Organophosphate and Carbamate Insecticides
Organochlorine insecticides have now largely been replaced by those that are less persistent and less subject to bioaccumulation. The most important classes of these newer insecticides are organophosphates and carbamates, both of which are discussed below.
Structurally, all organophosphate (OP) pesticide molecules can be considered as derivatives of phosphoric acid, 0=P(0H)3, and consist of a central, pentavalent phosphorus atom to which are connected
• an oxygen or sulfur atom doubly bonded to the P atom,
• two methoxy (—OCH3) or ethoxy (—OCH2CH3) groups singly bonded to the P atom, and
• a longer, more complicated, characteristic R group singly bonded to phosphorus, usually through an oxygen or sulfur atom, that differentiates one organophosphate insecticide from another.
The organophosphates are toxic to insects because they inhibit enzymes in the nervous system; thus they function as nerve poisons. In particular, organophosphates disrupt the communication that is carried between cells by the acetylcholine molecule. This cell-to-cell transmission cannot operate properly unless the bound acetylcholine molecule is destroyed after it has executed its function. Organophosphates block the action of the enzymes whose job it is to destroy the acetylcholine, by selectively bonding to them. (At the atomic level, it is the phosphorus atom of the organophosphate molecule that attaches to the enzyme and stays bound to it for many hours.) The presence of the insecticide molecule has the effect of suppressing the dissociation of the bound acetylcholine. Consequently, continuous stimulation of the receptor cell and its target muscle occurs. Normally the enzyme occurs in large excess and therefore some exposure to OPs occurs without immediate effects, but symptoms begin to appear if a majority of them
Field corn Cotton Other field crops* Fruits and nuts
Residential and commercial uses
Livestock and pets
Forestry and rangeland
Turf and ornamental plants m^m
Millions of lb applied per year ^Examples are canoJa and alfalfa.
FIGURE 10-6 Consumption of organophosphate pesticides by various crops in the United States. [Source: B. Hileman, "Reexamining Pesticide Risk/' Chemical and Engineering News (17 July 2000): 34;.
are inactivated. Since the affected nerves include those controlling gastrointestinal activities and bronchial secretions, massive gastric and respiratory secretions occur along with involuntary motions. Death ensues if 80-90% or more of the enzyme sites are inactivated.
The largest use of organophosphate insecticides is in agriculture (see Figure 10-6), but they also find many uses domestically. Organophosphates generally are nonpersistent; in this respect, they represent an environmental advance over organochlorines since they do not bioac-cumulate in the food chain and present a chronic exposure and health problem to us later. However, they are generally much more acutely toxic to humans and other mammals than are organochlorines. Many organophosphates represent an acute danger to the health of those who apply them and to others who may come into contact with them. Exposure to these chemicals by inhalation, swallowing, or absorption through the skin can lead to immediate health problems. However, organophosphates metabolize relatively quickly and are excreted in the urine.
After application, most organophosphates decompose within days or weeks and thus are seldom found to bioconcentrate appreciably in food chains. However, because they have a wide range of uses in homes, on lawns, in commercial buildings, and in agriculture, almost everyone is regularly exposed to them. There is some evidence that OPs cause chronic as well as acute health problems. Children in the United States generally have their greatest exposure to organophosphates through the food they eat, with exposure from drinking water only 1-10% of that from food. A study reported in 2003 concerning a group of preschool children in Seattle, Washington, who consumed mainly organic fruits, vegetables, and juices found that they had much lower exposure to organophosphates, as measured by metabolites in their urine, than children with conventional diets. A number of recent studies, none of them large enough to be statistically definitive, have found links between indoor use of insecticides— organophosphates especially—and childhood leukemia and brain cancer. The U.S. EPA has placed OPs in its highest priority group in its current re-examination of pesticides.
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