Insect Ecology and Integrated Pest Management –ENTO-231 (2+1) – Revised Syllabus

Dr. Cherukuri Sreenivasa Rao

Associate Professor & Head, Department of Entomology, Agricultural College, JAGTIAL

Lecture: 18
Chemical control- importance and ideal properties of insecticides, classification of insecticides on
the basis of origin, mode of entry, mode of action and toxicity-toxicity evaluation of insecticides,
acute and chronic toxicities, oral and dermal toxicities, LC50 (lethal concentration), LD50 (leathal
dose), ED50 (effective dose), LT50 (lethal time), KD50 (knockdown dose), KT50 (knockdown time)-
bioassay methods.

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

destroying, repelling or mitigating any pest.
• A pesticide may be a chemical substance, biological agent (such as a virus or
bacterium), antimicrobial, disinfectant or device used against any pest.
• Pests include insects, plant pathogens, weeds, molluscs, birds, mammals, fish,
nematodes (roundworms), and microbes that destroy property, spread disease or are a
vector for disease or cause a nuisance. Although there are benefits to the use of
pesticides, there are also drawbacks, such as potential toxicity to humans and other
animals.
• FAO has defined the term of pesticide 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.

Classification of Pesticides based on type of pest

Insecticides Substances that prevent, destroy, kill insects

Fungicides Substances that prevent destroy or inhibit the growth of fungi in crops.
Herbicides Substances used for preventing or inhibiting growth of plants or for killing weeds.
Rodenticides Substances that inhibit, destroy or kill rodents
Nematicides Substances that prevent, destroy, repel or inhibit the nematodes
Chemosterilants Substances that sterilize the insect pests.
Molluscicides Substances that prevent, repel, destroy or inhibit the growth of Mollusca.
Plant growth Substances that cause the acceleration or retardation of the rate of growth or rate
regulators of maturation of plants.
Defoliants Substances that cause the plant leave to die and fall away.
Desiccants Substances that cause to drain moisture out of the plants causing them to dry.
Attractants Substances that attract the insect pests
Repellents Substances that repel the insect pest from a treated plant.

Importance of Chemical control:

• Conventional insecticides are among the most popular chemical control agents because
they are readily available, rapid acting, and highly reliable.
• A single application may control several different pest species and usually forms a
persistent residue that continues to kill insects for hours or even days after application.
• Because of their convenience and effectiveness, insecticides quickly became standard
practice for pest control during the 1960's and 1970's.
• Pesticides are used both in agriculture and as vector-control agents in public-health
programmes.
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 Insect Ecology and Integrated Pest Management –ENTO-231 (2+1) – Revised Syllabus
Dr. Cherukuri Sreenivasa Rao
Associate Professor & Head, Department of Entomology, Agricultural College, JAGTIAL

• In the absence of agro-chemicals, crop production has become an impossible

proposition, because it is estimated that India approximately loses 18% of the crop
yield valued at Rupees 90,000 crores due to pest attack each year.
• The protection of humans from vectors such as mosquitoes is possible only due to
pesticides.
• Significant amounts are also used in forestry and livestock protection.
• Although there are benefits to the use of pesticides, there are also drawbacks, such as
potential toxicity to humans and other animals. Overuse, misuse, and abuse of these
chemicals have led to widespread criticism of chemical control and, in a few cases,
resulted in long-term environmental consequences.
• The effectiveness of an insecticide usually depends on when and where the pest
encounters it.

Properties of an ideal insecticide:

1. Should be freely available in market in different formulations
2. Should be selective
3. Should be toxic for the target pest
4. Should have quick knock down effect on target pest
5. Should have high tolerance limits
6. Should be safe to humans
7. Should be safe to fish
8. Should be safe to bees and pollinators
9. Should be safe to natural enemies
10. Should be safe to non-target organisms
11. Should have wide range of compatibility with other chemicals
12. Should be cheaper
13. Should be free from offensive smell/odor
14. Should be stable on application
15. Should be non-residual, preferably bio-degradable
16. Should work in multiple mode of action
17. Should not be phytotoxic
18. Should be easy to use
19. Should not allow the insect to develop resistance fast
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 Insect Ecology and Integrated Pest Management –ENTO-231 (2+1) – Revised Syllabus
Dr. Cherukuri Sreenivasa Rao
Associate Professor & Head, Department of Entomology, Agricultural College, JAGTIAL

Classification on
Insecticides

Based on Origin & Source of Supply

Inorganic Insecticides
Organic Insecticides
Natural
Animal Origin
Plant Orogin
Synthetic
Hydrocarbon oils
Mode of Entry
Contact Insecticides
Stomach Poisons
Fumigants
Systemic Insecticides
Mode of Action
Phyiscal Poisons
Protoplasmic Poisons
Respiratory Poisons
Nerve Poisons
Growth regulators

Classification based on origin and source of supply:

1) Inorganic insecticides:
• Inorganic insecticides are often derived from mineral origin, elemental sulphur, heavy
metals and arsenic and flourine-containing compounds.
• Types of inorganic insecticides include elemental sulphur as fungicide and acaricide,
boric acid, diatomaceous earth, silica gel, arsenic and fluorine-containing compounds
as insecticides, and zinc phosphide as rodenticide.
• Considered highly effective against insects.
2) Organic insecticides:
• It is possible to make an organic insecticide from a number of different substances/
sources.
• It is also possible to buy them commercially.
• It should be noted that many organic insecticides are meant to only target a certain
species or a few different species.
• Therefore, those who have a variety of insect species they wish to treat will likely need
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more than one type of organic insecticide.

• Organic insecticides can be classified based on the origin or source.
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 Insect Ecology and Integrated Pest Management –ENTO-231 (2+1) – Revised Syllabus
Dr. Cherukuri Sreenivasa Rao
Associate Professor & Head, Department of Entomology, Agricultural College, JAGTIAL

2a) Organic insecticides of animal origin:

• Nereistoxin isolated from marine annelid Lumbrineris heteropoda (cartap is the
synthetic analogue of neriestoxin, available in the market at Padan, Celedon, popular
against rice pests).
• Other examples are fish oil resin soap and whale oil against scale insects.
• The naturally occurring avermectins (abamectin, emamectin) are macrocyclic lactone
derivatives with potent anthelmintic and insecticidal properties, generated as
fermentation products by Streptomyces avermitilis, a soil actinomycete.
2b) Organic insecticides of Plant origin or Botanical insecticides:
Chemical Source
Azadirachtin All parts of Neem
Allicin Garlic
Pyrethrins Chrysanthemum
Jasmolins Flowers of Jasmine
Rotenone Stems of roots of several plants
Nicotine Tobacco
Ryania Stems of Ryania speciosa
Matrine, Oxymatrine Roots of Sophora
2c) Organic insecticides of Synthetic origin:
• All these are synthesized in chemical labs.
• Most of the available insecticides fall in this group.
Group Examples
Organochlorines DDT, BHC
Cyclodienes Endosulfan, aldrin, dieldrin, endrin, Heptachlor
Organo Phosphates Acephate, Phorate, Monocrotophos, Phosphamidon,
Parathion, Malathion, Nuvan, chlorpyriphos, methyl
demeton, ethion, phosalone, quinalphos, fenitrothion,
fenthion, phenamiphos, etc.
Organo Carbamates Carbaryl, carbofuran, carbosulfan, aldicarb, methomyl etc.
Synthetic pyrethroids Allethrin, permethrin, fenvalerate, fluvalinate,
deltemethrin, cyhalothrin, bifenthrin, cyfluthrin,
alphamethrin, cypermethrin etc.
Other groups Neonicotinoids (imidacloprid, thiacloprid, thiamethaxam,
acetamiprid, nitenpyram), pyrazoles (fipronil, ethiprole),
pyrroles (chlorfenapyr), Spiromesifens etc.
Growth regulators Chitin synthesis inhibitors (buprofezin, diflubenzuron,
triflubenzuron, lufenuron, novaluron), Juvenile hormone
mimics (methoprene, kinoprene, fenoxycarb)
2d) Hydrocarbon oils etc.

Classification based on Mode of Entry:

• Insecticides can be classified according to their mode of entry into the insect 1) contact
poisons, 2) stomach poison and 3) fumigants.
• However, many insecticides belong to more than one category when grouped in this
way, limiting its usefulness.
Contact poisons:
• These insecticides kill insects when they are hit by or come in contact with the poison.
• A chemical that kills insects through skin contact; it does not have to be ingested.
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 Insect Ecology and Integrated Pest Management –ENTO-231 (2+1) – Revised Syllabus
Dr. Cherukuri Sreenivasa Rao
Associate Professor & Head, Department of Entomology, Agricultural College, JAGTIAL

• Most insecticides are absorbed directly through an insect's exoskeleton. These

compounds are known as contact poisons because they are effective "on contact".
• These are capable of gaining entry into insect body either through spiracles or through
trachea or through cuticle. The insect does not have to consume it.
• Eg: DDT, BHC, methyl parathion, malathion, nicotine, pyrethroids, are contact
insecticides.
• Now highly refined oil sprays are excellent contact egg poisons.
• Most soft-bodied insects like aphids, mites, scales, and white flies are vulnerable to
contact insecticides.
• They must be sprayed directly on the insects to be effective; any insects that evade the
spray survive. These are sprayed or dusted on the insect's body.
Stomach poisons:
• These are applied on the surface of plants, fabrics, and wood, or are added to baits.
• The insecticide is eaten, along with the food material, by insects that chew, such as
ants, caterpillars, and grasshoppers, and must be taken into the alimentary canal to be
effective.
• The chemicals sprayed or dusted on plants, as the insect eats the plant, it is poisoned
through stomach.
• The best examples are Bacillus thuringiensis (Bt), rotenone, methyl parathion,
pyrethroids are stomach poisons.
Fumigants:
• Fumigants are insecticidal gases, actually contact poisons applied in gaseous form.
• They are released in the vapor state (as gases) and enter the insect's body through its
tracheal system.
• Fumigants are most effective when they are used in an enclosed area such as a
greenhouse, a warehouse, or a grain bin.
• The gases enter into system through breathing pores and kills the system either
hampering nervous system or respiratory system.
• Insects that are hard to reach by sprays are killed when they breathe or are exposed to
the gas.
• Fumigants are used by professional exterminators to kill the cockroaches, bedbugs and
to kill insect pests in grain bins/storage.
• The soil may be fumigated to destroy grubs that attack roots.
• Fumigants are highly toxic, and hence fumigation is a hazardous operation.
• Commonly used fumigants are
a) Aluminium phosphide (marketed as celphos tablets of 3g each used against storage pests @
4 tablets/ton. Each tablet produce 1g phosphine gas; ½ tablet is recommended for red palm
weevil in coconut).
b) Carbon disulphide (CS2) @ 8-20lbs/1000ft3
c) EDB (Ethylene Dibromide) @ 1lb/1000ft3
d) SO2 fumes by burning sulphur
e) DD mixture-a soil fumigant against nematodes
f) CCl4 (Carbon tetra chloride)
g) Napthalene
h) ED (Ethylene dibormide)
i) EDCT (Ethylene dibromide + carbon tetra chloride)
Systemic Insecticides
• These are absorbed by plant tissues, so that when insects feed on the sap they are
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poisoned.
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 Insect Ecology and Integrated Pest Management –ENTO-231 (2+1) – Revised Syllabus
Dr. Cherukuri Sreenivasa Rao
Associate Professor & Head, Department of Entomology, Agricultural College, JAGTIAL

• Systemic insecticides are a special type of "stomach poison" and are absorbed by the
tissues of a plant without any effects on the plant.
• Insect pests ingest the insecticide when they feed on the treated plant.
• Systemic insecticides can be incorporated into the soil around ornamentals or bedding
plants. The insecticides are absorbed by the roots and transferred to leaves, stems, and
flowers.
• Examples: Acephate systemic insecticide (Orthene, starthene) is an example of
systemic insecticide product used to protect lawns, shrubs, and ornamentals from
various insect pests such as white grubs, japanese beetles, and molecrickets. Other
examples include Phorate, imidacloprid, carbofuran, monocrotopohs etc., Merit
systemic granules give long term control of lawn pests such as white grubs, but the
timing of insecticide application is crucial to be effective in killing grubs that feed on
grass.
• Most of these insecticides applied as granules, but many also applied as foliar sprays.

Classification based on Mode of Action:

Insecticides can be classified according to their mode of action:
Physical poisons:
• These kill insects by exerting a physical effect.
• Eg: Heavy oils, tar oils, which causes death by asphyxiation (scales insects) by
exclusion of air.
• Others like inert dusts effect loss of moisture by their abrasiveness as in aliminium
oxide or absorb moisture from the body as does by drie-die, kaolin clay, charcoal.
• In the 1930's and 1940's it was common practice for a farmer to hitch a bale of chicken
wire behind a team of horses (or a tractor) and drive it through his fields to stir up dust.
When the dust settled, it gave the insect pests an abrasive coating that gradually
rubbed away their cuticular waxes and caused them to die from dehydration. Although
this is certainly not the most reliable method of pest control, it does explain why crops
planted near the edge of a well-travelled dirt road often have less insect injury than
those on the opposite side of the same field.
• A material called drie-die marketed in USA consists of highly porous, finely divided
silica gel which when applied to insects in field, abrades insect cuticle, resulting in
loss of moisture and finally death of insect. This material is mainly used in stored
grains.
• Similarly, kaolinic-clay after successive activation with acid and heat can be mixed
with stored grain. This clay material absorbs the lipid layer of insect in the
exoskeleton, thus allowing to loss moisture, leading to death of insect.
• Generally, at household level, to store the rice and other pulses, boric powder is added,
to avoid the pest infestation, which causes damage to insect exoskeleton.
Protoplasmic poisons:
• Cause precipitation of protein, especially destruction of protoplasm of midgut
epithelium.
• Eg: Arsenical and Flourides are the general protoplasmic poisons.
Nerve poisons:
• Chemicals which block different enzymes involved in nerve impulse transmission.
• Organo carbamates and organo phosphates block/effect acetyl choline esterase (AChE)
enzyme which is involved in synaptic transmission, pyrethrin, indoxacarb effect
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sodium channels in axonic transmission, organochlorines are agonists of GABA gated

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 Insect Ecology and Integrated Pest Management –ENTO-231 (2+1) – Revised Syllabus
Dr. Cherukuri Sreenivasa Rao
Associate Professor & Head, Department of Entomology, Agricultural College, JAGTIAL

chloride channels, avermectins are chloride channel activators in axon, neonicotinoids

agonize the nicotinic Ach receptors in synaptic transmission.
Respiratory poisons:
• Block cellular respiration process by reacting with enzymes involved in glycolysis,
krebs cycle and ETS.
• Eg: HCN, CO inhibit cytochrome oxidase. Rotenone and Phosphine inhibit ETS,
General poisons:
• Chemicals which mimic hormones, or inhibit the chitin synthesis in insects.
• Hormone mimics (JH mimic-methoprene, kinoprene, fenoxycarb), chitin synthesis
inhibitors (benzyl ureas-diflubenzuron, novoluron) etc.

Classification based on Toxicity:

• Insecticides can be classified according to their mode of action: Insecticide toxicity is
generally measured using LD50– the exposure level that causes 50% of the population
exposed to die.

Classification of Medium lethal dose Medium lethal dose Colour of

the Insecticides by the oral route by the dermal route identification Symbols and word in upper
acute toxicity dermal toxicity band on the triangle
LD 50 mg/kg.. body LD 50 mg/kg. label in lower
weight of test Body weight of test triangle
animals animals
Extremely toxic 1-50 1-200 Bright red POISON in red
skull and cross-bones
Highly toxic 51-500 201-2000 Bright yellow POISON in red
Moderately toxic 501-5000 2001-20000 Bright blue DANGER
Slightly toxic More than 5000 More than 20000 Bright green CAUTION

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