The Evolution of Chemical Pesticides
Modern pest management and control is an increasingly diverse science with thousands of different management strategies. Synthetic chemical pesticides, which were first deployed during the World War II era, are a relatively new development in an epic battle against pests and parasites. Prior to the development of synthetic pesticides, there was a slow, perpetual battle of simple tools and natural chemicals against the incessant onslaught of pests. More recently, pest control efforts have evolved into management strategies.
Chemical pest control methods encompass a large range of strategies from companion planting to chemical sterilization agents. The most common forms of chemical pest controls are pesticides, which are chemical or biological agents designed to deter, discourage, incapacitate, or kill a pest. Early pesticides included the use of botanicals and simple elements or compounds. Early Romans, for example, discovered that crushed olive pits could produce an oil called Amurea that was capable of killing pests. Subsequent scientific and cultural development led to the discovery and utilization of additional pesticide agents.
The earliest documented chemical pesticide compounds were elements such as sulfur, heavy metals and salt. The use of elemental compounds for pest control started at the dawn of agriculture and has continued, in some cases, through the present day. Elemental sulfur is believed to be one of the earliest chemical pesticides. Solutions of lime sulfur were once used as dips to destroy lice. Sulfur dioxide, which was generated by burning elemental sulfur, was used to inhibit the respiration of insects and other small pests. Applied as a liquid or powder, acidic solutions of sulfur discouraged the growth of molds. Even today, the use of sulfur as a pesticide persists in modern pest management.
The heavy metal compounds were probably first employed as pesticides because of their high toxicity. Arsenic compounds (particularly Arsenic (III) oxides) were found to be highly toxic to insects, bacteria and fungi. Arsenic (III) oxides, which combine rapidly with thiols found in biologically important molecules such as cysteine and coenzyme A, interrupt enzymatic activities that include ATP production. Arsenic compounds are still used today in wood treatment and preservative processes as well as some arsenate pesticides. Mercury compounds (primarily organic mercury compounds) also have a high affinity for thiols, similar to arsenic compounds, and can disrupt biological and enzymatic processes. Meanwhile, lead compounds act as a calcium analog and cause incomplete heme synthesis, which leads to anemia.
The benefit of these inorganic pesticides, at the time, was that they lasted a long time and were not easily degraded. Unfortunately, they often leached into the ecosystem, wreaking havoc on local wildlife and posing a health threat to its human inhabitants.
Pest Control from the Middle Ages to the Victorian Era
In the span from the Middle Ages to the Victorian era, science moved from the realm of religion and magic to practical study. The disciplines of chemistry and biology were embraced, opening up studies into chemical compounds, reactions, and chemical synthesis. Pest control methods definitely benefited from this pursuit of knowledge. Older methods of pest control were still in use (removal, barriers, botanicals, and elemental salts) but the mechanisms behind their efficacy were still being discovered.
The 19th century marked the dawn of manufactured chemical pesticides, when chemicals began to be extracted from their botanical sources and were purified in laboratories. It was at this time that nicotine compounds were purified from tobacco, pyrethrums were extracted from flowers, and rotenone isolated from roots. In addition, cyanides were recognized as toxic compounds in the pits of some fruits.
During this era, chemical compounds were blended and produced for the purpose of pest control. In 1814, an inorganic compound of copper (II) acetoarsenite called “Paris Green” was introduced as a pigment. By 1867, Paris Green was widely sold as an insecticide and rodenticide. In fact, Paris Green paints even continued to be produced up until the 1960s.
Similarly, the Bordeaux Mixture was developed in the late 19th century to fight the Great French Wine Blight. Its mixture of copper (II) sulfate and calcium hydroxide was designed to combat fungal and mildew infections in vineyards.
It was during the Victorian era that traditional methods of pest control were formally investigated and put to the scientific method. As a result, all of the chemical compounds that were historically available in their botanical forms (e.g., rotenone in roots and pyrethrums in chrysanthemums) were purified for commercial and home use, and elemental compounds were blended to create more efficient pesticides. The humble beginnings of simple, natural repellents and physical pest controls grew into chemical and agricultural industries seeking out new and improved methods.
The Advance of Pesticides Through the 20th Century
The primitive tools now had scientific reasoning to explain their efficacy and identify their chemical formulations, moving them from the realm of natural extracts to synthesized pesticides, and signaling the rise of the chemical pesticide revolution. Pest control, which had begun with simple tools and methods, was refined over centuries and completely reborn during World War II. The late 19th and early 20th century world of the first synthetic organic chemicals gave rise to the first modern synthetic pesticides in the form of organochloride compounds.
Many organochloride compounds, such as BHC and DDT, were first synthesized in the 1800s, but their properties as insecticides were not fully discovered and exploited until the late 1930s. BHC (Benzene hexachloride) was first produced by the English scientist Michael Faraday in 1825, but its properties as an insecticide were not identified until 1944. DDT (dichlorodiphenyltrichloroethane) was first prepared by Othmar Ziedler, an Austrian chemist, in 1825, but the Swiss chemist Paul Hermann Müller did not discover DDT’s insecticidal properties until 1939 — a discovery that led to Müller’s award of the Nobel Prize in 1948.
DDT’s use as a pesticide proved to be a huge boon to war efforts. Prior to the discovery of DDT, pyrethrins were among the major insecticides in use. But pyrethrins were extracted from natural sources, primarily from flowers of the genus Chrysanthemum (Pyrethrum), supplies of which were limited and insufficient to meet the demands of wartime use. It was due to this shortage that DDT, instead, became the Allied Forces’ insecticide of choice to control insects that were vectors for typhus, malaria and dengue fever.
At the time, DDT was seen as a broad-spectrum insecticide with low toxicity to mammals. It was inexpensive to produce, easy to apply to large areas, and was persistent, so that reapplication was generally not needed; DDT is insoluble in water and therefore not washed away by weather. The compound also appeared, at first, to be incredibly effective at eliminating the insect vectors of disease, which led it to be hailed as a wonder insecticide.
By 1945, DDT was made available for agricultural applications. But the first signs of insect resistance to DDT began to appear in the 1950s. In 1962 Rachel Carson, a marine biologist and conservationist, published Silent Spring, a book that highlighted the detrimental effects of pesticides on the environment. The widespread popularity of Carson’s book led to the establishment of influential grassroots organizations that called for greater environmental protections and stricter controls on pesticide use. Part of that call to change was the reduction or elimination of DDT and many other pesticides developed from the 1940s through the 1960s from the pest-fighting arsenal.
DDT remained in widespread use around the world until the 1980s, but its decline hastened once the U.S. Environmental Protection Agency (EPA) canceled most uses of DDT by 1972. Many other countries followed suit shortly thereafter by removing DDT from lists of approved agricultural applications. In 2004, the Stockholm Convention outlawed many persistent organic pollutants (POPs) and restricted the use of DDT to vector control (primarily for malaria). Despite increasing worldwide restrictions and bans on DDT, as of 2008, India and North Korea were still using DDT in agricultural applications. Today, India is the only country in the world still producing DDT.
Since the start of the production boom in the 1940s to present day, a huge catalog of thousands of insecticides, herbicides, and general pesticides was developed, including organochlorides (DDT, BHC), organophosphates (Parathion, Malathion, Azinophos Methyl), phenoxyacetic acids (2,4-D, MCPA, 2,4,5-T), Captan, Carbamates (Aldicarb, Carbofuran, Oxamyl, Methomyl), neonicotinoids (Imidacloprid, Acetamiprid, Clothianidin, Nitenpyram), and Glysophates.
The neonicotinoids are neuro-active insecticides, similar to nicotine compounds that were developed in the 1980s and 1990s. Of all the neonicotinoids, Imidacloprid has become one of the most abundantly used insecticides in the world. Patented in 1988 and registered with the EPA in 1994 by Bayer Crop Science, Imidacloprid works by disrupting the transmission of nerve impulses in insects by binding to an insect’s nicotinic acetylcholine receptors, resulting in paralysis and death. Imidacloprid is highly toxic to insects and other arthropods, including marine invertebrates. It is considered to be moderately toxic to mammals if ingested at high dosages.
The acute toxicity and environmental fate of Imidacloprid and other neonicotinoid pesticides have been greatly debated since their adoption in the 1990s. Many studies have examined the persistence of neonicotinoids in water supplies and their ecological impacts on other environmentally and economically important arthropods. Studies published within the last two decades have linked bee colony collapse disorders with Imidacloprid and other similar pesticides. The most toxic pesticide in the world today for honey bees (genus Apis) is also the most commonly used insecticide in the world: Imidacloprid.
If Imidacloprid is the most widely used insecticide in the world, Glyphosate is the most widely used herbicide on Earth. Glysophate was developed by a Monsanto chemist, John E. Franz, in 1970. Roundup, as it was trademarked, quickly became one of the most popular herbicides in the world among both agricultural enterprises and home users. The mode of action for Glyphosate is to inhibit a plant enzyme that is integral to the synthesis of aromatic amino acids. The inhibition of the amino acid production affects primarily the growing regions of the plants, killing plants in their growth cycle but not in their seed stage.
In 1994, the Roundup Ready Soybean was commercially approved in the United States. This genetically engineered soybean was created to be resistant to glyphosate. These types of crops allowed for the use of glyphosate to control other pest plants without endangering the crop. The list of glyphosate-resistant crops has grown since the introduction of the Roundup Ready Soybean to include corn, canola, alfalfa, cotton, and wheat.
GMOs: The Future of Pest Control?
Genetically modified organisms (GMOs) have become widely used in the United States since their initial introduction in the 1990s. Genetically modified crops are grown by millions of farmers in dozens of countries. The predominant modified crops are soybeans, corn or maize, cotton, and canola.
As of 2010, 93 percent of soybeans, 78 percent of cotton, and 70 percent of corn were herbicide-resistant GMOs. The United States is one of the leading proponents of research into GMOs and surpasses most other countries in their proliferation, growing 59% of the world’s GMO crops. The success of GMOs around the world has been mixed. Many European nations have experienced protests over GMOs and their safety. Most of the controversy surrounds the actual process of altering the genetic structure of plants and whether or not those modified plants should require labeling. Generally speaking, scientific opinion weighs in favor of the safety of GMO crops.
GMO labeling is required in many countries but not in the United States. Scientists and economists argue that the potential benefits of GMO crops include reductions in the use of pesticides and other hazardous pollutants and an increase in the nutritional value and production of agricultural products. Opponents of GMOs claim that all the latent risks have not been adequately identified, especially the potential long-term impact of GMOs on human health and the environment.
The issue is not a question of whether genetically modified organisms should be used as a pest strategy. The truth is that genetic modification is already well established as the newest tool in the arsenal of pest management. It will be up to future generations to decide if the history of this nascent pest-management strategy will go the way of heavy metal pesticides and DDT, or if GMOs will become the ultimate solution for pest control.
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