Introduction

Although responsible for saving and improving the quality of human life, pesticides have exerted a significant detrimental effect on the environment and have caused serious health problems, resulting in severe criticism of their use (Hayes, 1986). There is often a fundamental conflict between the need for a sustained level of biological activity of a pesticide in the environment and the requirement that the chemical should be degraded to non-toxic and ecologically safe products (Hill, 1978; Casida & Quistad, 1998). The era of modern synthetic pesticides largely dates from 1939 when the insecticidal properties of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) were discovered (Tessier, 1982). Unlike naturally occurring organic compounds, which are readily degraded upon introduction into the environment, some pesticides such as DDT are extremely resistant to biode-gradation by native microflora (Rochkind-Dubinsky, Sayler & Blackburn, 1987a). In most cases, the persistence can be explained by the chemical structure and by the degree of water solubility. In addition, some of these pesticides tend to accumulate in organisms at different trophic levels of the food chain. Chlorinated organic pesticides are one of the major groups of toxic chemicals responsible for environmental contamination and an important potential risk to human health (Kullman & Matsumura, 1996).

The most common pesticides are herbicides, insecticides and fungicides, where herbicides account for nearly 50% of all the pesticides used in developed countries and insecticides account for 75% of all pesticides used in developing countries.