Effects Of Carbamate And Pyrethroid Insecticides On Microbial Numbers And On Leucaena Decomposition In Soil

The effects of lannate (a carbamate) and karate (a pyrethroid) on the decomposition of Leucaena green manure were evaluated after establishing the inhibitory effects of these insecticides in vitro, on the heterotrophic micro-organisms (bacteria, fungi and actinomycetes). In this study, the manure, loosely-packed in perforated plastic litter bags, was buried (10 cm) in soil contained in wooden troughs (450 x 35 x 20 cm) and the insecticides in aqueous solution, were applied on the soil surface at the final concentrations of 0.5 and 1.5 ppm for lannate or 0.5 and 1.0 ppm for karate. The decomposition process was monitored at 0,1,3,5, and 7 weeks of incubation using various parameters, namely, residual dry matter, residual organic carbon and residual total nitrogen of the incorporated manure. The total heterotrophic microbial count was also determined in the treated soil at each sampling time. Results of these investigations indicated that when evaluated in vitro, lannate and karate each at 0.5 ppm concentration, depressed the growth of the heterotrophs. However, this inhibitory effect was surprisingly not evident at much higher (10x) insecticide concentrations. It was further observed that the two insecticides, when applied to soil even up to 2-3 times of their respective field recommended rates, did not affect the populations of the decomposer micro-organisms, nor did they influence the rates and magnitude of the Leucaena decomposition, regardless of the parameters used in assessing the decomposition process. It was, therefore, concluded that this type of insecticides could be used even at much higher concentrations, without upsetting the ecological balance or activity of the soil heterotrophic micro flora. TJFNC Vol. 76 2007: pp. 50-5