Persistence of Diazinon and Zinophos In Soils1

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Abstract
The persistence of C14-labeled diazinon and Zinophos® (O, O-diethyl O-2-pyrazinyl phosphorothioate) was determined in 4 soils under laboratory conditions at 25°C. Treated soils were extracted at intervals with equal amounts of acetone and 0.05N CaCl2 to remove the insecticide and its degradation products. The initial recovery of both insecticides was greater than 94%. The disappearance rate of C14-diazinon was similar in the 4 soils. One-half of the original applications was lost in 2 to 4 weeks and less than 8% remained after 20 weeks. The persistence of C14-Zinophos varied considerably in the 4 soils. The time required for 50% loss in an organic soil, sandy loam, silt loam, and clay loam was approximately 10, 6, 4, and 1.5 weeks, respectively. After 24 weeks 2 to 27% of the original applications remained. Nonextractable radioactivity at the end of the experiment represented 20 to 30% of the original applications. Carbon14 from soil treated with ethoxy-labeled diazinon and pyrazinyl-labeled Zinophos was lost to the atmosphere as C14O2 In field trials diazinon and Zinophos were applied to a silt loam at rates of 5 and 6 Ib per acre, respectively. Insecticide residues were measured at intervals by gas chromatography. A rapid loss of toxicant during the first 8 weeks after treatment was followed by a much slower decline with both insecticides. After 24 weeks approximately 10% of the original applications remained. Degradation curves for the laboratory and field experiments were simila.