Pesticides, classes insecticides

Pesticides are chemicals or biological substances used to kill or control pests. They fall into three major classes insecticides, fungicides, and herbicides (or weed killers). There are also rodenticides (for control of vertebrate pests), nematicides (to kill eelworms, etc.), molluscicides (to kill slugs and snails), and acaricides (to kill mites). These chemicals are typically manmade synthetic organic compounds, but there are exceptions which occur naturally that are plant derivatives or naturally occurring inorganic minerals. 

Biotransformation of certain chlorinated hydrocarbon insecticides results in their conversion to metabolites which are less polar than the parent chemical. Heptachlor and aldrin are converted to the more lipophilic compounds heptachlor epoxide and dieldrin, respectively, whereas DDT is converted to DDE. The primary residue of DDT, which persists to the present day in animals and humans after exposure over a decade ago, is DDE. Following biotransformation, these compounds distribute to tissues which are higher in neutral lipid content than are the major organs of metabolism and excretion, the liver and kidney. These lipid-rich tissues are relatively, deficient in the so-called mixed-function oxidase (MFO) enzyme systems necessary for biotransformation of the halogenated hydrocarbons to more polar and thus more easily excreted compounds. As a result, these lipophilic chemicals remain unchanged in adipose tissue with only limited amounts returning to the circulation for possible metabolism and excretion. Paradoxically, aldrin and heptachlor metabolism results in an increased rather than reduced body load. This is opposite of the pattern seen for most other pesticide classes. 

To undertake an assessment of the different approaches an experimental ecotoxicity data set was generated by randomly selecting fifty transformation products from the daphnid data set described previously. An assessment of the predictive performance of five techniques was undertaken by comparing predictions from each approach to the experimentally determined data. The data set included transformation product ecotoxicity data from a range of pesticide classes, i.e. insecticides, herbicides and fungicides and chemical classes, e.g. organophosphorus insecticides, sulfonylureas and azoles. 

Pesticides are further subdivided into classes of compounds. Historically, insecticides included the organochlorine, methyl carbamate, and organophosphate classes of pesticides. Herbicides comprise about 10—12 principal classes of compounds. Within each class of pesticide there may be several hundred active ingredients. 

The relative immobility of the chlorodioxins is expected, based on the very low solubility of these compounds in water (0.6 / g/liter). In contrast, the herbicide, 2,4,5-T, is relatively mobile in sandy soils, but movement decreases as soil organic matter increases. What does this information tell us, and how does it compare with other organic compounds A mobility scale has been devised for a large number of pesticides (3). Higher mobility numbers reflect increased compound mobility in soils. The dioxins would be in Class 1—i.e., they are immobile in soils and would compare with several chlorinated hydrocarbon insecticides. 

Disulfoton is a systemic insecticide/acaricide that belongs to the organophosphate class of pesticides. Pure disulfoton is a colorless oil with low volatility and water solubility, but is readily soluble in most organic solvents (Worthing 1987). Information regarding the physical and chemical properties of disulfoton is located in Table 3-2. 

Pesticides are classified according to the pests they control. Table 1 lists the various pesticides and other classes of chemical compounds not commonly considered pesticides but included among the pesticides as defined by U.S. federal and state laws [1]. The four most widely used types of pesticides are (a) insecticides, (b) herbicides, (c) fungicides, and (d) rodenticides [6]. 

Our discussion of pesticides has focused primarily on insecticides. In the United States the primary use of pesticides is in the form of herbicides, those pesticides used to control weeds. Approximately 70% of the pesticides used in the United States are herbicides and 20% are insecticides. The use and development of herbicides parallels that of insecticides. The first herbicides were inorganic metal compounds and salts. During World War II organic herbicides were synthesized and their use increased dramatically. One of the first major classes of herbicides synthesized in the mid-1940s was phenoxyaliphatic acids. As this name implies, the phenoxyaliphatic acids contain the benzene ring, oxygen, and an aliphatic acid. The two most common phenoxyaliphatic acids are 2,4 dichlorophenoxyacetic acid, called 2,4-D and 2,4,5 trichlorophenoxyacetic acid, known as 2,4,5-T (Figure 18.10). The numbers in these... 

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