**5. Pesticides residue in the environment**

Chemical pesticides are used to control target pests. Extensive use of pesticides after World War II has substantially increased the agricultural production. However non target organisms including human and wildlife are affected. Pesticides are bioactive molecules that interfere with vital biochemical and physiological processes in organisms. Some are lethal to exposed organisms and many can cause disorder at sub lethal level. Extensive research is necessary to clarify the side effects of pesticides on organisms. About 3 billion kg of pesticides is applied each year with a purchase price rose to \$47 billion in 2008, worldwide (Pimentel, 2009; Frabotta, 2009).

Ecological Impacts of Pesticides in Agricultural Ecosystem 157

Carbamate insecticides are a group of synthetic compounds derived from carbamic acid. The first carbamate carbaryl was an N-methyl carbamate with high insecticidal activity against many insect pests and ectoparasites of animals. Carbamates especially N-mathyl carbamates are extremely toxic to hymenoptera and are lethal to exposed foraging bees.

Oxime carbamates are a group of carbamate with systemic action. Aldicarb, an oxime carbamate is the most potent toxic substance (LD50=0.9 mg/kg) ever used in crop protection. Because of high toxicity it is used as granular formulation. Aldicarb sulfoxide is its oxidative metabolite that may undergo further oxidation to the sulfone. Oxidative residues and its parent compound (Tatal aldicarb) are toxic and highly mobile in the environment. Total aldicarb is detected especially in shallow ground water since 1979. Ground water quality monitoring has shown that many samples contain aldicarb residues and some of them exceeded maximum acceptable concentration (Priddle et al. 1989; Marade & Weaver 1994).

Pyrethroids are synthesized based on the model of naturally occurring pyrethrins with more stability to light and air. Pyrethroids are used in agriculture, homes, restaurants and hospitals. These compounds are readily metabolized by man but they are effective against insects. Most pyrethroids are esters however non-ester pyrethroids are discovered with good insecticidal activity and low mammalian toxicity. These readily penetrate insects and paralyze their nervous system (Reigart et al., 1999). Since pyrethroids are highly toxic to

Sunlight, microbial activity, heat, and moisture accelerate pyrethroids break down, hence in areas with limited sunlight, pyrethroids persist for a long time. After treatment in the home, cypermethrin persist for about three months (Wright et al, 1993). Pyrethroids are lipophilic compound that strongly absorb to colloids of soil. Dissipation of cypermethrin, fenvalerate, and deltamethrin, were investigated in yellow red soils. The half-life of theses compounds were 17, 19, 18 days in unsterilized, compared to 76, 92 and 80 days in sterilized soil (Gu et al, 2008). This experiment shows the effects of biodegradation in pyrethroids life span in

Neonicotinoids are similar to nicotine with the same mode of action. These insecticides have been used worldwide. Most neonicotinoids are absorbed and translocated to the tips of the plants. Imidacloprid is the first widely used insecticide of this group with relatively low mammalian toxicity. However, it is harmful to beneficial arthropods including bees (LD50=0.008 µg /bee). Imidacloprid and clothianidin are more toxic to bees as spray than as seed dressing (Tennekes, 2010). Most neonicotinoids are moderately soluble and so they are mobile in the environment. In ground water 18 feet below sandy loam soil concentrations of imidacloprid ranged from < 0.1 ppb to 1 Ppb (Bacey, J. 2000). This observation shows the potential of imidacloprid to leach downward into shallow groundwater. Imidacloprid has a moderate binding affinity to soil colloids. Half-life in soil varies under different conditions. The half-life of imidacloprid in soil was 48-90 days, depending on the ground cover (Scholz & Spiteller, 1992). Laboratory experiments showed that persistence of another neonicotinoid, thiamethoxam is highly depending on moisture and the half-life varied from

Carbamates biodegradation in environment is relatively rapid.

insects, both the beneficial and pest insects are affected.

**5.1.3 Carbamate Insecticides** 

**5.1.4 Pyrethroids** 

soil.

**5.1.5 Neonicotinoids** 

The environmental persistence is different from pesticide to pesticide. Some are persistent and remain in the environment either as a parent compound or transferred products. The fate of pesticides in soil depends on the value of Koc, carbon sorption coefficient. High values of Koc indicate a pesticide that strongly adsorbs to the soil particles and less likely to move with water. Moreover, soil composition, pH, moisture content and microbial activity affect pesticide persistence.
