Effects of organochlorine, organophosphorus and carbamate insecticides on the population structure and biomass of earthworms in a semi-arid tropical grassland

Abstract

Effects of normal (1 ml of endosulfan 35% E.C., 0.66 ml of methyl parathion 50% E.C. and 4.16 g ofcarbaryl (Sevin 50% W.D.P.), each dissolved in 1 litre of water and applied to 5 × 5 m plot) and high doses (3 times the normal dose) of endosulfan, methyl parathion and carbaryl on the population structure and biomass (wet) of earthworms of a semi-arid tropical grassland were investigated. The earthworm population and biomass (wet wt) were highest in the untreated control plots. No earthworm was recorded in plots treated with the high dose of endosulfan, until 80 days after treatment, while the earthworm adundance was reduced significantly in the plots treated with the normal dose. The adults, juveniles and total population reduced by 52–58% after 40 days of treatment with the high dose of methyl parathion and by 15–52% with the normal dose. However, the numbers of adults increased after 60 and 80 days of treatment at the high dose and the juvenile population reduced by 28% after 60 days, and by > 70% after 80 days of treatment ( P < 0.05). The number of adults increased after 60 days, and reduced after 80 days of treatment with the normal dose of methyl parathion. The adults, juveniles and total population was reduced by 15–50% after 40 days of treatment with high dose ofcarbaryl. However, the number of adults increased after 40 days of treatment with the normal dose, and after 60 and 80 days of treatment with the high dose. The biomass of the earthworms was significantly reduced with treatment of either dose by each of the insecticides.