Pesticides
Types Of Pesticides
With the Green Revolution food supply has become triple compared with the past, yet it is not enough to control the population’s food demands.
Increased yields have partly been due to the use of improved crop varieties but mainly due to the use of
better management practices and the use of agrochemicals such as fertilizers and pesticides.
Pesticides are classified depending upon the types of pests killed or controlled.
These are fungicides, weedicides/herbicides,
nematicides, rodenticides, insecticides, and biopesticides.
Fungicides
These substances eliminate the fungal infection on crops and destroy fungal pathogens. Inorganic
fungicides include Bordeaux mixture, Burgandy Mixture, sulfur, mercuric chloride, etc.
Organic fungicides include
Mercury Compounds (e.g., agrosan, tillex), Dithane M-22, Dithane Z-78, Dithane S-21, oxanthiins (e.g., vitavax), benzimidazole derivatives (e.g., benlate). Thiram and Ziram is fungicide but toxic to aquatic
zooplanktons. Phytochemical extraction, such as Neem oil containing Azadirachtin and Nimbin, is antifungal
properties. Fentin is another example of a fungicide.
Weedicides/Herbicides
Herbicides and weedicides kill unwanted plants or weeds on agricultural land. Depending upon the
mode of action, selective and nonselective herbicides, contact herbicides, translocated herbicides, foliage
applied, and soil-applied herbicides are used.
Nematicides
They kill or repel nematodes, e.g., Aldirab is an acetylcholine esterase inhibitor used to kill the nematodes, which
infect the tobacco production in farming.
A nematode Meloidogyne incognita infects the roots of tobacco plants
and causes a significant reduction in yield. For biological control agents (BCA), purpureocillium platinum is helpful. it infests
the Meloidogyne incognita.
One strain of P. lilacinum has been shown to produce proteases and a chitinase,
enzymes that could weaken a nematode egg shell to enable a narrow infection peg to push through.
Paecilomyces, nematophagous carnivorous fungi, can help control nematode attacks. Methyl bromide (MB),
ethylene dibromide (EDB), and chloropicrin are nematicides. Soil steam sterilization (SSS) or soil steaming
technique can disinfect the soil from nematode/pathogen by enzyme inactivation using heat treatment.
Rodenticides
It is commonly known as rat poison, e.g., Na+.
-fluoroacetate, warfarin, red squill, Zinc phosphide. Rodenticides have inhibitory effects on the Vitamin-K cycle in rodents and mammals, thus the death of pests. Vitamin D3, D2, and D cause
hypercalcemia in rodents. Strychnine, obtained from Strychnos nux-vomica tree or Semen nut tree, is a rodenticide
that causes asphyxia in rats, thus death. Chloralose is a chlorinated acetal derivative of glucose, used as rodenticide
and avicide. Combined with copper acetate, the arsenic trioxide forms Paris Green rodenticide,
also used as a blue colorant in fireworks.
Insecticides
Insecticides are stomach/alimentary canal poisons (poisonous on ingestion), contact poisons, or fumigants (inhaled)
to the insects. Natural Insecticides such as Azadirachta indica (Margosa/Neem), Peganum harmala, Chrysanthemum (pyrethrum), Derries (rotenone), and Boenighsausenia albiflora.
Azadirachta indica is helpful as a larvicide. Aquapac and Vectobac are larvicides.
The first commercial bioinsecticide is sporting developed in Germany.
Insects Growth Regulators (IGRs) are insecticides.
An example of IGR is azadirachtin, hydroplane, etc.
Synthetic insecticides can control insects present in Stored grains. Synthetic insecticides include organochlorines, carbamates, organophosphates, pyrethroids, and ethylene dibromide (EDB).
Chemical pesticides are the toxic chemicals used in killing pests. Based on chemical structure,
major pesticides are as follows: (i) Organochlorines, (ii) Organophosphates, (iii) Carbamates, (iv) Pyrethroids, and (v) Triazines.
(i) Organochlorines: These are chlorinated organic compounds. Organochlorines are
lipophilic and show much affinity for fatty tissue of animals. Organochlorines have very low bio-degradation, accumulate in the environment, and cause severe problems. Important examples of organochlorines are (a) DDT, (b)
BHC, (c) Aldrin, and (d) Endosulfan.
(a) DDT (Dichlorodiphenyltrichloroethane): DDT was first synthesized by a German chemist Othmar Zeidler in
Paul Muller discovered 1874 and its insecticidal value in 1939. DDT is the most famous pesticide globally and is a non-biodegradable pollutant. Spraying DDT on crops produces air, soil, and water pollution. In
In India, as a result of prolonged use of DDT, 13-31 ppm of DDT can be detected in the body fat of the people, the highest
in the world. DDT concentrates from water in the body and is magnified in more elevated food web members. DDT
tolerance level is 10ppm for a freshwater crustacean Daphnia, which means Daphnia will die beyond that
concentration. DDT has become ineffective for killing mosquitoes because of the development of adaptive
resistance. DDT does not inhibit cholinesterase activity and is relatively non-toxic to mammals, but in oil solution, it
is absorbed by the skin. Pesticide (DDT) is banned nowadays because DDT interacts with the food chain in our
ecosystem and causes severe damage and loss of biodiversity. For example, biomagnification,
or amplification, is the increasing concentration of a substance, such as a toxic chemical like DDT or mercury, in
the tissues of organisms at successively higher levels in a food chain. This happens because a poisonous substance accumulated by an organism cannot be metabolized or excreted and is thus passed on to the next higher trophic
level. In this manner, the concentration of DDT is increased at successive trophic levels; say, if it starts at 0.003 ppb
(parts per billion) in water, it can ultimately reach 25 ppm (parts per million) in fish-eating birds through
biomagnification. High concentrations of DDT disturb Ca2+
-metabolism in birds causes thinning of eggshells
and their premature breaking, eventually causing a decline in bird populations. Toxaphene, a cotton pesticide, is also
banned in the USA and causes serious health problems to the nervous system.
(b) Lindane: γ-hexachlorocyclohexane/Gammaxene/Lindane was 1
St synthesized by Michael Faraday in 1825, and
its insecticidal value was independently discovered by Dupire (1941) in France and Leicester (1942) in England. It
is the most common pesticide used in India and represents about 50% of the total volume of pesticides used in India. Lindane
can bioaccumulate in the food chain, thus more toxicant than DDT. Lindane is used in shampoos and lotion.
(c) Aldrin (Octalene): Aldrin is an insecticide named after German chemist Kurt Alder, applied to foundations of
buildings to prevent termites. It has been successfully used to control locusts and grasshoppers in Asian
countries. Aldrin, Dieldrin, and Endrin are very poisonous pesticides.
(d) Endosulfan (Thiodan) C9H6Cl6O2S: Endosulphan is a pesticide and is usefully used in agriculture in the
control of insect pests, including whiteflies aphids, leafhoppers, Colorado potato beetles, and cabbage worms. It is
also an endocrine disruptor and carcinogenic to humans.
(e) Mirex is an insecticide that kills fire ants in agricultural lands. It was banned in the USA because of
biomagnifications for turtles, coyotes, and other animals. It is a potent endocrine disruptor to animals, including
human beings.
(ii) Organophosphates: The insecticidal properties of organophosphates were discovered by Schrader.
Organophosphates are the pesticides most toxic to vertebrates. Organophosphates inhibit cholinesterase, an
enzyme essential for transmitting nerve impulses across a synapse. Malathion, parathion, and fenitrothion are the main
organophosphates used in Asian countries. Malathion is one of the two active ingredients in Flit, the second being
Pyrethrin is derived from Chrysanthemum cinerariifolium. Examples of pyrethroids are Allethrin, Cyclethrin and
Brethren are quick-acting broad-spectrum insecticides. Mosquito-repelling coils contain pyrethrin. Naled is
an insecticide used to control the spread of the Zika virus in the USA in 2015. Fashion insecticide is used to control
mosquitoes in India, but its manufacturing was banned in 2017 due to environmental impact.
(iii) Carbamates are carbonic acid derivatives and have an – OCON = group in the molecule.
Some commonly used carbamates are Carbofuran (Furadan), Propoxur (Baygon), and Aldicarb (Temik).
Derivatives of carbamates are also used as herbicides (phenyl carbamates, thiocarbamates) and Fungicides
dithiocarbamates. Carbamates are helpful in the control of nematodes and snails. The mode of action of carbamates is
quite similar to that of organophosphates. Methyl isocyanate gas caused the Bhopal gas tragedy in India on 3rd
Dec. 1984 is helpful as a raw material for synthesizing Carbaryl (Selvin).
(iv) Triazines : Triazines (Simazine, Atrazine, etc.) are a group of herbicides derived from urea.
Triazines are used for controlling weeds in tea, tobacco, and cotton.
Bordeaux Mixture: Millardet discovered (1882) the Bordeaux mixture in France. This mixture contains the solution of
40gm of copper sulfate and 40gm of calcium hydroxide in 5ltr of water. Bordeaux mixture is used primarily as a
fungicide; it was first used to control downy mildew of grape wine caused by a fungus, Plasmopara
viticola. Bordeaux mixture prevents potato blight and apple scab from Venturia inaequalis, an
ascomycetes fungi.
Algicide
It is a biocide used to eliminate the algal growth in water. Cupric sulfate or Bluestone is used as an algicide.
Diuron/DCMU (3-(3,4-chlorophenyl)-1,1-dimethylurea) is photosynthesis and electron transport chain
ETC-inhibitor is helpful as an algicide. Dichlorophen with toluene is helpful as an algicide, fungicide, and
homicide/helminthicide/nematocide.