, Research Paper
Introduction
What is the goal of agriculture? Mainly, it is to produce healthy food, affordable for consumers to purchase, while ensuring that farmers are able to earn a decent income. Recently, with greater environmental awareness in society in general, it is also now very important to protect the agricultural environment. This is a big challenge for farmers as they must still obtain reasonable yields and produce quality produce in order to meet the demands of the market. Both these can be severely affected by harmful organisms, commonly referred to as pests (weeds, disease, etc.) that compete against, infect or damage the cultivated crop in a detrimental manner. The most economic and effective way to handle these has been to employ pesticides, many of which are now composed of synthetic chemicals, and it is these substances – very beneficial from the economic and production aspects of farming – which can pose risks to human health and the environment if not properly used.
Pest problems are not new; in fact, they have been around as long as agriculture itself. But the pest pressure faced by farmers is now as great as it ever was: the world’s fast-growing human population needs to be fed from an always shrinking base of agricultural land, and the substantial damage that can be inflicted by pests (e.g. insects, diseases, weeds, rodents, birds) on crops is the margin between a good harvest and a bad one. Pests can reduce the quality of a harvest as well as its quantity. Since the quality of food is increasingly important to consumers, a pest could reduce the value of a crop or make it unsaleable. But it is important to keep in mind that pests are not the only cause of yield
reduction and of lower quality produce: factors such as soil fertility and availability of water may have a greater influence in a particular situation.
Hence, crop protection has always been an important component of agriculture, leading to the development and employment of measures that can limit damage, such as synthetic chemicals. Easily stored for long periods in a compact form, easily applied at very short notice (provided the machinery is available and the weather conditions are suitable), they are fast-acting and efficient. They can also be toxic, and the farmer must use pesticides wisely to make sure that they will not harm the applicator, the farm family and the surrounding environment.
What Is Pesticide?
Pesticide
biological, physical, or chemical agent used to kill plants or animals considered harmful to human beings; see FUNGICIDE; HERBICIDE; INSECTICIDE.
? Fungicide
substance used to prevent or destroy a fungus. Made from sulfur or copper compounds, organic salts of iron, zinc, and mercury, or other chemicals, fungicides are used on seeds, soil, wood (to prevent dry rot), and fabrics (to prevent mildew). Human fungal infections are treated with fungicides, called antifungals or antimycotics in medicine, such as nystatin and clotrimazole.
? Herbicide
substance that kills plants or inhibits their growth. Nonselective herbicides, generally toxic, are used to clear all plants from a broad area; selective herbicides attack weeds without permanently harming crops. Scientists are using genetic engineering to develop crop varieties with increased tolerance for herbicides. Inorganic compounds such as common salts have long been used as herbicides; c.1900 certain sulfates, ammonium and potassium salts, and other compounds began to be used as selective herbicides. The 1940s saw the development of 2,4-D (2,4-trichlorophenoxyacetic acid), an organic compound that is a highly selective systemic herbicide. Such herbicides are now widely used. Several such compounds, including 2,4,5-T (2,4,5-trichlorophenoxyacetic acid), have been banned by the Environmental Protection Agency as potentially dangerous. 2,4,5-T was used in Agent Orange (a defoliant employed by U.S. forces in Vietnam), which has been linked to some diseases suffered by veterans. Since Agent Orange, heightened awareness of possible ecological and health dangers attributable to herbicides has resulted in reevaluation of many compounds and has called indiscriminate use into question.
? Insecticide
agent used to kill insect pests. Insecticides have helped increase the yield and improve the quality of crops, but there has been concern about the dangers of chemical insecticide residues in the ecosystem and in foodstuffs. Such concerns have led to governmental regulation and the replacement of some toxic insecticides that persist in
the environment (e.g., DDT and other chlorinated hydrocarbons) with compounds that break down more quickly into less toxic forms. The liabilities of chemical insecticides have encouraged interest in biological controls, which turn natural processes and mechanisms against pest insects and have few if any harmful side effects. Biological controls include using predators, parasites, and pathogens to kill target insects or using synthetic hormones to disrupt pests’ normal life processes. Increasingly, biological and chemical methods are coordinated in Integrated Pest Management programs.
Pesticides are unique among the hazardous chemicals. They are specifically designed and produced to kill or disrupt biological organisms. Ideally, pesticides should be highly selective, effecting only targeted organisms, but unfortunately most are not. Herbicides, for example paraquat, not only kill green plants, but may also be acutely toxic to human beings. Others cause cancer, birth defects, genetic damage and changes in human and animal endocrine systems.
Effects on Agriculture
1. Concerns About Pesticides
? Pest resistance
Pests, after repeated exposure to a pesticide, can start to build a resistance against the effects of the materials, especially chemicals.
In every population of or plant weeds, there is a very small population which is immune to the effects of the pesticide. As the affected population is eliminated by the pesticide treatment, the immune portion of the population slowly, year after year, becomes the dominant segment of the pest population. As a result, the farmer must apply more and more pesticide to achieve the same effect, risking greater damage to health and the environment. Eventually, the pesticide becomes ineffective and can lead to the collapse of some agricultural systems with highly resistant pests and no natural enemies left to control them.
Left: Irrigation ditch, a potential
highway for pesticides.
? Contamination and toxicity
Pesticides can be toxic to the surrounding environment – the plants, fish, animals, certain useful insects such as bees as well as to the natural enemies of the pests. The consequences to these latter species can be particularly dramatic since the devastation of a natural control agent population by pesticide use may result in a resurgence of pest populations.
The danger of toxicity to humans who handle or are closely exposed to pesticides is also important, and a great deal of care must be taken when using pesticides; this also applies to household applications of chemical weed and insect pesticides. Because of the interaction between air, soil and water, a pesticide applied to one medium (i.e. to the soil
in a field) can contaminate elsewhere as the material is transported to other locations once mixed with another medium (i.e. rain water that runs off into a river.) Pesticides spread further afield than where they were applied, and the consequences of unanticipated pesticide contamination can be as harmful as they are unexpected.
2. Case Study : Fate of Insecticides Used for Termite Control in Soil
Termites cause substantial damage to residential and commercial buildings in the United States. It has been estimated that the annual cost for controlling termites and repairing their damage in the United States exceeds $1.7 billion. Subterranean termites, the most destructive of all termites, account for 95 percent of this damage.
Because subterranean termites are soil-inhabiting social insects living in complex colonies, the conventional control method is to establish an insecticide barrier between the termite colony (usually in soil) and wood in a building. Currently, Pest Control Operators (PCOs) who specialize in termite treatments have access to more than 11 insecticides (termiticides) for subsoil application. However, due to differences in soil characteristics, and physical and chemical properties of termiticides, PCOs and the
general public often question how the soil and termiticide will interact. How long will the insecticide be effective in the soil? Is the degradation rate the same in different soils? Will the termiticide leach through the soil and contaminate nearby water sources?
It is important to examine some of the soil factors and chemical properties of termiticides that affect the behavior of these compounds to better understand these issues.
Major factors influencing efficacy and persistence of termiticides are:
Soil Characteristics :
? Soil Texture
Clay and organic matter contents are important characteristics influencing termiticide sorption mechanism. Clay and organic matter in soil can vary from less than 1 percent in sand to well over 50 percent in heavy clay soils. The vertical and horizontal distribution of termiticides is dependent upon the interaction with soil particles through processes called adsorption and desorption.
Adsorption is the binding of a termiticide to the surface of soil particles, especially clay and organic matter. Desorption is the release of adsorbed chemicals from a soil particle surface. Depending on many factors in a soil profile, such as moisture, pH, temperature, etc., compounds may adsorb and desorb from soil particle surfaces as they migrate down through the soil. It is also important to consider the clay, sand and silt content of soils because insecticides generally do not migrate as readily in soils with high clay and organic matter contents. The mineral content of soil is also an important factor in
determining the persistence of termiticides by either catalyzing decomposition or affecting the adsorption rate. Because groundwater contamination is an extremely important issue to PCOs and the general public, understanding how compounds bind to soil particles is an important part of evaluating whether a termiticide will leach. However, there are no conclusive research data to determine how adsorption/desorption affects termiticide efficacy and application rate in different soils. It is generally assumed that since termites come in contact with soil particles, it may not be necessary to adjust termiticide dilution rates for most soils. Additional research is needed to accurately determine if variable rates are needed.
? Soil pH
The soil pH is known to have a major impact on performance of termiticides because it affects how rapidly a compound degrades. The pH is used to describe whether soil is acidic (pH less than 7) or alkaline (pH above 7). Most soils have pH values between 4 and 8. In general, termiticides used today persist longer in acidic soil than in alkaline soil.
? Soil temperature and moisture
For the most part, termiticides will remain more efficacious and persistent in soils with low temperatures and low moisture content. Warm soil temperatures and moist conditions can enhance the activity of insecticide-degrading microorganisms, thereby increasing degradation of compounds.
? Soil Microorganisms
Microbial degradation is another process in which soil microbes utilize insecticides as substrate (food source) for growth and maintenance. However, little information is available on how microbial degradation of registered termiticides occurs in various soils.
Chemical Characteristics :
The second major element affecting termiticide performance involves the chemical characteristics of each insecticide.
? Solubility
Solubility of termiticides in water is an important factor affecting their distribution and mobility in soil, but it is not necessarily the best indicator of performance. For example, soluble compounds may have strong affinity to adsorb to soil particles, subsequently limiting their mobility through soil. Ultimately, a combination of factors determines the termiticide mobility in soil.
? Degradation
Termiticide efficacy and persistence are primarily affected by the degradation rate of that compound. Once the termiticides are applied to soils, their fate relies on degradation processes. As the termiticide degrades, it is transformed into other compounds that may be more or less toxic than the parent insecticide. Photodegradation: The breakdown of chemicals due to exposure to sunlight is not a major factor because termiticides are usually applied below the soil surface. Chemical degradation: The most important
process affecting termiticides is chemical degradation which involves hydrolysis, oxidation and reduction. This process directly affects the half life or residual of insecticides in soil.
Hydrolysis is a chemical process in which an insecticide reacts with water, resulting in splitting of the water molecules to form less toxic compounds. There is generally enough moisture in soil to initiate this reaction.
Oxidation is a chemical reaction through which an oxygen atom is added to the parent molecule of an insecticide. Initially, it may not appear that an oxidation reaction results in degradation of insecticides, but a more oxidized form of a molecule may be necessary for further microbial or chemical degradation.
Reduction is the third aspect of chemical degradation. An insecticide molecule is considered to be reduced if its hydrogen content increases or its oxygen content decreases. However, similar to oxidation, reduction may be a preliminary step toward further degradation by other processes. Reduction reactions generally occur under conditions where oxygen is limited (anaerobic environment). Reduction reactions may increase if a soil becomes flooded. As a result, termiticides applied to water-saturated soils may degrade rapidly, rendering the treatment unsuccessful. Areas with excessive water problems should always be corrected prior to termiticide application.
? Volatilization
This process involves transforming chemicals from solid or liquid into a gas or vapor. Several factors influence the tendency of termiticides to volatilize and leave soil as a vapor. The structure of the chemical is important because this determines its vapor
pressure as well as its solubility in soil water and its tendency to be adsorbed. Cool and dry conditions in soils with high organic matter or clay content normally result in very little loss of even the most volatile chemicals from the soil. Conversely, warm and moist conditions contribute to great desorption and greater volatilization losses.
Many processes influence efficacy, persistence and movement of termiticide in soil. Knowledge of these processes can ultimately lead to better understanding of behavior and performance of termiticide products in various soils.
3. 2,4-D RESEARCH DATA
2,4-D, a member of the phenoxy family of herbicides, was the first selective herbicide developed. It was introduced in 1947, and rapidly became the most widely used herbicide in the world.
A selective herbicide is one that controls weeds in a crop without damaging that crop.
After 50 years of use, 2,4-D is still the third most widely used herbicide in the United States and Canada, and the most widely used worldwide. Its major uses in agriculture are on wheat and small grains, sorghum, corn, rice, sugar cane, low-till soybeans, rangeland, and pasture. It is also used on rights-of-way, roadsides, non-crop areas, forestry, lawn and turf care, and on aquatic weeds. A recently published eight-year U.S Department of Agriculture study (NAPIAP Report NO. 1-PA-96) concluded that, should 2,4-D no longer be available, the cost to growers and other users, in terms of higher weed control expenses, and to consumers, in the form of higher food and fiber prices, would total $1,683 million annually in the U.S. alone. The study also reviewed the 2,4-D
epidemiology and toxicology data packages and concluded (page2) that after 50 years of extensive use, “The phenoxy herbicides are low in toxicity to humans and animals (1,9). No scientifically documented health risks, either acute or chronic, exist from the approved uses of the phenoxy herbicides.”
A study entitled. “An economic assessment of the benefits of 2,4-D in Canada” done in 1988 under Canadian Government sponsorship, Concluded that the net benefits of 2,4-D in Canada totaled a third of a billon dollars annually. A worldwide study of the benefits of 2,4-D measured in terms of increased food production and lower food prices has never been done, although those benifits are known to be enormous. 2,4-D has for the past fifty years, been a major tool in the continuing fight to reduce world hunger.
2,4-D is the most thoroughly researched herbicide in the world.
? 2,4-Dichlorophenoxyacetic Acid Toxicology and Environmental Toxicology
The 114 research studies completed in these areas under the EPA reregistration program confirm the existing toxicology data package. Apart from the hundreds of unpublished studies required by various regulatory agencies around the world, there are more than 4,000 peer-reviewed, published studies on 2,4-D in the scientific literature. The recent studies reconfirm:
? 2,4-D has moderate to low acute toxicity. The LD50 (rats) ranges from 699 mg per kg of body weight (2,4-D acid) to >1000 mg/kg for ester and amine