Assessment of recovery of population, biomass and reproduction of the earthworm Drawida willsi following the application of malathion under field conditions

Abstract

 The recovery of population, biomass and reproduction of a dominant crop field earthworm (Drawida willsi, Michaelsen) was assessed following the application of two recommended agricultural doses of malathion (2.2 mg kg^–1– single dose and 4.4 mg kg^–1– double dose) in a rice field agroecosystem for a period of 105 days. The average worm population and biomass showed 12% and 21% decline, respectively, in plots treated with single and double doses of malathion, compared with control. The peak of worm population was observed after 60 days in control (plot I) and this peak value was shifted by 15 days (i.e. observed after 75 days) in malathion-treated plots (plots II and III) indicating delay in growth and maturation of the worms following the application of malathion. Inhibition in the total number of cocoons produced, and thus in the rate of reproduction, was also observed in malathion-treated plots. The peak rate of reproduction (1.71 cocoons adult^–1) was observed after 60 days in control plots and after 75 days (1.57 and 1.40 cocoons adult^–1) in single and double dose malathion-treated plots, respectively. One-way analysis of variance showed significant differences in the population size and biomass of D. willsi up to 75 days, in rate of reproduction up to 90 days, and no difference thereafter. This indicates that, under field conditions, D. willsi worms took about 75–90 days after application of malathion to resume normal population, biomass and reproduction, and hence we suggest that a second application of malathion in single and double agricultural doses should be avoided before this time.