Potential Impact of Cypermethrin on Selected Groups of Soil Microorganisms

Abstract

Applicationof cypermethrin as insecticide during agricultural practices is a globalconcern due to its toxicity and potential accumulation of its metabolites inthe aquatic environment. A mesocosm plot of agricultural land (500 m2)was demarcated and deliberately contaminated with cypermethrin to study itsimpact on the autochthonous microorganisms. Potential effects of3-phenoxybenzoic acid (3-PBA), a known suicide metabolite of cypermethrin, onhydrolytic activities were also investigated by using fluorescein diacetate assay(FDA). A significant increase in the abundance of total heterotrophic bacteriafrom 3.55 x 106cfu g-1to 1.30 x 108cfu g-1in12 weeks as against the control was induced by the insecticide in the mesocosm.The number of starch hydrolysing bacteria increased after the secondinsecticide treatment from 6.25 x 106cfu g-1to 2.70 x 107cfu g-1in three weeks. However, there was no significantincrease in the microbial count of fungi, presumptive actinomycetes andnitrogen fixing bacteria during the twelve weeks of experiment. The hydrolyticactivity of soil microorganisms was apparently inhibited insoil treated with differentconcentration of 3-PBAthan in untreated soil. These results suggest that theapplication of a commercial cypermethrin formulation increased the abundance ofselected soil microorganisms while3-PBA reduced the soil microbial hydrolyticactivity