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COURSE CODE: CRP 515 COURSE TITLE: PESTICIDES AND

POLLUTANTS LECTURER: DR. P.O. OGUNGBILE

MODULE 1

Pest and Pesticide

Unwanted organism interferes with food production, human health, and peace and quietness of

environment and causes economic harm.

The word pesticide itself means “pest killer”. Pests include bacteria, fungi, insects, weeds, rodents

and other living things that affect humans, animals, or plants adversely. Depending on the kind of

pest against which they are effective, pesticides are known as bactericides, fungicides, nematicides,

insecticides, herbicides, and so on.

A pesticide is any substance or mixture of substances intended for preventing destroying,

repelling, or mitigating any pest. Pests can be insects and insect-like organisms, mice and other

vertebrate animals, unwanted plants (weeds), or fungi, bacteria and viruses that cause plant

diseases. Though often misunderstood to refer only to insecticides, the term pesticide also applies

to herbicides, fungicides, and various other substances used to control pests.

Any material, whether naturally derived or not, that is sold or distributed with the intent to control

or eliminate any pest (weeds, insects, microorganisms, etc.) is classified as a pesticide. By their

very nature, pesticides create some risk of harm to humans, animals, or the environment because

they are designed to kill or otherwise adversely affect living organisms. Many household products

are pesticides. An ideal pesticide: - Kills target pest

Non-persistent, short lived

No adverse effects on other organisms

No genetic resistance

Less costly than economic losses

Types of Pesticides: -

Chemical, organism, facility or activity that kills pest organisms.

Insecticides

Herbicides

Fungicides Bactericides

Rodenticides

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Biopesticides

Biopesticides (also known as biological pesticides) are pesticides derived from such natural

materials as animals, plants, bacteria, and certain minerals. For example, canola oil and baking

soda have pesticidal applications and re considered biopesticides.

Types of Biopesticides

Microbial pesticides

Plant pesticides

Biochemical pesticides

Pesticide Characteristics Introduction

To understand how pesticides move in the environment, one must first understand certain physical

and chemical characteristics of pesticides, as well as how these characteristics determine a

pesticide’sinteraction with the environment.

Solubility

Stability is a measure of the ability of a pesticide to dissolve in a solvent, which is usually water.

Pesticides that are highly soluble in water dissolve easily. Such pesticides are more likely to move

with water in surface runoff or to move through the soil in water than less-soluble pesticides.

In the SDS, manufacturers use relative learns – such as miscible, dispensible, suspension,

emulsifiable, and water solubility – to describe their products solubility. Some manufacturers will

use a numerical value for this description, such as 2.9mg/L or ppm. Pesticides with a value of 100

ppm or less are considered relatively insoluble while pesticides with values greater than 1,000 ppm

are considered very soluble.

Adsorption

Adsorption is the process whereby a pesticide binds to soil colloids which are microscopic

inorganic and organic particles in the soil. Colloid is derived from the Greek term meaning glue-

like. These particles have an extremely large surface area in proportion to a given volume. It has

been calculated that 1 cubic inch of colloidal clay may have 200-500 square feet of particle surface

area.

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Adsorption occurs because of an attraction between the chemical and soil particles. Typically, oil-

soluble pesticides are more attracted to clay particles and to organic matter in soil than water-

soluble pesticides. Pesticide molecules with positive charges are more tightly adsorbed to

negatively charged soil particles. A pesticide that adsorbs to soil particles is less likely to move

from the application site than a chemical that does not adsorb tightly to the soil.

Persistent

Persistence is the ability of a pesticide to remain present and active in its original form during an

extended period before degrading. A chemical’s persistence is described in terms of its half-life,

which is a comparative measure of the time needed for the chemical to degrade. The longer a

pesticide’s half-life, themore persistent the pesticide. Persistent pesticide residues are sometimes

desirable because they provide long-term pest control and reduce the need for repeated

applications. However, some persistent pesticides applied to soil, plants lumber, and other surfaces

or spilled into water or on soil can later harm sensitive plants or animals, including humans. It is

especially important to prevent persistent pesticides from moving off-site through improper

handling, application, drift, leaching or runoff.

Application of persistent pesticides presents a hazard to persons and non-target animals entering

a treated area and may lead to the presence of illegal residues on rotational food or feed crops.

Check the label for statements about the persistence of the pesticide and for replanting restrictions.

The rate of pesticide degradation relates to the persistence of the pesticide.

Degradation processes break down pesticide compounds into simpler and often less-toxic

chemicals. Some pesticides break down rapidly – in a matter of days or even hours. Other

pesticides can be detected in the environment for a year or more.

Pesticides re degraded by the following processes

• Chemical degradation is the breakdown of chemicals by processes that do not involve

living organism, most commonly by hydrolysis, a reaction with water.

• Microbial degradation is the process in which chemicals are degraded by soil

microorganisms, such as fungi and bacteria.

• Photodegradation is the breakdown of chemicals in reaction to sunlight.

Water and temperature both affect the degradation of pesticides. Warm, wet conditions can

increase the speed of pesticide degradation, cool, dry conditions slow the degradation process.

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Volatility

Volatility is the tendency of a pesticide to turn into a gas or vapor. Some pesticides are more

volatile than others. The likelihood of pesticide volatilization increases as temperatures and wind

increase. Volatility is also more likely under conditions of low relative humidity.

The potential for a pesticide to volatilize is measured by its vapor pressure. This measurement

may be described in units of Pa (Pascals) or mmHg (millimeters of mercury). Pesticides that have

high vapor-pressure values are more volatile. Vapors from such pesticides can move off-site and

cause injury to susceptible plants. Some volatile pesticide products carry label statements that

warm handlers of the product’s potential for vapor movement.

Biological Properties of Insecticides

Most chemical insecticides act by poisoning the nervous system. The central and peripheral

nervous system of insects is fundamentally similar to that of mammals. A small amount of

pesticide can be fatal to an insect, primarily because of the insect’s small size and high rate of

metabolism. While that same amount will not be fatal to a person, it may still cause harm. The

similarities of nervous system structure make it nearly impossible to design insecticides that affect

only target insect pests; consequently, insecticides may affect non-pest insects, people, wildlife,

and pests. Some insecticides harm water quality or affect organisms in other ways; for example,

the insecticide carbaryl (a carbamate insecticide), is listed as a carcinogen by the state of California

and as a possible hormone disruptor by the state of Illinois EPA. The newer insecticides are

designed to be more specific and less persistent in the environment. The most prominent classes

of insecticides are organochlorines, organophosphates, carbamates, and pyrethroids.

Organochlorines

The chemical structure of organochlorines is diverse, but they all contain chlorine, which places

them in a larger class of compounds called chlorinated hydrocarbons. Organochlorines, which

include DDT, demonstrate many of the potential risks and benefits of insecticide use.

While organochlorine have the advantage of being cheap to manufacture and are effective against

target species, they have serious unintended consequences. Organochlorines disrupt the movement

of ions such as calcium, chloride, sodium and potassium into and out of nerve cells. Depending on

the specific structure of the organochlorine chemical,

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it may also affect the nervous system in other ways. At one time organochlorines were thought to

be ideal because they are very stable, slow to degrade in the environment, dissolve in fats (and or

therefore readily taken up by insects), and seemingly harmless to mammals. Unfortunately, it

eventually became clear that the attributes of persistence and fat solubility were actually very

undesirable; the organochlorines passed up the food chain, where they bioaccumulated in the fat

of large animals and humans and were passed on to nursing young. The global use and transport

of organochlorines resulted in the contamination of wildlife around the globe, including in Arctic

and Antarctic regions where these insecticides are rarely if ever used. A decline in the number of

birds that prey on animals exposed to DDT was one of the first signs of the unintended

consequences. Unexpectedly, DDT caused a thinning of the birds’ eggshells and resulted in the

death of their developing young.

Organochlorines like DDT are now largely banned in industrialized countries but they are still

manufactured and used in developing countries. (Banned pesticides are still manufactured in some

industrialized countries and exported). Organochlorine insecticides provide many important

lessons about the desirable and undesirable properties of pesticides. Organochlorines

H

Cl

Cl Cl Cl

DDT

Organophosphates and Carbamates

Organophosphates and carbamates have very different chemical structures, but share a similar

mechanisms of action and will be examined here as one class of insecticides.

Organphosphates were initially developed in the 1940s as highly toxic biological warfare agents

(nerve gases). Modern derivatives, including sarin, soman, and VX, were stockpiled by various

countries and now present some difficult disposal problems. Researchers created many different

organophosphates in their search for insecticides that would target selected species and would be

less toxic to mammals. When the organophosphate parathion was first used as a replacement for

DDT, it was believed to be better as it was more specific. Unfortunately, there were a number of

C

C

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human deaths because workers failed to appreciate the fact that parathion’s short-term (acute)

toxicity is greater than DDTs.

The problem with organophosphates and carbamates is that they affect an important

neurotransmitter common to both insects and mammals. This neurotransmitter, acetylcholine, is

essential for nerve cells to be able to communicate with each other. Acetylcholine released by one

nerve cell initiates communication with another nerve cell, but that stimulation must eventually be

stopped. To stop the communication, acetylcholine is removed from the area around the nerve

cells, and an enzyme, acetylcholinesterase, breaks down the acetylcholine. Organophosphates and

carbamates block the enzyme and disrupt the proper functioning of the nerve cells. Hence, these

insecticides are called acetylcholinesterase inhibitors.

Structural differences between the various organophosphates and carbamates affect the efficiency

and degree to which the acetylcholinesterase is blocked. Nerve gases are highly efficient and

permanently block aceylcholinesterase, while the commonly used pesticides block

aceylcholinesterase only temporarily. The toxicity of these pesticides presents significant health

hazards, and researchers continue to work to develop new insecticides that have fewer unintended

consequences.

Organophosphates

R O O or S

R O X

The R groups are usually methyl or ethyl and are the same molecules X is referred to as the leaving

group.

(1) Thiophosphates

R – O S

P

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P

R – O O - X Examples

are Bnomophos, parathion and pirimiphos

(2) If 2 oxygen are replaced, they are dithiophosphate.

R - O S

R - O S – X

Dimethioate, malathion and phorate

Carbamate Pesticides

O CH3

Carbocyclic O C N group

H or CH3

An important member of the group is subgroup.

H

O

Carboryl

The hydroxyl group is directly attached to a phenyl ring.

Carbofuran(Nematicide)

P

C N

O CH 3

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H

O

CH3

CH3

Classification of Organophosphorus Compounds into Subgroups

The golden rule in the choice of a pesticide is never to use a powerful undiscriminating weapon

when a mild selective one will do. It is always better to employ the safer and cheaper member of

the group.

Classification of Organophosphorus Compounds into sub groups

This classification is according to the practical use of organophosphorus compounds.

Sub group 1

1. Contact Pesticides

They are compounds of low chemical stability, soluble in water but more or less rapidly

hydrolysed by water. Examples are mevinphos, tetraethyl, phosphates and tetrachlorvinphos.

2. Persistent Contact Pesticides

They are compounds of moderate to high chemical stability. They are usually of low solubility in

water but soluble in oil. The compound do not travel around the plant and are activated before they

reach the site of action. Examples are malathion, methy parathion, diazion, trichlorophon.

3. Systemic Pesticides

They are compounds of moderate to high chemical stability. The systemic compounds are

activated before reaching their site of action. Their oil/water partition coefficients are such that

enables them bot to enter plants and to be translocated within them. Examples are Dimethoate,

demeton-methyl, phorate, formothion.

4. Fumigants: They are compounds with high vapour pressure and low chemical stability.

Example Dichlorvos.

C N

O CH 3

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Pyrethroids

One of the newer classes of insecticide, synthetic pyrethroids are loosely based upon the naturally

occurring pyrethrum found in chrysanthemum flowers. Synthetic pyrethroids were first developed

in the 1980s, but the naturally occurring pyrethrum was first commercially used in the 1800s. Their

use has increased significantly over the last 20 years. The chemical structure of pyrethroids is quite

different from that of organochlorines, organophosphates, and carbamates but the primary site of

action is also the nervous system. Pyrethroids affect the movement of sodium ions (Na+) into and

out of nerve cells, causing the nerve cells to become hypersensitive to neurotransmitters. Structural

differences between various pyrethroids can change their toxic effects on specific insects and even

mammals.

Synthetic pyrethroids are more persistent in the environment than natural pyrethrum, which is

unstable in light and breaks down very quickly in sunlight.

CH3 H H CH2 CH CH CH2

C = HC

R CH3 O

Pyrethnins

CH3

Cinerins

H H CH 2 CH CH CH 3

C = HC CH 3 O C

O O

CH 3 C

O

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Biological properties of Herbicides

Herbicides are used to kill or damage plants and are the most rapidly growing type of pesticide.

Prior to the 1930s, herbicides were nonspecific and often very toxic to humans as well as other

animals. In the 1930s, researchers discovered several chemicals that selectively killed plants while

developing new insecticides. These chemicals are now widely used to increase food production by

killing weeds that choke out or compete with food crops.

The most well-known herbicides are the chlorophenoxy compounds that include 2,4-D and 2, 4,

5-T. This herbicide mixture, sometimes called Agent Orange in the 1960s, was widely used to kill

broadleaf plants in agricultural fields, along roadsides, and on rights of way for power lines. It was

also extensively used as a chemical warfare agent to kill unwanted vegetation, for example in

jungles. The mechanism of action of this class of chemicals is poorly understood, but the herbicides

appear to interact with plant growth hormones (See Pesticides – History for discussion of the

contamination of 2, 4, 5-T with dioxin).

Paraquat and the related chemical diquat are nonselective herbicides that are also toxic to

mammals. Occupational or accidental exposure to paraquat can occur by ingestion, skin exposure,

or inhalation, all of which can cause serious illness or death. While seldom used in the United

States at this time, paraquat is still widely used in developing countries. At one time it was used in

marijuana plant eradication programs, but it was discontinued when a number of fatalities were

observed in smokers of paraquat-contaminated marijuana.

There are many other herbicides in widespread use, such as alachlor, glyphosate, and atrazine, and

they have a range of actions on plants and animals. Herbicides have become an essential part of

the agriculture business and are thought by some to be necessary for high crop yields. However, a

serious limitation of many herbicides is their lack of specificity; in other words, herbicides can

damage the crops of interest. The manufacturers of herbicides are working to address this problem

and are increasingly turning to biotechnology to create genetically modified crops that are

herbicide resistant. For example, Monsanto produces the glyphosatebased herbicide Roundup. The

company also manufactures a genetically modified soybean that is resistant to Roundup. This

allows farmers to use Roundup herbicide with the Roundup Ready soybean plants and not have to

worry about killing the soybean plants. The genetically modified Roundup Ready soybean is now

widely planted, though the practice has generated considerable controversy internationally.

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Mode of Action

Most people know that insecticides kill insects. However, the way in which these chemicals work

is a mystery to most of us. How an insecticide works is called its mode of action. A complete

understanding of the mode of action of an insecticide requires knowledge of how it affects a

specific target site within an organism. The target site is usually a critical protein or enzyme in the

insect, but some insecticides affect broader targets. For example, silica aerogels affect the entire

lipid layer on the insect cuticle. Although most insecticides have multiple biological effects,

toxicity is usually attributed to a single major effect. This fact sheet is intended to explain what

insecticides do in insects to cause toxicity and death (Table 2).

Insecticides can be classified according to their mode entry into the insect as 1) stomach poisons,

2) contact poisons, or 3) fumigants. However, many insecticides belong to more than one category

when grouped in this way, limiting its usefulness. Another way insecticides can be classified is by

their mode of action. Most insecticides affect one of five biological systems in insects. These

include 1) the nervous system, 2) the production of energy, 3) the production of cuticle, 4) the

endocrine system, and 5) water balance. This method of classification is preferred among

scientists.

Insecticides that Affect the Nervous System

Most traditional insecticides fit into this category. Pyrethroid, organophosphorus, and carbamate

insecticides all adversely affect the nervous system. Pyrethroids are synthetic chemicals whose

structures mimic the natural insecticide pyrethrin. Pyrethrins are found in the flower heads of plants

belonging to the family composite (e.g., chrysanthemums). These insecticides have a unique ability

to knock down insects quickly. Synthetic pyrethrins (also known as pyrethroids) have been

chemically altered to make them more stable Pyrethroids are axonic poisons (They poison the

nerve fiber). They bind to a protein in nerves called the voltagegated sodium channel. Normally,

this protein opens causing stimulation of the nerve and closes to terminate the nerve signal.

Pyrethroids bind to this gate and prevent it from closing normally which results in continuous

nerve stimulation. This explains the tremors exhibited by poisoned insects. They lose control of

their nervous system and are unable to produce coordinated movement.

Carbamate and organophosphorus insecticides also affect the nervous system. However, these

insecticides are synaptic poisons. The synapse is a junction between two nerves or a nerve

connection point (hence the name synaptic poison). Specifically, organophosphorus and carbamate

insecticides bind to an enzyme found in the synapse called acetylcholinesterase. This enzyme is

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designed to stop a nerve impulse after it has crossed the synapse. Organophosphorus and carbamate

insecticides bind to and prevent the enzyme from working. Therefore, poisoned synapses cannot

stop the nerve impulse. Consequently, continued stimulation of the nerve occurs as observed with

pyrethroids. Again, poisoned insects exhibit tremors and uncoordinated movement.

Avermectins belong to a group of chemicals called macrolactones. These chemicals are derived

form a fungus and also adversely affect the nervous system. Avermectins are axonic poisons (affect

the nerve fiber). They bind in another protein in the nerve fiber called the (gamma)amino butyric

acid (GABA)-gated chloride channel. This protein forms a channel within the nerve that attenuates

some nerve impulses. Avermectins block the channel causing nerve hyperexcitation. Again, the

result is that the nervous system becomes overexcited resulting in tremors and uncoordinated

movement.

Two new insecticides have been introduced recently that also cause toxicity by affecting the

nervous system. Imidacloprid belongs to the chloronicotinyl chemical class of insecticides.

Imidacloprid is also a synaptic nervous system poison. Specifically, this chemical mimics the

action of a neurotransmitter called acetylchlorine. Acetylcholine normally turns on a nerve impulse

at the synapse but its effects are terminated very quickly. Imidacloprid turns on the nerve impulse

but cannot terminate it because of its chemical structure. Therefore, the nervous system is

overexcited (as with organophosphates, carbamates, and pyrethroids), resulting in tremors and

uncoordinated movement. Imidacloprid is more specific for insect nervous tissue compared with

mammalian nervous tissue.

The other new insecticide that affects the nervous system is fipronil. Fipronil is a phenylpyrazole

chemical class insecticide. Its mode of action is similar to cyclodiene insecticides (e.g. chlordane

or aldrin), which were used extensively as termiticides during the 1960’s and 1970’s, and the

abamectins described above. These chemicals are axonic poisons that affect the GABA-gated

chloride channel.

Insecticides that Inhibit Energy Production

Only a handful of chemicals that inhibit the production of energy are currently in use as

insecticides. However, significant research and development of new chemicals with this mode of

action are currently under way by many pesticide manufacturers.

The most pervasive and well-known energy inhibiting insecticide is hydramethylnon, the active

ingredient in Amdro®, Siege Gel Bait®, and Combat®. This insecticide belongs to the chemical

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class amidinohydrazone. This chemical binds to a protein called a cytochrome in the electron

transport system of the mitochondrion. This binding blocks the production of ATP.

Insects killed by these chemicals die on their feet. They essentially “run out of gas.”

Another insecticide currently available that inhibits energy production is sulfluramid. This

insecticide belongs to the halogenated alkyl sulphonamide chemical class. It is the active ingredient

found in Raid Max® ant bait. Sulfluramid is made more toxic by the organism. The enzyme

affected is different.

Finally, the furmigantsulfuryl fluoride inhibits energy production. This chemical is very volatile

and typically used to fumigate houses for drywood termite infestations. Sulfluryl fluoride is fast

acting and its mode of action is similar to hydramethylnon and sulfluramid. However, the enzyme

affected is different.

Many new chemicals are being developed for use as energy production inhibitors. Chemicals in

the class pyrrole, thiourea, and quinazoline are showing great promise as pesticides that inhibit

energy production.

Insecticides that Affect the Insect Endocrine System

These chemicals are typically referred to as insect growth regulators, or IGRs. IGRs act on the

endocrine or hormone system of insects. These insecticides are specific for insects, have very low

mammalian toxicity are nonpersistent in the environment, and cause death slowly. Most of the

currently registered IGRs mimic the juvenile hormone produced in the insect brain. Juvenile

hormone tells the insect to remain in the immature state. When sufficient growth has occurred, the

juvenile hormone production ceases triggering the molt to the adult stage. IGR chemicals, such as

hydroprene, methoprene, pyriproxyfen, and fenoxycarb, mimic the action of juvenile hormone and

keep the insect in the immature state. Insects treated with these chemicals are unable to molt

successfully to the adult stage, and cannot reproduce normally.

Insecticides that inhibit Cuticle Production

These chemicals are known as chitin synthesis inhibitors or CSIs. They are often grouped with the

IGRs. The most notable chemical being used as a CSI is the benzoyphenylureas. This class of

insecticides includes lufenuron (Program®) which is a systemic insecticide used for flea control

(fed to your pet), diflubenzuron ((Dimilin®) used against fly larvae in manure, and hexaflumuron

(Sentricon®) used in a termite bait station. These chemicals inhibit the production of chitin. Chitin

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is a major component of the insect exoskeleton. Insects poisoned with CSIs are unable to

synthesize new cuticle, thereby preventing them from molting successfully to the next stage.

Insecticides Affecting Water Balance

Insecticides with this mode of action include boric acid, diatomaceous earth, and sorptive dusts.

Insects have a thin covering of wax on their body that helps to prevent water loss from the cuticular

surface. Silica aerogels (sorptive dusts) and diatomaceous earth are very effective at absorbing

oils. Therefore, when an insect contacts one of these chemicals it absorbs the protective waxy

covering on the insect resulting in rapid water loss from the cuticle and eventually death from

dessication. Unfortunately, insects that live in environments with high relative humidity, or that

have ready access to a water source. Show an increased tolerance to silica aerogels and

diatomaceous earth. This is because water loss can be minimized by either of these conditions, and

the insect may survive despite the absence of a wax layer.

Borate-containing insecticides also disrupt water balance in insects. The exact mode of action

(more specifically) the target site) of borate containing insecticides is not currently known.

Impact of pesticides use in agriculture: their benefits and hazards

The term pesticide covers a wide range of compounds including insecticides, fungicides,

herbicides, rodenticides, molluscicides, nematicides, plant growth regulators and others. Among

these, organochlorine (OC) insecticides, used successfully in controlling a number of diseases,

such as malaria and typhus, were banned or restricted after the 1960s in most of the technologically

advanced countries. The introduction of other synthetic insecticides – organophosphate (OP)

insecticides in the 1960s, carbamates in 1970s and pyrethroids 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 lethal 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. The rampant use of these chemicals, under the adage, “if little is good, a lot more

will be better” has played havoc with human and other life forms.

The primary benefits are the consequences of the pesticides’ effects – the direct gains expected

from their use. For example the effect of killing caterpillars feeding on the crop brings the primary

benefit of higher yields and better quality of cabbage. The three main effects result in 26 primary

benefits ranging from protection of recreational turf to saved human lives. The secondary benefits

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are the less immediate or less obvious benefits that result from the primary benefits. They may be

subtle, less intuitively obvious, or of longer term. It follows that for secondary benefits it is

therefore more difficult to establish cause and effect, but nevertheless they can be powerful

justifications for pesticide use. For example the higher cabbage yield might bring additional

revenue that could be put towards children’s education or medical care, leading to a healthier,

better educated population. There are various secondary benefits identified, ranging from fitter

people to conserved biodiversity.

Improving Productivity

Tremendous benefits have been derived from the use of pesticides in forestry, public health and

the domestic sphere – and, of course, in agriculture, a sector upon which the Indian economy is

largely dependent. Food grain production, which stood at a mere 50 million tons in 1948-49, had

increased almost fourfold to 198 million tons by the end of 1996-97 from an estimated 169 million

hectares of permanently cropped land. This result has been achieved by the use of high-yield

varieties of seeds, advanced irrigation technologies and agricultural chemicals (Employment

Information: Indian Labour Statistics, 1994). Similarly outputs and productivity have increased

dramatically in most countries, for example wheat yields in the United Kingdom, corn yields in

the USA. 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. Warren (1998) also drew attention to the spectacular increases in crop yields

in the United States in the twentieth century. Webster et al. (1999) stated that “considerable

economic losses” would be suffered without pesticide use and quantified the significant increases

in yield and economic margin that result from pesticide use. Moreover, in the environment most

pesticides undergo photochemical transformation to produce metabolites which are relatively non-

toxic to both human beings and the environment.

Protection of crop losses/yield reduction

In medium land, rice even under puddle conditions during the critical period warranted an

effective and economic weed control practice to prevent reduction in rice yield due to weeds that

ranged from 28 to 48%, based on comparisons that included control (weedy) plots. Weeds reduce

yield of dry land crops by 37-79%. Severe infestation of weeds, particularly in the early stage of

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crop establishment, ultimately accounts for a yield reduction of 40%. Herbicides provided both an

economic and labour, 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. In 2004, Bhatia wrote that 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 are crucially important also for livestock.

Quality of food

In countries of the first world, it has been observed that a diet containing fresh fruit and vegetables

far outweigh potential risks from eating very low residues of pesticides in crops (Brown, 2004).

Increasing evidence shows that eating fruit and vegetables regularly reduces the risk of many

cancers, high blood pressure, heart disease, diabetes, stroke, and other chronic diseases.

The nutritional properties of apples and blueberries in the US diet and concluded that their high

concentrations of antioxidants act as protectants against cancer and heart disease. Lewis attributed

doubling in wild blueberry production and subsequent increases in consumption chiefly to

herbicide use that improved weed control.

Otherareas – 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

woodboring insects.

Hazards of pesticides Direct Impact on humans

If the credits of pesticides include enhanced economic potential in terms of increased production

of food and fibre, and amelioration of vector-borne diseases, then their debits have resulted in

serious health implications to man and his environment. There is now overwhelming evidence that

some of these chemicals do pose a potential risk to humans and other life forms and unwanted side

effects to the environment. No segment of the population is completely protected against exposure

to pesticides and the potentially serious health effects, though a disproportionate burden, is

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shouldered by the people of developing countries and by high risk groups in each country (WHO,

1990). The world-wide deaths and chronic diseases due to pesticide poisoning number about 1

million per year.

The high risk groups exposed to pesticides include production workers, formulators, sprayers,

mixers, loaders and agricultural farm workers. During manufacture and formulation, the possibility

ofhazards may be higher because theprocesses involved are not risk free. In industrial settings,

workers are at increased risk since they handle various toxic chemicals including pesticides, raw

materials, toxic solvents and inert carriers.

OC compounds could pollute the tissues of virtually every life form on the earth, the lakes and the

oceans, the fishes that live in them and the birds that feed on the fishes. The US National Academy

of Sciences stated that the DDT metabolite DDE causes eggshell thinning and that the bald eagle

population in the United States declined primarily because of exposure to DDT and its metabolites.

Certain environmental chemicals, including pesticides termed as endocrine disruptors, are known

to elicit their adverse effects by mimicking or antagonizing natural hormones in the body and it

has been postulated that their long-term, low-dose exposure is increasingly linked to human health

effects such as immune suppression, hormone disruption, diminished intelligence, reproductive

abnormalities and cancer.

A study on workers (N = 356) in four units manufacturing HCH in India revealed neurological

symptoms (21%) which were related to the intensity of exposure. The magnitude of the toxicity

risk involved in the spraying of methomyl, a carbamate insecticide, in field conditions was assessed

by the National Institute of Occupational Health (NIOH). Significant changes were noticed in the

ECG, the serum LDH levels, and cholinesterase (ChE) activities in the spraymen, indicating

cardiotoxic effects of methomyl. Observations confined to health surveillance in male formulators

engaged in production of dust and liquid formulations of various pesticides (malathion, methyl

parathion, DDT andlindane) in industrial settings of the unorganized sector revealed a high

occurrence of generalisedsymptoms (headache, nausea, vomiting, fatigue, irritation of skin and

eyes) besides psychological, neurological, cardiorespiratory and gastrointestinal symptoms

coupled with low plasma ChE activity.

Data on reproductive toxicity were collected from 1,106 couples when the males were associated

with the spraying of pesticides (OC, OP and carbamates) in cotton fields. A study in malaria

spraymen was initiated to evaluate the effects of a short-term (16 week) exposure in workers

(N=216) spraying HCH in field conditions.

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A study on those affected in the Seveso disaster of 1976 in Italy during the production of

2,4,5&, a herbicide, concluded that chloracne(nearly 200 cases with a definite exposure

dependence) was the only effect established with certainty as a result of dioxin formation. Early

health investigations including liver function, immune function, neurologic impairment, and

reproductive effects yielded inconclusive results. An excess mortality from cardiovascular and

respiratory diseases was uncovered, possibly related to the psychosocial consequences of the

accident in addition to the chemical contamination. An excess of diabetes cases was also found.

Results of cancer incidence and mortality follow-up showed an increased occurrence of cancer of

the gastrointestinal sites and of the lymphatic and haematopoietic tissue. Results cannot be viewed

as conclusive, however, because of various limitations; few individual exposure data, short latency

period, and small population size for certain cancer types. A similar study in 2001 observed no

increase in all-cause and all-cancer mortality. However, the results support the notion that dioxin

is carcinogenic to humans and corroborate the hypotheses of its association with cardiovascular-

and endocrine-related effects. During the Vietnam War, United States military forces sprayed

nearly 19 million gallons of herbicide on approximately 3.6 million acres of Vietnamese and

Laotian land to remove forest cover, destroy crops, and clear vegetation from the perimeters of US

bases.. Various herbicide formulations were used, but most were mixtures of the phenoxy

herbicides 2,4-dichlorophenoxyacetic acid (2.4-D) and 2,4,5-

trichlorophenoxyacetic acid (2,4,5-T). Approximately 3 million Americans served in the armed

forces in Vietnam during the Vietnam War. Some of them (as well as some Vietnamese

combatants and civilians, and members of the armed forces of other nations) were exposed to

defoliant mixtures, including Agent Orange. There was evidence on cancer risk of Vietnam

veterans, workers occupationally exposed to herbicides or dioxins (since dioxins contaminated the

herbicide mixtures used in Vietnam), and of the Vietnamese population.

Conclusion

The data on environmental-cum-health risk assessment studies may be regarded as an aid towards

a better understanding of the problem. Data on the occurrence of pesticide-related illnesses among

defined populations in developing countries are scanty. Generation of base-line descriptive

epidemiological data based on area profiles, development of intervention strategies designed to

lower the incidence of acute poisoning and periodic surveillance studies on high risk groups are

needed. Our efforts should include investigations of outbreaks and accidental exposure to

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pesticides correlation studies, cohort analyses, prospective studies and randomized trials of

intervention procedures. Valuable information can be collected by monitoring the end product of

human exposure in the form of residue levels in body fluids and tissues of the general population.

The importance of education and training of workers as a major vehicle to ensure a safe use of

pesticides is being increasingly recognised.

Because of the extensive benefits which man accrues from pesticides, these chemicals provide the

best opportunity to those who juggle with the risk-benefit equations. The economic impact of

pesticides in non-target species (including humans) has been estimated at approximately $8 billion

annually in developing countries. What is required is to weigh all the risks against the benefits to

ensure a maximum margin of safety. The total cost-benefit picture from pesticide use differs

appreciably between developed and developing countries. For developing countries it is imperative

to use pesticides, as no one would prefer famine and communicable diseases like malaria. It may

thus be expedient to accept a reasonable degree of risk. Our approach to the use of pesticides should

be pragmatic. In other words, all activities concerning pesticides should be based on scientific

judgement and not on commercial considerations. There are some inherent difficulties in fully

evaluating the risks of human health due to pesticides. For example there is a large number of

human variables such as age, sex, race, socio-economic status, diet, state of health, etc. – all of

which affect human exposure to pesticides. But practically little is known about the effects of these

variables. The long-term effects of low level exposure to one pesticide are greatly influenced by

concomitant exposure to other pesticides as well as to pollutants present in air, water, food and

drugs.

Pesticides are often considered a quick, easy, and inexpensive solution for controlling weeds and

insect pests in urban landscapes. However, pesticide use comes at a significant cost. Pesticides

have contaminated almost every part of our environment. Pesticide residues are found in soil and

air, and in surface and ground water across the countries, and urban pesticide uses contribute to

the problem. Pesticide contamination poses significant risks to the environment and non-target

organisms ranging from beneficial soil microorganisms, to insects, plants, fish, and birds. Contrary

to common misconceptions, event herbicides can cause harm to the environment. In fact, weed

killers can be especially problematic because they are used in relatively large volumes. The best

way to reduce pesticide contamination (and the harm it causes) in our environment is for all of us

to do our part to use safer, non-chemical pest control (including weed control) methods.

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The exercise of analysing the range and nature of benefits arising from pesticide use has

been a mixture of delving, dreaming and distillation. There have been blind alleys, but also positive

surprises. The general picture is as we suspected; there is publicity, ideological kudos and scientific

opportunity associated with ‘knocking’ pesticides, while praising them brings accusations of

vested interests. This is reflected in the imbalance in the number of published scientific papers,

reports, newspaper articles and websites against and for pesticides. The colour coding for types of

benefit, economic, social or environmental, reveals the fact that at community level, most of the

benefits are social, with some compelling economic benefits. At national level, the benefits are

principally economic, with some social benefits and one or two issues of environmental benefits.

It is only at global level that the environmental benefits really come into play.

There is a need to convey the message that prevention of adverse health effects and promotion of

health are profitable investments for employers and employees as a support to a sustainable

development of economics. To sum up, based on our limited knowledge of direct and/or inferential

information, the domain of pesticides illustrates a certain ambiguity in situations in which people

are undergoing life-long exposure. There is thus every reason to develop health education packages

based on knowledge, aptitude and practices and to disseminate them within the community in order

to minimise human exposure to pesticides.

Environmental impact of Pesticides

The environmental impact of pesticides consists of the effects of pesticides on non-target species.

Over 98% of sprayed insecticides and 95% of herbicides reach a destination other than their target

species, because they are sprayed or spread across entire agricultural fields. Runoff can carry

pesticides into aquatic environments while wind can carry them to other fields, grazing areas,

human settlements and undeveloped areas, potentially affecting other species. Other problems

emerge from poor production, transport and storage practices. Over time, repeated application

increases pest resistance, while its effects on other species can facilitate the pest’s resurgence.

Each pesticide or pesticide class comes with a specific set of environmental concerns. Such

undesirable effects have led many pesticides to be banned, while regulations have limited and/or

reduced the use of others. Over time, pesticides have generally become less persistent and more

species-specific, reducing their environmental footprint. In addition the amounts of pesticides

applied per hectare have declined, in some cases by 99%. However, the global spread of pesticide

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use, including the use of older/obsolete pesticides that have been banned in some jurisdictions, has

increased overall.

Agriculture and the environment

The arrival of humans in an area, to live or to conduct agriculture, necessarily has environmental

impacts. These range from simple crowding out of wild plants in favor of more desirable cultivars

to larger scale impacts such as reducing biodiversity by reducing food availability of native species,

which can propagate across food chains. The use of agricultural chemicals such as fertilizer and

pesticides magnify those impacts. While advances in agrochemistry have reduced those impacts,

for example by the replacement of long-lived chemicals with those that reliably degrade, even in

the best case they remain substantial. These effects are magnified by the use of older chemistries

and poor management practices.

History

While concern ecotoxicology began with acute poisoning events in the late 19th century; public

concern over the undesirable environmental effects of chemicals arose in the early 1960s with the

publication of Rachel Carson’s book, Silent Spring. Shortly thereafter, DDT, originally used to

combat malaria, and its metabolites were shown to cause population-level effects in raptorial birds.

Initial studies in industrialized countries focused on acute mortality effects mostly involving birds

or fish.

Data on pesticide usage remain scattered and/or not publicly available. The common practice of

incident registration is inadequate for understanding the entirety of effects.

Since 1990, research interest has shifted from documenting incidents and quantifying chemical

exposure to studies aimed at linking laboratory, mesocosm and field experiments. The proportion

of effect-related publications has increased. Animal studies mostly focus on fish, insects, birds,

amphibians and arachnids.

Since 1993, the United States and the European Union have updated pesticide risk assessments,

ending the use of acutely toxic organophosphate and carbamate insecticides. Newer pesticides aim

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at efficiency in target and minimum side effects in nontarget organism. The phylogenetic proximity

of beneficial and pest species complicates the project.

One of the major challenges is to link the results from cellular studies through many levels of

increasing complexity to ecosystem.