Disappearance of dimethoate, methamidophos and pirimicarb in lettuce

Foliar sprays of dimethoate at 150 or 300 g a.i./ha, methamidophos at 450 or 900 g a.i./ha and pirimicarb at 140 or 280 g a.i./ha were applied for control of the green peach aphid, Myzus pericae (Sulzer), and the lettuce aphid, Nasonovia ribisnigri ( Mosley ), about 2 weeks before the lettuce started heading, and again about 1 week 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 (less than 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.