Persistence of four pyrethroid insecticides in a mineral and an organic soil.

Permethrin, cypermethrin, fenpropanate and fenvalerate (emulsifiable concentrates) were applied at 280 g AI/ha and incorporated into mineral and organic soil contained in small field plots. Radishes and carrots were grown to serve as indicators of insecticide uptake. Similar plots were treated with the same insecticides at 140 g AI/ha and the soil surface was left undisturbed following application. Soil cores were removed at appropriate intervals and the crops were harvested when mature. The insecticide concentration in all samples was determined by gas-liquid chromatography. The amount of insecticide in the soil declined rapidly to less than 50% of the initial value in 1 month or less for most material-soil-treatment combinations and within 2 months for all cases. Concentrations remained in excess of 0.01 ppm in the organic soil for at least six months for all material-treatment combinations but fell below this level over 2-5 months in the mineral soils. Organic soil incorporated fenvalerate was the most persistent combination overall with 25, 17 and 7% remaining at 6, 18 and 28 months respectively. The trans-isomers of permethrin and cypermethrin disappeared more quickly than the cis-isomers but the insecticidally active IR isomers were not preferentially degraded relative to the inactive 1S. No residues (less than 0.01 ppm) were found in the radish or carrot crops. First order disappearance rates were not constant for any of the combinations. A comparison of partial rate constants showed: 1) 0-1 mo rates were generally greater in mineral than organic soil, 2) 1-6 mo rates in organic soil were lower than 0-1 mo rates, 3) 1-6 mo rates for surface applications to organic soil were generally less than for incorporated applications. In laboratory experiments, 0-1 mo rates for fenvalerate disappearance in a mineral soil were 2-3x greater for 0.5 ppm than for 10 ppm while 1-6 mo rates were independent of insecticide concentration but were 1.5x greater for 0.5% moisture than for 5%.