The Only Control Agent That Reproduces: Entomopathogenic Fungi, Autodissemination and the Aldehyde Problem

1. Introduction: an agent that multiplies

Every control agent examined in this journal so far is finite. A gram of gel bait contains a fixed quantity of active ingredient. A residual deposit decays. A dust is depleted as it is removed. What is applied is all there will ever be.

Entomopathogenic fungi are the exception. They infect the host, kill it, and then grow out of the cadaver to produce new infective spores.

1.1 Why this belongs in a structural pest journal

Because the approach has moved from agricultural research into commercial structural products within roughly a decade, and the evidence base behind it is younger and thinner than for anything else covered in this journal. It deserves examination before rather than after widespread adoption.

2. The infection cycle

The process is well characterised and divides into discrete stages.

The invasive and developmental processes can be delineated into six key stages: attachment of conidia to the host, germination, appressorium formation and penetration, fungal growth within the haemolymph, conidia production on the host, and transmission and dispersal.⁶

A three-phase summary appears elsewhere: spore adhesion to the cuticle with germination and formation of appressoria that penetrate it; penetration into the haemocoel with rapid propagation and neutralisation of the immune response; and development of fungal reproductive structures.⁷

2.1 The additional routes

Direct cuticular penetration is primary, although recent studies indicate these fungi can also utilise oral and respiratory routes to infect their hosts.⁶

3. Attachment

The first stage determines whether anything else happens, and the chemistry of it is unusual.

Conidia attach to the insect integument via non-specific hydrophobic interactions, mediated by a rodlet layer on the conidium's surface containing hyper-hydrophobic proteins. These weak bonds are later replaced with more specific protein-mediated interaction.⁸

3.1 Why hydrophobicity is the right property

An insect cuticle is coated in wax, as the desiccant dust article in this journal established. A hydrophobic spore adheres to a hydrophobic surface without requiring any receptor or recognition event, which is why the initial attachment is described as non-specific.

Specificity arrives afterwards, with surface proteins on the spores recognising compounds specific to insects.¹⁰

3.2 The grooming question

Attachment strength determines whether the insect can remove the spore before it germinates, and here one commercial strain is reported to perform notably well.

Beauveria bassiana attaches very securely to the cuticle of the targeted insect pest and is typically not removed by the grooming activities thereof. This may account somewhat for the high virulence of the fungus.⁹

Grooming is a substantial defence in social and fastidious insects, and resisting it is a meaningful property rather than a marketing point.

3.3 The dual function

The hydrophobic nature of the conidia serves not only to promote attachment to the insect cuticle but also to facilitate dispersion via water.⁶

4. Germination and penetration

This is the stage where the environment intervenes, and §13 returns to it at length.

After active attachment, germination on the host integument occurs, which requires high relative humidity conditions.⁸

4.1 The decision to form an appressorium

The forming, or lack of forming, of an appressorium is induced via a set of chemical and tactile cues: physical contact; deficiency in nutrients; and epicuticular lipids and cuticular composition.⁸

The third of those is worth noting. The fungus reads the lipid composition of the surface it has landed on as part of deciding whether to invade, which means cuticular chemistry influences infectivity before any immune response is involved.

4.2 How the cuticle is breached

Germ tubes and appressoria form, enabling enzymatic and mechanical penetration.¹⁰ These fungi secrete an arsenal of chitinases that hydrolyse the cuticle, facilitating host penetration and colonisation.¹¹

Chitinase secretion is an interesting juxtaposition with the preceding article in this journal on chitin synthesis inhibitors. One approach prevents the insect building chitin; this one digests the chitin it has already built.

4.3 The appressorium

Appressoria form at the tips of fungal germ tubes, displaying apical swelling structures adhering to the host cuticle.⁶ The structure is sometimes described as a penetration peg,¹² which captures the mechanics: the fungus builds a rigid anchored point and pushes through.

5. Inside the insect

Once through, the fungus is in a nutrient-rich space it must first make safe.

The fungus grows and proliferates in the haemocoel and produces blastospore toxins, ultimately leading to insect death.¹² Hyphal bodies multiply within the haemocoel, colonise tissues, and kill the host.¹⁰

Hyphal bodies or segments of the hyphae distribute throughout the haemocoel, filling the dying insect with mycelium.⁹

5.1 The immune contest

After penetration the fungi come into contact with the insect haemolymph, and host cellular and humoral immune responses are activated.¹⁰ The second phase of infection involves rapid propagation and neutralisation of the immune response.⁷

The fungus may employ various secondary metabolites to overcome the host's response.⁸

5.2 The nutritional strategy

Inside the haemocoel the fungi metabolise host nutrients while exhibiting limited dependence on less preferred carbon sources including chitin. Once host nutrients are depleted, they switch to the saprophytic stage, growing on the cadaver, at which point they have a higher capacity to metabolise the less preferred sources.¹¹

This suggests a distinction between chitinase-mediated virulence and chitin metabolism.¹¹ The same enzyme family serves two purposes at two different stages.

6. Out the other side

The stage that makes this category self-propagating.

After host death the fungus switches to saprophytic growth,⁸ and emergence hyphae grow out through the insect's integument and produce spores on the external surface of the host. These conidia are dispersed and capable of infecting new host insects.⁹

Under high relative humidity, above 95 per cent, hyphae will break through.⁸ Initially the fungal hyphae appear white, but as conidia form and mature they often take on a characteristic olive-green colour.¹²

6.1 The genetic economy

Genes used for primary penetration and invasion are reused during the later cuticle degradation.¹³

The fungus uses the same enzymatic toolkit to break in and to break out, which is an elegant piece of biological economy and also a target: work on B. bassiana found that mutants lacking a particular kinase could grow hyphae when injected into the haemocoel but were incapable of emerging from the surface of insect cadavers.⁶

6.2 Protecting the corpse

One detail is worth recording for its strangeness. Some Metarhizium species produce and accumulate the mycotoxin swainsonine, which protects the insect cadavers from being consumed by birds or other animals.⁶

The fungus defends its own food and spore source from scavengers, which is a level of sophistication not usually associated with a pest control product.

7. The cuticle as an immune organ

A point the literature makes about its own blind spot, and one that connects to the resistance material elsewhere in this journal.

Beneath the cuticle, the epidermis synthesises antimicrobial peptides and activates the stress management apparatus. These front line defences are augmented with humoral and cellular responses within the haemolymph.¹⁴

The innate immune responses within the haemocoel have received comparatively more attention than the cuticle, in spite of the latter being the primary and most important barrier to disease-causing agents.¹⁴

7.1 The time cost of delay

Delaying penetration of the cuticle predisposes the fungal inoculum to other biotic and abiotic factors that are deleterious to the pathogen, including low or fluctuating humidity, rainfall which washes off spores, and ultraviolet radiation.¹⁴

So a thicker or more resistant cuticle does not merely slow the fungus. It exposes the spore to environmental destruction for longer.

7.2 The resistance connection

This journal has documented cuticular thickening as a bed bug resistance mechanism against pyrethroids, and tolerance to desiccant dusts in a pyrethroid-resistant strain attributed to the same modification.

Whether cuticular thickening also confers tolerance to fungal penetration is an obvious question that we have not found answered. Given that the cuticle is described as the primary barrier to infection,¹⁴ it is a question worth someone's attention.

8. The first bed bug study

The application to structural pests is recent enough that the founding paper states its own novelty.

To date, there are no published studies on the efficacy of entomopathogenic fungi against bed bugs.¹

8.1 The design

Residual biopesticide treatments of Beauveria bassiana were tested against Cimex lectularius. An oil formulation of conidia was applied to different substrates. Bed bugs were exposed for one hour, transferred to an unsprayed environment, and monitored for mortality. Separate bioassays evaluated the effect of strain, sex, life stage and exposure substrate.¹

The application rate was three million conidia per square centimetre applied using an airbrush sprayer.³

8.2 The results

Rapid mortality was observed in all bioassays, with bed bugs exposed to treated jersey knit cotton dying most rapidly. The fungus proved pathogenic to bed bugs and was effective regardless of feeding status, sex, and strain, with jersey cotton providing a slightly faster rate of kill than paper substrate.¹

8.3 Why the substrate result matters

Independence from strain is the most commercially significant finding, because it implies the pyrethroid resistance that defeats conventional bed bug treatment does not transfer here.

The fabric effect is also practical. A fungal treatment performs better on textile than on hard smooth surfaces, which is the opposite of the substrate relationship established for liquid residuals in the formulation article published in this journal, where non-porous surfaces were favourable.

We would offer a mechanical explanation, flagged as ours. A conidium is a discrete particle roughly a few micrometres across that must contact and adhere to a passing insect. A textile presents an enormous surface area of fibres standing proud of the plane, each capable of holding spores in the path of an insect brushing past, where a smooth surface presents only the plane itself. The same geometry that makes fabric hard to clean makes it an effective spore reservoir.

If that reading is right, it has an agreeable consequence: the materials bed bugs prefer to harbour in, being textile, are the materials best suited to holding the agent that kills them.

9. Autodissemination

The property that distinguishes this approach operationally.

A further assay demonstrated efficient autodissemination of conidia from exposed bed bugs to unexposed bed bugs within artificial harbourages.¹

9.1 The visual confirmation

Use of a fluorescent dust provided visual confirmation that contaminated bed bugs transfer dust to untreated bed bugs in harbourage. This result is important because bed bugs live in hard-to-reach places and interaction between conspecifics can be exploited for delivery and dissemination of management products.¹

9.2 The experimental arrangement

In the transmission experiment, ten bed bugs were exposed to treated or untreated jersey cotton for one hour, then returned to diet cups to comingle with ten clean unexposed bed bugs, with a sterile filter paper harbourage provided and twenty bed bugs in each artificial harbourage.³

Mean survival of bed bug populations within an artificial harbourage significantly decreased as a result of horizontal transmission of conidia, further dependent on the type of substrate surface treated.¹⁵

9.3 Why the inoculum does not dilute

There is a mathematical difference between this and every other transfer mechanism in this journal worth making explicit.

The horizontal transfer article described indoxacarb passing through three levels of a cockroach colony, killing 81 per cent of adults that ate nymphs killed by an adult that ate bait. Impressive, and finite: each step divides a fixed quantity of toxicant among more insects, and the cascade terminates when the dose per recipient falls below lethal.

A fungal conidium is not divided. An infected insect does not carry a share of the original inoculum; it carries an organism that has multiplied inside it and is producing fresh spores from its own cadaver.⁹ In principle the second generation of infections is as potent as the first.

We say in principle because §12 shows the practical cascade is limited by who contacts whom, and §14 by what the harbourage contains. The point stands as a difference in kind rather than degree.

9.4 The connection to the transfer literature

This is the same logic as the horizontal transfer article published in this journal, with a crucial difference. A bait transfers a fixed quantity of toxicant that dilutes at each step. A fungus transfers an organism that multiplies.

In principle the cascade does not attenuate. In practice §12 shows it does, for behavioural reasons.

10. Which fungus works

Not all entomopathogenic fungi are candidates, and a direct comparison established that clearly.

Laboratory bioassays were used with Isaria fumosoroseus, Lecanicillium muscarium and Beauveria bassiana to evaluate virulence to the bed bug. Only B. bassiana significantly affected bed bug survival, and the effect was dependent on dose and formulation.²

10.1 The Metarhizium position

Beauveria bassiana has been reported as a suitable fungus, while Metarhizium anisopliae has in laboratory studies been found effective only under moist conditions and consequently is suspected to have limited effect in relatively dry urban environments such as human bedrooms.²

10.2 The dose ceiling

A two per cent B. bassiana oil formulation induced horizontal transfer to elevate mortality in a ten-day arena bioassay, and increasing the dose from two to four per cent did not increase mortality.²

That is a useful and unusual finding. Doubling the concentration achieved nothing, which indicates the limiting factor is not the quantity of inoculum but something else, plausibly the contact and germination steps rather than availability.

11. Speed

The performance figure that makes this category commercially plausible.

Beauveria bassiana was found to be highly virulent to both adult and nymphal stages of C. lectularius, usually resulting in 100 per cent mortality by three to five days post-exposure, and can be transferred horizontally among conspecifics from treated to untreated bed bugs within harbourage sites.⁴

11.1 Comparison across the journal

Three to five days after a single hour of exposure compares favourably with the fourteen days recorded for diatomaceous earth in the desiccant article, and sits close to the three to four days recorded for silica gel.

It is slower than a neurotoxin and faster than every other non-neurotoxic approach this journal has examined, which places it in a useful position.

11.2 The agricultural benchmark

For context from another target, the fungus works rapidly enough that 80 to 90 per cent kill of the adult boll weevil occurs within three to ten days of application.⁹

12. Who transfers to whom

The transfer result carries an important restriction.

Horizontal transfer mainly occurred between adults, and only partly between adults and nymphs.²

12.1 Why this is a problem

The bed bug behaviour article published in this journal established that nymphs are a substantial fraction of any established population, and the horizontal transfer article established for cockroaches that reaching sedentary immature stages is the central difficulty of any transfer-based approach.

A mechanism that moves efficiently between adults and only partly to nymphs will suppress the adult population while leaving a developing cohort comparatively less exposed.

12.2 What does not influence transfer

The same study found that distribution between harbourages was not affected by carbon dioxide activation, level of infection, or the biopesticide, and that horizontal transfer was not dependent on the degree of aggregation.²

The last finding is counterintuitive. One would expect denser aggregation to produce more contact and therefore more transfer. It did not, which suggests transfer is governed by encounter behaviour rather than by crowding.

12.3 The activity observation

Bed bugs showed activity peaks during the night, and activity was increased by elevated levels of carbon dioxide.²

That is the same finding reported in the desiccant dust article, where dust alone cleared no dormitories and dust with carbon dioxide cleared all five. A passive deposit of any kind, chemical or biological, depends on the insect moving across it.

13. The humidity problem

Sections 13 to 16 cover the constraints, and it is worth seeing them together before taking them one at a time, because they are cumulative rather than alternative. A spore must survive all of them in sequence.

The first is the constraint that has kept this category out of mainstream use.

Germination on the host integument requires high relative humidity conditions,⁸ and the requirement for high humidity negates integration into conventional agriculture.⁸

13.1 The formulation response

Formulation is the lever being used against this. Researchers formulated Metarhizium brunneum conidia in an oil-in-water Pickering emulsion and compared it with aqueous formulation under low or high humidity. The emulsion dispersed conidia more efficiently and caused two-fold more adhesion of conidia to host cuticle, with mortality significantly higher than other treatments, reaching 86.5 per cent under high humidity.⁸

Note the qualifier. The best result was still under high humidity. Oil formulation improved dispersal and adhesion, which are the stages before germination.

13.2 Why oil formulations dominate

The bed bug studies used oil formulations of conidia,¹² and advanced delivery systems such as oil-based dispersions are described as enhancing conidial performance.¹⁰

An oil carrier holds moisture at the spore and slows evaporation, which is a reasonable inference from the mechanism though we have not found it stated in exactly those terms.

13.3 The unresolved question

Whether the microclimate inside a bed bug harbourage, a tight crevice occupied by a cluster of respiring insects, is humid enough to support germination even when room humidity is low, is a question we could not find answered and consider central to the whole approach.

14. The aldehyde problem

The most elegant finding in this literature, and it comes directly from material covered elsewhere in this journal.

Suspected antifungal properties, specifically inhibition of growth and elastase production, of the aggregation chemicals (E)-2-octenal and (E)-2-hexenal, contribute to uncertainty with respect to true efficiency.²

Published work specifically examined inhibition of the entomopathogenic fungus Metarhizium anisopliae in vitro by the bed bug defensive secretions (E)-2-hexenal and (E)-2-octenal,⁵ and further work examined the effect of bed bug aldehydes on efficacy of fungal biopesticides.⁵

14.1 Why this is more than a curiosity

Autodissemination depends on infected insects returning to harbourage and contacting nestmates, as §9 established. Harbourage is precisely where aldehyde concentration is highest, because that is where the deposits accumulate.

So the fungal approach delivers its inoculum into the one location in the building where an antifungal compound is concentrated by the target's own behaviour.

14.2 The honest position

The inhibition is documented in vitro.⁵ Whether it materially reduces field efficacy is described in the literature as a source of uncertainty,² not as an established failure, and the observed transfer within artificial harbourages¹ indicates it is not prohibitive.

It is a reason for caution rather than dismissal, and it is the kind of interaction that would not be discovered without understanding the chemical ecology.

15. The insecticide residue problem

A practical constraint that follows from how these products are actually deployed.

Residues of a majority of 22 insecticides used for bed bug control resulted in reduced spore viability when the fungal product was applied to the surface.⁴

15.1 The sequencing consequence

A unit that has already received conventional treatment carries residues on the surfaces a fungal product would be applied to. Applying the biological agent over that residue reduces the viability of the spores.

Since almost every bed bug situation in which a professional would consider a biological approach is one where conventional treatment has already been tried, this is not an edge case.

15.2 The practical reading

Biological and conventional approaches are not simply additive. The order matters, the surfaces matter, and a plan combining both requires the interaction to be considered explicitly rather than assumed benign.

We have not found guidance on how long residues remain inhibitory, which would be the number a practitioner needs.

16. Behavioural avoidance

A further interaction concerns where the insects will go.

Efficacy of a fungal biopesticide for bed bug management is influenced by the toxicity and associated behavioural avoidance of harbourages on insecticide-impregnated box spring covers.⁵

16.1 Why avoidance undermines a transfer-based agent

An approach relying on infected insects returning to harbourage and contacting nestmates depends on harbourage being used. If a treated encasement causes avoidance, the aggregation the fungus needs to exploit is dispersed.

This is the same argument this journal has made repeatedly against combining repellent chemistry with bait, arriving here through a different mechanism. Anything that scatters the population works against anything that relies on the population being together.

17. Age and physiology

Later work identified host condition as a significant variable, with an encouraging implication.

Substrate, dosage, application strategy, and bed bug physiology are important factors to consider for optimal efficacy and safe indoor control with insect pathogenic fungi.⁵

17.1 The senescence finding

Mortality rates increased with a moderate degree of senescence, and a mixed-age structure was always observed in established field populations. Death after exposure to a mortal dose of conidia may therefore occur even faster in adult bed bugs found in natural field populations, because adults in natural populations are more likely to be reproducing and older than most of the individuals investigated.⁵

17.2 A rare direction of bias

This is unusual in the literature reviewed across this journal. Almost every other laboratory result reviewed here overstates field performance. Here the authors argue their laboratory conditions may have understated it, because they used young laboratory adults where field populations contain older reproducing ones.

We note this is the authors' reasoning rather than a field measurement, and that only adult insects were used in those experiments.⁵

18. The commercialisation gap

The distance between the biology available and the biology used is substantial.

There are an estimated 700 species of entomopathogenic fungi in approximately 90 genera. Over 170 products have been developed based on at least 12 species, and most commercially produced fungi are species of Beauveria, Metarhizium, Lecanicillium and Isaria, which are relatively easy to mass produce.⁷

18.1 What determines commercialisation

Ease of mass production, not virulence. Attention has focused predominantly on technical aspects of biopesticide development such as mass production and formulation, and selection of strains with rapid kill. Production requirements include reasonable cost, long-term stability and, most importantly, consistent efficacy under field conditions.⁷

18.2 The implication

Twelve of seven hundred species are commercialised, and the filter is manufacturability. There is no reason to assume the four commercialised genera are the most effective against structural pests; they are the ones that can be grown at scale and stored.

That is a familiar pattern from the registration economics article in this journal, where the compounds that reach market are selected by the cost of getting them there rather than by performance.

19. Where the evidence is thin

A limitation stated plainly in the literature itself and worth foregrounding.

As of the 2018 review, studies on fungi and bed bugs had only been conducted in small petri dish-sized containers.²

19.1 Why container size matters here more than usual

A petri dish removes the variable that §12 and §16 identify as decisive: whether the insect encounters the treated surface, and whether it returns to harbourage afterwards.

The desiccant dust article in this journal found dust alone clearing none of six dormitories while the same dust with carbon dioxide activation cleared all five. Arena results and room results diverge for behavioural reasons, and a category depending on contact and transfer is maximally exposed to that divergence.

19.2 The acceptance question

Biological control against bed bugs is not commonly accepted as an expedient method, though insect pathogenic fungi may contribute to management strategies.²

The authors' own framing is modest: as part of an integrated strategy, the consistent mortality and horizontal transfer may contribute to elevated population mortality and improved control by reaching hidden or passive individuals.²

20. Other structural targets

Bed bugs dominate the recent literature but they are not the only application.

Patent work exists on controlling cockroaches, carpenter ants and pharaoh ants using strains of Beauveria bassiana.¹⁶

20.1 The social insect case

That source notes pharaoh ants have been described as the most persistent and difficult of all house-infesting ants to control or eradicate, and that this tropical species has extended its range to more temperate regions by establishing colonies in heated buildings.¹⁶

That last observation is precisely the indoor thermal refuge argument this journal has made for cockroaches, pharaoh ants and termites, appearing independently in a 1990s patent.

20.2 Why social insects are attractive targets

A colony with intense social contact and inaccessible reproductives is the ideal case for an agent that transmits between individuals, for the same reasons set out in the horizontal transfer article for baits.

The same source notes that for carpenter ants, effective biological control agents had not been found and a need clearly existed for one,¹⁶ which given the moisture association established in the carpenter ant article suggests a target whose habitat is already damp enough for germination.

We are not aware of commercial structural products against these targets and flag this as a research direction rather than an available option.

21. What this means for practice

Expect days, not hours. Complete mortality typically at three to five days after exposure.⁴

Resistance status may not matter. Efficacy was independent of strain in the founding study.¹

Textile substrates outperform hard ones. Jersey knit cotton killed fastest, and transfer depended on treated substrate type.¹¹⁵

Do not apply over insecticide residue. Most of 22 tested products reduced spore viability.⁴

Avoid combining with anything that disperses the population. Behavioural avoidance of treated harbourages reduces efficacy.⁵

Do not expect nymphs to be reached efficiently by transfer. Transfer was mainly between adults.²

Raising the dose may not help. Doubling from two to four per cent did not increase mortality.²

22. Limitations and open questions

The structural evidence base is small and young. The founding bed bug paper dates from 2012 and stated no prior published studies existed.¹

Container scale is a real limitation. Stated in the literature as of 2018.²

The humidity question is unresolved for indoor use. Our §13.3 point about harbourage microclimate is a question we are raising, not one we can answer.

Several connections are ours. The link between cuticular thickening and fungal tolerance in §7.2, the oil carrier reasoning in §13.2, and the harbourage aldehyde concentration argument in §14.1 are our inferences from cited findings.

Patent sources have an interest. The grooming resistance claim and the structural pest applications come from patent material.⁹¹⁵¹⁶

We have not evaluated human safety in detail. These are living organisms applied indoors, and the regulatory and health assessment of that is outside what we have reviewed. Anyone using such a product should work from its label and registration.

No Canadian or Manitoba data. As with every topic in this journal, and here the humidity dependence makes local climate particularly relevant.

We do not currently offer fungal treatments. Readers should weigh that in both directions.

23. Conclusion

Entomopathogenic fungi penetrate the insect cuticle directly, using a hydrophobic rodlet layer to adhere, an appressorium to breach, and chitinases to digest their way in.⁶⁸¹¹ They proliferate in the haemocoel, neutralise the immune response, kill the host, and then grow out through the integument to produce new infective conidia on the cadaver.⁷⁹ No other control agent in this journal reproduces.

Against bed bugs, only Beauveria bassiana of three fungi tested significantly affected survival,² producing complete mortality in adults and nymphs typically within three to five days of exposure,⁴ effective regardless of feeding status, sex or strain,¹ and transferring efficiently from exposed to unexposed individuals inside harbourages.¹

Three constraints are serious. Germination requires high relative humidity,⁸ which conflicts with dry indoor environments. Residues of a majority of 22 bed bug insecticides reduced spore viability.⁴ And the aggregation pheromone components (E)-2-hexenal and (E)-2-octenal, which draw bed bugs into the harbourage where transfer must happen, have documented antifungal activity.²⁵

That last point is the one worth sitting with. The insect aggregates using compounds that inhibit the organism we would use to exploit its aggregation. It is not an obstacle anyone would have predicted from the chemistry of the fungus or the toxicology of the product, and it was found only because someone had worked out what the pheromone was made of. Whatever else this literature establishes, it makes the case that knowing the animal's chemical ecology is not an academic luxury. It is where the failure modes live.

References

1. Barbarin, A.M. et al. A preliminary evaluation of the potential of Beauveria bassiana for bed bug control. Journal of Invertebrate Pathology. Source for the statement that no published studies existed on entomopathogenic fungi against bed bugs at the time; the residual treatment design applying an oil formulation of conidia to different substrates with one hour exposure followed by transfer to an unsprayed environment; rapid mortality in all bioassays with jersey knit cotton killing fastest; efficacy regardless of feeding status, sex and strain; the demonstration of efficient autodissemination of conidia from exposed to unexposed bed bugs within artificial harbourages; and the fluorescent dust confirmation that contaminated bed bugs transfer material to untreated bed bugs in harbourage. https://www.sciencedirect.com/science/article/abs/pii/S0022201112001152
2. Aak, A., Hage, M. & Rukke, B.A. (2018). Insect pathogenic fungi and bed bugs: behaviour, horizontal transfer and the potential contribution to IPM solutions. Journal of Pest Science, 91(2), 823–835. Source for the comparison of Isaria fumosoroseus, Lecanicillium muscarium and Beauveria bassiana with only the latter significantly affecting survival dependent on dose and formulation; the two per cent oil formulation inducing horizontal transfer in a ten-day arena bioassay with no increase in mortality from doubling to four per cent; horizontal transfer occurring mainly between adults and only partly between adults and nymphs; nocturnal activity peaks increased by elevated carbon dioxide; distribution between harbourages being unaffected by carbon dioxide activation, infection level or biopesticide and transfer not depending on degree of aggregation; the finding that Metarhizium anisopliae is effective only under moist conditions and suspected of limited effect in dry urban environments; the suspected antifungal properties of the aggregation chemicals (E)-2-octenal and (E)-2-hexenal; the note that studies had only been conducted in petri dish-sized containers; and the observation that biological control against bed bugs is not commonly accepted as an expedient method. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5847144/
3. Compositions and methods for bed bug control using entomopathogenic fungi. United States patent. Source for the horizontal transmission experiment protocol including the application rate of three million conidia per square centimetre using an airbrush sprayer, and the arrangement in which ten exposed bed bugs were returned to diet cups to comingle with ten unexposed bed bugs with a sterile filter paper harbourage, twenty insects per artificial harbourage across three replicates. Note the commercial interest inherent in patent material. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/10085436
4. Dery, M. & Choe, D-H. (2022). Effect of bed bug (Hemiptera: Cimicidae) aldehydes on efficacy of fungal biopesticides. Journal of Economic Entomology, 116, 40–46. Source for the statement that Beauveria bassiana is highly virulent to both adult and nymphal stages of C. lectularius, usually resulting in 100 per cent mortality by three to five days post-exposure and transferring horizontally among conspecifics within harbourage sites citing Barbarin et al. (2012) and Aak et al. (2018); and for the finding of Shikano et al. (2021) that residues of a majority of 22 insecticides used for bed bug control resulted in reduced spore viability when the fungal product was applied to the surface. https://my.ucanr.edu/sites/ucrurbanpest/files/380610.pdf
5. Aak, A. et al. (2023). Biological control of Cimex lectularius with Beauveria bassiana: effects of substrate, dosage, application strategy, and bed bug physiology. Pest Management Science, 79(11), 4599–4606. doi:10.1002/ps.7659. Source for the conclusion that substrate, dosage, application strategy and bed bug physiology are important for optimal efficacy and safe indoor control; the finding that mortality rates increased with a moderate degree of senescence with mixed-age structure always observed in established field populations and the inference that death may occur faster in natural populations; the note that only adult insects were used; and the cited work of Shikano (2020) on efficacy being influenced by toxicity and behavioural avoidance of harbourages on insecticide-impregnated box spring covers, Ulrich et al. (2015) on inhibition of Metarhizium anisopliae in vitro by the bed bug defensive secretions (E)-2-hexenal and (E)-2-octenal, and Dery and Choe (2022). https://scijournals.onlinelibrary.wiley.com/doi/full/10.1002/ps.7659
6. A life-and-death struggle: interaction of insects with entomopathogenic fungi across various infection stages. Frontiers in Immunology. doi:10.3389/fimmu.2023.1329843. Source for the six key stages of attachment, germination, appressorium formation and penetration, growth within the haemolymph, conidia production on host, and transmission and dispersal; direct cuticular penetration as the primary mode with oral and respiratory routes also reported; appressoria forming at germ tube tips with apical swelling structures; the dispersal of regenerated conidia by non-biological means with hydrophobicity serving both attachment and water dispersion; the mutant conidia lacking a kinase that grow hyphae when injected but cannot emerge from cadavers; the production of swainsonine by some Metarhizium species protecting cadavers from consumption by birds and other animals; and the prominence of Hypocreales genera in commercial development. https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2023.1329843/full
7. Entomopathogenic Fungus overview. ScienceDirect Topics. Source for the three-phase division of host infection into spore adhesion with germination and appressorium formation, penetration into the haemocoel with rapid propagation and neutralisation of the immune response, and development of fungal reproductive structures; the estimate of 700 species in approximately 90 genera; the development of over 170 products based on at least 12 species citing Faria and Wraight (2007); the dominance of Beauveria, Metarhizium, Lecanicillium and Isaria as relatively easy to mass produce; and the production requirements of reasonable cost, long-term stability and most importantly consistent efficacy under field conditions. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/entomopathogenic-fungus
8. Not Only a Formulation: The Effects of Pickering Emulsion on the Entomopathogenic Action of Metarhizium brunneum. PMC. Source for the statement that the requirement for high humidity negates integration into conventional agriculture; conidial attachment via non-specific hydrophobic interactions mediated by a rodlet layer containing hyper-hydrophobic proteins later replaced by specific protein-mediated interaction; the requirement of high relative humidity for germination on the host integument; the induction of appressorium formation by physical contact, nutrient deficiency, and epicuticular lipids and cuticular composition; the use of secondary metabolites to overcome host response; the switch to saprophytic growth after host death with hyphae breaking through above 95 per cent relative humidity; and the Pickering emulsion result of more efficient dispersal, two-fold more adhesion to host cuticle, and mortality reaching 86.5 per cent under high humidity. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8307842/
9. Biopesticide composition and process for controlling insect pests. United States patent. Source for the description of infection initiated by a germinating conidium attaching to and penetrating the cuticle; the claim that the fungus attaches very securely and is typically not removed by grooming activities, possibly accounting for high virulence; the description of invasive hyphae ramifying through the haemocoel and filling the dying insect with mycelium; emergence hyphae growing out through the integument to produce spores on the external surface capable of infecting new hosts; and the reported 80 to 90 per cent kill of adult boll weevil within three to ten days of application. Note the commercial interest inherent in patent material. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/5413784
10. Endophytic entomopathogenic fungi: the next frontier in mycological biocontrol. PMC. Source for the four-stage infection cycle of adhesion via hydrophobic interactions and mucilage, germination and penetration by germ tubes and appressoria enabling enzymatic and mechanical penetration, proliferation and host death through hyphal bodies colonising tissues, and sporulation from the cadaver releasing new conidia; the recognition by surface proteins on fungal spores of compounds specific to insects; the activation of host cellular and humoral immune responses after cuticular penetration; and the role of advanced delivery systems such as oil-based dispersions. https://pmc.ncbi.nlm.nih.gov/articles/PMC13242415/
11. Chitinases in entomopathogenic fungi. Frontiers in Fungal Biology. Source for the secretion of an arsenal of chitinases that hydrolyse the cuticle facilitating host penetration and colonisation; the metabolism of host nutrients inside the haemocoel with limited dependence on less preferred carbon sources including chitin; the switch to the saprophytic stage once host nutrients are depleted with higher capacity to metabolise less preferred sources; and the suggested distinction between chitinase-mediated virulence and chitin metabolism. https://www.frontiersin.org/journals/fungal-biology/articles/10.3389/ffunb.2026.1732437/xml
12. Interactions between Entomopathogenic Fungi and Insects and Prospects with Glycans. PMC. Source for the description of penetration through spore adhesion and conidial germination followed by appressorium formation and cuticular penetration, growth and proliferation in the haemocoel with production of blastospore toxins leading to insect death; the description of appressoria as penetration pegs; and the observation that hyphae initially appear white before conidia form and mature to a characteristic olive-green colour. https://pmc.ncbi.nlm.nih.gov/articles/PMC10219344/
13. Entomopathogenic fungi and their relevance in sustainable agriculture: a review. Cogent Food & Agriculture. doi:10.1080/23311932.2023.2180857. Source for the sequence of conidial germination, appressorium formation, mycelial passage into the procuticle and arrival at the haemolymph with blastospore formation, and the finding that genes used for primary penetration and invasion are reused during later cuticle degradation, together with the role of water and wind in conidial dispersal. https://www.tandfonline.com/doi/full/10.1080/23311932.2023.2180857
14. Metarhizium brunneum infection dynamics differ at the cuticle interface of susceptible and tolerant morphs of Galleria mellonella. PMC. Source for the statement that delaying penetration of the cuticle predisposes the fungal inoculum to deleterious biotic and abiotic factors including low or fluctuating humidity, rainfall and ultraviolet radiation; the synthesis of antimicrobial peptides by the epidermis beneath the cuticle with activation of the stress management apparatus; the augmentation of these front line defences by humoral and cellular responses within the haemolymph; and the observation that haemocoel immune responses have received comparatively more attention than the cuticle despite the latter being the primary and most important barrier to disease-causing agents. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8647853/
15. Compositions and methods for bed bug control using entomopathogenic fungi, patent application. Source for the finding that mean survival of bed bug populations within an artificial harbourage significantly decreased as a result of horizontal transmission of Beauveria bassiana conidia, further dependent on the type of substrate surface treated, and for the observation that mean survival decreased with exposure to increased temperature. Note the commercial interest inherent in patent material. https://patents.google.com/patent/US20130195945A1/en
16. Controlling cockroaches, carpenter ants, and pharaoh ants using strains of Beauveria bassiana. United States patent. Source for the application of the fungus to structural pest targets; the observation that effective biological control agents for carpenter ants had not been found and a clear need existed; the description of pharaoh ants as the most persistent and difficult of house-infesting ants to control or eradicate; and the note that this tropical species has extended its range to more temperate regions by establishing colonies in heated buildings. Note the commercial interest inherent in patent material. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/5683689