PESTICIDES
Definition of Pesticide
A pesticide is a substance or any number of substances mixed together and used to repel or kill a pest. A pesticide may be a chemical substance or a biological agent. A pest, which is any organism that destroys property, spread or is a vector for disease of crops and livestock or cause a nuisance, includes insects, plant pathogens, weeds, birds, mammals, nematodes, and microbes.
The Food and Agriculture Organisation of the United Nations (FAO) defines pesticide in a wide context as:any substance or mixture of substances intended for preventing, destroying or controlling any pest, including vectors of human or animal disease, unwanted species of plants or animals causing harm during or otherwise interfering with the production, processing, storage, transport or marketing of food, agricultural commodities, wood and wood products or animal feedstuffs, or substances which may be administered to animals for the control of insects, arachnids or other pests in or on their bodies.
The term includes substances intended for use as a plant growth regulator, defoliant, desiccant or agent for thinning fruit or preventing the premature fall of fruit, and substances applied to crops either before or after harvest to protect the commodity from deterioration during storage and transport.
Although there are benefits to the use of pesticides, such as reducing diseases and increasing food supplies throughout the world, there are also drawbacks, such as environmental pollution and potential toxicity to humans and other animals.
NAMING OF PESTICIDES
Pesticides are named by trade name of the pesticide or by common name of active ingredient of the pesticide, or by chemical name of active ingredient of the pesticide.
1. The trade name (or brand name) of pesticide is the brand name given to the pesticide by the manufacturer. It is the prominent name on the front of a pesticide label. The first letter of a trade name is normally upper case.
2. The common name of active ingredient is the common name of the chemical active ingredient that controls the pest. It is written beside the guarantee on a pesticide label. The first letter of an active ingredient name is normally lower case.
3. The chemical name of active ingredient is the name of the chemical structure of the active ingredient and is used by scientists.
Examples:
Roundup: an herbicide
Roundup = trade name of a herbicide glyphosate = active ingredient (common name) in Roundup N-(phosphonomethyl) glycine = chemical name of the active ingredient in Roundup
Quadris or Heritage: a fungicide
Quadris or Heritage = trade name
azoxystrobin = common name
Methyl (E)-2-{2-[6-(2-cyanophenoxy) pyrimidin-4-yloxy] phenyl}-3-methoxyacrylate = chemical name
Assail: an insecticide
Assail = trade name
Acetamiprid = common name
(E) - N-[(6-Toxic-3-pyridinyl) methyl] - N-cyano-N-methylethanimidamide = chemical name.
Types/Groups/Classifications of Pesticides
Pesticides are grouped or classified according to
- The pests they control.
- Their chemical structure.
- How/when they work and
- Their mode of action (or site of action).
Classification by Target Pest Species
Most pesticides may be classified according to the pests they kill. The suffix -cide of pesticide means to kill. The following types of pesticides are used to kill specific kinds of pests:
PESTICIDE TARGET PEST
Algicide e.g chlorohexidine -- Algae
Avicide e.g 3-chloro-p-toluidine hydrochloride -- Birds
Bactericide e.g Triclosan -- Bacteria
Fungicide e.g copper sulfate pentahydrate (CuSO4) -- Fungi
Herbicide e.g 2,4 dichlorophenoxyl acetic acid -- Weeds
Insecticide e.g parathion -- Insects
Miticide e.g Acramite -- Mites
Nematicide e.g methyl bromide -- Nematodes
Rodenticide e.g Bromadiolone -- Rodents
Classification by Chemical Structure
Pesticides can be categorized according to chemical structure. Pesticides with similar structures have similar characteristics and usually have a similar mode of action. Pesticide active ingredients are either inorganic or organic. Inorganic pesticides do not contain carbon and are usually derived from mineral ores extracted from the earth.
Examples of inorganic pesticides include copper sulphate, ferrous sulphate, copper and sulphur. Organic pesticides contain carbon in their chemical structure.
Most organic compounds are created from various compounds, but a few are extracted from plant material and are called 'botanicals'. Examples of organic pesticides include: captan, pyrethrin, and glyphosate. Organic pesticides with similar structures are grouped into families of chemicals.
Examples of inorganic pesticides include copper sulphate, ferrous sulphate, copper and sulphur. Organic pesticides contain carbon in their chemical structure.
Most organic compounds are created from various compounds, but a few are extracted from plant material and are called 'botanicals'. Examples of organic pesticides include: captan, pyrethrin, and glyphosate. Organic pesticides with similar structures are grouped into families of chemicals.
Grouped by How or When They Work
Pesticides can also be categorized according to how or when they work:
1. Contact pesticides control a pest as a result of direct contact. Insects are killed when sprayed directly or when they crawl across surfaces treated with a contact insecticide. Weed is killed when enough surface area is covered with a contact herbicide. An example is the herbicide Glufosinate ammonium.
2. Systemic pesticides are pesticides which are absorbed by plants or animals and move to untreated tissues. Systemic or translocated herbicides may kill weeds with only partial spraying. Some pesticides have a specific direction of movement within the plant, either up or down. For example some insecticides only move upwards in plants; and so if applied to the root zone, it will travel throughout the plant, but if applied to the leaves it will not move throughout the plant. Such an insecticide may not effectively control pest when applied to the leaves of a plant. An example is the insecticide Acephate.
3. Foliar pesticides are applied to plant leaves, stems and branches. They may be either a contact pesticide or a systemic pesticide. An example is the systemic herbicide Triclopyr.
4. Soil-applied pesticides are applied to the soil. Some are taken up by roots and translocated inside the plant. Others kill weed seedlings by contact with young shoots or leaves as they grow through the soil. An example is the herbicide Atrazine.
5. Fumigants are applied as toxic gas or as a solid or liquid which forms a toxic gas. The gas will penetrate cracks and crevices of structures or soil or the spaces between products stored in containers. An example is methyl bromide.
6. Selective pesticides will only control certain pests. An example is the fungicide thienylurea derivatives.
7. Non-selective pesticides will control a wide range of pests. An example is Roundup
8. Residual pesticides do not break down quickly and may control pests for a long time after application (i.e. several weeks or a year). An example is the insecticide diazinon.
9. Non-residual pesticides are quickly made inactive after application and do not affect future crops. An example is the insecticide pyrethrin.
Grouped by Mode of Action (Site of Action)
Types of Pesticide Formulations
When a pesticide active ingredient (a.i.) is manufactured, it is not in a usable form. The a.i. may not mix well with water or may be unstable. Therefore the a.i. is mixed with other compounds to improve its effectiveness, safety, handling and storage. The other compounds, often called adjuvants, can include solvents, mineral clays, stickers or wetting agents. This mixture of a.i. and inert (inactive) ingredients is called a pesticide formulation. Thus pesticide formulations may come as solid, liquid or gas.
STRUCTURES OF SOME PESTICIDES
Structure of 2,3,4,5,6-pentachlorophenol (PCP) (Used as a fungicide, insecticide, mollusicide, and algicide. But it is mainly used as an industrial and commercial wood preservative for wooden fence posts, boats, furniture and log homes).
2,4-Dichlorophenoxyacetic acid (2,4-D) (Used as an herbicide for the control of weed)
Parathion (Used as an insecticide)
Dichlorodiphenyl Trichloroethane (DDT) (A banned insecticide because of its harmful environmental effects.
INSECTICIDES
Insecticides are being used worldwide to increase food production and eradicate insect-borne diseases. Insecticides have therefore helped to minimise food shortages and starvation throughout the world; and also to minimise the incidence of, say malaria, especially in sub-Saharan Africa where it is mostly endemic.
Besides these obvious advantages the use of insecticides is causing a lot of problems in the environment, such as disruption of the balance in the ecosystem, be it aquatic or terrestrial, pollution, diseases in humans when the flesh of polluted organisms are consumed, etc.
Many insecticides act by interfering with the electron transport chain, e.g, rotenone inhibits an enzyme of the electron transport chain. Many others are closely related to the action of nerve gases by killing insects through inhibiting the enzyme acetylcholinesterase, e.g, parathion.
Some insecticides are not active until they have undergone bioconversion, i.e., metabolism in the insect tissues. An example of bioconversion or bioactivation is found in parathion and malathion. In this bioconversion a relatively non-toxic P=S group is converted to a highly toxic P=O group in the tissues of the insect.
Insecticides that resemble nerve gases in their action do not persist in the soil for a long time because they are quickly hydrolyzed to harmless compounds in the presence of moisture in the soil. In contrast, the chlorinated hydrocarbons which include DDT, benzene hexachloride (BHC), dieldrin, aldrin, chlordane and lindane etc, are difficult to degrade in the soil and so tend to accumulate in animal tissues. Their toxic effects are spread to other organisms through the food chain.
HERBICIDES
Herbicides are weedkillers used to boost agricultural yield. Some are used as defoliants by the military, etc. However, the use of herbicides, like insecticides, has negative environmental consequences. They have the potential, especially those recalcitrant to degradation, to disrupt the ecosystem, and also to be carcinogenic (cancer-inducing) in humans when they enter the food chain.
Many herbicides act by inhibiting steps in the photosynthetic process.
Herbicides are highly toxic to plants, but completely non-toxic to animals which do not have photosynthetic systems.
Herbicides are selective, i.e., they kill the weeds but not the crops. This is achieved because of the differences in the metabolic systems of different plants, such as monocotyledons versus dicotyledons, c3 plants versus c4 plants, etc.
An example of a very successful herbicide is 2,4-dichlorophenoxyacetic acid. It inhibits certain glycolytic enzymes, uncouples oxidative phosphorylation and stimulates protein biosynthesis by mimicking the action of the naturally-occurring plant hormone indolacetic acid, resulting in unregulated growth and rapid death.
FACTORS IN THE DESIGN AND USE OF PESTICIDES
Factors that must be borne in mind when designing and using pesticides include the following:
- It should be specific.
- It should be readily biodegradable.
- It should be inexpensive to produce and use.
- Pesticides should be used in an integrated manner by incorporating the use of other approaches in the control of pests, such as the biological methods.
BIODEGRADATION OF PESTICIDES
Although insects, earthworms and plants play roles in the biodegradation of pesticides, miroorganisms are by far the most important.
The rhizosphere of the soil, which is the layer of soil in which plant roots are very active, is the most important part of soil as far as biodegradation of pesticides is concerned. Plants and microorganisms work in a mutually beneficial way in the rhizosphere to bring about degradation of pesticides.
Besides providing a good biological surface for the growth of microorganisms, carbohydrates, amino acids and root-growth-lubricant mucigel secreted from the roots provide nutrients for the growth of microorganisms. Plants benefit from the activities of microorganisms.
The ability of a microorganism to degrade pesticide depends on whether it is able to synthesize the appropriate enzymes required. Most times degradation is brought about as a result of two or more microorganisms acting sequentially. Examples of pesticide-decomposing microorganisms are Pseudomonas, Bacillus, Flavobacterium, Achromobacter, Nocardia and Aspergillus, etc.
The following classes of pesticides are presented in order of increasing recalcitrance, i.e., in order of increasing difficulty or resistance to biodegradation:
- aliphatic acids
- organophosphates, e.g, parathion
- long-chain phenoxyaliphatic acids, e.g, 2-(4-chlorophenoxycaproate) (C6)
- short-chain phenoxyaliphatic acids, e.g, 2-chlorophenoxyacetate (C2)
- monosubstituted phenoxyaliphatic acids
- disubstituted phenoxyaliphatic acids
- trisubstituted phenoxyaliphatic acids
- dinitrobenzene
- chlorinated hydrocarbons, e.g, DDT
There are many mechanisms of microbial transformations of chemicals involved in the biodegradation of pesticides. These are summarised as follows:
REACTIONS EXAMPLES
Dehalogenation
RCH2C1 Õ RCH2OH ------ Propachlor (C,S)
ArC1 Õ ArCH ------ Nitrofen (S)
ARC1 Õ ArH ------ Pentachlorophenol (S,C)
Ar2CHCC13 Õ Ar2CHCHC12 ---- DDT (C,S)
Ar2CHCC13 Õ Ar2C=CC12 ---- DDT (S,C)
Deamination
ArNH2 Õ ArOH ---- Fluchloralin (S)
Decarboxylation
ArCOOH Õ Ar4 ---- Biofenox (S)
RCH(CH3)COOH Õ RCH2CH2 ---- Dichlorfop-methyl (S)
ArN(R)COOH Õ ArN(R)H ---- DDD(S)
RCH3 Õ RCH2OH Õ RCHO Õ RCOOH ---- Bromacil (S)
Hydroxylation and ketone formation
ArH Õ ArOH ---- Benthiocarb (S)
R(R’)CHR" Õ R(R’)CHOH(R") ---- Bux insecticide (S)
R(R’)(R")CCH3 Õ R(R’)(R")CCH2OH --- Denmert (S)
B-oxidation
ArO(CH2)nCH2CH2COOH Õ
ArO(CH2)nCOOH --- Dichlorphenoxyacetic acids(S,C)
R(R’)NR" Õ R(R’)N(O)R" --- Trideomorph (S)
Sulfur Oxidation
RSR’ Õ RS(0)R’ Õ RS(O2)R’ ---- Aldicarb (S,C)
=S to =0 (AlkO)2P(S)R Õ
(AlkO)2P(O)R ---- Parathion (S,C)
Sulfoxide reduction
RS(O)R’ Õ RSR’ ---- Phorate (S)
Double bond reduction
Ar2C=CH2 Õ Ar2CHCH3 ---- DDT (C)
Hydration of double bond
Ar2C=CH2 Õ Ar2CHCH2OH ---- DDT (C)
Nitro Metabolism
RNO2 Õ ROH ---- Nitrofen (S)
Oxime metabolism
RCH=NOH Õ RC=N ---- Aldicarb (S,C)
Note: S = reaction observed in soils, C = reaction observed in cultures.
There are also mechanisms which are not strictly degradative; they form conjugates between the pesticide and other chemicals or chemical groups and so modify the chemical. These may be more or less harmful than the original compounds. These reactions include methylation, ether formation, N-acylation (ArNH2 Õ ArNHC(O)H), nitration, N-nitrosation [(Alk)2NH + NO2 --- (Alk)2NNO], dimerization, and nitrogen heterocycle formation.
References
- Adams, Robert W. 1995. Handbook for Pesticide Applicators and Dispensers. Fifth Edition, Victoria, B.C. BC Environment.
- Bhatia S.C (2006), Environmental Chemistry, CBS Publishers and Distributors, India, pg 288 – 305.
- Wood E.J, Pickering W.R (1982), Introducing Biochemistry, John Murray Ltd, London, pg 236 – 239.
ASSIGNMENT
- Compare and contrast the common definition of pesticide with the FAO definition of pesticide.
- Identify the trade name, the common name and the chemical name of the following pesticides: a) N-(phosphonomethyl) glycine, Roundup, glyphosate b) Azoxystrobin, Methyl (E)-2-{2-[6-(2-cyanophenoxy)pyrimidin-4-yloxy]phenyl}-3- methoxyacrylate, Heritage c) Assail, (E) - N-[(6-Toxic-3-pyridinyl) methyl] - N-cyano-N-methylethanimidamide, Acetamiprid.
- Give one example each of the following pesticides: d) algicide e) fungicide f) insecticide g) herbicide h) bactericide
- Giving examples differentiate between contact pesticides and systemic pesticides.
- Giving examples differentiate between residual pesticides and non-residual pesticides.
- Identify the benefits derivable from the use of adjuvants in pesticide formulation.
- Draw the structures of any five pesticides known to you.
- Briefly discuss “Insecticides”.
- Briefly discuss “Herbicides”.
- List and explain the factors that are crucial in the design and use of pesticides.
- Explain why the rhizosphere is the most important part of the soil as far as biodegradation of pesticides is concerned.
- What do you understand by the term “Recalcitrance” of pesticides?
- Outline a few examples of the degradative and conjugation mechanisms for the detoxification of pesticides in the environment.
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