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Organophosphate Insecticides
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An ad for Malathion from the 1950s. Notice the implied emphasis on safety and the lack of integrated pest management. Credit: The Internet Antique Shop
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As part of our series on insecticide groups, we will next be looking at organophosphates. This group, while as old as organochlorines, did not see widespread use until later in the 20th century and is much more widely used to this day. As with many other early insecticides, they also have a checkered past and can be very toxic to humans.
The group of chemicals known as organophosphates was first synthesized almost two centuries ago in 1837, although it would take several decades until researchers discovered their ability as an insecticide. Even then, products such as tetraethylpyrophosphate broke down too quickly in the environment to be of much use. In the 1930s researchers in Germany noted several compounds synthesized in this group and the negative neurological effects they seemed to have on biological life. This in turn led to the development of chemical weapons such as Sarin gas used in WWII which were later used by allied scientists for further investigation.
In 1950 the first recognizable organophosphates were developed in the forms of Malathion and Parathion, with several other closely related products following closely on their heels. During this time, organochlorines such as DDT remained much more popular until they began to be banned in the 1970s. Following this, the use of organophosphates exploded in the United States and abroad until 2002 when they began to be phased out for residential use.
Although organophosphates provide strong pest control, they are also extremely acutely toxic to humans and other life, and as of 2010 were responsible for more deaths in the US than any other pesticide group. At that time they also accounted for nearly double the amount of poisonings as the second most dangerous group in use, carbamates.
Organophosphates work by binding to and inhibiting acetylcholinesterase, which is the enzyme responsible for breaking down acetylcholine. This chemical is responsible for transmitting nerve signals between cells. With the enzyme unable to break down AcH, the nerve enters a permanent state of overstimulation which leads to a rapid death for the insect. Organophosphates also tend to break down quickly in the environment which made them more attractive when they were first formulated.
In humans, exposure to organophosphates can lead to muscle tremors, confusion, sweating, and excessive salivation. In high exposure scenarios, it can also lead to death. While they are now greatly phased out in residential settings, organophosphates remain in use in industrial settings in the US with a significant number of injuries each year. Developing countries continue to bear the brunt of the cases as usage in these places remains high since the product is cheap to produce.
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A normal nerve cell firing. Acetylcholine (green) is released and received by a neighboring nerve. This triggers a nerve impulse on the new nerve and continues the signal down the line.
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A normal nerve cell resetting. Acetylcholinesterase (blue) is released to break down the messenger acetylcholine (green). This opens up receptor sites on the nerve and allows it to reset and stop firing.
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An inhibited nerve being impacted by an organophosphate product. The organophosphate (yellow) binds to the enzyme acetylcholinesterase (blue) thus rendering it unable to break down the acetylcholine (green). Since the acetylcholine remains in the receptor sites, the nerve is unable to reset and continues firing.
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Compiled by David Olson, Forest Health Specialist
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