A comparison of the effects of cypermethrin, parathion-methyl and DDT on cereal aphids, predatory beetles, earthworms and litter decomposition in spring wheat

Abstract

Abstract Cypermethrin (Ripcord 100 g a.i./ha), parathion-methyl (1000 g a.i./ha) and DDT (1000 g a.i./ha) were each applied to small replicated barriered plots of spring wheat. Residues of cypermethrin in soil declined to half their initial value within 6 weeks compared with 3 weeks for parathion-methyl and 22 weeks for DDT. No effects attributable to any of the three treatments were observed on earthworm populations or leaf litter decomposition. DDT was the only compound found to accumulate in the body tissues of earthworms. After application few cereal aphids were found in parathion-methyl- and cypermethrin-treated plots, whereas heavy infestations quickly built up in both the control and DDT-treated plots. One week after application the number of predatory beetles (mainly four species of Carabidae and three genera of Staphylinidae) in parathion-methyl-treated plots fell to about 10% of that found in the untreated control. A decrease in predatory beetles to about 50% of that in the control was observed in both cypermethrin and DDT-treated plots. A substantial recovery in the numbers of predatory beetles occurred in all treated plots between 4 and 6 weeks after application. A further fall in numbers of predatory beetles was observed 8–12 weeks after application in plots treated with cypermethrin and parathion-methyl. This reduction was attributed to an indirect effect of the treatments, operating via a removal of the predators' food supply (mainly cereal aphids).