LABORATORY STUDIES ON THE PERSISTENCE AND BEHAVIOUR IN SOIL OF FOUR PYRETHROID INSECTICIDES

In direct contact toxicity tests with four pyrethroid and three standard insecticides, using 24–48 h old crickets, Acheta pennsylvanicus (Burmeister), as test insects, carbofuran was most toxic > cypermethrin > fensulfothion > chlorpyrifos > fenvalerate > fenpropanate > permethrin. In moist (5% water) Plainfield sand, using the same test insect, chlorpyrifos was most toxic > carbofuran > fenpropanate > cypermethrin > permethrin > fenvalerate > fensulfothion, i.e. the pyrethroids, although quite strongly adsorbed by soil, were still moderately active as soil insecticides. All pyrethroids were less toxic in muck soil as compared with Plainfield sand, and in air-dry (0.5% water) as compared with moist (5% water) Plainfield sand, with the effects being less on cypermethrin and fenvalerate and greater on permethrin and fenpropanate. None of the pyrethroids was affected by soil type and moisture to the same extent as was carbofuran. Permethrin, fenvalerate, and cypermethrin showed a negative temperature coefficient of toxicity in moist Plainfield sand, being from 1.4 to 1.9× more toxic at 15° than at 32°C. Fenpropanate, like carbofuran, was slightly more toxic at 32° than at 15°C. Generally, soil type, moisture, and temperature had minimal effects on the toxicity of the four pyrethroid insecticides. In persistence studies over 48 weeks, using Plainfield sand, the pyrethroids were more persistent than chlorpyrifos, but less persistent than dieldrin. Under these laboratory conditions, permethrin and fenvalerate were slightly more persistent, while fenpropanate and cypermethrin were as persistent as carbofuran. Trans-isomers of permethrin and cypermethrin declined more quickly in the sand than did the cis-isomers and of the trans-isomers the 1S,trans declined much faster than the 1R,trans while little difference was observed for the corresponding 1S- and 1R,cis-isomers.