resistance,insecticides,pests,types of resistance ,mechanism of insecticide resistance,insects,genetic,metabolic ,

1. ENT 508 TOXICOLOGY OF INSECTICIDES (2+1)
Pest resistance to insecticides,
mechanisms and types of resistance
MANIKANTHA J
1ST YEAR M.Sc
ID NO : 202O505104
DEPT : ENTOMOLOGY

3. Pests resistance to insecticides
• Definition
– Resistance is the development of ability in a strain of insects to tolerate a dose
of insecticide, which would prove lethal to majority of individuals in a normal
population of the same species
– “An inherited ability to tolerate dosage of insecticide that would be lethal to
the majority of individuals in a normal wild populations of same species.”
(WHO, 1957)
• This developed ability
– Result of selecting individuals with heritable capacity to withstand toxicant
– NOT due to action of insecticide on individual insect
– Depends on genetic variability of insects

4.  Population of a single kind of pests is made up of biotypes of that organism.
 A biotype is the same organism but has genetic difference.
 A pest can also have different genetics within the same species.
 Pesticides resistance is the natual ability of a biotype of an organism to
survive exposure to a pesticide that would normally kill an individual of
that species.
 Pesticide resistance at a population level, as opposed to just a few individual
pests within a species, can occur after repeated exposure to a single type of
pesticide
 This is because only the resistant organisms are left to reproduce with other
resistant organisms.
 The new resistant biotype (with the natural ability to survive a pesticide
exposure) then becomes the dominant biotype of the pest population
How?

5. History
• First resistance report-
– A. L. Melander in 1914 – Sanjose scale insects to lime Sulphur
– Quayle in 1916 – California red scale to HCN
• Between 1914 and 1946 – some reports – 10 or more cases of resistance to
inorganic pesticides like arsenicals, selenium, cyanide
• 1947 - Housefly resistance to DDT
• 1971 – Tribolium castaneum resistence to DDT which is the first among stored
pest
• 1985 – field resistance against Bt delta-endotoxins
• 1990 – resistance in diamondback moth
India
• 1963 – Singhara beetle, Galerucella birmanica resistant to DDT, HCH.
• 1965 – Spodoptera litura to HCH
• 1990s – American bollworm resistant to syn. Pyrethroids.
Do you know which insect has developed resistance to maximum no. of insects??

6. Insecticides resistance in public health insects
 Resistance to DDT was first noticed in India in the year 1952 in the Mosquitoes Culex
fatigans
Inseticides resistance in household pests
 The Bed bug Cimex lectularis ,was reported to be resistance to DDT in 1953 and
subsequently to HCH and OP componds.
 Human body louse Pediculus humanus corporis develepoed resistance to HCH in 1952
and Rat flea Xenopsylla cheopsis to DDT in 1961.
 The house fly Musca domestica nebulo is resistance to HCH and DDT throughout the
country.

7. Insecticides resistance in veterinary pests
 So far only Boophilus microplus infesting cattle has shown resistance to Lindane in
1963 in Mukteshwar
Insecticides resistance in Agricultural pests
 First resistance has been noticed in Singhara beetle at delhi to HCH
and DDT in 1963
 Later reported, resistance in Spodoptera litura to HCH in1965
 Lipaphis erysimi has been reported to be rsistance to endosulfan and
tolerance to malathion and dimethoate

8. Causes
 Many pest species produce large numbers of offspring, for example insect
pests produce large broods. This increases the probability of mutations and
ensures the rapid expansion of resistant populations.
 Pest species had been exposed to natural toxins long before agriculture
began.
For example, many plants produce phytotoxins to protect them from
herbivores. As a result, coevolution of herbivores and their host plants required
development of the physiological capability to detoxify or tolerate poisons

9.  Using one type of pesticide over and over again can produce resistance in a
population of insects, fungi, weeds, or other organisms.
 For example ; the picture represents a corn field that is infested with green
worms feeding on the corn plants.
 First, the grower decides to spray a certain chemical to get rid of those worms.
 The pesticide will kill the majority of green worms , but there may be some
part of the population that is not affected by the chemical, just as in humans
some chemicals may affect certain people whereas those same chemicals may
not affect other people. These unaffected worms are represented above in red.

10.  Let’s say that out of thousands and thousands of green worms a few have the
genetic ability to survive the pesticide application.
 These worms keep on eating corn plants and can eat as much as they want
because most of the competition they had in the field is now dead and the
insecticide has not killed them.
 The surviving worms are not killed by a subsequent application of the same
chemical and they reproduce,offspring have the same genetic composition as
their parents and are able to withstand the insecticide.
 The insecticide is no longer useable as a viable method of pest management for
this organism since the surviving biotype is now resistant to the pesticide

11. In this diagram, the first generation happens to have an insect with a heightened resistance
to a pesticide (red). After pesticide application, its descendants represent a larger
proportion of the population, because sensitive pests (white) have been selectively killed.
After repeated applications, resistant pests may comprise the majority of the population.

12. Contd..(causes)
• Humans often rely almost exclusively on pesticides for pest
control. This increases selection pressure towards resistance.
• Insect predators and parasites generally have smaller populations
and are less likely to evolve resistance than are pesticides' primary
targets, such as mosquitoes and those that feed on plants.
Weakening them allows the pests to flourish.
• Pests with limited viable range (such as insects with a specific diet
of a few related crop plants) are more likely to evolve resistance,
because they are exposed to higher pesticide concentrations
• Pests with shorter generation times develop resistance more
quickly than others.

13. Resistance reported from other countries.
• Studies in America have shown that fruit flies that infest orange groves
were becoming resistant to malathion.
• In Hawaii, Japan the diamondback moth evolved a resistance to Bacillus
thuringiensisabout three years after it began to be used heavily.
• In England, rats in certain areas have evolved resistance that allows them to
consume up to five times as much rat poison as normal rats without dying.
• The cabbage looper is an agricultural pest that is becoming increasingly
problematic due to its increasing resistance to Bacillus thuringiensis, as
demonstrated in Canadian greenhouses.
• DDT is no longer effective in preventing malaria in some places.

14. Insect and mite species reported to be resistant to insecticides
Order Agricultur
al
Medical/
Veterinar
y
Beneficia
l
Total Per cent
Diptera 23 132 1 156 35
Lepidoptera 67 - - 67 15
Coleoptera 64 - 2 66 15
Homoptera 46 - - 46 10
Heteroptera 16 4 - 20 4
Others 12 19 3 34 8
Mites 36 16 6 58 13
Total 264 171 12 447
Percent 59 38 3
Resistance seen mostly in Diptera-globally on mosquitoes

15. Chemicals Agrl, forest,
ornamental
pests
Medical,
veterinary
pest
Natural
enemies
Pollinator
s
Total
OP 715 358 52 - 1135 (44.1%)
OC 484 329 10 2 840 (32.6%)
Carbamates 35 132 57 - 219 (8.5%)
IGRs 10 16 - - 21 (0.8%)
Formamidines 2 4 2 - 6 (0.2%)
Neonicotinoids 1 2 1 - 3 (0.1%)
Phenylpyrazole
s
Pyrethroids
1
133
-
74
1
11
-
1
1 (0.4%)
219 (8.5%)
Category of resistant pests

16. Dynamics of increase of the resistant species of arthropods
in the world.

17. Resistance is Biphasic
Phase I - Due to selection of variants in the population according
to genetic principle, the resistance which is initially present in the
population is expressed.
Phase II - Acceleration of resistance takes place by induction of
pre-existing detoxifying enzymes towards enhanced activity,
resulting in faster breakdown of the chemicals.
Saxena, 1996

18. Resistant development is faster in..
• Insects
– Higher reproductive rates
– Shorter generation times
– Greater numbers of progeny
– More genetically varied local populations
• Use of related insecticides
• More persistent insecticides
• Selection pressure (high doses)

19. Resistance in insects
1)Preadaptive or genetic resistance
• Insecticides resistance genes in Houseflies : Kdr0 in chromosome 3 for DDT
resistance,Kdr NPR in chromosome 5 for pyrethroids
• Drosophila has single dominant resistance gene on chromosome 2 to DDT and
BHC
• Mutation in only a single gene can lead to the evolution of a resistant organism.
In other cases, multiple genes are involved. Resistant genes are usually
autosomal. This means that they are located on autosomes
2)Post adaptive or physiological mechanism
• Conversion of insecticides to non toxic metabolites
• excretion of toxicants
• Dietary factors
• Storage of toxicants
• Behavioural and ecological aspects
Eg: Blowfly larva produces an enzyme that confers resistance to organochlorines

21.  Genetics of resistance
1.Preadaptation : Resistance is preadaptive
Eg: Resistance to DDT in House flies was eight times higher than
in original strain
2.Gene frequency :
 Low in original natural population (0.0001 to 0.01%)
 High in wild & resistance population
Eg.: Mosquito in Nigeria – Dieldrin-R gene @ 0.4-0.6%
(Simon, 2008)

23. Dominance and number of genes
Resistant gene can be dominant, recessive, incomplete
dominant or incomplete recessive.
 Carbmates & OP’s Dominant or incomplete
dominant
 DDT, Bt & Spinosyns Recessive
 Dieldrin Incomplete dominant
 Pyrethroid Incomplete recessive
 Single gene High resistance

24. Mechanisms of resistance
• Resistant- reduction in sensitivity of insect population to
insecticides
• In several ways insects become resistant to insecticides
Major mechanisms
• Behavioural resistance
• Penetration resistance
• Knockdown resistance
• Metabolic resistance
• Altered site of action and
• Increased rate of excretion

25. Changing of susceptible to resistant

26. Mechanisms of resistance
Behavorial Resistance
Behavioural resistance describes any modification in insect
behaviour that helps to avoid the lethal effects of insecticides
• Resistant insects may detect or recognize a danger and avoid toxins
like OCs, OPs, Carbamates, Synthetic Pyrethroids
• Avoidance of treated surface
• Decreased period of contact
• For example, some Anopheles mosquitoes evolved a preference for
resting outside that kept them away from pesticide sprayed on
interior walls
• Insects may
- Stop feeding
- Leave sprayed area
- eg. Leg autotomy in DBM – against fenvalerate residues

27. Table:Leg-drop and activity status of diamondback adults at 48
h after walking on fenvalrate residues (1,000 mg/cm2 ) for 5
min

29. • Penetration Resistance (per gene)
Modifications in the insect cuticle or digestive tract linings that
prevent the absorption or penetration of insecticides can be found is
some strains of resistant insects.
– Resistant insects may absorb toxin more slowly than susceptible
insects
– Insect’s outer cuticle - barriers that slow absorption of chemicals
• Altered target-site resistance
– Target site modified thereby reducing effect of toxin
Knock down resistance (kdr gene) on chromose no.3-in Houseflies
– DDT, Pyrethroids – act on Na channel
• The number and/or sensitivity of biochemical receptors that bind to the pesticide
may be reduced

30.  This is the second most common resistance mechanism encountered in insects is
target site resistance. Insecticides generally act at a specific site within the insect,
typically within the nervous system (e.g. OP, carbamate, and pyrethroid
insecticides).
 The site of action can be modified in resistant strains of insects such that the
insecticide no longer binds effectively. This results in the insects being unaffected,
or less affected, by the insecticide than susceptible insects.
 Several mutated forms of AChE have been found which result in reduced
sensitivity to inhibition by these insecticides; resistance to OPs in Culex spp.
typically results from this mechanism.
 Similarly, a mutation (known as kdr) in the amino acid sequence in the voltage
gated sodium channels of nerve cell membranes leads to a reduction in the
sensitivity of the channels to the binding of DDT and pyrethroid insecticides.
 Reduced susceptibility to pyrethroids conferred by kdr mutations has been
confirmed in Anopheles gambiae in West, Central and East Africa.

31. Target site insensitivity
DDT resistant strains
Point mutation in Na channel gene
Replacement of aspartic acid by asparagine
Modified affinity of receptor site for insecticide

32. • OP compounds
– Point mutation in AchE Altered target site
insensitvity to OP
• Reduction in midgut target site binding
– involved in Cry proteins resistance
– reduced affinity of receptors in brush border membrane
vesicles for toxins

33. Metabolic resistance
This mechanism is based on the enzyme systems which all insects possess to
help them detoxify naturally occurring foreign materials. Three categories of
enzymes typically fulfil this function, namely esterases, monooxygenases and
glutathione S-transferases.
Detoxification enzymes detoxify toxins
• Microsomes yield enzymes like mixed function oxidases or monoxygenases to
oxidise lipophic substances.
• MFOs metabolise many foreign substances,they impart resistance.
• Esterases, Glutathione-S-transferases, Cytochrome P450 monooxygenases- OP
and Carbamates.
• Eg: DDT in resistance houseflies converted into DDE by the process of
dehydrochlorination in the presence of enzyme DDT dehydrochorinase.
Synergists are inhibitors of Microsomes,so in other way it will
inhibit metabolism of insecticide

34. Dehydrochlorination of DDT to DDE in resistant
houseflies

35.  Major factors that influence resistance development
1.Frequency of application
 How often an insecticide or control tactic is used is one of the most important
factors that influence resistance development. With each use, an advantage is
given to the resistant insects within a population
2.Dosage and persistence of effect
 Products which provide a persistent effect provide continual selection pressure in
a similar manner to multiple treatments.
 For example, a space spray will persist for a very short time and will select only
against a single generation of mosquitoes.
 In contrast, a residual wall application or a bednet treatment will persist for
months or years providing a selection pressure against many generations of the
same insect.
3.Rate of reproduction
 Insects that have a short life cycle and high rates of reproduction are likely to
develop resistance more rapidly than species which have a lower rate of
reproduction, as any resistance genes can rapidly spread throughout the population.

37. Other factors are…
• Prolonged exposure to single insecticide
• High selection pressure
• Large coverage area
• Insecticides related to the one used earlier
• Treatment triggered by low number of insect.
• Insects multiply by asexual means
• Insects highly mobile or migratory
• Selection at every stage of insect life cycle

38. Simple resistance
• Resistance restricted to only one insecticide and not to related ones
Eg: malathion resistant houseflies are not resistant to other op
compounds
Cross resistance
• Pest that becomes resistant to one insecticide becomes resistant to
related ones
Eg: DDT resistant houseflies are resistant to other organochlorines
Multiple resistance
• Pests are resistant to more than one class of pesticides
Types of Resistances

39. Cross Resistance Multiple Resistance
Single defense mechanisms
against various insecticides
Eg. Diamond back moth
resistant to various pyrethroids
Different defense mechanisms
against various insecticides
Eg. Mosquito resistant to OP,
OC, Carbamates

40. References
• Insect resistance to insecticides by R L Metcalf
• Evolution of insect resistance to insecticides by J A Lockwood and
TC Sparks
• Rapid evolution in insect pests by MS Crossley
• Role of behavior in evolution of insect adaption to insecticides by
F Gould