**5.1.4 Pyrethroids**

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 insects, both the beneficial and pest insects are affected.

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 soil.

#### **5.1.5 Neonicotinoids**

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

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less adverse effects in the environment.

**7. References** 

45 to 300 days (Gupta et al. 2008). The half-life of neonicotinoids increases with increasing soil colloids. Overall, neonicotinoids have a low potential to persist in soil and accumulate in the environment.
