35 Pesticides

Prof. K. Maharaj Kumari

Contents

1. What are pesticides?
2. Where are pesticides used?
3. Classification of pesticides

                         Insecticides

                      Fungicides

                     Fumigants

                     Herbicides

                     Insect repellents

4. Damaging effects of pesticides on humans

                       Through food

                    Water contamination

                    Soil contamination

                    Air contamination

5. Effect on Beneficial Micro-organisms
6. Effect on Non-targeted Vegetation
7. Pesticides: Indian Scene

What is a pesticide?

Pesticides are the materials used to control pests. A  pesticide  may  be  a  chemical  substance,  biological   agent (such  as virus  or bacteria), antimicrobial,  disinfectant or device these chemicals may be natural or man-made. Application of pesticides must be done safely to reduce potential injury to people and the environment.

The term pesticide covers a wide range of compounds including pesticide covers a wide fungicides, herbicides, rodenticides, molluscicides, nematicides, plant growth regulators and others in advanced countries Organochlorine (OC) insecticides used in controlling diseases, such as malaria and typhus were banned or restricted  after  the  1960s.The introductin of other  synthetic  insecticides–Organophosphate (OP) insecticides in the 1960s, Carbamates in 1970s and Parathyroids in 1980s and the introduction of Herbicides  and  Fungicides  in  the  1970s–1980s contributed  greatly  to  pest  control  and  agricultural output ideally a pesticide must be iethal to the  targeted pests, but not to non-target species, including man. Unfortunately, this is not the case, so the controversy of use and abuse of pesticides has surfaced. Insecticides have been used by society for thousands of years. The major motivation for using insecticides was their role in controlling vector borne diseases like malaria, yellow fever etc. The other  principal motivation was use of insecticides to prevent destruction of food crops (about one-third of the world’s crop yield) by pests or weeds during growth, harvesting and storage.

Benefits of pesticides

Worldwide approximately 9,000 species of insects and mites, 50,000 species of plant pathogens, and 8,000 species of weeds damage crops. Insect pests cause an estimated 14% of loss, plant pathogens cause a 13% loss, and weeds a 13% loss. Pesticide is indispensable in agriculture. About one-third of the agricultural products are produced by using pesticides. Without pesticide application the loss of fruits, vegetables and cereals from pest injury would reach 78%, 54% and 32% respectively. Crop loss from pests declines to 35% to 42% when pesticides are used (Zhang et al, 2011). As a consequence of pesticide usage both primary and secondary benefits result. Primary benefits are the direct gains expected from their use and the secondary benefits are the less immediate or less obvious benefits that result from the primary benefits. Pesticides allow agricultural producers to improve the quality, quantity, and diversity of food supply.

The main benefits of pesticide usage are:

Improving productivity

Increases in productivity have been due to several factors including use of fertilizer, better varieties and use of machinery. Pesticides have been an integral part of the process by reducing losses from the weeds, diseases and insect pests that can markedly reduce the amount of harvestable produce.

Protection of crop losses

Weeds reduce yield of dry land crops by 37–79%. Severe infestation of weeds, particularly in the early stage of crop establishment, ultimately accounts for a yield reduction of 40%. Herbicides provide both an economic and labor benefit.

Vector disease control

Vector-borne diseases are most effectively tackled by killing the vectors. Insecticides are often the only practical way to control the insects that spread deadly diseases such as malaria, resulting in an estimated 5000 deaths each day. Malaria is one of the leading causes of morbidity and mortality in the developing world and a major public health problem in India. Disease control strategies using insecticides are crucially important.

Other areas – transport, sport complex, building

The transport sector makes extensive use of pesticides, particularly herbicides. Herbicides and insecticides are used to maintain the turf on sports pitches, cricket grounds and golf courses. Insecticides protect buildings and other wooden structures from damage by termites and wood boring insects.

Natural Pesticides

Several  significant  classes  of  insecticides  are  derived  from  plants.  These  include  nicotine from tobacco, rotenone extracted from certain legume roots and pyrethrins.           As these substances are biodegradable they do not cause water pollution. Pyrethrins and their synthetic analogs have been known  for  their  insecticidal  properties  for  a  long  time.  Pyrethrins  have  several  advantages  as insecticides, including facile enzymatic degradation, which makes them relatively safe for mammals, ability to rapidly paralyze flying insects and good biodegradability.

Synthetic Pesticides

The use of synthetic pesticides began in the 1930s and became widespread after World War II.

By 1950, pesticide was found to increase farm yield far beyond pre-World War II levels. Farmers depend heavily on synthetic pesticides to control insects in their crops. Today, it is one of the most commonly used methods in controlling insects. The use of synthetic pesticides in agriculture comes with a cost for the environment, and the health of animals and humans.

Danger of synthetic pesticide

The synthetic pesticide DDT was widely used in urban aerial sprays to control urban mosquito, gypsymoth, Japanese beetle and other insects in the 1940’s. By 1972, DDT was banned from the United States due to widespread development of resistance to DDT and evidences that DDT use was increasing pre-term births and also harming the environment. DDT was found to cause behavioral anomalies and eggshell thinning in populations of bald eagles and peregrine falcons. Although DDT is banned in the US and many other countries, DDT continues to be manufactured and applied in underdeveloped nations.

Types of synthetic pesticide

There are many classes of synthetic pesticides: Insecticides, Herbicides, Fungicides, Fumigants and insect repellents. The main classes of insecticides are: organochlorines, organophosphates, carbamates, and pyrethroids. Exposure to pesticides can cause acute (short term) or chronic (long term) effects on animals and humans, especially in the reproductive, endocrine, and central nervous systems. Today, there are more than 500 species of insects and mites that are resistant to some form of pesticides. As a result of the increasing resistance, countries have started to apply more products, combine pesticides, increase applications, or substitute with more toxic replacements.

Persistent pesticides

Some pesticides, when they are exposed to sunlight and rain, quickly breakdown into less toxic materials. Persistent pesticides are those that do not breakdown in the environment, but remain toxic to fish, animals, and humans for many years. They can bioaccumulate, their toxic effects multiplying as they move up the food chain from plants, to animals and finally humans. Some examples of persistent pesticides are –Aldrin which is used to kill insects, dieldrin-used to preserve wood, chlordane-to kill insects in crops, DDT- to kill mosquitoes etc.

Organochlorines

Chlorinated hydrocarbon or organochlorine insecticides are hydrocarbon compounds in which hydrogen atoms have been replaced by Cl atoms. Commercial chlorohydrocarbon insecticides, apart from DDT, include heptachlor, lindane, dieldrin, chlorodane, aldrin, endrin and mirex. Within this group, there is a subgroup described as the cyclodiene group that has properties enabling a distinction to be made from other members. Heptachlor, dieldrin, chlorodane, aldrin are usually included in this group.

DDT (1,1,1-trichloro-2,2-bis (p-chlorophenyl) ethane was first synthesized in 1874 and used to kill moths and carpet beetles . It has a low toxicity to mammals, although there is evidence that it might be a carcinogen. It was used extensively from 1940s to 1960s in: agriculture and forestry, building and structural protection from termites, disease prevention in humans. It was very effective against the mosquitoes that carry malaria and yellow fever, against body lice that can transmit typhus and against plague –carrying fleas. However, because of it’s over usage, especially in agriculture, its concentration increased rapidly in the environment and began to effect the reproduction of birds which indirectly incorporated it into their bodies. It is a very persistent pesticide and accumulates in food chain and hence has been banned. Metoxychlor was a popular substitute of DDT as it is biodegradable and has low toxicity to mammals. Structurally similar heptachlor, dieldrin, chlorodane, aldrin and endrin all share characteristics of high persistence and suspicions of potential carcinogenicity.

Another example of this group is hexachlorobenzene (HCB) C6Cl6. It is stable and easy to prepare from chlorine and benzene. Since it is extremely stable it remains widespread in the environment. It is of concern because it causes liver cancer in laboratory rodents and so perhaps also in humans.

Mode of Action

The mode of action of the compounds in this group is not fully clear. They are neurotoxins since they act on the nervous system, producing tremors followed by loss of movement, convulsions and death. It is presumed they dissolve in the fatty membrane surrounding nerve fibre and interfere with the transport of ions in and out of the fibre.

Organophosphates

Organophosphates are non-persistent and do not bioaccumulate in food chain; in this respect they are better than organochlorines. However, they are much more toxic to humans and other mammals than organochlorines. In recent years, a wide range of organophosphate insecticides have been developed that are acceptable for agricultural use. The organophosphate pesticides all have the following general formula:

Most common organophosphate insecticides are Malathion, Parathion and chlorpyrifos which have S rather than O bonded to P

One of the most common and most toxic insecticides for both insects and humans are the organophosphates All chemicals in this group act by inhibiting the action of several ester- splitting enzymes present in living organisms, and they are particularly active in inhibiting acetylcholine, which is generated in the transmission of nerve impulses. Acetylcholine contains an ester grouping that is the focus of the action of the enzyme. The body responds by twitching, convulsing and seizing. High exposure can kill.

Organophosphates can result in acute poisoning from a single large exposure or several small exposures over time can lead to chronic pesticide poisoning. The toxicities of organophosphate insecticides vary a great deal. A dose of 120 mg of Parathion can kill a human adult where as a small dose of 2 mg is sufficient to kill a child. Several hundred people have been killed by Parathion. In contrast Malathion has two corboxy ester linkages which are hydrolysable by carboxylase enzyme to form relatively nontoxic products. This specific enzyme which hydrolyzes Malathion is present in mammals but not in insects; therefore malathion does not cause toxicity to mammals. Unlike the Organochlorines, Organophosphates are biodegradable (little significance to water pollution) and do not bioaccumulate.

Carbamates

Derivatives of carbamic acid are collectively known as carbamates. Carbamate pesticides are widely used because they are more biodegradable than organochlorine pesticides and have lower toxicities than common organophosphate pesticides. Their general formula is

Carbaryl is widely used as an insecticide in gardens and lawns. It has low toxicity to mammals. Carbofuran has high water solubility and acts as a plant systemic insecticide. The toxic effects of carbamates are due to the fact that these compounds inhibit acetylcholinesterase. Like organophosphates carbamates interfere with the enzyme cholinesterase in the nervous system and can lead to acute poisoning. But, carbamates do not “add up” in the body. Eventually, the body will break down and expel the carbamates.

Herbicides

Herbicides are chemicals used to manipulate or control undesirable vegetation. The most frequent application of herbicides occurs in row-crop farming, where they are applied before or during planting  to maximize crop productivity by minimizing other vegetation. They also may be applied to crops in the fall, to improve harvesting. In suburban and urban areas, herbicides are applied to lawns, parks, golf courses and other areas. Herbicides are applied to water bodies to control aquatic weeds that impede irrigation withdrawals or interfere with recreational and industrial uses of water .The potential effects of herbicides are strongly influenced by their toxic mode of action and their method of application. Herbicides can act by inhibiting cell division, photosynthesis, or amino acid production or by mimicking natural auxin hormones, which regulate plant growth and cause deformities. Methods of application include spraying onto foliage, applying to soils, and applying directly to aquatic systems. The use of herbicides in agriculture has human and mechanical weeding. Herbicides can be classified depending on selectivity, mode of action, relative time of application and chemical composition.

According to chemical composition Herbicides can be classified as

Algicides

Algicides are chemical substances that are specifically used to control or kill algae. Algicides include copper sulfate, copper chelates (ethanolamines, ethylene diamines, triethanolamines, triethanolamine + ethylene diamine, and copper citrate/gluconate), endothall (as the mono (N,N-dimethylalkylamine) salt), and formulations containing the active ingredient sodium carbonate peroxyhydrate. Similar to herbicides, algicides must come in contact with and enter algal cells to be effective. Algicides vary in their mechanism of action, but they are all considered “contact” pesticides, meaning they cause injury to only the algal cells or filaments that come in contact with or are exposed to dissolved algaecide, with little intercellular movement. Algicides are used primarily to control algal growth in lakes, ponds, reservoirs, stock tanks, and irrigation conveyance systems.

Molluscicides

Molluscicides are chemical substances or biocides developed specifically for destroying molluscs. Molluscicides are typically classified as either oxidizing or non-oxidizing compounds. Oxidizing chemicals include chlorine, chlorine dioxide, chloramines, ozone, bromine, hydrogen peroxide, and potassium permanganate. Non-oxidizing molluscicides include – Quaternary and Polyquaternary Ammonium Compounds, Aromatic Hydrocarbons, Endothall as the mono (N,N-dimethylakylamine) salt, Metals and their salts, and Niclosamide. The mode of action of many of these compounds is stress to the water balance system of mollusk species.

Rodenticides

Rodenticides are pesticides that kill rodents. Rodents include not only rats and mice, but also squirrels, woodchucks, chipmunks, and other animals. Although rodents play important roles in nature, they sometimes require control. They can damage crops, transmit disease, and in some cases cause ecological damage. Rodenticides are usually formulated as baits, which are designed to attract animals. Many rodenticides stop normal blood clotting; these are called anticoagulants eg.Bromadiolone, chlorophacinone, difethialone, diphacinone, brodifacoum, and warfarin. There are a number of rodenticides that are not anticoagulants (zinc phosphide, bromethalin, cholecalciferol, and strychnine.) and they work differently. All rodenticides can be toxic when eaten. Most rodenticides are also toxic when inhaled and when they come into contact with skin. The exceptions include warfarin, which is low in toxicity when inhaled or if skin contact occurs.

Bioaccumulation of pesticides: Bioaccumulation is the process by which the concentration of pesticidein an aquatic organism achieves a level that exceeds the level in the water, because of chemical uptake through all possible routes (e.g., dietary absorption, transport across the respiratory surface, dermal absorption, and inhalation). Bioaccumulation is a combination of chemical bioconcentration and biomagnification. Biomagnification is increase in concentration of a pollutant from one link in a food chain to another.In order for biomagnification to occur, the pollutant must be long-lived, mobile, soluble in fats and biologically active. If a pollutant is short-lived, it will be broken down before it can become dangerous.

DDT is a classical example of Biomagnification. Source: www.tutorvista.com

If it is not mobile, it will stay in one place and is unlikely to be taken up by organisms. If the pollutant is soluble in water it will be excreted by the organism. Pollutants that dissolve in fats, however, may be retained for a long time. If a pollutant is not active biologically it won’t cause any problems.

Damaging effects of pesticides on humans

Direct impact

There is increasing evidence that pesticides pose a potential risk to humans and other life forms and unwanted side effects to the environment. Most pesticides are not spontaneously generated. Most of them   are   high   toxic   to   humans   and   the  environment.   Pesticides and their degraded  products  would  flow  into the  atmosphere,  soils and rivers, resulting in the accumulation of toxic substances and thus threatening human health and the environment. The nervous system of insects  and  humans  has  some  common  characteristics. Therefore, pesticides targeted at the insect nervous system can affect humans in the same way.  Some people are more at risk than others, depending on their age, gender, and individual sensitivity.  There are four ways pesticides can enter the body. The  major  routes  of  exposure are ingestion (oral),  Inhalation  (lungs) eyes and dermal  Absorption    (Skin).     The    symptoms    for   acute pesticide poisoning vary with the type of pesticide and the method of exposure. Symptoms may appear immediately after exposure, or in a few hours. Some symptoms of acute pesticide poisoning are itching, nausea, weakness, burning lungs, and eye irritation. It involves dermal and ocular irritation, upper and lower respiratory tract irritation, asthma, gastrointestinal symptoms: usually vomiting, diarrhoea and abdominal pain neurological symptoms: excitatory signs in the case of exposure to organochlorines, lethargy and coma.

Inhalation of spray mist or dust from fungicides may cause throat irritation, sneezing, and coughing. The most serious pesticide poisonings usually result from acute exposure to organophosphate and carbamate insecticides. All life forms with cholinesterase in their nervous system, such as insects, fish, birds, humans, and other mammals, can be poisoned by these chemicals.

Pesticide contamination can be through the following routes:

Through food

People have environmental exposures to pesticides mainly through diet. Pesticide residues in food commodities are usually much higher than in water and air. Contamination of fruits and vegetables may result from treatment as well as from improper use of pesticides. Some pesticides are soluble in water so they get distributed in fruits and vegetables.

Water Contamination

Pesticides are found in every source of water including rain and groundwater. Pesticides may reach water through various sources; they may percolate or leach through the soil, carried to the water as runoff, spilled accidentally or through neglect in water. Streams and rivers are frequently more polluted than groundwater and more near the areas with substantial agricultural and urban land use. Rapid transport to groundwater may be caused by heavy rainfall shortly after application of the pesticide to wet soils. Most frequently detected pesticides are herbicides (atrazine, simazine, alachlor, metolachlor and trifluralin), insecticides (diazinon, parathion methyl), and organochlorine compounds (lindane, endosulfan, and aldrin) due to their long persistance.

Soil contamination

Pesticides enter the soil via spray drift during foliage treatment, wash-off from treated foliage, from treated seeds in soil. Some pesticides such as soil Fumigants and Nematocides are applied directly into soil to control pests and plant diseases. Soil properties are affected by past land use, current activities on the site, and nearness to pollution sources. Human activities have intentionally added substances such as pesticides and fertilizers to soil. Carbon containing pesticides can undergo chemical changes or degrade into products that may be more or less toxic than the original compound. Soil characteristics affect the fate of contaminants and whether they can be readily taken up by plants or animals. These characteristics include: Soil texture, pH (acidity) of the soil, amount of organic matter in the soil, moisture levels and temperature.

Air contamination

Pesticides can contribute to air pollution. Pesticide drift occurs when pesticides suspended in the air as particles are carried by wind to other areas. Volatile pesticides applied to crops will volatilize and are blown by winds to nearby areas posing a threat to wildlife. Droplets of sprayed pesticides can stick to particles in the wind, like dust and pollen and be deposited on non target areas.

Effect on beneficial Micro-Organisms

Pesticides have diverse toxic effects on beneficial microorganisms, they affect the survival of a range of life cycle stages, reductions in reproductive capacity, changes in the suitability of hosts for parasitising or predation, and reduced emergence of parasitoids from sprayed host eggs. Pesticides in soil hinder nitrogen fixation, which is required for the growth of higher plants. Insecticides DDT, methyl parathion, and especially pentachlorophenol found to interfere with legume-rhizobium chemical signaling. Reduction of this symbiotic chemical signaling results in reduced nitrogen fixation and thus reduces crop yields.Fungicides are found to be toxic to soil fungi and actinomycetes. Pesticides like benomyl and dimethoate inhibits functioning of symbiotic mycorrhizal fungi which are responsible for plant nutrient uptake.

Effect on Non-targeted Vegetation

Pesticides are highly toxic chemicals and may not be absolutely specific in their actions. Impact of pesticide on plant depends upon its absorption, translocation and metabolism within the plant. Pesticides are usually detoxified in plants through series of degradation reactions and conjugation processes forming bound or insoluble residues. Although, the purpose of pesticides is to protect crop against pests without disturbing the ecological balance of nature r damaging the plants of interest ,sometimes they become injurious to the non-target plants directly or indirectly by affecting the environment or plants as such. Since the chemicals belonging to different groups of pesticides (insecticides, fungicides, herbicides, rodenticides, miticides, hormones, antibiotics etc.) are basically toxic in nature and are meant to kill pests, may not be absolutely specific in their action. Utmost care is taken to develop highly specific, less persistent and environmental friendly pesticide and to use of natural products like antibiotics, plant products etc. instead of synthetic compounds for pest control.

Exposure of a sensitive plant to a growth-regulating chemical might cause remarkable growth distortions, yield loss and outright death of the plant depending on the dose received. With other types of pesticides, the effect may be much subtler and less obvious. As such till today there is no alternative than to use synthetic pesticides for pest control in economically backward countries. Knowledge of pesticide-plant interaction will help us to identify the problems with non-target plants and adequate measures may be taken to overcome the risks involved in their use. Pesticides when used wisely and judiciously much of the problems created by these toxic chemicals to non-target plants could be avoided and the benefits will certainly outweigh the risks involved in their use.

Pesticides: Indian Scene

India, being predominantly an agricultural country, pesticide is an important tool for productivity enhancement and crop protection. Use of pesticides in India began in 1948 when DDT was imported for malaria control and BHC for locust control. India started pesticide production with manufacturing plant for DDT and benzene hexachloride (BHC) in the year 1952. In 1958, India was producing over 5000 metric tonnes of pesticides. There are more than 234 registered pesticides in India and the Indian pesticide industry includes more than 125 large and medium scale producers of more than 500 pesticide products. The pesticide consumption during 2005 to 2010 is presented in Figure. However, exposure to pesticides both occupationally and environmentally causes a range of human health problems. It has been observed that the pesticides exposures are increasingly linked to immune suppression, hormone disruption, diminished intelligence, reproductive abnormalities and cancer. Currently, India is the largest producer of pesticides in Asia and ranks twelfth in the world for the use of pesticides. Although Indian average consumption of pesticide is far lower than many other developed economies, the problem of pesticide residue is very high in India. Unfortunately, India is one of the few remaining countries still producing and using some of the chlorinated pesticides such as DDT and lindane (Abhilash and Singh, 2009). Among the various states, Uttar Pradesh is the largest consumer followed by Punjab, Haryana and Maharashtra. Regarding the pesticide share across agricultural crops, cotton account for 45% followed by rice (25%), chillies/vegetables/fruits (13-24%), plantations (7-8%), cereals/millets/oil seeds (6-7%), sugarcane (2-3%) and other (1-2%) in India, 51% of food commodities are contaminated with pesticide residues and out of these, 20% have pesticides residues above the maximum residue level values on a worldwide basis. In this light, problems of pesticide safety, regulation of pesticide use, use of biotechnology, and biopesticides, and use of pesticides obtained from natural plant sources such as neem extracts are some of the future strategies for minimizing human exposure to pesticides.

References:

• Abhilash PC, Singh N Pesticide use and application: an Indian scenario. Journal of Hazardous Materials, 2009; 165(1-3):1-12.
• Environmental Chemistry by Baird Published by W. H. Freeman
• Environmental Chemistry by Stanley E. Mahanan Published by Lewis Publishers
• Environmental  Chemistry by A. K.  De Published  by  New Age International