The Ingredient That Is Not an Insecticide: What Piperonyl Butoxide Does, What It Does Not Fix, and the Finding That It Can Make Things Worse

1. Introduction: the ingredient nobody explains

Read the guarantee panel on a great many insecticides and one of the listed actives is piperonyl butoxide, often at a higher concentration than the insecticide itself. It kills nothing.

It is there to disable the insect's ability to survive the other ingredient, and how well it does that is one of the more interesting open questions in this subject.

1.1 What the word synergist conceals

It sounds like a booster, something that makes the insecticide stronger. It is not. It makes the insect weaker, by disabling a defence, and the distinction matters because a defence the insect does not happen to be using cannot be disabled to any effect.

1.2 Why this article matters practically

Because a synergised product is often chosen specifically because a population is resistant, which makes what the synergist actually delivers a live question on a job rather than an academic one. It is also one of the few things in a product's composition that a technician can read off the container and reason about before deciding whether to use it.

2. What a synergist is

The definition, which is precise and worth quoting.

The insecticide synergist by itself does not harm insects at low concentrations; instead, it enhances insecticidal toxicity by inhibiting metabolic enzyme activities.³

To enhance the killing power of insecticides, synergized insecticides were developed by mixing insecticide synergists with pyrethroids, with citations to work from the late 1940s and early 1950s.³

2.1 The age of the idea

Discovered in the 1940s, piperonyl butoxide is one of the earliest insecticide synergists,⁶ and is included in over 2500 insecticide formulations and used in agriculture, veterinary medicine, and the prevention of vector-borne diseases.²

2.2 Why the idea took hold

Because it is elegant. Rather than searching for a new molecule that kills, you disable the machinery the insect uses to survive the molecule you already have, which extends the useful life of an existing chemistry at low cost.

That is the same logic this journal found behind every other response to resistance, and §18 shows it arriving at the same destination.

2.3 Which makes the evidence position strange

An ingredient in that many products, in use for eighty years, whose performance against resistant populations is described in current literature as unexpectedly weak and whose mechanism in that respect is stated to be unknown.² Section 15 is where that lands.

3. The enzymes it blocks

What is being inhibited.

Piperonyl butoxide is the cytochrome P450 inhibitor, and it works by inhibiting the detoxifying cytochrome P450 enzymes, thus reducing the capacity of insects to metabolize and resist insecticides.²

Those enzymes are responsible for the biotransformation of xenobiotics.²

3.1 Why these enzymes exist at all

Not for insecticides. The cytochrome P450 family is ancient and present across the tree of life, including in us, where it handles most drug metabolism.

3.2 Xenobiotic means foreign compound

The system exists because insects encounter plant defensive chemistry constantly and must transform it into something excretable. It was not built to deal with insecticides; insecticides arrived into machinery that was already there.

Which is why resistance can appear quickly: the capacity is present and selection needs only to increase it, rather than invent it. That reading is ours.

4. The two kinds of resistance

The distinction the whole article turns on.

Studies have demonstrated target site insensitivity due to mutations in the voltage-gated sodium channel gene, also known as knockdown resistance, and separately that the overproduction of metabolic detoxification enzymes, such as cytochrome P450 monooxygenases, plays a critical role in insecticide resistance.³

4.1 Both are established

Target site insensitivity in the sodium channel has spread across the world,⁶ and metabolic detoxification plays a critical role alongside it.³ Neither is speculative and neither is rare.

4.2 Two entirely different problems

Target site insensitivity changes the lock. Metabolic resistance destroys the key before it arrives.

A synergist addresses only the second. An insect whose sodium channel no longer binds the compound is unaffected by anything that improves delivery to that channel.

4.3 And insects carry both

Insects can develop several of these mechanisms simultaneously, becoming super-insecticide-resistant.²

This journal's article on layered resistance in bed bugs described exactly that accumulation. The present article is about what happens when you attack one layer of it.

5. Why the distinction matters

The practical consequence.

A synergised product used against a population whose resistance is largely metabolic should work considerably better than the unsynergised one. Used against a population whose resistance is largely target site, it should work about the same.

5.1 The asymmetry in the error

Choosing a synergised product where resistance is metabolic is correct. Choosing one where resistance is target site wastes money and delivers nothing, but it does not make the situation worse than the unsynergised product would have been.

Except, possibly, in the light of §15. That is the only circumstance in the article where the choice could actively cost something, and it is a recent finding rather than an established one.

5.2 And nobody knows which they have

Not without testing. This journal's article on molecular resistance diagnostics found no routine testing available to this trade, and the consequence appears here: the decision to use a synergised product is made without the information that determines whether it will help.

6. The three synergists

The toolkit, which is larger than most people realise.

One study used triphenyl phosphate for carboxylesterase inhibition, diethyl maleate for glutathione S-transferase inhibition, and piperonyl butoxide for cytochrome inhibition, with insecticides and synergists mixed in a 1:4 ratio.⁷

6.1 Why most people only know one

Because piperonyl butoxide is the one that appears on consumer labels. The other two are research reagents rather than registered product components, which is why a technician can work for years knowing the name of one synergist and not the concept.

6.2 Three enzyme families

Cytochrome P450 monooxygenases, carboxylesterases and glutathione S-transferases are the three principal detoxification systems, and each has a specific inhibitor.

6.3 The ratio is worth noticing

One part insecticide to four parts synergist.⁷ The non-insecticidal component is the bulk of the mixture, which matches what a product label often shows and explains why the synergist frequently appears at the higher concentration.

7. Synergists as diagnostics

The use this journal finds most interesting, and it is not pest control at all.

Researchers used the inhibitor to assess the role of P450s in deltamethrin resistance in three field-collected bed bug strains.¹ A separate study inferred that resistance in certain populations is likely due to metabolic resistance mediated by cytochrome P450 because the inhibitor synergised.⁷

7.1 Why this is the most useful thing in the article

Because it turns a product additive into an instrument. A synergist bioassay answers a question about a specific population in a specific building, which is the question that determines what chemistry to reach for.

7.2 The logic

If blocking an enzyme family restores susceptibility, resistance was running through that family. If it does not, resistance is somewhere else.

Three synergists therefore partition resistance between three mechanisms, using a bioassay rather than a sequencing facility. For a trade with no access to molecular diagnostics, that is a considerably more attainable method than the one this journal previously described.

7.3 What it cannot tell you

The proportion attributable to target site insensitivity, except by subtraction. Whatever resistance remains after all three enzyme families are blocked is attributable to something else, and inferring a quantity by residual is weaker than measuring it.

It also assumes each inhibitor blocks its family completely, which §14 suggests is optimistic when a single family may contain ten overexpressed genes with different properties.

8. The bed bug experiment

The principal evidence, and it is directly about this trade's pest.

Researchers used the inhibitor to assess the role of P450s in deltamethrin resistance in three field-collected bed bug strains, LA-1, CIN-1 and WOR-1, and additionally exposed two highly resistant strains, CIN-1 and WOR-1, resistance ratio greater than 2,500-fold, to dry residues of piperonyl butoxide-synergized pyrethroid formulations to determine the utility of synergism.¹

8.1 Three strains from the field

LA-1, CIN-1 and WOR-1, collected from real infestations rather than maintained in a laboratory.¹ That is the right population to test, because a laboratory strain has been selected by whatever the laboratory did to it.

It is also why the variability in §9 is meaningful rather than an artefact: three separate infestations produced three separate resistance profiles.

8.2 Two thousand five hundred fold

Meaning the dose required to achieve the same effect was over two and a half thousand times the dose required against a susceptible strain.

Numbers at that scale are not a reduction in performance. They are a chemistry that has stopped being an option, which is the position this journal's articles on bed bug control have described throughout.

9. What the synergistic ratios show

The headline result.

Piperonyl butoxide synergized deltamethrin in all three strains, but its impact was variable. The synergistic ratio varied from 40 in CIN-1 to 176 in WOR-1.¹

9.1 Every strain responded

Synergism occurred in all three, which establishes that cytochrome P450 enzymes were contributing to deltamethrin resistance in all three.¹ The question was never whether the mechanism was present; it was how much of the problem it accounted for.

9.2 What a synergistic ratio of 176 means

That adding the inhibitor made the insecticide 176 times more effective against that strain. In isolation that is a spectacular number.

9.3 The variability is the finding

Forty against one hundred and seventy-six, between two strains of the same species, tested in the same laboratory in the same experiment.

A four-fold difference in how much help the synergist provides means the answer to does it work is a property of the population in the building rather than of the product in the can. Two clients with the same pest, treated with the same product by the same technician, can get different outcomes for reasons neither of them can see.

10. What was left afterwards

The number that matters operationally, and it inverts the previous section.

The resistance ratio for each strain after piperonyl butoxide treatment was 174 and 39, respectively.¹

10.1 How the two numbers relate

Resistance ratio before, divided by synergistic ratio, gives resistance ratio after. More than 2,500 divided by 40 is more than 60, and the reported figure is 174.¹

The arithmetic does not close on the greater-than figure, which is expected: greater than 2,500 is a floor rather than a value, and the true starting ratios evidently differed between the two strains. We note it rather than resolve it.

10.2 The inversion

The strain the synergist helped least, with a ratio of 40, was left 174-fold resistant. The strain it helped most, at 176, was left 39-fold resistant.

Those are not the same ranking read twice; they are two different questions. How much did it help is not the same as how resistant is it now, and only the second determines whether the treatment works.

10.3 Thirty-nine fold is still resistant

The best outcome in the experiment was a strain requiring thirty-nine times the dose of a susceptible one.

A product applied at label rate is applied at label rate. Nothing about a synergist permits the rate to be multiplied, which the label article in this journal established is a binding condition rather than a starting point. So a thirty-nine-fold gap between what the label delivers and what the insect needs is not a gap anybody is permitted to close.

11. The authors' conclusion

Stated carefully by the people who ran the experiment.

The results suggest that P450s have some involvement in deltamethrin resistance, but other resistance mechanisms must be involved as well. And addition of PBO to pyrethroids is not a comprehensive solution to pyrethroid resistance because strains vary in both overall resistance level and the proportion of that resistance attributable to P450s.¹

11.1 Two variables, not one

Overall resistance level, and the share of it running through the enzymes the synergist blocks. Both vary between populations and neither is knowable without testing.

A product choice that depends on two unmeasured quantities is a guess with a mechanism attached, and that is our characterisation rather than theirs.

12. The formulated product result

A detail with a direct commercial implication that is easy to miss.

No significant synergistic effect of formulated deltamethrin was observed with the addition of synergized pyrethrins or formulated piperonyl butoxide in the CIN-1 strain, but synergism occurred in the WOR-1 strain.¹

12.1 Dry residues specifically

The exposure was to dry residues of the synergised formulations,¹ which is how a residual product is actually encountered: deposited, dried, and crossed some time later.

Piperonyl butoxide is volatile enough that its persistence in a dried deposit is a fair question, and if the synergist leaves faster than the insecticide then a dry residue is an unsynergised residue. We found no source addressing that and raise it as a hypothesis.

12.2 Technical material against formulated product

The synergistic ratios in §9 come from assessing the role of the enzymes. This result concerns dry residues of actual commercial formulations,¹ which is what a technician applies.

In one of the two strains, the commercial synergised products produced no significant effect. The mechanism was demonstrable and the product still did not deliver it.

12.3 Why those can differ

Formulation, deposit, availability from a dry residue and the ratio present in a registered product are all different from a laboratory exposure. This journal's articles on formulation and on substrate dealt with that gap at length.

The lesson is the one those articles reached: a mechanism established in a bioassay is a hypothesis about a building, and the step from one to the other is where most of what this trade believes gets lost.

13. The bednet evidence

The largest-scale deployment of this idea, and it is more positive.

In one mosquito population, pre-exposure to piperonyl butoxide followed by permethrin exposure in bottle bioassays led to partial restoration of susceptibility, the synergist being described as one that can block pyrethroid-metabolizing enzymes in a mosquito.⁴

The work investigated resistance in two species during a cluster randomized trial, and describes successful deployment of PBO-treated bednets.⁴

13.1 Why bednets are the best test available

A cluster randomised trial with an epidemiological endpoint is a stronger design than anything available in structural pest control, where §22 of the rotation article explained why the equivalent experiment cannot be funded.

So the mosquito literature is where the real answers about synergists will come from, even though the insect is not ours.

13.2 Partial restoration

The phrase does real work and is consistent with §10: better, not fixed.

13.3 The resistance characterisation

Diagnostic dose bioassays with intensity assays suggest pyrethroid resistance that is both strong and very common, but not extreme.⁴

Not extreme is the operative qualifier, and it may be why the synergist worked here and less well against bed bug strains at over 2,500-fold. A partial fix applied to a moderate problem can be sufficient; applied to an extreme one it is not. That inference is ours.

14. How many enzymes are involved

A detail that explains why the answer is never simple.

Transcriptomic analysis found multiple P450 genes over expressed, naming six in one mosquito species and four in another.⁴

14.1 Ten genes across two species

Six overexpressed cytochrome P450 genes named in one mosquito species and four in another.⁴ Different sets in each, which means the resistance is not one solution that spread but several arrived at independently.

14.2 Not one enzyme

Resistance mediated by cytochrome P450 is not a single gene switched on. It is a set of related enzymes upregulated together, which is why the trait behaves as the polygenic character this journal's rotation article described rather than as a single mutation.

It also means a synergist has to inhibit a family rather than an enzyme, and how completely it does so across ten different gene products is not something a single figure captures.

15. The unexpected finding

The reason this article exists in its current form.

Recent studies, however, reveal an unexpectedly weak restoration of pyrethroid efficacy by PBO, but the underlying mechanism is unknown.² The same work notes that recent studies raise questions about the unexpectedly weak performance of PBO in enhancing the pyrethroid efficacy without providing an explanation thereof.⁵

15.1 The phrasing worth noticing

Unexpectedly weak.² Not absent, and not disputed. The field result was smaller than the mechanism predicted, and the gap was large enough that people went looking for an explanation.

15.2 The experiment

Researchers exposed susceptible mosquitoes of a standard laboratory strain sequentially for 1 hour to piperonyl butoxide, at 0.1 to 4 per cent, and then for 1 hour to filter papers impregnated with a type II pyrethroid, and repeated the design in a second laboratory in another country at a different insecticide concentration.²

15.3 Why using a susceptible strain is the clever part

A susceptible strain has no metabolic resistance to relieve. If the synergist works only by removing detoxification, it should do nothing at all here.

Testing the synergist where it has no job to do isolates whatever else it is doing, and the answer appears to be that it is doing something. Running it in two laboratories in different countries at different insecticide concentrations is the other thing a sceptical reader would ask for.

16. The proposed mechanism

What the researchers concluded.

Here, we demonstrate that the PBO-induced inhibition of cytochrome P450 impedes the effect of deltamethrin by mainly affecting its interaction with the inactivated state of voltage-gated sodium channels. The results reveal a novel mechanism of insect adaptation to stress conditions that decreases insecticide efficacy.²

16.1 Why anyone would look for this

Because a discrepancy existed. The metabolic argument predicts that relieving detoxification should restore efficacy substantially, and field performance did not match that.⁵

A mechanism was then sought for the gap rather than the gap being attributed to field conditions, which is the harder and better move.

16.2 Reading the word impedes

Not fails to help. Impedes.² The claim is that inhibiting the enzyme reduces the insecticide's effect through its action at the target site.

16.3 Why the inactivated state matters

A sodium channel cycles between resting, open and inactivated conformations, and pyrethroids bind preferentially to particular states. This journal's article on layered resistance in bed bugs dealt with the sodium channel as a target; the claim here is that P450 inhibition changes how much of the channel population sits in the state the insecticide needs.

If that is right, the synergist has a second effect on the target that partly cancels its first effect on metabolism, which would explain a weak net result without anything being wrong with either mechanism individually.

17. How to hold that result

The scepticism this deserves, stated before anybody acts on it.

It is a single research group's finding, reported in preprint and subsequently in a journal version, on one insect species with one insecticide class.²⁵

17.1 The preprint question

We cite both a preprint and a journal version of the same work.²⁵ The journal version means it has been through review, which is reassurance about the reporting rather than about whether the finding will replicate.

17.2 What it does not overturn

The bed bug experiment, which found genuine synergism in all three strains including ratios of 176.¹ Whatever the new mechanism is, it did not prevent synergism there.

Nor the bednet deployment, described as successful.⁴

17.3 What it would explain

Why the field performance of synergised products has been disappointing relative to what the metabolic argument predicts, which is a puzzle the literature states plainly rather than one this journal has invented.²

17.4 What we would want next

The same sequential-exposure design run on a metabolically resistant strain and a susceptible one side by side, which would separate the relief of detoxification from the effect at the channel. We do not know whether that has been done.

18. Resistance to the synergised product

The development that should surprise nobody who has read this journal's resistance articles.

Work exists on emerging mosquito resistance to piperonyl butoxide-synergized pyrethroid insecticide and its mechanism.³

18.1 Why this was predictable

A synergist creates selection on the enzymes it inhibits and on anything that reduces its uptake or increases its own metabolism. Adding a compound to a mixture does not exempt that compound from selection.

18.2 What that means

A combination product designed to defeat resistance is itself being resisted. Adding a second compound produced a new selection pressure and the population responded to it, which is the sequence this journal documented for every other chemistry.

It also means the synergist is not outside the resistance treadmill. It is another rung on it, and the treadmill this journal described for anticoagulants, for pyrethroids and for phosphine has the same shape here.

19. Metabolic cross-resistance

The finding with the widest implications, from work on entirely different chemistry.

Researchers found that the inhibitor significantly enhanced the efficacy of fenpyroximate and tolfenpyrad, fully restoring mortality in one resistant strain, and concluded that the Complex I inhibitors tested are susceptible to metabolic cross-resistance and may lack efficacy in controlling pyrethroid resistant mosquitoes.⁸

19.1 The screening context

That work was developing new vector control products and testing whether they would survive existing resistance, concluding that the assays used are a useful guide in the development of new vector control products.⁸

So the cross-resistance was found before deployment rather than after, which is the right order and is not how this journal's resistance articles usually end.

19.2 What that sentence establishes

Compounds from a completely different mode of action group, acting on mitochondrial respiration rather than sodium channels, were metabolised by the same enzymes that metabolise pyrethroids.⁸

The resistance crossed the mode of action boundary, because the detoxification system does not care what class a molecule belongs to. It cares whether the molecule is a substrate.

20. What that does to rotation

The consequence for the strategy this journal examined a few articles ago.

The rotation article identified cross resistance as one of four parameters driving whether a resistance management strategy works, and noted that two products in different named groups sharing a detoxification mechanism are not a rotation in any meaningful sense.

20.1 Why the two taxonomies differ

Mode of action groups are organised by target: sodium channel, acetylcholinesterase, chitin synthesis, mitochondrial complex. Detoxification enzymes are organised by chemistry: what a molecule looks like and whether it can be oxidised, hydrolysed or conjugated.

Two compounds can hit completely different targets and present the same handle to the same enzyme, or hit the same target and be handled quite differently. Nothing requires the two classifications to agree.

20.2 This is the demonstration of that

A new class, developed specifically because the old one had failed, turning out to be a substrate for the enzymes that caused the old one to fail.⁸

Rotating out of pyrethroids into that class would have looked like sound resistance management and would have delivered a compound the population was already equipped to handle.

20.3 The uncomfortable general point

Mode of action group tells you how a compound kills. Metabolic resistance is about how a compound is destroyed, and the two are not the same taxonomy.

A rotation built on mode of action groups is managing target site resistance and may be doing nothing at all about metabolic resistance. That is our conclusion and we think it is the most important sentence in this article.

21. What we take from this

A synergist is not an insecticide. It removes a defence rather than adding a weapon.³

It addresses one of two resistance mechanisms. Target site insensitivity is untouched.³

It helps variably and incompletely. Two strains, ratios of 40 and 176, left at 174-fold and 39-fold.¹

Formulated products may not deliver the laboratory effect. One strain showed no significant effect from synergised formulations.¹

It is most useful as a diagnostic. Three synergists partition resistance across three enzyme families without a laboratory.⁷

And metabolic resistance crosses classes. Which means a rotation by mode of action group may not be a rotation at all.⁸

Do not assume a synergised product solves a failure. If a pyrethroid has stopped working, the reason may be the mechanism the synergist cannot touch.¹

And read the guarantee panel. A product whose largest listed active kills nothing is telling you something about the problem it was built for.

22. Limitations and open questions

The bed bug study is from 2009. Bed bug resistance has developed further since, and the strains tested are specific field collections rather than a sample.¹

We read abstracts. The synergistic ratios, resistance ratios and formulated product results come from abstract text, and we have not seen the concentrations, exposure times or statistical detail behind them.¹

The impeding result is recent and narrow. Recorded in §17, and we are not in a position to evaluate the electrophysiology behind it.²⁵

Most of the evidence is mosquito work. Vector control drives this literature, and the applicability of bednet findings to structural treatment of a different insect is an assumption rather than a demonstration.³⁴⁸

The synergist diagnostic is untested here. Section 7 describes a method used by researchers on collected strains. Whether it could be operationalised for a pest control company is something we have not established and would not assert.

Nothing on non-target or human exposure. An ingredient that inhibits cytochrome P450 enzymes raises obvious questions about other organisms that possess them, and we have not examined that literature at all. It deserves its own paper.

Sections 3.1, 4.1, 5, 7.1, 7.2, 9.2, 10.1, 11.1, 12.2, 13.2, 14.1, 16.2, 17.3 and 20 are our reasoning. The xenobiotic argument, the lock-and-key distinction, the practical consequence, the diagnostic logic and its limit, the variability reading, the inversion, the two-variable characterisation, the bioassay-to-building point, the not-extreme inference, the polygenic connection, the channel state reasoning, the experiment we would want, and the conclusion about rotation are ours rather than sourced positions.

23. Conclusion

Piperonyl butoxide is in over two and a half thousand formulations and does not kill anything.² It blocks the enzymes insects use to metabolise foreign compounds, which is one of the two principal routes to pyrethroid resistance and is not the only one.³

Against three field-collected bed bug strains it synergised deltamethrin every time, with ratios from 40 to 176, and left two strains that had been more than 2,500-fold resistant at 174-fold and 39-fold.¹ Those are real gains and they are not enough, and the researchers said so: not a comprehensive solution, because strains differ both in how resistant they are and in how much of it runs through the enzymes being blocked.¹ In one of those strains, the commercial synergised formulations produced no significant effect at all.¹

The part we will be thinking about is §19. Compounds from a different mode of action group, developed because pyrethroids had failed, were metabolised by the same enzymes that metabolise pyrethroids.⁸ Mode of action describes how a molecule kills; detoxification does not sort molecules that way. A rotation built on mode of action groups is managing the resistance you can name and may be doing nothing whatever about the resistance you cannot.

References

1. Romero, A., Potter, M. F. and Haynes, K. F. (2009). Evaluation of Piperonyl Butoxide as a Deltamethrin Synergist for Pyrethroid-Resistant Bed Bugs. Journal of Economic Entomology, 102(6), 2310 to 2315. doi:10.1603/029.102.0637. Principal source. Used for the statement that an understanding of insecticide resistance mechanisms in the bed bug has the potential to lead to new approaches for control of resistant populations; for the study design in which the cytochrome P450 monooxygenase inhibitor piperonyl butoxide was used to assess the role of those enzymes in deltamethrin resistance in three field-collected strains, and in which two highly resistant strains with a resistance ratio greater than 2,500-fold were exposed to dry residues of piperonyl butoxide-synergized pyrethroid formulations to determine the utility of synergism; for the results that piperonyl butoxide synergized deltamethrin in all three strains but with variable impact, that the synergistic ratio varied from 40 in one strain to 176 in another, and that the resistance ratio for each strain after piperonyl butoxide treatment was 174 and 39 respectively; for the finding that no significant synergistic effect of formulated deltamethrin was observed with the addition of synergized pyrethrins or formulated piperonyl butoxide in one strain while synergism occurred in the other; and for the authors' conclusions that the results suggest cytochrome P450 enzymes have some involvement in deltamethrin resistance but that other resistance mechanisms must be involved as well, and that addition of the synergist to pyrethroids is not a comprehensive solution to pyrethroid resistance because strains vary in both overall resistance level and the proportion of that resistance attributable to those enzymes. https://academic.oup.com/jee/article-abstract/102/6/2310/2199344
2. Cytochrome P450 inhibition impedes pyrethroid effects on insects through Nav channel regulation. Preprint. Used for the statements that massive use of pyrethroids led to the emergence and spread of resistance through behavioural, physiological and genetic mechanisms including target site modifications, that insects can develop several of these mechanisms simultaneously and so become highly insecticide-resistant, and that upregulation of detoxifying cytochrome P450 enzymes responsible for the biotransformation of xenobiotics is one of the most prevalent mechanisms of pyrethroid resistance; for the description of piperonyl butoxide as a cytochrome P450 inhibitor widely used as a synergist in combination with pyrethroids to enhance their overall effectiveness, developed in the 1940s and included in over 2,500 insecticide formulations used in agriculture, veterinary medicine and the prevention of vector-borne diseases; for the statement that recent studies reveal an unexpectedly weak restoration of pyrethroid efficacy by the synergist with the underlying mechanism unknown; for the experimental design in which susceptible mosquitoes of a standard laboratory strain were sequentially exposed for one hour to the synergist at 0.1 to 4 per cent and then for one hour to filter papers impregnated with a type II pyrethroid, replicated at two institutions in different countries at different insecticide concentrations; and for the conclusion that the synergist-induced inhibition of cytochrome P450 impedes the effect of deltamethrin by mainly affecting its interaction with the inactivated state of voltage-gated sodium channels, revealing a mechanism of insect adaptation to stress conditions that decreases insecticide efficacy. https://www.biorxiv.org/content/10.1101/2025.02.03.636193v1.full
3. Emerging Mosquito Resistance to Piperonyl Butoxide-Synergized Pyrethroid Insecticide and Its Mechanism. Journal of Medical Entomology, 59(2), 638. Used for the statements that studies have demonstrated target site insensitivity due to mutations in the voltage-gated sodium channel gene, also known as knockdown resistance, and that overproduction of metabolic detoxification enzymes such as cytochrome P450 monooxygenases plays a critical role in insecticide resistance in mosquitoes; for the account that to enhance the killing power of insecticides, synergized insecticides were developed by mixing insecticide synergists with pyrethroids, with citations to work from the late 1940s and early 1950s; for the definition that the insecticide synergist by itself does not harm insects at low concentrations but instead enhances insecticidal toxicity by inhibiting metabolic enzyme activities; and for the existence of work on emerging resistance to the synergized combination itself. https://academic.oup.com/jme/article/59/2/638/6512082
4. Expression of pyrethroid metabolizing P450 enzymes characterizes highly resistant Anopheles vector species targeted by successful deployment of PBO-treated bednets in Tanzania. PubMed Central PMC8786186. Used for the account of a mosquito population in which pre-exposure to piperonyl butoxide followed by permethrin exposure in bottle bioassays led to partial restoration of susceptibility, with the synergist described as one that can block pyrethroid-metabolizing enzymes; for the investigation of insecticide resistance profiles and underlying mechanisms in two vector species during a cluster randomized trial; for the finding from diagnostic dose bioassays together with intensity assays suggesting pyrethroid resistance that is both strong and very common but not extreme; and for the transcriptomic analysis finding multiple cytochrome P450 genes overexpressed, six named in one species and four in another. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8786186/
5. Cytochrome P450 inhibition impedes the insecticide pyrethroid effects on insects through Nav channels regulation. PubMed Central PMC13270567, journal version of the preprint. Used for the statements that insecticides in various formulations are indispensable for agricultural pest control and the prevention of vector-borne diseases; that the synergist is widely used to enhance the effectiveness of insecticides, notably pyrethroids, by inhibiting detoxifying cytochrome P450 enzymes and thus reducing the capacity of insects to metabolise and resist insecticides; that recent studies have revealed an unexpectedly weak enhancement of pyrethroid efficacy by the synergist which may have serious implications for insect control, with the underlying mechanism unclear; and that recent studies raise questions about the unexpectedly weak performance of the synergist in enhancing pyrethroid efficacy without providing an explanation. https://pmc.ncbi.nlm.nih.gov/articles/PMC13270567/
6. Emerging mosquito resistance to piperonyl butoxide-synergized pyrethroid insecticide and its mechanism. Preprint version. Used for the statements that target site insensitivity due to sodium channel mutations has spread across the world, that overproduction of metabolic detoxification enzymes also plays a critical role, and that piperonyl butoxide, discovered in the 1940s, is one of the earliest insecticide synergists. https://www.biorxiv.org/content/10.1101/2021.09.29.462303.full.pdf
7. Repository record of the bed bug synergist study together with citing research on synergist-based resistance characterisation. Used for the description of synergists used in a resistance study, being triphenyl phosphate for carboxylesterase inhibition, diethyl maleate for glutathione S-transferase inhibition and piperonyl butoxide for cytochrome inhibition, with insecticides and synergists mixed in a one to four ratio; and for the inference in that work that resistance in the populations studied is likely due to metabolic resistance mediated by cytochrome P450 because the inhibitor synergised. https://www.researchgate.net/publication/41011291_Evaluation_of_Piperonyl_Butoxide_as_a_Deltamethrin_Synergist_for_Pyrethroid-Resistant_Bed_Bugs
8. New insecticide screening platforms indicate that Mitochondrial Complex I inhibitors are susceptible to cross-resistance by mosquito P450s that metabolise pyrethroids. PubMed Central PMC7530702. Used for the context that the mainstay of bednet insecticides has been members of the pyrethroid class of voltage-gated sodium channel modulators, due to rapid knockdown effect against mosquito vectors and low toxicity to humans; for the finding that the cytochrome P450 inhibitor piperonyl butoxide significantly enhanced the efficacy of fenpyroximate and tolfenpyrad, fully restoring mortality in one resistant strain exposed to fenpyroximate; and for the conclusion that the Complex I inhibitors tested are susceptible to metabolic cross-resistance and may lack efficacy in controlling pyrethroid resistant mosquitoes. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7530702/