Certain insects carry disease, while others prey upon crop plants. The use of chemical insecticides helps keep these harmful insects under control. Chemists are producing new insecticides, always looking for one that will be safe, specific, and even more effective. It is also possible to create plants with built-in insect resistance, using genetic modification.
However, the widespread use of insecticides has been associated with many problems. They kill useful insects, as well as harmful ones, and also cause damage to other wildlife. Insects often evolve a resistance to insecticides and become even harder to kill, so farmers are on a kind of treadmill, trying out one chemical after another on their crops. Some insecticides are difficult to break down in the environment and may travel long distances, accumulating in the food chain. The long-term impact of many insecticide residues on human health remains unknown.
Historical Background and Scientific Foundations
Pests are organisms that damage resources, the most important of which are food crops. Insects are the most frequent crop pests. Of the million, or more, species of insect, around 10,000 are crop eating and, of these, around 700 species cause the majority of damage to crops, whether growing in the field or in storage after harvesting. Meanwhile, some insects act as vectors for the transmission of disease-causing microbes. Perhaps the most notorious is the Anopheles mosquito that carries the malaria parasite. The tsetse fly carries sleeping sickness, which is another major tropical disease.
The earliest insecticide dates back to around 1000 BC, when burning sulfur was used to drive insects away. Tobacco, vinegar, arsenic, brine, turpentine, and hydrogen cyanide gas have also been tried. The modern insecticide era opened with the discovery of DDT, dichloro-diethyl-trichloroethane, by the Swiss chemist Paul Müller (1899–1965) in 1939. DDT was widely used during World War II (1939–1945) to protect troops from insect-borne disease. DDT is cheap, easy to apply, and very effective. It kills a wide variety of insects. By the 1950s, it was widely used to combat insects, being sprayed freely on houses and crops and even dusted on people and animals. Müsller’s discovery was considered so important that he was awarded the Nobel Prize for physiology or medicine for his work in 1948.
Insecticides are a major group of pesticides, which are inorganic or organic chemicals, either natural or synthetic, that kill pests. There are several major classes of insecticide. Inorganic pesticides include compounds of sulfur, mercury, arsenic, or lead that are often used to dust seeds to ward off insect attack. These are toxic to those handling them, as well as to the pest. Natural organic insecticides are compounds that come from plants, such as nicotine from tobacco, pyrethrum from Chrysanthemum cinerariaefolium, and rotenone from the root of the derris plant. Some are toxic to organisms other than humans. For instance, rotenone kills fish. Fumigants are small, volatile molecules like carbon tetrachloride and methylene bromide, which can be used to sterilize soil and stored grain. Many fumigants are now heavily restricted or even banned because of the danger they pose to those handling them.
Then there are the organochlorines. Besides DDT, well-known insecticides in this group include aldrin, dieldrin, toxaphene, lindane, and paradichlorobenzene, the latter being used in mothballs. They block nerve signal transmission in insects and act very fast. The organochlorines are extremely toxic to some other organisms, such as fish. They also persist for a long time in the environment. Because of these drawbacks, many
WORDS TO KNOW
DDT: One of the earliest insecticides, dichloro-diphenyl-trichloroethane, used until banned by many countries in the 1960s after bird populations were decimated by the substance, and other negative environmental consequences occurred. Lately, selective DDT use has returned in targeted areas in Africa in order to eliminate high concentrations of the mosquitoes that carry the parasite that causes malaria.
FOOD CHAIN: A sequence of organisms, each of which uses the next lower member of the sequence as a food source.
INTEGRATED PEST MANAGEMENT: an ecological pest control strategy involving minimal application of pesticides.
of the organochlorines are now banned in much of the world. The organophosphate insecticides emerged from military research into nerve gases and include parathion, malathion, and dichlorvos. They block the enzyme cholinesterase, which is needed for the proper operation of the synapses, the junctions between neurons. Many of these compounds are also toxic to fish, birds, and mammals, but are less persistent in the environment than the organochlorines. The carbamates are broadly similar to the organophosphates in their mode of action, toxicity, and environmental effects and include carbaryl and aldicarb. Finally, the halogenated pyrroles are a new class of insecticide that are based on a naturally occurring microbial toxin.
The bacterium Bacillus thuringiensis (bt) produces a toxin that ruptures the gut of certain insects. Therefore, biological control can be achieved if crops are inoculated with the bacterium. A more advanced way of employing the toxin is to insert the bt gene into the plant itself via genetic modification (GM). Bt versions of corn, cotton, and other crops are now widely planted in some parts of the world, although the GM technology remains controversial among the general public.
Pests probably reduce crop yields by about a third and maybe another third is lost in storage, although it is hard to come by accurate data. Therefore, modern insecticides increase the supply and quality of our food. Meanwhile, insecticidal control of malaria is estimated to have prevented at least 50 million deaths in the last 50 years. In Sri Lanka, there were two million deaths a year from the disease in the 1950s. Introduction of DDT spraying reduced the rate to almost zero. The spraying stopped in 1964 and within three years, the malaria incidence was back to more than one million cases a year. Accordingly, the Sri Lankan government went back to limited use of DDT.
Impacts and Issues
The first sign that insecticides had drawbacks came when various carnivorous birds, like the bald eagle, began to disappear from North America in the 1960s. DDT was being concentrated in the food chain, and causing thinning of the shells of eggs of these birds of prey. Biologist Rachel Carson published Silent Spring in 1962 as a wakeup call to the dangers of indiscriminate use of insecticides. Many of them have unintended adverse effects on non-target species such as birds and fish. They also kill off useful insects like bees, which pollinate plants, and wasps, which kill small insect pests.
The stability, high solubility, and high toxicity of DDT and other organochlorines create a persistent environmental problem. They have been found at high levels in the fat of top predator animals like whales, trout, eagles, and polar bears. Canadian researchers found unexpectedly high levels of chlorinated hydrocarbons in the breast milk of Inuit mothers living in remote Arctic villages. The long-term human health effects of insecticide exposure is not known, although they have been tentatively linked to birth defects and neurological problems.
Insects also tend to evolve resistance to insecticides. Among a population of insects being sprayed, a few will always survive because they are naturally resistant and will go on to breed more resistant insects. Therefore, farmers need to turn to more toxic insecticides when their preferred ones stop working. Insecticides undoubtedly play an important role in agriculture and health, but care must be taken that these benefits really outweigh the risks. A start has been made in banning some of the more harmful insecticides. Those that remain can be used in a more appropriate way. In integrated pest management, for instance, chemical insecticides are applied sparingly along with other methods of pest control, such as enhancing the plant’s defenses and letting their natural enemies, such as other insects, do their work.
Primary Source Connection
The potential environmental damage posed by insecticides, as well as the need for economic regulation of their sale, was recognized by the U.S. federal government in the first decade of the twentieth century. The initial pesticide control legislation in 1910 was designed primarily to protect people from faulty products and misleading labeling. A more comprehensive safety-oriented legislation called the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) was passed in 1947.
The 1947 version of the insecticide act and subsequent amendments have helped curb the misuse and unauthorized use of insecticides. This has benefited both those who could be exposed to the chemicals and, more broadly, the environment.
An example of the strength of the act is the 1964 amendment, which allows unsafe or ineffective pesticides (and more specifically insecticides) to be denied the registration that is required for their sale and use.
Since 1970, the administration of the use of insecticides has been under the auspices of the U.S. Environmental Protection Agency (EPA). Now, insecticides are assessed more from an environmental viewpoint than an economic viewpoint.
Major amendments to FIFRA in 1972 and in 1996 mandated the EPA to regulate the manufacture and sale of agents, including insecticides, and enforce compliance with agents that are banned from use. This includes inspecting facilities that use insecticides to ensure that the chemicals being used are approved for use and that the application of the insecticides is being done in accordance with the regulations.
FEDERAL INSECTICIDE, FUNGICIDE, AND RODENTICIDE ACT
CHAPTER 6—INSECTICIDES AND ENVIRONMENTAL PESTICIDE CONTROL
SUBCHAPTER II—ENVIRONMENTAL PESTICIDE CONTROL
Sec. 136. Definitions
For purposes of this subchapter—
(a) Active ingredient
The term “active ingredient” means—
- in the case of a pesticide other than a plant regulator, defoliant, desiccant, or nitrogen stabilizer, an ingredient which will prevent, destroy, repel, or mitigate any pest;
- in the case of a plant regulator, an ingredient which, through physiological action, will accelerate or retard the rate of growth or rate of maturation or otherwise alter the behavior of ornamental or crop plants or the product thereof;
- in the case of a defoliant, an ingredient which will cause the leaves or foliage to drop from a plant;
- in the case of a desiccant, an ingredient which will artificially accelerate the drying of plant tissue; and
- in the case of a nitrogen stabilizer, an ingredient which will prevent or hinder the process of nitrification, denitrification, ammonia volatilization, or urease production through action affecting soil bacteria.
The term “Administrator” means the Administrator of the Environmental Protection Agency.
The term “adulterated” applies to any pesticide if—
- its strength or purity falls below the professed standard of quality as expressed on its labeling under which it is sold;
- any substance has been substituted wholly or in part for the pesticide; or
- any valuable constituent of the pesticide has been wholly or in part abstracted.
The term “animal” means all vertebrate and invertebrate species, including but not limited to man and other mammals, birds, fish, and shellfish.
(e) Certified applicator, etc.
- Certified applicator. The term “certified applicator” means any individual who is certified under section 136i of this title as authorized to use or supervise the use of any pesticide which is classified for restricted use. Any applicator who holds or applies registered pesticides, or uses dilutions of registered pesticides consistent with subsection (ee) of this section, only to provide a service of controlling pests without delivering any unapplied pesticide to any person so served is not deemed to be a seller or distributor of pesticides under this subchapter.
- Private applicator. The term “private applicator” means a certified applicator who uses or supervises the use of any pesticide which is classified for restricted use for purposes of producing any agricultural commodity on property owned or rented by the applicator or the applicator’s employer or (if applied without compensation other than trading of personal services between producers of agricultural commodities) on the property of another person.
- Commercial applicator. The term “commercial applicator” means an applicator (whether or not the applicator is a private applicator with respect to some uses) who uses or supervises the use of any pesticide which is classified for restricted use for any purpose or on any property other than as provided by paragraph (2).
- Under the direct supervision of a certified applicator. Unless otherwise prescribed by its labeling, a pesticide shall be considered to be applied under the direct supervision of a certified applicator if it is applied by a competent person acting under the instructions and control of a certified applicator who is available if and when needed, even though such certified applicator is not physically present at the time and place the pesticide is applied.
The term “defoliant” means any substance of mixture of substances intended for causing the leaves or foliage to drop from a plant, with or without causing abscission.
The term “desiccant” means any substance or mixture of substances intended for artificially accelerating the drying of plant tissue.
The term “device” means any instrument or contrivance (other than a firearm) which is intended for trapping, destroying, repelling, or mitigating any pest or any other form of plant or animal life (other than man and other than bacteria, virus, or other microorganism on or in living man or other living animals); but not including equipment used for the application of pesticides when sold separately therefrom.
(i) District court
The term “environment” includes water, air, land, and all plants and man and other animals living therein, and the interrelationships which exist among these.
The term “fungus” means any non-chlorophyll-bearing thallophyte (that is, any non-chlorophyll-bearing plant of a lower order than mosses and liverworts), as for example, rust, smut, mildew, mold, yeast, and bacteria, except those on or in living man or other animals and those on or in processed food, beverages, or pharmaceuticals.
(l) Imminent hazard
The term “imminent hazard” means a situation which exists when the continued use of a pesticide during the time required for cancellation proceeding would be likely to result in unreasonable adverse effects on the environment or will involve unreasonable hazard to the survival of a species declared endangered or threatened by the Secretary pursuant to the Endangered Species Act of 1973 [16 U.S.C. 1531 et seq.].
(m) Inert ingredient
The term “inert ingredient” means an ingredient which is not active.
(n) Ingredient statement
The term “ingredient statement” means a statement which contains—
- the name and percentage of each active ingredient, and the total percentage of all inert ingredients, in the pesticide; and
- if the pesticide contains arsenic in any form, a statement of the percentages of total and water soluble arsenic, calculated aselementary arsenic.
The term “insect” means any of the numerous small invertebrate animals generally having the body more or less obviously segmented, for the most part belonging to the class insecta, comprising six-legged, usually winged forms, as for example, beetles, bugs, bees, flies, and to other allied classes of anthropods whose members are wingless and usually have more than six legs, as for example, spiders, mites, ticks, centipedes, and wood lice.
U.S. CODE. “FEDERAL INSECTICIDE, FUNGICIDE, AND RODENTICIDE ACT (FIFRA).” TITLE 7, CHAPTER 6, SUBCHAPTER II. JUNE 25, 1947.
Cunningham, W. P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.
American Society of Plant Biologists. “Gatehouse J, Biotechnological Prospects for Engineering Insect Resistant Plants.” 2008. http://www.plantphysiol.org/cgi/content/full/146/3/881 (accessed March 26, 2008).
University of Minnesota. “Ware GW, Whiteacre GM, An Introduction to Insecticides, 4th ed. (extracted from The Pesticide Book).” 2004 http://ipmworld.umn.edu/chapters/ware.htm (accessed March 27, 2008).