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, LD 50 =1200 mg/kg Mechanism of action: Cholinesterase inhibitor Parathion (a phosphorothioate) Malathion (a phosphorodithioate) Dichlorvos (a phosphate)

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

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 Also formed in the postcombustion zones of incinerators directly from carbon or chlorinated phenols, via reactions catalyzed by fly ash (metal salts). Indoleacetic acid Which are mimics for the plant growth hormone: Effective on broad-leafed plants, but not harmful to grasses.

Table 8-4 Variation in TCDD Toxicity AnimalLD 50 (mg/kg) Guinea pig Mink 4 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, Toxicity? 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.

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