2. Alternatives to Organochlorine Insecticides
Organophosphate Insecticides
• Strongly active against a wide variety of species
• Break down rapidly in the environment
• Exhibit high acute toxicity to verterbrates as well as insects
- Exception: Malathion, LD50 =1200 mg/kg
• Mechanism of action: Cholinesterase inhibitor
Dichlorvos
(a phosphate)
Parathion
(a phosphorothioate)
Malathion
(a phosphorodithioate)

3. Some Considerations:
• Choice of appropriate type of pesticide for a given application:
 Stability, solubility, toxicity
 Natural vs. synthetic
• Use of pheromones or hormones where feasible
• Biological control (via predators, parasites pathogens; examples: Bacillus thuringiensis (BT) or Entomophaga maimaiga for the gypsy moth)
• Using farming methods that do not provide favorable conditions for pests
• Using resistant plant strains; genetic engineering
• Using physical methods to reduce pest population (e.g., traps)
• Keeping new species from entering an area

4. Formed as an impurity in the synthesis of
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)
Formed as an impurity in the synthesis of
2,4-dichlorophenoxyacetic acid (2,4-D) and
2,4,5-trichlorophenoxyacetic acid (2,4,5-T):
2,4-D
2,4,5-T
Which are mimics for the plant growth hormone:
Indoleacetic acid
Effective on broad-leafed plants, but not harmful to grasses.
Also formed in the postcombustion zones of incinerators directly from carbon or chlorinated phenols, via reactions catalyzed by fly ash (metal salts).

5. Variation in TCDD Toxicity Animal LD50 (mg/kg) Guinea pig 0.6-2.5
•Listed as “known to be a human carcinogen” in January, 2001 by the U.S. National Toxicity Program.
•Oil contaminated with some dioxins was applied to a horse arena in Missouri. As little as 32g dioxins per kg soil was sufficient to kill birds, cats, dogs and horses that used the arena. Reactions in humans were far less severe: One child developed a variety of skin lesions.
•Toxicity in mammals occurs through binding of Ah (aryl hydrocarbon receptor; affects translocation.
•Exhibits antiestrogenic activity; may cause endometriosis.
Table 8-4
Variation in TCDD Toxicity
Animal LD50 (mg/kg)
Guinea pig
Mink
Rat
Monkey <70
Rabbit
Mouse
Dog >100-<3000
Hamster
Source: Adapted from D.J. Hanson, Dioxin toxicity: New studies prompt debate, regulatory action, Chem. Eng. News, p. 8, Aug. 12, 1991.

7. Bailey, Clark, Ferris, Krause and Strong, “Chemistry of the Environment,” second edition (San Diego: Academic Press) 2002.

9. Bailey, Clark, Ferris, Krause and Strong, “Chemistry of the Environment,” second edition (San Diego: Academic Press) 2002.