Classification of pesticides can be based on their mode of action. Some major classifications include nerve poisons like organophosphates and carbamates which inhibit the enzyme acetylcholinesterase, metabolic poisons which disrupt mitochondrial processes, and insect growth regulators which inhibit development. The most common modes of action target the nervous system by affecting acetylcholinesterase or sodium channels. Natural pesticides derived from plants can also act as feeding deterrents or growth regulators. The main target sites for many synthetic insecticides involve acetylcholinesterase and voltage gated sodium channels in the nervous system.

1. Classification of Pesticide Based on Mode of Action
Presenter:
Dharma Raj Katuwal
Department of Entomology
Tribhuvan University, Kathmandu, Nepal

2. Outline of presentation
➢Introduction
➢History of pesticide in world
➢History of pesticide in Nepal
➢Classification
➢Summary

3. Introduction
➢ Any substance or mixture of substances intended for preventing,
destroying or controlling any pest (FAO, 2013)
➢ Pest + cide = Pesticide
The term includes ( Pesticide management act, 2076)
➢ Plant growth regulator
➢ Defoliant
➢ Desiccant
➢ Disinfectant
➢ Thinning agent
➢ Crop protectant

4. History of pesticide use in world
➢4500 years ago Sumerians used Sulphur compounds to control insects and
mites (Unsworth, 2010).
➢15th century: Arsenic, mercury, lead
➢17th century: Nicotine sulfate
➢19th century: Pyrethrum and rotenone
➢1939 A.D. : Discovery of pesticidal properties of DDT
➢1962 A.D. : “Silent Spring”
➢China is the largest user, producer, and exporter (Workman, 2020)

5. History of pesticide use in Nepal
➢Nepal started use of pesticide very late in around 1950s.
➢In 1952: DDT imported by Ministry of health
➢In 1956: DDT imported by Ministry of Agriculture for pest control purpose
➢In 1977: Establishment of Nepal Pesticide and Chemical Industries Private Ltd.
(NEPCIL) at Bahadurganj, Kapilvastu
Crop Pesticide use (a.i. kg/ha)
Brinjal 3.34
Tomato 1.95
Potato 1.05
Cole crops 0.70
Table: Use of pesticide in different crops
Source: PQPMC, 2018
Status of pesticide use in Nepal
➢Average use: 396 a.i. gm/ha (World : 500
a.i. gm/ha)
➢ > 45% used in Kavrepalanchok, Dhading
and Nuwakot (Prasain, 2019).
➢> 85 % used in vegetables.
➢High use II category of WHO Class.

6. Cont…
Insecticide Fungicide Bactericide Herbicide Rhodenticide Bio pesticide
2070/71(2013/14) 162326 192203.63 31.25 90126.99 9836.36 71.74
2071/72(2014/15) 156041.9 250807.8 25.88 133857.4 9180.382 51.778
2072/73(2015/16) 181270.87 247475.27 11.36 134232.37 11007.74 63.33
2073/74(2016/17) 169360 347707.22 6.7 105445.44 12077.31 1125.25
2074/75(2017/18) 215891.91 267034.99 37.66 142142.7 6171.21 866.56
162326
192203.63
31.25
90126.99
9836.36
71.74
156041.9
250807.8
25.88
133857.4
9180.382
51.778
181270.87
247475.27
11.36
134232.37
11007.74
63.33
169360
347707.22
6.7
105445.44
12077.31
1125.25
215891.91
267034.99
37.66
142142.7
6171.21
866.56
PESTICIDE
IN
A.I
(KG)
2070/71(2013/14) 2071/72(2014/15) 2072/73(2015/16) 2073/74(2016/17) 2074/75(2017/18)
Figure. Total pesticides imported and formulated in Nepal in past five
years
Source: PQPMC (2018)

7. Cont…
Figure: Total pesticide use in Nepal from 2008-2017 Source: FAOSTAT, 2017

8. According to Larry P. Pedigo and Marlin E. Rice (2008)
a) Nerve poisons
b) Metabolic poisons
c) Alkylating poisons
d) Muscles poisons
e) Physical toxicants
Classification

9. Cont…
A) Nerve poison
I) Narcotic poison
➢Mode of action is mostly physical.
➢Many fumigants (particularly those containing CI, Br, and F)
➢Fat-soluble and lodge in fatty tissues, including nerve sheaths and
lipoproteins of the brain.
➢Action is reversible: if a human being shows early symptoms of poisoning,
removing him or her from the source of the fumigant allows recovery. This
also means that poisoned insects can recover if the fumigated space is
ventilated too quickly.

10. Cont…
II. Axonic poisons
➢Act primarily by interrupting normal axonic transmission of the nervous system.
➢These impulses are electrical and arise from the flow of positive sodium and
potassium ions through the cell membrane, creating a wave- like action potential
the impulse.
➢ All the chlorinated hydrocarbons and pyrethroids.
➢Cyclodienes and pyrethroids induce changes in one membrane permeability,
causing repel the discharges. Such discharges eventually result in convulsions,
paralysis and death.
Figure: Schematic representation of axonic transmission of nerve cell

11. Cont…
III. Synaptic poisons
➢Act by interrupting normal synaptic transmission of the nervous system.
➢Both organophosphates and carbamates inhibit acetyl cholinesterase.
➢Nerve firing, restlessness, hyper excitability, tremors, convulsions, paralysis, and
death.
Acetyl choline
(CNS)
Acetic acid
Choline
Figure: Schematic representation of inhibition by synaptic
poison

12. Cont…
B) Metabolic poisons
➢Disrupt a wide range of metabolic processes in insect cells.
➢The site of the disruptions is the mitochondrion of the cell.
➢Many of these poisons disrupt the process, blocking respiration and causing death of the insect.
➢Eg: rotenone, pyrroles, pyrazoles, and pyridazines.
C) Alkylating poisons
➢They attack loci on the nucleic acid molecule and deactivate critical enzymes, preventing the
synthesis of nucleic acids.
➢Eg: methyl bromide, ethylene dibromide, and some chemosterilants.

13. Cont…
D) Muscle poisons
➢Direct influence on muscle tissue.
➢ Eg: ryania.
E) Physical toxicants
➢Block a metabolic process by physical, rather than chemical means.
➢Eg: Oils, abbrssive dust
Alkaloid
(ryanodine,
Sabadilla)
Muscle
Tenfold increase
in oxygen
consumption
Paralysis and
death
Oils Clogged spiracles Suffocation
Abrasive dust Adsorb waxes
from cuticle
Desiccation,
Dehydration
Death of an insect

14. Cont...
According to Ware, 1975
i. Physical insecticides
➢E.g.: Heavy mineral oils and inert dusts.
➢Mineral oil exert a purely asphyxiant effect and dusts affect loss of moisture by
abrasion (aluminum oxide) or by absorbing moisture (charcoal).
ii. Protoplasmic insecticides
➢Cause cellular destruction of the midgut epithelium.
➢E.g.: inorganic insecticides.
iii. Respiratory poisons
➢Block cellular respiration.
➢E.g.: Fumigants

15. Cont…
iv. Cholinesterase (CHE) inhibitors
➢Inactivate the cholinesterase causing sickness
➢OP and Carbamates
V. Enzyme inhibitors
➢Inhibit enzymes necessary for normal metabolism.
➢Eg: flourides, arsenates
Vi. Insect growth regulators
a. Chitin synthesis inhibitors such as diflubenzuron
b. the antijuvenile hormone such as Methoprene

16. According to Radcliffe et al. (2009)
Insecticide Type Mode of Action
Organochlorine
Causing a sodium/potassium imbalance preventing normal transmission of nerve
impulses.
Organophosphate
Cause acetyl cholinesterase (AChE) inhibition and accumulation of acetylcholine at
neuromuscular junctions causing rapid twitching of voluntary muscles and
eventually paralysis.
Carbamates Cause acetyl cholinesterase (AChE) inhibition leading to central nervous system
Cont…

17. Cont…
Pyrethroids
Acts by keeping open the sodium channels in neuronal membranes affecting both the
peripheral and central nervous systems causing a hyper-excitable state.
Nicotinoids
Act on the central nervous system causing irreversible blockage of the postsynaptic
nicotinergic acetylcholine receptors.
Botanicals Pyrethrum – affects both the central and peripheral nervous systems, stimulating nerve cells
to produce repetitive discharges and eventually leading to paralysis.
Nicotine – mimics acetylcholine (Ach) in the central nervous system ganglia, causing
twitching, convulsions and death.
Rotenone – acts as a respiratory enzyme inhibitor.
Limonene – affects the sensory nerves of the peripheral nervous system
Neem – reduces feeding and disrupts molting by inhibiting biosynthesis or metabolism of
ecdysone, the juvenile molting hormone
Fumigants
Act as narcotics that lodge in lipid-containing tissues inducing narcosis, sleep or
unconsciousness.

18. Summary
Mode of action of insecticides involves mainly two target sites in the nervous
system:
➢Acetyl cholinesterase, an enzyme of critical importance in the transmission of
nerve impulse (organ phosphorus and carbamates) and
➢Voltage gated sodium channels across the nerve membrane (pyrethoids)
➢Among the insecticides derived from natural sources, feeding deterrent and an
insect growth regulator that suppresses fecundity, moulting, pupation and adult
emergence.

19. Thank you