Introduction

Two Nostoc species were used to study the toxic effects of two commonly used insecticides: sumi-alpha [(s)-α-cyano-3-phenoxybenzyl (s)-2(4-chlorophenyl)-3methylbutyrate] and Dursban [O,O-diethyl-O-(3,5,6-trichloro-2-Pyridyl) phosphorothioate) ] to soil microflora. The heterocystic filamentous Nostoc minutum Desm. ex. Born. et Flah. & Nostoc puniforme Ag. ex. Born. et Flah. were exposed to different concentrations of sumi-alpha and dursban. Differential growth effects were observed on the two cyanobacterial species exposed to different insecticide concentrations. Selected parameters of the tested cyanobacteria (dry weight, Chl a, photosynthetic O2 evolution, total soluble sugars, nitrogenase activity, total soluble proteins, total lipids and fatty acid contents of the total lipids) were measured and examined. The data revealed a marked inhibitory effects in all measured parameters in both Nostoc species under different insecticides concentrations of sumi-alpha and dursban except the total lipids. This suggests a potential hazard of these insecticides on soil microflora and subsequently natural ecosystem. Key words, Cyanobacteria , Insecticide Toxicity, Sumi-alpha, Dursban. Introduction Much more attention has been paid to the effects of various insecticides on microorganisms. The contamination of aquatic environments with both sumialpha and dursban insecticides is a long-term problem because of the chemical stability and potent toxicity of these pollutants. Of the many organophosphorous (dursban) and pyrethrins (sumi-alpha) are among the most ubiquitous and are the most prominent contaminants in the aquatic biota. Contamination of aquatic habitats by sumi-alpha and dursban is an unavoidable characteristic of aerial spraying of forests. These insecticides enters the aquatic system primarily via direct application by spray air craft or indirectly from spray drift and surface runoff (Abd-El Hafez, et. al., 1996). In Egypt the surface-water concentrations of studied insecticides ranged from 1L to 600mL per Fadan for both dursban and sumi-alpha, respectively. Several detailed reviews (Kunert & Boger, 1981, Nikolenko, and Amirkhanov, 1993, and Kent & Caux, 1995) addressed sumi-alpha and dursban degradation and persistence in aquatic environments. These provide the necessary context for their exposure, duration profile and subsequent potential toxic hazard to phytobiota.