Organochlorine and Organophosphate Insecticides
The insecticides that have caused the greatest water pollution problems, and that still do in developing countries, are the organochlorine insecticides. The crazy thing is that, if it is banned in the U.S., companies just sell it cheap to third world countries! There were many of these, best exemplified by DDT and chlordane. DDT was sprayed widely in the environment and helpful in killing mosquitoes that carried malaria. Highly persistent chlordane was the most effective insecticide against termites and was buried around buildings to prevent termite infestation.[6]
Organochlorine insecticides were dominant from the 1940s until the 1960s. They were not particularly toxic to humans and other animals. However, DDT and related compounds have even more detrimental characteristics because of their tendency to undergo bioaccumulation in fish and other organisms, concentrating in fat tissue. As these organisms are eaten by other organisms in higher trophic levels, the organochlorine compounds become concentrated in fat tissue, a process called biomagnification. This made the organochlorine insecticide go out of favor. This is best illustrated in the unit by the egg-shell-thinning lab experience students are engaging in at the latter portion of each class period that is related to thinning of egg shells of bald eagles caused by DDT. Today there are still remnants of these chemicals. I am not sure if you have heard of nurdles or not; they are small resin pellets that have not been made into plastic products yet. They are said to be 10 percent of the plastic trash in the ocean. These nurdles are attracting DDT and dioxins so that the nurdles actually have a concentration a million times higher than the surrounding water. This is really a bad thing since they are the size of fish eggs and they are ingested by organisms in the ocean.[15]
Organophosphate insecticides, organic derivatives of phosphoric acid, H 3PO 4, came into common use when organochlorine compounds were found to bioaccumulate. These had a big advantage in being biodegradable with no tendency to undergo bioaccumulation. Since then, organophosphates have been used extensively due to their effectiveness on a large variety of pests and because pests do not seem to develop a resistance to the chemicals in this class. Organophosphates are extremely toxic, particularly in the concentrated forms that they are sold in commercially.
These chemicals are more persistent than other classes that are currently used in the landscape. Organophosphates do eventually break down chemically under the influence of environmental factors such as sunlight, air, rainfall, and soil moisture, or biologically through plants, animals and microorganisms like bacteria and fungi.
Organophosphates can be absorbed directly through the skin or through the lining of the stomach or respiratory tract following ingestion or inhalation. They affect the nervous system by attaching to the enzyme acetylcholinesterase. When functioning normally, nerves transmit messages through the production of a chemical called acetylcholine (ACh). After a message is sent, the enzyme acetylcholinesterase breaks down the ACh to end stimulation of the nerve and return it to its normal state. Organophosphates inhibit the enzyme, causing the accumulation of ACh and over stimulating the nerves. This causes the insect to loose control of their nervous system, resulting in paralysis and respiratory failure.
The human nervous system also relies on acetylcholinesterase, and, therefore, is also susceptible to organophosphates. Just as in insects, if humans inhale, ingest, or absorb enough organophosphate pesticides through the skin, they are at risk of experiencing adverse health effects. An acute poisoning causes symptoms such as stomach cramps, sweating, muscle contraction, twitching, or just overall weakness. Chronic poisoning, from long term exposure, could result in general feelings of illness, loss of appetite, anaemia, or liver, kidney or nerve damage.
Two examples that are commonly used are methyl parathion and Malathion. Parathion, once the most widely used organophosphate pesticide, is very effective since it affects the nervous system by attaching to the enzyme acetylcholinesterase like the military poison "nerve gases" and a significant number of fatal poisonings occurred due to parathion exposure. Parathion is now banned. Malathion remains on the market and is only about 1/100 as toxic to mammals. This is due to the differences in structure of the two molecules; Malathion can be cleaved with addition of water by enzymes possessed by humans and other mammals, but not insects.[6]
Green chemistry has advanced this class of chemicals with the development of spinetoram from Dow AgroSciences. This is made from naturally occurring fermentation products spinosyns J and L that are modified with a low impact synthesis in which catalysts and most reagents and solvents are recycled. Spinetoram is having a big benefit for human health since it replaces the existing organophosphate insecticides azinphos-methyl and phosmet which are used on pome fruits, stone fruits, and tree nuts. The mammalian acute toxicity of spinetoram is more than 1,000 times lower than azinphos-methyl and 44 times lower than phosmet. There is also a lower impact to the environment since its toxicity to non-target species is low and its persistence in the environment is much less. Spinetoram is effective at use rates 10-34 times lower than the organophosphate pesticides it replaces. It is projected by Dow AgroSciences that in the first 5 years of its use it will eliminate about 1.8 million pounds of organic pesticides used in the fruit industry.[16]
Commonly used organophosphate insecticides to use for review of the labels and MSDS sheets, to fill in the product profile worksheet at the end of the unit and to discuss with the students include Acephate Pro 75 (Acephate), Malathion (Malathion), and Supracide 2E (Methidathion). Students should contrast and compare the different insecticides and determine the use limitations of each of the chemicals. Their Excel sheet should be filled out for each of these chemicals with their half-life in the environment, water solubility, partition coefficient (K O C), and calculated GUS.
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