**5.1.1 Chlorinated insecticides**

The first synthetic insecticide was DDT with a wide spectrum of insecticidal action that was used in agriculture and against insect vectors of deadly diseases. DDT solubility in water is very low, about 0.006 mg/l, which makes it one of the most hydrophobic insecticides. DDT residues either as parent compound or its metabolites DDD and DDE are stable and have high persistence in the environment. It has a great tendency to be stored in fatty tissue of different organisms. After the introduction of DDT, HCH was marketed. HCH has eight isomeric forms of which γ-isomer is called Lindane. Lindane is a volatile insecticide and was used against agricultural and households pests. Lindane is less persistent than the other organochlorine insecticides especially under moist conditions. The cyclodiens are stable organochlorine soil applied insecticides. These included aldrin, dieldrin, endrin, chlordane, heptachlor and endosulfan. Cyclodiens are environmentally persistent compounds that have raised concern about adverse effects on human health and wildlife. Residues of DDT and its metabolites DDD and DDE, dieldrin and heptachlor epoxide were detected in high percentage of soil and water samples from agricultural areas decades after their use were banned. Extensive studies on organochlorine pesticides has shown the environmental persistence of these Compounds (LeaMond et al. 1992; Reiser & O'Brien, 1999).

#### **5.1.2 Organophosphorus Insecticides**

Organophosphorus insecticides replaced persistent organochlorine compounds. Utilization of these insecticides increased rapidly and for several decades comprised high proportion of total insecticide use. Organophosphates are unstable compounds, however some of these insecticides are more acutely toxic to invertebrate than chlorinated insecticides. Parathion was the first marketed product that was effective against a wide variety of pests. Some organophosphates caused severe toxicity associated with many deaths especially in developing country, whereas a few compounds such as malathion are relatively safe to mammals and degrade fairly rapidly in the environment. Most organophosphates are harmful to beneficial arthropods, though few compounds such as phosalone and dimethoate are considered as harmless compounds.

The occurrence and movement of some organophosphate pesticides are reported in rivers and streams. Several studies conducted to find out the presence of organophosphate residues in California rivers during 1993-1994. Diazinon, methidathion, dimethoate and chlorpyrifos residues were detected in water samples. The detection occurred mostly during rainy season, showing how run off influences the presence of pesticide residues in rivers and streams (Ganapathy et al. 1997).

#### **5.1.3 Carbamate Insecticides**

156 Pesticides in the Modern World - Risks and Benefits

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

The most toxic and environmentally persistent compounds are found among insecticides;

The first synthetic insecticide was DDT with a wide spectrum of insecticidal action that was used in agriculture and against insect vectors of deadly diseases. DDT solubility in water is very low, about 0.006 mg/l, which makes it one of the most hydrophobic insecticides. DDT residues either as parent compound or its metabolites DDD and DDE are stable and have high persistence in the environment. It has a great tendency to be stored in fatty tissue of different organisms. After the introduction of DDT, HCH was marketed. HCH has eight isomeric forms of which γ-isomer is called Lindane. Lindane is a volatile insecticide and was used against agricultural and households pests. Lindane is less persistent than the other organochlorine insecticides especially under moist conditions. The cyclodiens are stable organochlorine soil applied insecticides. These included aldrin, dieldrin, endrin, chlordane, heptachlor and endosulfan. Cyclodiens are environmentally persistent compounds that have raised concern about adverse effects on human health and wildlife. Residues of DDT and its metabolites DDD and DDE, dieldrin and heptachlor epoxide were detected in high percentage of soil and water samples from agricultural areas decades after their use were banned. Extensive studies on organochlorine pesticides has shown the environmental

therefore, the emphasis is on groups of insecticides that have been studied in detail.

persistence of these Compounds (LeaMond et al. 1992; Reiser & O'Brien, 1999).

Organophosphorus insecticides replaced persistent organochlorine compounds. Utilization of these insecticides increased rapidly and for several decades comprised high proportion of total insecticide use. Organophosphates are unstable compounds, however some of these insecticides are more acutely toxic to invertebrate than chlorinated insecticides. Parathion was the first marketed product that was effective against a wide variety of pests. Some organophosphates caused severe toxicity associated with many deaths especially in developing country, whereas a few compounds such as malathion are relatively safe to mammals and degrade fairly rapidly in the environment. Most organophosphates are harmful to beneficial arthropods, though few compounds such as phosalone and

The occurrence and movement of some organophosphate pesticides are reported in rivers and streams. Several studies conducted to find out the presence of organophosphate residues in California rivers during 1993-1994. Diazinon, methidathion, dimethoate and chlorpyrifos residues were detected in water samples. The detection occurred mostly during rainy season, showing how run off influences the presence of pesticide residues in rivers

affect pesticide persistence.

**5.1.1 Chlorinated insecticides** 

**5.1.2 Organophosphorus Insecticides** 

dimethoate are considered as harmless compounds.

and streams (Ganapathy et al. 1997).

**5.1 Insecticides** 

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. Carbamates biodegradation in environment is relatively rapid.

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).
