Different insecticide group act on different target site and mechanism of their toxicity lies in differential actions on the target receptors/channels. To understand the mode of action of insecticides that target the insect nervous system, it is important to have a basic understanding of how the nervous system operates. In insects, the nervous system is composed of a series of highly specialized, interconnected cells, along which travel electrical charges called impulses. A nervous system is essential for the passage of information through the body by means of electrical signals. Impulses are driven by the movement of electrically charged sodium, potassium and chloride ions into and out of nerve cells. The uninterrupted transmission of impulses along this series of cells is required for a nervous system to function properly. In insects, prolonged or irreversible disruption of a normal-functioning nervous system will result in death.

1. Biochemical and Physiological target
sites of insecticides in insects
Presented by
Nikita Negi
Dept. of Agril. Entomology
IGKV, Raipur
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2. Insect Nervous system
1. Sensory neuron
2. Interneuron
3. Motor neuron
Passage of information (electrical signal)
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3. INSECT NERVOUS SYSTEM
Insect Neuron
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4. Mechanism of Impulse conduction
Axonic conduction
• Ionic composition (Na+ and K+) varies between inside and
outside of axon resulting in excitable conditions, which leads to
impulse conduction as electric response
Synaptic conduction
• Neurochemical transmitters are involved in the impulse
conduction through the synaptic gap. Neurotransmitters (Ach,
GABA, Octapomine) and the type of reactions help in the
impulse conduction
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5. Axonic conduction
Action Potential
STAGES OF NERVE CONDUCTION
1. Resting potential
2. Depolarisation
3. Repolarisation
4. Hyperpolarisation
(more influx of negative ion) it is required to inhibit the
hyperexcitation of nerve 5
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6. SYNAPTIC CONDUCTION
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7. 1. Insecticide affecting voltage gated sodium
channels
• DDT group
• Pyrethroids
• Indoxacarb
• Sabdilla
• Metaflumizone
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8. MOA of DDT group and Pyrethroids,
• DDT and Pyrethroid Insecticides bind to sodium
channels
• Delay sodium channel closing
• Axon doesn’t readily recover to its resting stage
• Repetitive discharges of axonal action potential
• Excessive neuroexcitation cause Hyperactivity, tremors
and rigid paralysis (Matsumura.,1985)
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9. DDT
Repetitive discharges of axonal action potential
Axon doesn’t readily recover to its resting stage
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10. Pyrethroids
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11. Cross section of Sodium gate
Acting site of DDT and Pyrethroid
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12. Type I Pyrethroid Type II Pyrethroid
Repetitive discharge of neurons Do not induce repetitive discharge
- Cause slow depolarization of nerve
membrane
- Reduce amplitude of the action potential
Negative temperature coefficient Positive temperature coefficient
Behavioral arousal, Body tremor Profuse salivation and tremors leading to
seizures
Examples-Permethrin Examples- Cypermethrin
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13. Sabadilla
Veratridine Cevadine
Prolong the opening of Sodium channels by delaying the channel closing
(Bloomquist.,1999)
Sabadilla is an insecticide
produced from the seeds of the
sabadilla plant, Schoenocaulon
officinale
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14. Indoxacarb
• Metabolized by esterases/amidase to N
decarbomethoxylated metabolite (DCJW)
• Metabolite is Sodium channel blocker in
insect
• Binds to sodium channel and prevents
sodium ions from flowing into Axon
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15. Metabolite
Indoxacarb is proinsecticide that is readily metabolized by an esterases to DCJW
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16. 2. Insecticide affecting Ryanodine receptors
Ryanodine
• RyRs -Class of intracellular calcium
channels
• Muscle contraction is mediated by
calcium channels
• Depolarization activates these
channel and stimulate the release of
chemical transmitter, Amino acid
glutamate
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17. How muscle contraction take place?
Glutamate
Synaptic cleft
Bind to receptor
influx of sodium and calcium ion
sarcoplasmic reticulum calcium
channel
calcium into protein filaments
Muscle contraction 17
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18. Flubendamide (pthalic diamide insecticide) and
chlorantraniliprole (Anthranilic diamide)
• Chlorantraniliprole – Selective RyRs
activator
• It activates RyRs through binding to receptor
site
• Stimulate release of ca ion cause impaired
regulation of muscle contraction
• It affect cardiac muscle causing feeding
cessation and immobility
• Cyantraniliprole- share same mode of action
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20. 3. Insecticide inhibiting acetylcholinesterase
Organo- phosphates
- Binding of OP
- Phosphorylation of Ache
- Dephosphorylation
NOTE: Dephosphorylation is slow
Reversible
Days or even weeks
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21. Carbamates
-Binding of carbamate
-Carbamylation of Ache
-Decarbamylation
NOTE: Decarbamylation is very fast
Irreversible
Minutes
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22. 4. Insecticide interfering with chloride channels
1. GABA gated chloride channel
2. Glutamate –gated chloride channel
γ-Aminobutyric
acid, or GABA
Reducing neuronal
excitability throughout the
nervous system.
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23. • Increase Cl permeability results in Hyperpolarisation
• Effect of hyperpolarization is to maintain the membrane at its
resting value so that excitability decreased
• Cyclodeines (BHC) and Phenyl Pyrazole (Fipronil) Bind and
block activation by GABA- Hyperexcitation CNS
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24. Avermectins
• Open Cl channel by binding to GABA
• The avermectins open the GABA receptor
chloride channel by binding to the GABA
recognition site (receptor protein) and act as
partial agonists (Abalis et al., 1986).
• Continuous flow of Chloride – This chloride
permeability increase can significantly
hyperpolarize (make more negative) the
membrane potential- Loss of sensitivity-
Paralysis
• Emmamectin benzoate and milbemectin share
same mode of action
Streptomyces avermitilis
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25. Glutamate –gated chloride channel
• Influx of chloride ion – hyperpolarize the membrane potential –
maintain membrane at resting value
• Signs of poisoning- Ataxia( poor mucle control) paralysis and death
• Fipronil – block glutamate gated chloride channels
GABA
Both Invertebrate and
Vertebrate
Glutamate –gated chloride
channel
Invertebrate
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26. 5. Insecticide that bind to nicotinic
acetylcholine receptors
• nAChR – both presynaptic and post synaptic nerve
• Examples: nicotine, neonicotinoids (immidacloprid,
acetamiprid, thiamethoxam, clothianidin), Sulfoxaflor
(Sulfoxamine) and Spinosyn (Spinosad and
Spinotram)
• Mimic ach (Agonist)- activate nAChR
• Activation cause influx of Sodium ion and generation
of action potential 26
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27. Overstimulation
• Hyperexcitation
• Convulsion
• Paralysis
• death
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28. 6. Insecticide interfering with respiration
• Cell respiration describes the metabolic reactions and
process that occur in cell to produce ATP
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29. Inhibitors of the mitochondrial electron transport system
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30. Oxidative phosphorylation
• Sulfluramid is first N-deethylated by cytochrome P450
monooxygenases to become perfluorooctane
• This oxidative metabolite acts as a potent inhibitor
(uncoupler) of oxidative phosphorylation, disrupting the proton
gradient, thus inhibiting ATP production (Schnellman and
Manning, 1990)
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31. • Chlorfenapyr is activated by herbivores via oxidative removal of the N-
ethoxymethyl group to become CL 303,268, which acts as an uncoupler of
oxidative phosphorylation.
• It disrupts the proton gradient across mitochondrial membranes and
subsequently impairs the ability of mitochondria to produce ATP, which in
turn leads to the death of affected cells and ultimately to the death of the
organism (Hunt and Tracy, 1998)
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32. 7. INSECTICIDES DISRUPTING INSECT
MIDGUT
• Bt ( Bacillus thuringiensis)
By disturbing
cell osmotic
balance
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33.  In experiments with gypsy moth larvae, Broderick et
al. (2006) proposed a new mode of action for Bt.
According to these authors, δ-endotoxin causes the
formation of pores in the larval digestive tract. These
pores allow naturally occurring enteric bacteria such
as Escherichia coli and enterobacter to enter the
hemocoel where they multiply in the hemolymph and
causes sepsis.
 Midgut bacteria are required for Bt insecticidal
activity. Thus, Bt induces death by septicemia.
 Septicemia is a term referring to the presence of
pathogenic organisms in the bloodstream, leading
to sepsis.
 Sepsis is the condition or syndrome caused by the
presence of microorganisms or their
toxins in the bloodstream or the tissue. 33
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34. CHITIN
SYNTHESIS
8. Insecticides affecting chitin biosynthesis
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35. 8. Insecticides affecting chitin biosynthesis
• Benzoylphenylureas(diflubenzuron,hexaflumur
on, teflubenzuron, flufenoxuron, and lufenuron)
and thiadiazines (buprofezin) are inhibitors of
chitin biosynthesis.
• These insecticides inhibit the final step of chitin
synthesis, that is, the polymerization of N-
acetylglucosamine
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36. • Affect the elasticity and firmness of the endocuticle.
• As a result, the cuticle is unable to support the insect and withstand the
rigors of molting, leading to the death of the insect.
• Example- in cabbage white butterfly (Pieris brassicae) larvae,
diflubenzuron treatment causes an obliteration of the connection
between epidermal cells and cuticular layers. The space in
between contains scattered globules of apparently coagulated material
(Gijswijt et al., 1979).
• It was reported that chitin synthesis was blocked within 15 min after the
application of diflubenzuron in the cuticle of Pieris brassicae larvae
(Deul et al., 1978).
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37. • Cyromazine does not inhibit the biosynthesis of chitin, but it is a molting disruptor. It
affects cuticle sclerotization by increasing the cuticle stiffness in insects.
• It has been reported that the cuticle of cyromazine-treated tobacco hornworm larvae
rapidly becomes less extensible and is unable to expand to the degree normally
observed. The larvae show impaired growth, eventually developing cuticular lesions,
and die after a period of days
(Reynolds and Blakey, 1989; Reynolds and Kotze, 1992).
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38. 9. INSECTICIDE ACTING AS JUVENILE
HORMONE
Juvenile hormone
• It prevent premature metamorphosis
• JH tends to drop when as larva proceed to next instar
• In adults insects it control reproduction i.e. oocyte
development and vitellogenin production
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39. Effects of the juvenile hormone (JH) analog insecticide methoprene on mosquito
development. Under normal conditions 4th instar larval mosquitoes undergo a larval-
pupal molt following a rapid reduction in haemolymph JH levels and concurrent spikes
in molting hormone.
When 4th instar Cx. quinquefasciatus are exposed to methoprene at exceptionally low
levels (60 to 120 ng liter−1) unique morphologies are observed including larval-pupal
monsters (A) and insects that are unable to complete pupal-adult eclosion (B).
Kamita et al., 2011
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40. • JH mimics (methoprene, hydroprene, fenoxycarb, and pyriproxyfen)
act like JH and exhibit the highest toxicity when applied at the onset
of metamorphosis, that is, in the last larval or early pupal stage
• Pyriproxyfen affects the hormone balance in a wide range of insects,
resulting in the inhibition of embryogenesis, metamorphosis, and
adult formation (Ishaaya, 2001).
• It strongly supports the notion that JH analogs act as JH agonists
(Wilson, 2004).
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41. Morphological abnormalities
observed in ACP adults caused by
exposure of late instars (fourth and
fifth) to the two higher
concentrations of pyriproxyfen (32
and 64 µg mL−1). a–f, twisted wings
(tw); a–d, unable to emerge
completely from exuvia (ex); c and d,
wide abdomen (wa); e, deformed
(thicker) antennae (da); f, deformed
body (db).
(Boina et al., 2010)
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42. 10. Insecticide acting as ecdysone agonists
• The ecdysone receptor (EcR) is a nuclear receptor that is involved in
insect development
• Diacylhydrazine insecticides (tebufenozide, methoxyfenozide,
halofenozide, and chromafenozide) act as nonsteroidal ecdysone
agonists.
• They bind to specific ecdysteroid receptor binding proteins,
interrupting the normal events from proceeding. Consequently, they
induce an incomplete precocious molt, resulting in the mortality of the
larva (Wing, 1988; Wing et al., 1988).
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43. • Azadirachtin affects the brain–corpus
cardiacum complex by inhibiting the
release of the morphogenetic brain
peptide, prothoracicotropic hormone
(PTTH), and other peptide
hormones related to molting, such as
eclosion hormone or bursicon, thereby
blocking molting hormone activity. As
a result, it suppresses fecundity,
molting, pupation, and adult formation
(Luntz et al., 2005).
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44. 11. INSECTICIDE ABRADING INSECT
CUTICLE
• Boric acid is an absorber of insect cuticle wax and a stomach
poison.
• Its mode of action on insects has not been clearly established.
Two major hypotheses have been proposed:
1. Abrasion of the cuticle followed by slow desiccation (Ware
and Whitacre, 2004)
2. Destruction of the cellular lining of foregut after ingestion,
causing the death of the insect by starvation (Cochran, 1995).
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45. Examples
• Cochran reported that when feeding boric acid to German cockroaches, the
foreguts became empty and slightly enlarged on days 1 and 2 after
treatment. By day 3 after treatment, the foregut cells of treated insects were
completely destroyed, and by day 4, only the basement membrane
remained. The author concluded that the most probable cause of death from
ingested boric acid is starvation due to destruction of the foregut.
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46. • Habes et al. (2006) also reported that dietary boric acid
altered the midgut epithelial cells of the German cockroach.
• Sumida et al. (2010) found that boric acid caused cell death in
the midgut epithelium of the leaf-cutting ant
Conti…
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47. • Silica aerogels kill insects by absorbing
waxes from the insect cuticle, resulting in
slow desiccation and death by dehydration.
• The wax layer normally prevents insects
from losing water through their exoskeleton
and desiccating. By adsorbing the wax layer,
silica gel increase the permeability of the
exoskeleton, resulting in insect death by
dehydration
• Pesticidal soaps disrupt the cuticle and break
down cell membranes, resulting in rapid
death of insects
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48. 12. INSECTICIDE ACTING AS SELECTIVE
FEEDING BLOCKERS
• Pymetrozine (pyridine azomethine) is a selective feeding blocker against
plant-sucking insects such as aphids, whiteflies, leafhoppers, and
planthoppers.
• In aphids (Myzus persicae) pymetrozine prevents insects from inserting
their stylet into the plant vascular system. The feeding disruption appears
to be related to the nervous regulation of feeding behavior, which
consequently results in death due to starvation (Harrewijn and Kayser,
1997)
• Pymetrozine also found to inhibit phloem ingestion in the rice brown
planthopper, Nilaparvata lugens (He et al., 2011).
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49. Morita et al. 2007
Flonicamid inhibits aphid feeding by
inhibition of stylet penetration to
the plant tissue
The feeding disruption appears to be
related to the neurological effect of
flonicamid on aphids, resulting in death
from starvation.
Flonicamid-treated aphids also displayed
uncoordinated locomotion, walking
staggeringly with their legs stretched
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50. 13. INSECTICIDES CAUSING
SUFFOCATION
• The primary mode of action of pesticide
oils such as paraffinic oil is through
suffocation.
• The spray forms a coating of oil on the
insect’s body, which blocks the spiracles or
breathing opening, resulting in the death of
the insect by suffocation.
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51. Target site
Insecticide/Insecticide class
Mode of action
Voltage- gated Na
channels
DDT, Pyrethroids, Sabadilla Na channel modulator
Indoxacarb, Metaflumizone Na channel blocker
AChE Organophosphates and
carbamates
AChE inhibitors
GABA-gated chloride
channels
Cyclodienes,
Phenylpyrazoles, lindane
Antagonism
Glutamate- gated
chloride channel
Avermectins, Emamectin
benzoate, milbemectin
Activation
Fipronil Channel blocker
Biochemical and physiological target sites of insecticides on
insects
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52. Target site Insecticide/insecticide
class
Mode of action
RyRs Ryanodine,flubendamide,
chlorantraniliprole,
Cyantraniliprole
Modulator
Mitochondrial electron
transport system
Rotenone, HCN,
Tolfenpyrad, Fenazaquin,
Phosphine, pyrimidifen,
Tebufenpyrad,
Fenpyroximate
Inhibition
Sulfluramid, Chlofenapyr Uncouplers of oxidative
phosphorylation
Midgut cell membrane Bacillus thuringiensis Midgut membrane
disruption
Insect cuticle Benzophenylureas,
Buprofezin
Inhibition of chitin
biosynthesis
Cryomazine Molting disruption
Boric acid, Silica aerogels Abrasion of cuticle
Biochemical and physiological target sites of insecticides on
insects
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53. Target site Insecticide/insecticide
class
Mode of action
JH receptors Juvenoids, fenoxycarb,
pyriproxyfen
JH mimic
Azadirachtin Molting disruption
Nervous system Pymetrozine, Flonicamid Selective feeding blocker
Biochemical and physiological target sites of insecticides on
insects
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