As long as humans have existed they have been plagued by pests. Today when we observe animal species rolling in mud or primates grooming each other, we can imagine our ancient ancestors performing similar activities to combat pests. Agriculture as a means of obtaining food started approximately 10,000 years ago, and this increased the number of species that could be classified as pests. Several methods can be used to control pests. These include physical, genetic, biological, and chemical methods.
The word "pesticide" is a collective term in that it includes numerous other "cides" such as herbicides, insecticides, rodenticides, fungicides, bactericides, and avicides.
HISTORY
The use of chemical pesticides can be traced back several thousand years. The Sumerians, occupying present-day Iraq, burned sulfur compounds to produce fumigants. The Greeks also employed this practice as early as 1000 B.C. Evidence exists that the Chinese extracted pesticides from plants as early as 1200 B.C.
The first pesticides used widly in recent history were poisonous metals. In France in the early 18th century a mixture of copper sulfate (CuSO4) and lime (CaCO3) was sprayed on grapes to make them unappealing to thieves. It was noted that this mixture also protected the grapes from certain fungi, and thus a pesticide known as Bordeaux mixtures came into existance. Paris green, a compound of copper and arsenic, was a common pesticide used throughout the 18th century. Lead arsenate (Pb3(AsO4)2, and several metal compounds containing antimony and mercury started to replace Paris green at the beginning of the 20th century.
Metal, inorganic pesticides were common throughout the first half of the 20th century. Sodium fluoride and boric acid, B(OH)3, were commonly used as ant and cockroach poisons; hydrogen cyanide (HCN) was a fumigant. Nicotine solutions made from soaking tobacco leaves in water have been used for several hundred years as insecticides and rodenticides.
INORGANIC PESTICIDES The common inorganic compounds used as pesticides have several problems. Although they kill the target pest, they tend to be very lethal to humans and other mammals. Pesticides that kill indiscriminately are classified as broad spectrum pesticides. This means they do not distinguish between target and helpful species. Another major problem with the inorganic pesticides is that they are persistent in the enviroment, lasting years and even decades. A final major detriment to the widespread use of inorganic metal compounds is that they are expensive.
To try to alleviate some of these problems and produce large quantities of pesticides that were economically feasible, chemists turned to sybnthetic organic pesticides in the 1930s. In 1939, while working for the Geigy chemical company, the Swiss chemist Paul Müller (1899-1965) discovered that the compound dichlorodiphenyltrichloroethane (DDT) was an effective insecticide. (figure 1)
Figure 1
DDT had first been synthesized in 1873, but it was Müller who discovered its efficacy as an insecticide. DDDt was initially marketed in 1941, and found it first widespread use in World War II. During World War I several million deaths, including 150,000 soliers, were attributed to typhus. There are several forms of typhus, but the most common form is due to bacteria carried by lice. During World War II, fearing a repeat of typhus outbreaks. the Allied forces used DDT to combat not only typhus but also malaria, yellow fiver, and other diseases carried by insects. Soldiers liberaaly applied talcum powder with 10% DDT to clothes and bedding to kill lice. American and European allies were relatively free from typhus and other diseases, while the Germans who did not use DDT had many more noncombat deaths due to infectious diseases. In liquid form, DDT was used in the Pacific Theater to prevent malaria and yellow fever.
In addition to its use in the war, DDT was employed in tropical areas as a generic insecticide to prevent infectious diseases, especially malaria. Once the war ended, its use to advance public health in tropical undeveloped countries was expanded for use in agriculture in developed countries. Paul Müller was awarded the Nobel Prize in medicine in 1948 for his discovery of the insecticide potential of DDT. By 1950 DDT and several related compounds were viewed as miracle insecticides that were inexpensive and that could be used indiscriminately.
Even though DDT seemed to be a cheap and effective pesticide, enough was known in its early development to raise concerns. DDT is a persistent chemical that lasts a long time in the environment. DDT is fat soluble and not readily metabolized by higher organisms. This meant that DDT accumulated in the fat tissues of higher organisms. As organisms with longer life spans residing higher on the food chain continually fed on organisms lower on the food chain, DDT would accumulate in their tissues. For example, the concentration of DDT in a lake might be measured in parts per trillion, plankton in the lake may contain DDT in parts per billion, fish a few parts per million, and birds feeding on the fish from the lake several hundred parts per million. The acccumulation of a chemical moving up the food chain is a process known as biological magnification.
Another concern raised was that certain pests seemed to develop an immunity to DDT and the appplication rate had to be increased to combat insects. This was because natural selection favored insects that had the genetic characteristics to survive DDT and passed this ability on to their offspring. Direct deaths of birds and fish had been also observed in areas with heavy DDT use. The problems associated with DDT and other post-WWII organic pesticides became a national concern with Rachael Carson's (1906-1964) publication of Silent Spring in 1962. Carson's book alerted the public to the hazards of insecticides, and while not calling for ban, challenged the chemical and agricultural industry to curtail its widespread use of chemical pesticides.
Most developed countries started to ban the used of DDT and related compounds in the late 1960s. DDT was banned in the United States in 1973. Although banned in developed countries, its use to improved public health in undeveloped countries continues. The World Health Organization estimates that DDT has saved 25 million lives from malaria and hundreds of million of other lives from other diseases.
DDT belongs to a group of chemical insecticides known as organochlorides. These contain hydrogen, carbon, and chlorine and kill by interfering with nerve transmission; hence, they are neurotoxins. Organochlorines were the dominant type of chemical insecticides used from 1940 to 1970. Some common organochlorines besides DDT are chlordane, heptachlor, aldrin, and dieldrin. Because of their problems, and subsequent ban in many regions, numerous other classes of insecticides have been synthesized to replace organochlorines. Several major groups include organophosphates, carbamates, and pyrethroids.
Organophosphates were originally discovered in Germany during World War II while researchers were searching for a substitute for nicotine. Nicotine extracts obtained by soaking tobacco in water were commonly used as an insecticide as early as 1700. Organophosphates are derived from phosphoric acid, H3PO4, and like organochlorides, act on the nervous system (Figure 2
Figure 2
Organophosphates such as Malathion are derived from phosphoric acid.
They work by interfering with specific enzymes in the nervous system. Organophosphates are more lethal to humans and other mammals than organochlorides. Because of this, chemical weapons collectively known as nerve gas are organophosphates. The chief advantage of organophosphates is that they are unstable and break down in days to weeks in the environment. Although this reduces the toxicity to humans on food due to pesticide residues, organophosphates pose a hazard to agricultural workers and nontarget species. Because of this, several countries and different states in the United States ban specific organophosphates. Some common organophosphates are parthion, malathion, and diazinon.
Carbamates act similarly to organophosphates by interfering with nerve system enzymes. They are derived from carbamic acid, H2NCOOH, and share many of the properties of organophosphates. The carbamate known as carbaryl (commercial trade name is Sevin) was synthesized in 1956 and has been used extensively since that time. Carbaryl's advantages are that its toxicity to mammals is slow and it kills a broad number of insects. It is widely used in a number og household lawn and garden products. Other common carbamates include aldicarb (Temik) and carbofuran (Furadan).(Figure 3
Figure 3
Carbamates such as carbaryl are derived from carbamic acid.
Pyrethroids are based on mimicking the structure of the natural insecticide pyrethrin. Pyrethins are found in the flowers of chrysanthemums. Ground flowers were traditionally used to kill lice in the early 1800s. Synthetic pyrethins were first produced in the early 1970s. The exact nature of how pyrethroids work is unknown, but because they paralize insects it is speculated that they affect the nervous or muscular system. Pyrethroids are effective in low dosages and are nonpersistent.
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 first herbicides were inorganic metal compounds and salts. During World War II organic herbicides were synthesized and their used 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 4).
Figure 4
The two most common phenoxyaliphatic acids
The numbers in these compounds refer to the carbon to which the chlorine is attached. Dichlorophenoxyacetic acid works by producing excess growth hormones in plants. Plants die due to their inability to acquire sufficient nutrients. The herbicide 2,4,5-T acts as a defoliant that causes plants to shed leaves. During the Vietnam War, 2,4,-D and 2,4,5-T were combined into a formulation called Agent Orange. Agent Orange was applied extensively to forests in Vietnam to expose enemy positions. Subsequent birth defects in children of both Vietnam and American soldiers exposed to Agent Orange became a controversial topic in postwar years. Research has shown that in the manufacture of 2,4,5-T side reactions occur that produce small quantities of dioxins. Dioxins are compounds that are characterized by the dioxin structure:
Dioxins have been shown to cause birth defects in tests with laboratory animals. Long-term effects include certain cancers. Because of the health concerns associated with 2,4,5-T, this herbicide was banned in the United States.
Another class of herbicides is called triazines. Triazine compounds consist of a benzene-like structure with alternating nitrogen and carbon atoms in a hexagonal ring.
Amino groups are attached to two of the carbons in the ring and chlorine is attached to the third. The most common triazine is called atrazine. Atrazine disrupts photosynthesis in plants. Atrazine is widely used on corn plants because corn has the ability to deactivate atrazine by removing the chlorine atom.
Another common herbicide is paraquat. Paraquat belongs to a group of herbicides called bipyridyliums. Paraquat works rapidly by breaking down the cells responsibe for photosynthesis. It is used as a preemergent herbicide, which means it is applied to soil before plants emerge. Paraquat has been widely used to destroy marijuana plants. Its use to combat marijuana crops has led to speculation that some marijuana users may be susceptible to lung damage from marijuana contaminated with paraquat.
Since their development, synthetic pesticides have increased thirty-fold. Worldwide use of pesticides is approximately 3 million tons, with half of this in the United States and Europe. The use of synthetic chemical pesticides continue to be a controversial topic. There are strong arguments both for and against the use of pesticides. Positive aspects of pesticide use include their ability to save lives and reduce human suffering, increase food supplies, and lower food costs because they are relatively cheap compared to other forms of pest control. On the negative side, pesticides pose health risks to humans—both short term and long term—kill nontarget species, have uncertain ecological impacts, and produce hardier pests over time. Since the creation of the EPA in 1970, the federal government has heavily regulated the use of synthetic chemical pesticides. Many pesticides have been banned since then. Certain pesticides are available only to commercial users who have the credentials to apply them. New pesticides must undergo a rigorous screening that includes various laboratory tests that seek to ascertain the potential human and environmental risks of the pesticide. Currently, several hundreds different active ingredients are approved by the EPA for pesticide use. Still, even with extensive federal regulations many people feel oversight is lax and poorly enforced. Most of the chemicals approved by the EPA have never undergone extensive testing.
Scientists continue to explore the production of safer pesticides and other methods to augment chemical pesticides. Integrated Pest Management is an approach for controlling pests that utilize multiple techniques in an ecological systematic fashion. In this multifacetad approach, techniques such as cultivation methods, genetic engineering, natural biological controls, and insect sterilization are combined with chemical techniques to control pests. As IPM methods develop during the 21st century, the role of synthetic chemical pesticides in modern society will also change.