Potential Impact of Cypermethrin on Selected Groups of Soil Microorganisms

Abstract

Application of cypermethrin as insecticide during agricultural practices is a global concern due to its toxicity and potential accumulation of its metabolites in the aquatic environment. A mesocosm plot of agricultural land (500 m2) was demarcated and deliberately contaminated with cypermethrin to study its impact on the autochthonous microorganisms. Potential effects of 3- phenoxybenzoic acid (3-PBA), a known suicide metabolite of cypermethrin, on hydrolytic activities were also investigated by using fluorescein diacetate assay (FDA). A significant increase in the abundance of total heterotrophic bacteria from 3.55 x 106cfu g-1to 1.30 x 108cfu g-1in 12 weeks as against the control was induced by the insecticide in the mesocosm. The number of starch hydrolysing bacteria increased after the second insecticide treatment from 6.25 x 106cfu g-1to 2.70 x 107 cfu g-1in three weeks. However, there was no significant increase in the microbial count of fungi, presumptive actinomycetes and nitrogen fixing bacteria during the twelve weeks of experiment. The hydrolytic activity of soil microorganisms was apparently inhibited insoil treated with different concentration of 3-PBAthan in untreated soil. These results suggest that the application of a commercial cypermethrin formulation increased the abundance of selected soil microorganisms while3- PBA reduced the soil microbial hydrolytic activity.