Synergists are chemicals that enhance the toxicity of insecticides when combined, even though the synergists themselves are not toxic. They help overcome insect resistance by inhibiting enzymes that detoxify insecticides. Common synergists include PBO, TPP, and DEM. Case studies show synergists can help determine resistance mechanisms, such as a study where PBO synergism helped determine elevated MFO enzyme levels were causing spinosad resistance in Helicoverpa armigera. Synergists also help manage resistance by allowing lower insecticide doses through increased efficacy.

2. Role of Synergists in Combating Insect Resistance to Pesticides
JAYANT YADAV
2017A39D
2
ENT 606
ADVANCED INSECTICIDE TOXICOLOGY

3. CONTENTS
SUMMARY6
RESISTANCE MANAGEMENT2
MODE OF ACTION3
CASE STUDIES4
COMMON SYNERGISTS1
PRACTICAL LIMITATIONS5
3

4. SYNERGISTS
 Any chemical which in itself is not toxic to insects as
dosages used, but when combined with an insecticide
greatly enhances the toxicity of insecticide
 Process of activation is synergism
 Help in penetration and stabilization of insecticides, and
prevent the detoxification of insecticides
(Kuhr and Dorough, 1976)
4

5. ()
Pyrethroid
synergists
Carbamate
synergists
OP synergists
Sesamin
Sesamolin
sesamex
Piperine
Piperonyl
cyclonene
MGK 264
PBO
Sesamex
SKF 525-A
PBO
Sesamex
Piperonyl
cyclonene
Propyl isome
N-isobutyl
undecylene
amide
PBO
5
(Robert and Metcalf,1967)
COMMON SYNERGISTS
5

6. 6
STRUCTURES OF DIFFERENT SYNERGISTS

7. 7
STRUCTURES OF DIFFERENT SYNERGISTS

8. 8
STRUCTURES OF DIFFERENT SYNERGISTS

9. NEGATIVE SYNERGISM
 ANTOGONIST : A substance, that has the opposite effect
i.e. which reduces the toxicity of an insecticide
Example:
 Piperonyl butoxide - Malathion
(Srivasthava,1996)
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10. AUTOSYNERGISM
 AUTOSYNERGIST : A compound synergizes its own
toxicity
 Forging a synergophore and toxophore in to the same
molecule
 Example :
3,4- methylenedioxyphenyl methyl carbamate
(Kuhr and Dorough,1976)
10

11. 11
 QUASI-SYNERGISM: Increase in the toxicity through the
increase in cuticular penetration
 PSEUDO-SYNERGISM: Increase in the efficiency of insecticides by
stabilizing the droplet size of their sprays
Quasi and Pseudo-Synergism

12. QUANTIFICATION OF SYNERSISM
LC50 /LD50 of insecticide alone
SR ratio =
LC50 /LD50 0f insecticide +synergist
≥1.05 = Synergism
0 – 0.95 = Antagonism
0.95 – 1.05 = Additive action
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13. ROLE OF SYNERGISTS IN INSECT
RESISTANCE MANAGEMENT
13

14. 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 less persisting spray will persist for a very
short time and will select only against a single generation of insects.
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 resistance genes can rapidly
spread throughout the population.
14

15. 15
Possible Scenario for Resistance Development in an Insect
Population

16. Exoskeleton
Barrier
By
MFO
 HYDROLASES
 GSTs
Modified
AchE
Biochemical Mechanisms Conferring Resistance
Reduced
penetration
Altered site
action
DetoxificatiOn
16

17. 17
Fig: Schematic illustration of different possible resistance mechanisms known in insects:
1) penetration resistance; 2) enzymatic cleavage; 3) sequestration; 4) conjugation; 5)
excretion; 6) target-site modification.
INSECTICIDE RESISTANCE MECHANISM

18. RESISTANCE MANAGEMENT
18

19. Determine the insecticide
classes for which the resistance is
present in insect population
Synergist –insecticide
combination
Quantification by SR ratio
A. Analytical Tool
19
(Raffa and Priester,1985)

20. 20
The first step is to determine the insecticide classes for which resistance is
present in an insect population.
For each appropriate insecticide category, a corresponding group of
synergists are tested.
Each result either provides a presumed mechanism or suggests another
synergist-insecticide combination.
In some cases, both positive and negative data by different synergists are
required to demonstrate a particular form of resistance.
After the insecticide-synergist combination yields a presumed mechanism,
more specific biochemical techniques can be applied for confirmation.
 Use of Synergists as Analytical Tool

21. 21
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B. PREVENTION AND MANAGEMENT OF RESISTANCE
 Prevention:
Reducing the selection pressure by insecticides.
 Management:
Apply commercially available synthetic synergists like;
Synergy plus®, Syner pro PBO®, Tween ®20, Tween ® 80

22. 22
Resistance Management
A synergist can be defined as a chemical that, while not possessing significant inherent
pesticidal activity, but promote or enhance the effectiveness of a particular insecticide.
At non-toxic concentrations, insecticide synergists act by inhibiting certain enzymes
naturally present in insects that would otherwise breakdown and detoxify insecticide
molecules.

23. 23
Contd….
Synergists enhance the effect of several classes of insecticide, including the
pyrethroids, organophosphates and carbamates.
This is achieved by inhibiting the enzymes that metabolise insecticides, P450s and
esterases, within the insect.
In susceptible insects, these metabolic enzyme systems are at a ‘baseline level’,
whereas in resistant insects they are at an elevated level.
Thus in susceptible insects, insecticides are already working at near maximum effect
and the use of synergists may provide minimal enhancement.
Synergists have also been reported to be capable of delaying control failure, due to
insecticide resistance, in an agricultural setting.
Synergists have been used successfully in mosquito control programmes for over 50
years, to increase the efficacy of sprays. For example: the combined use of PBO with
natural pyrethrins.
The addition of PBO can provide increased mortality and efficacy at a reduced cost.
In some situations, the addition of a synergist can reduce the required rate of
insecticide by up to a half without a decrease in efficacy.

24. 24
Experimental Proofs
• Synergism of methomyl activity on methomyl-resistant Heliothis virescens larvae.
Raffa and Priester, 1985

25. 25
•Synergism of fenvalerate activity on pyrethroid-resistant Spodoptera exigua larvae.
Contd…
Raffa and Priester, 1985

26. 26
Mode Of Action Of Synergists
Inductive effects
Residual
effect
Enhanced cuticular
penetration
Non-inductive
effects
Activation of own
enzyme system
Enzymatic
inhibition
Quasi-synergism
Pseudo-
synergism

27. 1. NON-INDUCTIVE EFFECT
27

28. TABLE
Detoxification
mechanism
Synergist Insecticide
MFO PBO
Sesamex
Sulfoxide
MGK264
Fenvalerate,Carbaryl,methomyl,
permethrin,Parathion,malathion,
dimethoate,DDT
Carboxyesterase TBPT, TPP Malathion
Phosphatase TBPT, TPP Malathion, Dimethoate
GS-T Dimethylmaleate Malathion, Dimethoate
Dehydrochlorinase DMC,FDMC,
WARF-
antiresistant
DDT
28 (Raffa and Priester,1985)28
Major detoxification pathways and specific synergist inhibitors

29. TYPES OF NON-INDUCTIVE EFFECT
 BASED ON INHIBITION MECHANISM
 Inhibition of MFO
 Inhibition of hydrolases
 Inhibition of GS-T
 Analogue synergism
 Target site synergism
(Perry et al.,1950)
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30. (i) INHIBITION OF MFO
 Mainly by PBO and MGK 264
 Best synergism occurred with pyrethroids
 Synergists have a higher affinity for the active site of
MFO system
 Inhibit the oxidative metabolism
(Perry et al.,1950)
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31. 31

32. (ii) INHIBITION OF HYDROLASES
 Hydrolysis -Main route of detoxification in op
compounds and carbamates
 Carboxyesterase – Prominent hydrolase
 EPN,TPP,TBPT are the important inhibitors of
carboxyesterase by phosphorylation of the active site
(Robert and Metcalf,1967)
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33. (iii) INHIBITION OF GS-T
 Organothiocyanates act as synergists for carbamates and
pyrethrins
 Release hydrogen cyanide by metabolism with Glutathion –S-
Transferases, which contributes to the insecticidal activity
RCH2SCN HCN+ RCH2SSG RCH2SH+GSSG
(Perry et al.,1950)
GST
GSH GSH
33

34. (iv) TARGET SITE SYNERGISM
 Two compounds interact with the different sites of the target.
 Combination of an aryl N-methyl carbamate with an N-propyl
carbamate overcomes the aryl N- methyl carbamate resistance
in rice green leaf hopper
 Resistance due to modified AchE insensitive to n-methyl
carbamates and the N- propyl carbamates are more inhibitory
to the modified AchE
 Both components of the combination interact with the
preferred inhibition sites
(Yamamoto et al.,1983)
34

35. (v) ANALOGUE SYNERGISM
 Alternate substrate synergism by structurally related
synergists
 Non– toxic compound compete with a toxic compound
for the same site on the detoxifying enzyme system
 Reduce the amount of detoxification of toxic compound
Examples:
 Phenyl dibutyl carbamate – isolan synergism
 Butyl-2-methyl-carbanilate – carbaryl synergism
35
(Perry et al.,1950)

36. 2. QUASI-SYNERGISM
 Increase in the toxicity through the increase in
cuticular penetration
 Synergists which enhance penetration, transport or
accessibility of insecticides without any inhibition
of detoxifying system
 Example:
 Carbaryl-Thanite synergism
 Tween ® 20 , Tween ® 80
(Sun and Johnson, 1972)
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37. 3. PSEUDO-SYNERGISM
 Increase in the efficiency of insecticides by stabilizing
the droplet size of their sprays
 Ensuring better and persistent contact of insecticides
with the insect body
Example:
Oleic oil – Pyrethroids synergism
( Srivasthava,1996)
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37

38. C. NATURAL ENEMY CONSERVATION
 A feature of insecticide synergists that is highly
compatible with IPM is reduced mortality to beneficial
and non target species because of the lowered
insecticide rates needed to control the pest.
 Lower rates of Pyrethroid-PBO synergism to
Hymenopteran parasites and Neuropteran predators
than to their Lepidopteran host.
(Plapp, 1979)
38
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39. 39
This added control due to preserved natural enemies could be another
incentive for a grower to use synergists early in the life of an insecticide.
Contd…….

40. CASE STUDIES
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40

41. 1. Resistance selection and biochemical characterization
of spinosad resistance in Helicoverpa armigera
(Lepidoptera: Noctuiidae)
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Wang et al.,2009

42. Compound Strain LD 50 (µg/g) RR
Spinosad S
R
1.26
30.3
1.00
24.1
Spinosad + TPP S
R
1.08
17.6
1.00
16.4
Spinosad + DEM S
R
1.18
28.7
1.00
24.2
Spinosad + PBO S
R
0.518
3.97
1.00
7.66
4242
Toxicity of spinosad to the susceptible and resistant
strains of Helicoverpa armigera after synergism

43. 2. Selection for imidacloprid resistance in Nilaparvata
lugens: cross resistance patterns and possible
mechanisms
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Zewen et al., 2003

44. Strain Treatment LD50
(ng/pest)
SR
S
Imidacloprid alone
Imidacloprid+PBO
Imidacloprid+TPP
Imidacloprid+DEM
0.120
0.101
0.139
0.131
-
1.2
0.9
0.9
F
Imidacloprid alone
Imidacloprid+PBO
Imidacloprid+TPP
Imidacloprid+DEM
0.770
0.552
0.667
0.719
-
1.4
1.1
1.1
R
Imidacloprid alone
Imidacloprid+PBO
Imidacloprid+TPP
Imidacloprid+DEM
8.740
2.979
8.402
7.836
-
2.9
1.0
1.1
44
Synergistic Effects of PBO,TPP, DEM on Imidacloprid
44

45. 3. Resistance selection and mechanisms of oriental
tobacco budworm to indoxacarb
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45
Pang et al., 2012

46. Compound Strain LD50 (mg/l) SR
Indoxacarb S
R
2.31
9.68
-
-
Indoxacarb+ PBO S
R
1.55
6.38
1.49
1.52
Indoxacarb+ TPP S
R
1.32
2.36
1.75
4.10
Indoxacarb+ DEM S
R
1.46
3.51
1.58
2.76
46
Toxicity of indoxacarb with and without synergists
46

47. 47
4. Insecticide resistance and effect of piperonyl butoxide as a
synergist in three strains of Aedes aegypti (Linn.) (Diptera:
Culicidae) on insecticides permethrin, cypermethrin, and d-
allethrin.
Astari and Ahmad, 2005

48. 48
Insecticides Strain
Without PBO With PBO
n KT100(Min.) n KT100(Min.)
D- Allethrin
NAMRU*
IPB
ITB
95
71
10
8.99
5.00
5.00
67
61
10
5.45
5.00
5.00
Permethrin
NAMRU*
IPB*
ITB
86
69
10
9.82
8.04
7.50
65
67
12
8.23
6.12
8.33
Cypermethrin
NAMRU
IPB*
ITB*
87
67
10
9.02
9.10
10.00
68
65
10
9.26
5.00
7.00
-n: No. of mosquitoes tested
-* Shows the reduction of KT100 after the addition of PBO

49. 49
ROLE OF SYNERGISTS IN ELUCIDATING INSECT
RESISTANCE MECHANISM
STEPS:
An insect strain resistant to particular insecticide is taken.
Insecticide for which we want to elucidate the resistance mechanism, is applied with
different synergists on that insect strain.
Synergist which gives enhanced mortality when applied with that insecticide is taken into
consideration.
The mode of action of that synergists confirms actually what is the mechanism of
resistance development in that particular insect.
For example: that synergist inhibits the action of MFO’s. Thus by blocking the effect of
MFO’s with that particular synergist, we get enhanced mortality in resistance insect strain.
So we can say that MFO’s imparts resistance to that insect against our insecticide.

50. 50
One of the major contributions of synergists may be in bridging the gap between
biochemical/physiological studies and population genetics.
Physiological investigations can provide a precise description of a particular form of
resistance, but they cannot relate these mechanisms to population phenomena.
Questions about the survival value and spread of resistance mechanisms in the field
require large sample sizes which exceed the logistic constraints of biochemical assays
but Population genetics, on the other hand, address these questions directly, but they
must usually rely on toxicological data such as LD50 or RR50 (resistance ratio) values.
But several different forms of resistance can together be contributing to these values in
a field population. In the case of carbamate-resistance, both metabolism and an altered
target site could be responsible. Application of an appropriate synergist would allow only
those individuals with the altered target site to survive.
Thus they can be used to distinguish resistance mechanisms and their frequencies can
be individually assessed.
Role of Synergists in Population Genetics

51. 51
The simplest use of synergists in resistance management is direct application to
resistant populations. By this means, the resistant strains can be rendered susceptible.
Although this is the most powerful and straight forward use of synergists, it is not a
totally reliable one as there is no guarantee that anyone synergist-insecticide
combination will work on all strains of an insect species.
So synergist-insecticide applications run a high risk of being ineffective in some cases.
So most appealing prospects for synergists is the prevention of resistance development
in the first place.
According to this view, exposing susceptible populations to an insecticide-synergist
mixture would remove the selective advantage of certain metabolic alterations.
So those insects with alleles conferring resistance to the insecticide would die in equal
proportions as susceptible types. Therefore, the resistance evolving would be greatly
diminished.
Prevention Is Better Than Cure

52. Limitations In Use Of Synergists
 There is the added cost to already expensive insecticides. Whether synergists can be
discovered, developed, and produced as major cost-effective products remains to be
seen.
 In some instances, formulation can be a critical problem. This is particularly true
where the insecticide and synergist have different polarities.
 The rates that can be applied to yield synergism are also a limiting factor. In a matrix
of synergist concentration and insecticide concentration, we typically find two
threshold values: a synergist dose, below which the insecticide cannot be synergized
regardless of its concentration, and an insecticide dose, below which no amount of
synergist is effective.
 A potentially important, but little investigated, phenomenon is the differential impact
of synergized mixtures on different signs of toxicity in insects. The SRs are based on
mortality (LD50s or LD95s), and do not evaluate behavioral signs, antifeedant
properties, or hormonal disruptance caused by the synergized versus the non
synergized insecticide.
 There is only limited information on long-term effects of constant use of synergists
with the different groups of insecticides.
52

53. Contd…..
 Little can be done to overcome altered target site resistance mechanisms, other than
using an insecticide with an alternative mode of action. However the effect of
metabolic resistance can be reduced with the use of synergists.
 Synergists increase the activity of certain insecticides on insects with specific
resistance mechanisms and prolong the useful life of those insecticides where
resistance is developing. However there is currently insufficient evidence to
determine whether synergists can influence the frequency of resistance genes in a
vector population and hence no recommendations relating to resistance management
can be made at this stage.
 For a grower to employ a synergist on susceptible populations, there must be an
added incentive. For example, we have found that certain compounds that synergize
pyrethroid activity on Lepidoptera can also improve acaracidal activity. In such case
the synergist may prove economically acceptable.
53

54. SUMMARY
 Synergism is the increase in the toxicity of a insecticide after
addition of a non-toxic compound i.e. synergist.
 The commonly known synergists are piperonyl butoxide,
sesamin, sesamolin, sesamex, piperine, piperonyl cyclonene,
propyl isome, N-isobutylundecyleneamide, MGK264, SKF525A.
 Synergists mainly act by inhibiting the detoxifying enzymes.
 Classification based on its inhibition mechanism on detoxifying
enzymes.
 Help in penetration and stabilization of insecticides.
 Most potential tool for insect resistance management and
Compatible tool in IPM.
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