Effect of four insecticides on the pasture ecosystem

Abstract

Four insecticides (fensulfothion, fenitrothion, carbofuran, and DDT) were applied as granules to the surface of a pasture at 2.24 kg a.i./ha. An apparatus for wet sieving and floating earthworms and arthropods from soil samples is described, as is a procedure for kerosene-separation of arthropods. Earthworms were initially reduced to less than 50% of the control population in carbofuran-treated plots, but recovered, apparently by immigration, between 5 and 18 weeks after treatment. This movement of earthworms shows that large plots are needed to study the medium-term effects of worm-toxic pesticides, though a study of movement of worms into small plots would indicate the persistence of the biological activity of a pesticide. The other insecticides had only a slight or no effect on earthworm populations. The aphid population in fenitrothion-treated plots was less than the control population 5 weeks after treatment, but, 27 weeks after treatment, populations of Chironomidae were higher in the fenitrothion- and carbofuran-treated plots than in the control.