The Same Product, Twice the Result: Substrate, Formulation and Why a Residual Application Fails on Concrete

1. Introduction: the variable nobody specifies

A residual application is normally described by three things: the active ingredient, the concentration, and the area treated. The label specifies all three. What the label cannot specify is the surface, and the surface is frequently the variable that decides the outcome.

This journal has examined why treatments fail for reasons of scope, behaviour, resistance and timing. This paper covers a simpler and more mechanical failure: the product was applied correctly at the correct rate, and the insect walked across it without picking up enough to die.

1.1 Why this is under-discussed

Efficacy data supporting registration is generated under controlled conditions on standardised substrates. The field is not standardised. A technician treating a commercial kitchen crosses stainless steel, sealed tile, bare concrete, painted drywall and untreated timber within a few metres, and the same product behaves differently on each.

2. The rove beetle comparison

The study that supplies the opening figure is a clean design and worth setting out in full.

The contact toxicity of four insecticide formulations, deltamethrin, fipronil, fenitrothion and imidacloprid, applied on three different substrates, tile, plywood and concrete, was evaluated against the adult rove beetle Paederus fuscipes.¹

2.1 Speed of action

The relative order of speed of killing effects was deltamethrin, then imidacloprid, then fipronil, then fenitrothion.¹

2.2 The substrate result

Although deltamethrin showed the fastest action, the recovery rate of rove beetles at 48 hours post-treatment was moderate, approximately 25 per cent, on the tile surface, and high, approximately 80 per cent, on the plywood surface. Thus it is likely that the insects did not pick up the lethal dose especially on porous surfaces.¹

2.3 Why knockdown is misleading here

The distinction between knockdown and mortality is the important one. Deltamethrin was the fastest acting compound tested, so an observer watching the assay on plywood would have seen insects collapse promptly and concluded the treatment worked.

Four fifths of them then recovered.¹ A technician who judges a treatment by whether insects drop is measuring the wrong endpoint, and on a porous surface that error is systematic rather than occasional.

3. Thirty eight weeks against a few

The stored product literature supplies the widest reported gap.

Work on liposcelid psocids reported that pyrethroids had much higher long-term toxicity effects, up to 38 weeks, when applied to galvanised steel, a non-porous surface, compared with concrete, a porous surface.¹

3.1 What that difference means in practice

Thirty eight weeks is most of a year. A programme built around a pyrethroid residual on steel and one built around the same product on concrete are not the same programme, and no reasonable service interval reconciles them.

3.2 The general finding in stored product work

Residual efficacy is not a property of the product alone. It is influenced by insect biology, formulation characteristics, treated surface, food availability, and time after treatment.²

Porous substrates such as wood may sequester lipophilic residues and reduce transfer to insects, whereas concrete may further alter persistence through physicochemical interactions, and such substrate-driven effects are repeatedly reported in surface-residual bioassays.²

The word sequester is precise. The active ingredient has not degraded. It is present and unavailable.

4. A year on plastic, almost nothing on cement

A third line of evidence comes from vector control, where residual persistence is measured because it determines spray cycle length.

Researchers investigated the residual efficacy of an insecticide paint containing two organophosphates, chlorpyrifos and diazinon, together with the insect growth regulator pyriproxyfen, on commonly used porous surfaces, cement and stucco, and non-porous surfaces, softwood and hard plastic, against two mosquito species. They reported high mortality even one year after treatment on non-porous surfaces, but low mortality on concrete.⁴

4.1 Why a paint formulation is informative

An insecticide paint is designed to hold active ingredient in a film on the surface, which is the most favourable possible case for a porous substrate. It still failed on concrete.

4.2 The malaria programme perspective

A test house trial in the Brazilian Amazon evaluated six formulations, four pyrethroids, a carbamate and an organophosphate, applied to painted wood, unpainted wood, plastered cement and unplastered cement.⁶

The study framing is worth quoting because it states why this matters institutionally: it is essential to understand the residual efficacy of insecticides on different surfaces to determine spray cycles, ensure their rational use, and prevent wastage.⁶

The finding was low residual bioefficacy of pyrethroids on cement surfaces compared with wooden ones, confirming previous observations, and it was noted that the activity of pyrethroids can be compromised by rapid degradation on porous surfaces with high absorption.⁶

5. The absorption mechanism

The physical explanation is straightforward and has been understood for a long time.

The reduction in insecticide toxicity on porous surfaces might be because of rapid absorption of insecticide into the porous substrate.¹

With a more porous surface, the absorption of a freshly applied insecticide will be higher, resulting in less insecticide available on the surface to contact the insect.³

5.1 The age of the observation

This is not new. The literature includes work from 1949 on loss of insecticides by absorption into mud and vegetation, and from 1952 on factors affecting the availability of contact insecticides.³

Seventy five years of published work on the availability of contact insecticides, and the practical specification of a residual treatment still stops at active ingredient and rate.

5.2 The substrate ranking

In the triatomine work, highest mortality rates above 50 per cent were observed on wood blocks up to three months post-spraying. Mud was the substrate on which treatments showed lowest persistence, with the other two substrates showing intermediate residual efficacy. Porous surfaces, especially mud, showed most variability presumably due to absorption, while the less porous surfaces, wood and lime-coated mud, kept mortality rates high for longer post-treatment irrespective of the insecticide concentration used.³

5.3 The rate does not rescue it

That final clause deserves emphasis. Persistence on the less porous surfaces held up regardless of concentration, which is the inverse of the intuition that a difficult surface can be compensated for by applying more.

Where the mechanism is sequestration rather than degradation, increasing the rate increases what is absorbed as well as what remains available, and the ratio does not necessarily improve.

6. Concrete as a chemical environment

Concrete presents a second problem independent of porosity.

The alkaline nature of concrete likely slows down the effect of an insecticide.¹

6.1 Why alkalinity matters

Many insecticidal actives are susceptible to alkaline hydrolysis, meaning the molecule is broken down by reaction in a high pH environment. Fresh concrete is strongly alkaline and remains so for a long period.

So a deposit on concrete faces two independent losses: physical absorption into the pore structure, and chemical degradation at the surface it remains on.

6.2 The compounding

This is why concrete recurs as the worst case across these studies rather than simply appearing partway along a porosity gradient. Mud is more absorbent, but concrete adds a chemistry problem that mud does not.

7. Sorption kinetics and the reservoir effect

Work measuring the movement of a pyrethroid into concrete adds detail that complicates the simple picture.

A study of sorption and desorption of permethrin on concrete surfaces reported rapid sorption of the pyrethroid due to the concrete pores.⁴

7.1 The biphasic loss

The loss of recovery appears to be biphasic, in which the initial loss rate, in the first hour, is fast and is followed by a slower secondary loss rate.⁴

Most of the damage is done in the first hour. That has a direct practical implication: the interval between application and the surface drying is when the substrate takes its share, and anything that shortens or alters that window changes how much remains available.

7.2 The reservoir

The more surprising result is what stays. After 300 hours of desorption tests, more than 20 per cent of the initial concentration remained on concrete, suggesting that concrete may act as an insecticide reservoir.⁴

7.3 Why both matter

For efficacy, absorption is a loss. For exposure and environmental persistence, it is a store.

A programme that repeatedly treats the same concrete floor is achieving diminishing insecticidal return while accumulating a reservoir, which is an unfavourable combination on both axes and worth stating plainly given that repeated floor and perimeter spraying is standard practice in many commercial contracts.

8. Where the contradictions are

The literature is not unanimous and it would be misleading to present it as such.

There are cases where insecticides were more effective on concrete than on other surfaces.⁴

One malaria test house study found low pyrethroid bioefficacy on cement compared with wood, confirming previous observations, but diverging from the findings of another study.⁶

8.1 How to hold the disagreement

Substrate effects depend on the specific active, the specific formulation, the specific substrate preparation and the target species. Concrete that is sealed, painted, polished or aged behaves differently from fresh bare concrete, and studies rarely characterise it in enough detail for comparison.

8.2 The defensible general claim

What survives the disagreement is narrower than the strongest version of the argument: substrate materially affects residual efficacy, frequently by large margins, and the direction of the effect is usually but not always unfavourable on porous surfaces.

That is enough to justify treating substrate as a specification variable, which is the argument of this paper. It is not enough to justify a rule of thumb applied without checking.

9. Formulation: what the water leaves behind

If the problem is that active ingredient goes into the substrate, the obvious countermeasure is a formulation that keeps it on top. This is precisely what formulation chemistry does, and it is why the same active is sold in several forms.

Pesticides are formulated into baits, concentrated emulsions, dusts, emulsifiable concentrates, fumigants, gels, granules, microencapsulations, granules, suspension concentrates, suspoemulsions, tablets, water dispersible granules and wettable powders, among others. A pesticide is rarely suitable for application in its pure form, and other substances are added to permit ease of application, handling, transportation and storage, and maximum pesticide activity.⁷

9.1 The organising question

For residual performance, the question that matters is what remains after the carrier evaporates and where it sits relative to the surface plane.

A dissolved active carried in solvent goes wherever the solvent goes. A solid particle suspended in water is left behind when the water leaves. That difference drives most of what follows.

10. Emulsifiable concentrates

The most familiar formulation is also the most vulnerable to porous substrates.

An emulsifiable concentrate contains liquid active ingredient, one or more petroleum-based solvents, and an agent that allows the formulation to be mixed with water to form an emulsion.⁸ These comprise a concentration of pesticide, from about 50 to about 500 grams per litre, dissolved in a carrier that is either a water miscible solvent or a mixture of water-immiscible organic solvent and emulsifiers, with useful solvents including aromatics such as xylenes and high-boiling petroleum fractions.⁹

10.1 The advantages

Emulsifiable concentrates are widely used because they are relatively easy to prepare and can be easily used by dilution with water.¹⁰ They leave little visible residue on treated surfaces.⁸

10.2 The disadvantages

Since organic solvents are used, they have problems in points of toxicity and inflammability. Further, in the case of active ingredients which are very poorly soluble in organic solvents, formulating them into emulsifiable concentrates has been virtually impossible.¹⁰ They may also be corrosive.⁸

10.3 The porosity problem

The little visible residue property is an advantage aesthetically and a liability mechanically. A formulation that leaves nothing visible on a non-porous surface has, on a porous surface, largely gone somewhere.

We should be clear this is our reasoning rather than a directly cited finding. The sources establish that porous substrates absorb applied insecticide¹³ and separately that emulsifiable concentrates deliver the active in solution with little visible surface residue.⁸ The inference that solution-based formulations are most affected by absorption follows, and §13 provides supporting comparative data.

10.4 The knockdown advantage of solvents

One property of solvent-based formulations is worth isolating. Oil solutions of a pesticide usually provide faster knockdown and kill than other formulations, due to the solvents themselves having pesticidal action and to dissolution of the waxy covering of the integument increasing the speed of uptake.¹¹

This connects directly to the resistance architecture discussed elsewhere in this journal, where cuticular thickening reduces penetration rate. A solvent that dissolves cuticular wax is acting against that specific defence.

11. Wettable powders

The formulation with the opposite profile.

Wettable powders comprise an intimate mixture of the pesticide, a carrier, and surfactants, with pesticide concentration usually from about 10 to about 90 per cent by weight. The carrier is usually chosen from attapulgite clays, montmorillonite clays, diatomaceous earths or purified silicates, and surfactants comprise about 0.5 to 10 per cent.⁹

11.1 The physical state

In a wettable powder the active ingredient is not dissolved in petroleum solvents but rather very finely ground and diluted with an inert powder, with wetting agents added to aid mixing. Wettable powders must be continuously agitated in the spray tank because they are not in solution or emulsion, but rather held in suspension through agitation.¹²

11.2 Why that matters on a porous surface

A suspended solid particle cannot wick into a pore the way a solution can. When the water evaporates, the particle is left sitting on the surface.

That is the mechanical basis of the crossover result in §13, and it is why wettable powders remain the standard recommendation for porous substrates despite their handling disadvantages.

11.3 The costs

Wettable powders can be formulated even with active ingredients whose solubility in organic solvents is very poor, and since they ordinarily require no organic solvents the inflammability problem is eliminated. However, since they are in powder form, scattering may occur when they are formulated or diluted, leading to risk of operators inhaling the powder.¹⁰

They also leave visible residues, which is frequently unacceptable in occupied premises and is a substantial part of why the trade drifted toward less visible formulations.

12. Suspension concentrates

The intermediate case, and the one most commonly used in structural work today.

A flowable or liquid formulation combines many of the characteristics of emulsifiable concentrates and wettable powders. Manufacturers use these when the active ingredient is a solid that does not dissolve in either water or oil. The active ingredient, impregnated on a substance such as clay, is ground to a very fine powder, which is then suspended in a small amount of liquid, producing a thick product.⁸

12.1 The profile

Flowables share many features of emulsifiable concentrates and have similar disadvantages. They require moderate agitation to remain in suspension and leave visible residues similar to those of wettable powders.⁸

12.2 Why they dominate

They are easy to handle and apply,⁸ they avoid the inhalation hazard of dry powder handling, and they avoid the solvent load of an emulsifiable concentrate.

The trade-off is that the particle is finer and the liquid phase larger than a wettable powder, which on the §11.2 argument should make them somewhat more susceptible to substrate absorption. The next section indicates that this is what the data show.

13. The crossover finding

The triatomine study produced the most operationally useful formulation result in this literature.

Highest mortality rates above 50 per cent were observed for deltamethrin 2.5 per cent SC and lambda-cyhalothrin 10 per cent WP on wood blocks up to three months post-spraying.³

During the first 30 days, WP formulations were not as effective as SC flowable formulations, but overall in the longer term, WP gave greater mortality rates of nymphs exposed at up to six months post-spraying.³

13.1 What the crossover means

The two formulations have different decay curves. The suspension concentrate delivers more available active early and declines; the wettable powder starts lower and persists.

Neither is better in general. Which is better depends entirely on the timeframe over which the treatment has to work, which is a specification question rather than a product question.

13.2 The practical rule this suggests

Where the requirement is rapid reduction of an active population, the flowable performs better in the first month. Where the requirement is a long-lived barrier against reintroduction, the wettable powder performs better at six months.

A programme that needs both is not served by choosing one, and that is an argument for sequenced rather than uniform application, though we have not found a study testing that directly.

14. Microencapsulation

The formulation developed specifically to solve the residual problem.

Manufacturers cover liquid or dry pesticide particles in a plastic coating to produce a microencapsulated formulation. The formulated product is mixed with water and applied as a spray. Once applied, the capsule slowly releases the pesticide, and the encapsulation process can prolong the active life of the pesticide by providing a timed release of the active ingredient.¹³

14.1 How it reaches the insect

The pickup mechanism is different from every other formulation and is the key to understanding it.

Generally, when insects pass over a zone treated with microcapsules, these adhere to the insects' hair, legs and antennae. Once released, the insecticide diffuses from the capsule surface and is absorbed.¹⁴

14.2 Why this helps on difficult substrates

A capsule is a discrete solid object resting on the surface. It cannot wick into a pore as a solution can, and the polymer wall isolates the active from the substrate chemistry discussed in §6.

The stated purpose in the patent literature is explicit. Research into microencapsulation became intensive because enhancing residual efficacy with conventional emulsifiable concentrate, solubilised emulsion concentrate and oil solution formulations requires a high application dosage, which may bring about problems in safety.¹⁵

15. Tuning a capsule

The performance of an encapsulated product is a design parameter rather than a property of the active ingredient, which is unusual.

Pesticides can be microencapsulated by suspending particles or droplets in plastic polymers of various types. By altering the chemistry of the polymer or by changing factors in the processing, microcapsules can be formed of various sizes, solubility, wall thicknesses, and degrees of penetrability. These factors govern the speed with which the active ingredient within is released, which in turn affects the residual performance, speed of action, and odour of the product.¹¹

15.1 A worked specification

A patent for a microencapsulated cockroach-controlling composition specifies an insecticide having a 3-phenoxybenzyl group encapsulated in microcapsules having a polyurethane wall, with an average particle diameter of not more than 80 micrometres, a wall thickness of not more than 0.3 micrometres, and a value of average particle diameter divided by wall thickness of 100 to 400.¹⁵

That level of specificity exists because it has to. Residual efficacy of microencapsulated insecticides varies depending upon the particle diameter and the wall.¹⁵

15.2 Why two products with the same active differ

This explains something practitioners observe and generally attribute to marketing. Two microencapsulated products containing the same active at the same concentration can perform differently because the capsules are engineered differently, and neither the label nor the safety data sheet describes the capsule.

The variable determining release rate is not disclosed in anything a purchaser reads.

16. The pollinator problem

The property that makes microencapsulation effective creates a specific hazard.

Some bees may pick up the capsules and carry them back to their hive, where the released pesticide may poison the entire hive.⁸

For this reason, regulations require long restricted-entry intervals for some microencapsulated formulations.⁸

16.1 The same mechanism, the wrong recipient

The adherence to hair and legs that delivers a dose to a cockroach delivers capsules to a foraging bee, and a bee returns to a colony. The horizontal transfer that makes the formulation effective operates identically in a hive.

16.2 The practical consequence

This is a strong argument against microencapsulated products in exterior applications where flowering vegetation is present, and it is a consideration the anticoagulant and secondary poisoning article in this journal would recognise: a property that improves efficacy against the target has moved the same material into a non-target population.

17. Sealing the substrate

If the problem is porosity, the direct remedy is to remove it, and this has been tested.

Published work examined the residual efficacy of cyfluthrin emulsifiable concentrate and wettable powder formulations on porous concrete and on concrete sealed with commercial products prior to insecticide application.³

17.1 Why this is the structural answer

Sealing converts a porous substrate into a non-porous one. On the evidence in §2 to §5, that should move a surface from the unfavourable end of the range to the favourable end, and it does so permanently rather than for the duration of one application.

It also aligns with the argument this journal has made repeatedly in other contexts: the durable fix is usually a building intervention rather than a chemical one.

17.2 The commercial reality

Sealing a warehouse floor or a plant room is a capital expenditure that a pest control contract does not cover, and recommending it means recommending something the client pays someone else for.

It is nonetheless the correct recommendation where a porous floor is the reason a programme underperforms, and the wall surface data⁶ suggest the same logic applies to painted versus unpainted masonry.

18. Formulating for the surface

The alternative approach is to engineer the product around the substrate, and vector control provides a documented example.

Two experimental hut trials were conducted on broflanilide, a meta-diamide insecticide. The first investigated residual efficacy of three concentrations of a prototype formulation on concrete and mud substrates. The second compared two formulations, with the latter developed specifically to improve the residual efficacy of the insecticide on mud surfaces.⁵

18.1 The result

The second trial with an improved formulation showed prolonged residual efficacy of the 100 milligram per square metre concentration to five to six months on mud, and mosquito mortality on the concrete surface ranged between 94 and 100 per cent for the full duration of the trial.⁵

18.2 Why this matters

It demonstrates that the substrate problem is tractable by formulation rather than being an inherent limit, and that a manufacturer can target a specific difficult surface as a design objective.

It also contains a useful counterweight to §4 and §6: mortality of 94 to 100 per cent on concrete for a full trial duration shows concrete is not universally hostile. The outcome depends on the chemistry and the formulation, not on porosity alone.

18.3 A resistance note

The same work found broflanilide effective against both susceptible and pyrethroid-resistant mosquito strains, demonstrating absence of cross resistance between broflanilide and pyrethroids, and observed no blood-feeding inhibition or insecticide-induced exiting effects.⁵

Absence of an exiting effect is relevant to the repellency arguments made elsewhere in this journal, since a non-irritant residual does not disperse the population it is meant to control.

19. The other variables

Two further factors modify residual performance and both are within a client's control rather than an applicator's.

19.1 Food availability

Food presence or absence can substantially modify insect susceptibility, often by reducing contact with treated surfaces or enabling physiological recovery after sublethal exposure.²

Both halves are significant. An insect with an alternative resource crosses treated surfaces less, and an insect that feeds after a sub-lethal exposure may recover from it.

This supplies a mechanistic basis for something usually asserted as a platitude: sanitation is not merely about removing attraction, it directly determines whether a residual treatment achieves a lethal dose.

19.2 Species

Efficacy of surface-applied insecticides is significantly influenced by insect species, surface type, and exposure duration.¹⁶

In one comparison, spinosad exhibited the most consistent and rapid action across all surfaces and species, while alpha-cypermethrin proved highly effective particularly on non-porous materials. In contrast, pirimiphos-methyl required prolonged exposure to achieve complete control, especially against Tribolium castaneum on porous surfaces like concrete.¹⁶

19.3 Exposure duration

The exposure duration variable interacts with behaviour. A species that rests on treated surfaces accumulates dose; one that crosses them briefly does not.

The delayed mortality observed across treatments in that study was noted as supporting use of these products in areas where insects may migrate from treated to untreated zones,¹⁶ which is the same carried-dose logic as §14.1.

20. What this means for specification

Drawing the practical conclusions together.

Record the substrate. If residual efficacy varies by an order of magnitude between steel and concrete,¹ a service record that does not note what was treated cannot explain a failure.

Match formulation to surface. Wettable powders and other particulate formulations for porous substrates; the finer flowables and solution-based products are more exposed to absorption.³¹²

Match formulation to timeframe. Suspension concentrate led at thirty days, wettable powder at six months.³

Do not raise the rate to beat a substrate. Persistence on less porous surfaces held irrespective of concentration,³ and increasing rate on a porous one increases absorption too.

Judge by mortality, not knockdown. Four fifths of knocked-down beetles recovered on plywood.¹

Recommend sealing where it is the binding constraint. It is tested, and it is permanent.³

Treat sanitation as part of the dose. Food availability alters both contact and recovery.²

Avoid microencapsulation near flowering vegetation. Capsules are carried to hives.⁸

21. What this means for Manitoba buildings

Three local applications.

Unfinished basements are the worst case. Bare concrete floor and foundation wall is the substrate combination that performs poorly across nearly every study cited here,¹⁴⁶ and it describes a large share of older Winnipeg housing stock.

Older masonry and mortar joints behave like the difficult substrates. The malaria work distinguishing plastered from unplastered cement⁶ maps onto the difference between finished and unfinished interior masonry here.

Commercial food premises contain both extremes within metres. Stainless steel and sealed tile sit alongside bare concrete and untreated timber shelving, and a single product applied uniformly across that range will deliver very different results across it.

This also connects to the food premises drain article published in this journal. Where the durable fix is mechanical, the residual application is a secondary measure, and on a porous floor it may be a considerably weaker secondary measure than the invoice implies.

22. Limitations and open questions

The literature disagrees and we have said so. Section 8 sets out that some studies find insecticides more effective on concrete⁴ and that test house findings diverged from earlier work.⁶

Most target species are not structural pests. The evidence comes from rove beetles, psocids, triatomines, mosquitoes and stored product beetles.¹³⁵¹⁶ The physical mechanism should generalise; the specific magnitudes may not.

Section 10.3 is our inference. That emulsifiable concentrates are most affected by substrate absorption follows from combining sourced facts rather than from a single reported comparison, and we have labelled it.

Several formulation sources are patents. Patent background sections describe prior art accurately as a rule but are written by parties with an interest in establishing the inadequacy of existing formulations.¹⁰¹¹¹⁵

We have not read the primary sorption paper. The permethrin sorption and desorption figures are cited through a review.⁴

No Canadian or structural pest substrate data. We have found no study measuring residual efficacy across common Canadian building substrates against German cockroach, bed bug or commensal rodent targets, which given how routinely these products are applied to those surfaces is another instance of the pattern this journal keeps documenting.

We sell residual applications. This article argues that a substantial fraction of them underperform for physical reasons and that the durable remedy is sometimes a floor sealer bought from someone else.

23. Conclusion

The same active ingredient at the same rate produced roughly 25 per cent recovery on tile and roughly 80 per cent on plywood, with the authors concluding the insects likely did not pick up a lethal dose on the porous surface.¹ Pyrethroids retained toxicity up to 38 weeks on galvanised steel against far less on concrete.¹ An insecticide paint gave high mortality a year after treatment on non-porous surfaces and low mortality on concrete.⁴

The mechanism is physical and has been documented since at least 1949. A more porous surface absorbs more of a freshly applied insecticide, leaving less available on the surface to contact the insect,³ and porous substrates sequester lipophilic residues and reduce transfer.² Concrete adds alkaline degradation on top.¹ Loss is biphasic with most occurring in the first hour, and yet more than 20 per cent of applied permethrin remained after 300 hours of desorption, so concrete simultaneously starves the treatment and stores the chemical.⁴

Formulation is the lever. Solution-based products carry active wherever the solvent goes; particulate and encapsulated products leave solids sitting on the surface. In direct comparison the suspension concentrate outperformed at thirty days and the wettable powder outperformed at six months,³ which means neither is correct in general and the right answer depends on what the treatment is for. And an improved formulation of a novel active reached five to six months on mud and 94 to 100 per cent mortality on concrete for a full trial,⁵ demonstrating the problem is a design constraint rather than a law of nature.

What follows is unglamorous. Record the surface. Choose the formulation for it and for the timeframe. Judge the result by mortality rather than knockdown. Recommend sealing when the floor is the reason the programme is failing. None of that appears on a label, and all of it decides whether the product does anything at all.

References

1. Contact Toxicity and Residual Effects of Selected Insecticides Against the Adult Paederus fuscipes (Coleoptera: Staphylinidae). Journal of Economic Entomology, 106(6), 2530. Source for the evaluation of deltamethrin, fipronil, fenitrothion and imidacloprid on tile, plywood and concrete; the speed of killing order; the recovery rate at 48 hours of approximately 25 per cent on tile against approximately 80 per cent on plywood with the conclusion that insects likely did not pick up the lethal dose especially on porous surfaces; the attribution of reduced toxicity to rapid absorption into the porous substrate; the observation that the alkaline nature of concrete likely slows insecticide effect citing White 1982; and the cited finding that pyrethroids had much higher long-term toxicity up to 38 weeks on galvanised steel compared with concrete against liposcelid psocids. https://academic.oup.com/jee/article/106/6/2530/814313
2. Surface- and food-mediated effects on residual insecticide efficacy against key stored-product insects. Journal of Stored Products Research. Source for the statement that residual efficacy is influenced by insect biology, formulation characteristics, treated surface, food availability and time after treatment; that porous substrates such as wood may sequester lipophilic residues and reduce transfer to insects while concrete may alter persistence through physicochemical interactions; and that food presence or absence can substantially modify susceptibility by reducing contact with treated surfaces or enabling physiological recovery after sublethal exposure. https://www.sciencedirect.com/science/article/abs/pii/S0022474X26000937
3. Rojas de Arias, A. et al. Comparative evaluation of pyrethroid insecticide formulations against Triatoma infestans (Klug): residual efficacy on four substrates. Memorias do Instituto Oswaldo Cruz. Source for highest mortality above 50 per cent for deltamethrin 2.5 per cent SC and lambda-cyhalothrin 10 per cent WP on wood blocks up to three months post-spraying; mud showing lowest persistence with less porous surfaces maintaining mortality longer irrespective of concentration; the finding that WP formulations were less effective than SC flowables during the first 30 days but gave greater mortality at up to six months; the explanation that higher porosity means higher absorption and less insecticide available on the surface to contact the insect; and the cited work of Barlow and Hadaway (1949, 1952) on absorption losses and Arthur (1979, 1994) on cyfluthrin EC and WP on porous concrete and on concrete sealed with commercial products prior to application. https://www.scielo.br/j/mioc/a/w68qXxsjb5t3CgWpTWLXvmp/?format=pdf&lang=en
4. Sorption and Desorption of Pyrethroid Insecticide Permethrin on Concrete, and associated review material. Source for Jiang et al. (2011) reporting rapid sorption of permethrin due to concrete pores; the biphasic loss of recovery with a fast initial rate in the first hour followed by a slower secondary rate; the finding that more than 20 per cent of initial concentration remained after 300 hours of desorption suggesting concrete may act as an insecticide reservoir; Mosqueira et al. (2010) on insecticide paint containing chlorpyrifos, diazinon and pyriproxyfen giving high mortality one year after treatment on non-porous surfaces but low mortality on concrete; and the observation that there are cases where insecticides were more effective on concrete than on other surfaces. https://www.researchgate.net/publication/49660046_Sorption_and_Desorption_of_Pyrethroid_Insecticide_Permethrin_on_Concrete
5. Efficacy of indoor residual spraying with broflanilide (TENEBENAL), a novel meta-diamide insecticide, against pyrethroid-resistant anopheline vectors in northern Tanzania: an experimental hut trial. PMC. Source for the two trials on concrete and mud substrates, the second formulation developed specifically to improve residual efficacy on mud, the prolonged residual efficacy of the 100 milligram per square metre concentration to five to six months on mud, mosquito mortality on concrete of 94 to 100 per cent for the full trial duration, effectiveness against both susceptible and pyrethroid-resistant strains demonstrating absence of cross resistance, and the absence of blood-feeding inhibition or insecticide-induced exiting effects. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7928474/
6. Efficacy of insecticides used in indoor residual spraying for malaria control: an experimental trial on various surfaces in a test house. Malaria Journal. doi:10.1186/s12936-019-2969-6. Source for the evaluation of six formulations comprising four pyrethroids, a carbamate and an organophosphate on painted wood, unpainted wood, plastered cement and unplastered cement; the stated rationale of determining spray cycles, ensuring rational use and preventing wastage; the observation that pyrethroid activity can be compromised by rapid degradation on porous surfaces with high absorption; and the finding of low residual bioefficacy of pyrethroids on cement compared with wooden surfaces, confirming previous observations while diverging from another published study. https://link.springer.com/article/10.1186/s12936-019-2969-6
7. Pesticide formulation overview, patent background. Source for the list of formulation types including baits, concentrated emulsions, dusts, emulsifiable concentrates, fumigants, gels, granules, microencapsulations, suspension concentrates, suspoemulsions, tablets, water dispersible granules and wettable powders, and for the statement that a pesticide is rarely suitable for application in pure form and requires other substances to permit ease of application, handling, transportation, storage and maximum activity, referencing CropLife International Technical Monograph no. 2. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/8685423
8. Pesticide Formulations. University of Florida IFAS Extension, publication PI231 and School IPM materials. Source for the composition of emulsifiable concentrates and their little visible residue and corrosive properties; the description of flowable and liquid formulations as combining characteristics of emulsifiable concentrates and wettable powders with active ingredient impregnated on clay, finely ground and suspended, requiring moderate agitation and leaving visible residues; the description of microencapsulated formulations as pesticide particles surrounded by a plastic coating that slowly releases the pesticide providing timed release and prolonged active life; and the hazard that bees may pick up capsules and carry them back to the hive, with regulations requiring long restricted-entry intervals for some microencapsulated formulations. https://ask.ifas.ufl.edu/publication/PI231
9. Pesticidal compositions and processes related thereto. Patent background. Source for the composition of wettable powders as an intimate mixture of pesticide, carrier and surfactants with pesticide concentration from about 10 to 90 per cent by weight, carriers selected from attapulgite clays, montmorillonite clays, diatomaceous earths or purified silicates, and surfactants comprising about 0.5 to 10 per cent; and for emulsifiable concentrates comprising about 50 to 500 grams per litre of pesticide dissolved in a water miscible solvent or a mixture of water-immiscible organic solvent and emulsifiers, with useful solvents including xylenes and high-boiling petroleum fractions. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/9226500
10. Pesticidal composition containing a microencapsulated organo-phosphorus or carbamate in a pyrethroid dispersion. Patent background. Source for the listing of typical formulation forms; the wide use of emulsifiable concentrates due to ease of preparation and dilution together with their toxicity and inflammability problems from organic solvents; the statement that formulating very poorly soluble active ingredients as emulsifiable concentrates has been virtually impossible; the advantages of wettable powders in formulating poorly soluble actives without organic solvents; and the inhalation risk from powder scattering during formulation or dilution. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/5178872
11. Pesticidal compositions and related methods. Patent background. Source for the description of microencapsulation by suspending pesticide particles or droplets in plastic polymers, with capsules formed of various sizes, solubility, wall thicknesses and degrees of penetrability by altering polymer chemistry or processing, and the statement that these factors govern release speed which in turn affects residual performance, speed of action and odour; and for oil solution concentrates providing faster knockdown and kill due to solvents having pesticidal action and dissolving the waxy covering of the integument to increase uptake speed. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/9497966
12. Factors Affecting Pesticide Behavior and Breakdown, publication MF958. Kansas State University Research and Extension. Source for the description of wettable powders as formulations in which the active ingredient is not dissolved in petroleum solvents but finely ground and diluted with an inert powder, with wetting agents added, and requiring continuous agitation because they are held in suspension rather than solution or emulsion; and for dust formulations being similar to wettable powders but applied dry. https://bookstore.ksre.ksu.edu/pubs/factors-affecting-pesticide-behavior-and-breakdown_MF958.pdf
13. Pesticide Formulations. University of Florida School IPM. Source for the description of microencapsulated formulations as particles of liquid or dry pesticide surrounded by a plastic coating, mixed with water and applied as a spray, with the capsule slowly releasing the pesticide and the encapsulation process prolonging active life by providing timed release. https://schoolipm.ifas.ufl.edu/newtechp5.htm
14. Encapsulation: an Effective Environmentally Friendly approach. CABI Digital Library. Source for the mechanism by which microencapsulated formulations release active ingredient at target sites, with capsules adhering to insects' hair, legs and antennae when they pass over a treated zone, after which the insecticide diffuses from the capsule surface and is absorbed; and for the comparison of microencapsulated permethrin against an emulsifiable concentrate applied to cattle against stable flies. https://www.cabidigitallibrary.org/doi/pdf/10.5555/20113334987
15. Microencapsulated cockroach-controlling composition. Patent. Source for the specification of an insecticide having a 3-phenoxybenzyl group encapsulated in polyurethane-walled microcapsules with average particle diameter not more than 80 micrometres, wall thickness not more than 0.3 micrometres and a diameter to wall thickness ratio of 100 to 400; the observation that enhancing residual efficacy with conventional emulsifiable concentrate, solubilised emulsion concentrate and oil solution formulations requires high application dosage with attendant safety problems; and the statement that residual efficacy of microencapsulated insecticides varies depending upon particle diameter and wall thickness. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/5306499
16. Surface Application of Different Insecticides Against Two Coleopteran Pests of Stored Products. Applied Sciences, 15, 8306. doi:10.3390/app15158306. Source for the conclusion that efficacy of surface-applied insecticides is significantly influenced by insect species, surface type and exposure duration; the finding that spinosad exhibited the most consistent and rapid action across all surfaces and species while alpha-cypermethrin was highly effective particularly on non-porous materials and pirimiphos-methyl required prolonged exposure especially against Tribolium castaneum on porous concrete; and the observation that delayed mortality supports use where insects may migrate from treated to untreated zones. https://doi.org/10.3390/app15158306