Studies on Relative Toxicities of Six Insecticides on Epigeic Earthworm, Perionyx excavatus

Abstract

The 96 h LC50 values of six insecticides were determined on a non-target epigeic earthworm Perionyx xcavatus under laboratory conditions. Cypermethrin was found most toxic to P. excavatus (LC50-0.008 mg/kg), followed by endosulfan (LC50-0.03 mg/kg), carbaryl (LC50- 6.07 mg/kg), chlorpyrifos (LC50-7.3 mg/kg), aldicarb (LC50-10.63 mg/kg) and monocrotophos (LC50-13.04 mg/kg). When these LC50 values were compared with their respective recommended agricultural doses, aldicarb and carbaryl appeared more dangerous than other pesticides because of their lower LC50 values than their respective recommended agricultural dose. Mean lethal time to cause 50% mortality at recommended agricultural dose (LT50) also that aldicarb achieved the fastest LT50 (26 h) followed by endosulfan (38 h) and carbaryl (44 h) indicating the danger of these pesticides to P. excavatus. Earthworms are considered as ecosystem engineers (Lavelle et al. 1998) as they produce pronounced effects on soil structure by their burrowing activities, ingestion of soil and production of castings (Lavelle and Spain 2002).Pesticides, that are applied in agriculture fields, may affect the non-target soil organisms including earthworms and significantly damage the ecosystem (Reinecke and Reinecke 2007). Since earthworms are preferred as food by amphibians (Lescure 1966), reptiles (Catling and Freedman 1980), birds (Harlin 1977) and mammals (Churchfield 1979) there is a possible risk of these pesticides reaching higher trophic levels (Marino et al. 1992). Several researchers have advocated the use of earthworms as ecotoxicological model for risk assessment and bioassay of pesticides (Edwards 1990; Edwards and Bohlen 1992). In the present study experiments were made to evaluate the effects of six insecticides on an earthworm Perionyx excavatus. This species is widely distributed in India and is considered as an ecologically soil organism for its role in increasing soil fertility and for it’s potential of being used in the production of vermicompost (Ismail 1997). The insecticides tested in the present study include two carbamates (carbaryl and aldicarb), two organophosphates (monocrotophos and chlorpyrifos), one organochlorine (endosulfan) and one pyrethroids (cypermethrin). The purpose of these experiments was to evaluate the comparative toxicity of these insecticides on P. excavatus and their potential to damage the soil ecosystem.