INSECTICIDES - THEIR EFFECT ON MICROORGANISMS AND PERSISTENCE IN RICESOIL

A field experiment was conducted to investigate the effect of four ins ecticides, HCH, phorate, carbofuran and fenvalerate, at recommended do ses on the preponderance of bacteria, actinomycetes and fungi. We also measured the persistence of the insecticides in the rhizosphere soil of rice. HCH and fenvalerate stimulated the proliferation of all of th e microorganisms significantly. Phorate increased the population of ba cteria and actinomycetes. Carbofuran accentuated the preponderance of actinomycetes in soil. Insecticides, in general, did not have marked i nfluence on the proliferation of Bacillus, Streptomyces, Aspergillus a nd Fusarium in soil. However, we observed a stimulation of growth of S taphylococcus, Proteus and Sarcina with HCH, Pseudomonas, Corynebacter ium, Erysipelothrix and Rhizopus with phorate, Serratia, Corynebacteri um, Klebsiella, Escherichia, Rhizopus and Humicola with carbofuran, an d Staphylococcus, Sarcina, Klebsiella and Nocardia with fenvalerate. O n the other hand, there was an inhibition in growth of Pseudomonas, Mi crococcus, Nocardia and Penicillium with HCH, of Pseudomonas, Micrococ cus and Penicillium with carbofuran, and of Pseudomonas, Micrococcus a nd Micromonospora with fenvalerate. Different types of insecticides ex hibited differential patterns of dissipation in soil. HCH had the high est persistence followed by phorate, carbofuran and fenvalerate, respe ctively.