Dichlorvos Vapor for Insect Control in a Rice Mill1

An average dose of 63.5 g/1000 m3 dispensed over 6 h once a week for 2 weeks in each of 7 rooms of a rice mill produced average concentrations of 4.52 to 6.26 µg of dichlorvos/liter of air. This concentration caused higher mortalities among caged test insects than a routine pyrethrins fog: 3-4.5 times higher among adult cigarette beetles, Lasioderma serricorne (F.); 48-92 times higher among adult red flour beetles, Tribolium castaneum (Herbst); 3-5.5 times higher among adult lesser grain borers, Rhyzopertha dominica (F.); and 5-7 times higher among larvae of the almond moth, Cadra cautella (Walker). The dichlorvos treatments caused a decrease in the insect population of the mill to ¼ the pretreatment level. Residues on bagging materials exposed to as many as 13 successive daily treatments (by moving the materials from room to room) were relatively low on cotton (≤531.8 µg/dm2) and paper (≤669.5 µg/dm2), but high on burlap (as much as 3752.4 µg/dm2). Bulk products exposed similarly had residues as follows: 4.48 ppm on rough rice, 1.29 ppm on brown rice, 1.18 ppm on milled rice, and 8.26 ppm on bran. Milled rice and bran exposed in burlap bags had only ½ as much residue, milled rice exposed in cotton bags had about the same amount, and milled rice exposed in paper bags had about 1/3 as much residue. Spillage and sweepings exposed to single treatments had residues as follows: 0.67 ppm on rough rice, 0.56 ppm on brown rice, 0.72 ppm on hulls, 1.00 ppm on broken milled rice, and 7.12 ppm on bran.