The Last Fumigant: Phosphine Resistance, the DLD Gene, and a Global Survey in Which Three Populations in Four Had Already Failed

1. Introduction: a single point of dependence

Most resistance problems examined in this journal concern one option among several. A pyrethroid fails and there are other modes of action. An anticoagulant fails and there is a more potent one, at a cost examined elsewhere here.

Stored grain protection has no such depth. Residue and environmental risks associated with sulfuryl fluoride and methyl bromide have left phosphine as the only general use fumigant.⁶ It is described elsewhere as the sole fumigant registered for the routine protection of stored grain from insect pests.⁷

1.1 Why a Manitoba pest control journal covers this

Because grain storage is not agriculture happening somewhere else. It is structural pest management in elevators, terminals, mills and warehouses, much of it in this province, and the species involved are the same stored product pests that appear in commercial food premises. The resistance status of a beetle in a Manitoba elevator is a structural pest management question.

2. Why phosphine and nothing else

The dependence was not chosen so much as arrived at.

Phosphine use intensified with the global phasing out of methyl bromide due to its ozone depletion characteristics.¹ Phosphine is the most widely used fumigant for stored grains due to a lack of better alternatives, all of which have serious shortcomings that restrict their use.²

2.1 The properties that make it irreplaceable

The gas is an ideal fumigant for control of insect infestations in stored commodities because of the low cost of application, ease of use and lack of chemical residue, and because it does not affect seed viability.⁶

That final property deserves attention because it is the one hardest to replace. Grain is not merely a commodity to be protected; much of it is seed, and a treatment that impairs germination destroys the value it was meant to preserve.

2.2 The structural risk

A single registered chemistry protecting a global commodity, with resistance already widespread, is a fragile arrangement. The analysis of Canadian registration economics published elsewhere in this journal explains why replacements are slow to arrive: the cost of registration is largely fixed and falls on whoever would bring an alternative to market.

3. How the gas works

Phosphine, hydrogen phosphide, is the most common insecticide applied to durable stored products worldwide and is routinely used for treatment of bulk stored cereal grains and other durable stored products.²

Its mechanisms of action vary, spanning disruption of metabolism, oxidative stress and neurotoxicity.² It is an effective metabolic toxin that kills actively aerobically respiring organisms, but which spares grain because grain is metabolically dormant.⁷

3.1 Why that matters for resistance

If the mode of action is metabolic, the escape route is metabolic too. An organism that can reduce or reorganise its energy metabolism under exposure has a path to survival that does not require detoxifying anything. Section 6 shows that this is precisely what happened.

4. The global picture

Phosphine resistance was subjected to a globe-wide survey in the 1970s, but without a subsequent global update. A recent effort undertook a comprehensive review of the resistance literature published from the initial 1975 survey through 2021, then used meta-analysis to synthesise and quantify resistance within the main stored product pest species.¹

Forty-six papers were identified, surveying 13 stored product insect species and encompassing 980 populations around the world. Of these, 72.96 per cent exhibited phosphine resistance.¹

4.1 How to read that number

Some caution is warranted and we want to state it before drawing conclusions. Populations enter the resistance literature because somebody had a reason to test them, and control failure is a common reason. The surveyed set is therefore likely enriched for problem populations relative to a random sample of world grain storage.

That said, 980 populations across 13 species and five decades is a substantial body of testing, and a figure near three quarters is not the profile of an isolated problem. The honest reading is that resistance is the normal condition of tested populations rather than an exception.

4.2 The species involved

Strong resistance has been selected among major insect pests of grain including the flat grain beetle Cryptolestes ferrugineus, the lesser grain borer Rhyzopertha dominica, the rust red flour beetle Tribolium castaneum, psocids of the genus Liposcelis, and the rice weevil Sitophilus oryzae.⁶

The first of those is the one that matters most on the Canadian prairie, and we return to it in §12.

5. The genetics

Phosphine resistance is genetically inherited and is mediated by the mitochondrial dihydrolipoamide dehydrogenase complex.² Two major genes, rph1 and rph2, confer resistance, with rph2 variants strongly associated with it.²

Detailed genetic studies identified the two loci as interacting synergistically to create a strong resistance phenotype.⁴

5.1 The division of labour between the loci

The rph2 gene codes for the strong phosphine resistant phenotype recorded in resistant populations of the lesser grain borer, red flour beetle and rice weevil. A weak resistance phenotype also occurs and is associated with the other gene, rph1, and a different mechanism.¹

The rph2 gene has been shown responsible for high level resistance in independent field isolates of R. dominica, while rph1 is a common contributor to the evolution of resistance in independent field isolates of the same species.⁵

5.2 The dominance problem

One detail has outsized management significance. The strong phosphine resistance phenotype is expressed when in homozygosity or even heterozygosity.¹

A resistance allele that expresses in heterozygotes is exposed to selection from the first generation it appears in. It does not hide in carriers while frequency builds quietly. This is the same dominance property that makes glucose aversion spread so quickly in cockroaches, as discussed elsewhere in this journal, and it has the same consequence: the window in which a programme might notice a developing problem is short.

6. The DLD mechanism

The molecular story is unusually well resolved for an applied resistance problem.

Dihydrolipoamide dehydrogenase is a flavin-dependent oxidoreductase crucial for energy metabolism and important in phosphine resistance. It comprises a reactive disulfide and a flavin adenine dinucleotide cofactor directly involved in electron transfer in mitochondria.³

One mechanism of resistance is a single point mutation in the DLD gene, which reduces the production of reactive oxygen species under phosphine exposure, as observed with the rph2 gene.¹

6.1 Why reducing ROS is the escape

If part of phosphine's toxicity operates through oxidative stress,² then an alteration that reduces reactive oxygen species generation under exposure reduces the damage without requiring the insect to break the molecule down.

This is a categorically different strategy from the metabolic detoxification seen in bed bugs and described elsewhere in this journal. There, enzymes dismantle the toxicant. Here, the organism alters the pathway through which the toxicant does harm.

6.2 The conserved target

The same gene underlies resistance across taxa. Mutations in the dihydrolipoamide dehydrogenase gene, called dld-1 in the nematode Caenorhabditis elegans and rph2 in pest insects, cause phosphine resistance in insects and in the nematode.⁶

That a free-living nematode and a grain beetle arrive at resistance through the same gene tells you the target is fundamental rather than incidental, which is both scientifically interesting and practically discouraging.

7. Exclusion and the physiology of survival

Beyond the genetics, resistant insects differ physiologically in ways that compound the problem.

Resistant insects usually absorb less phosphine than susceptible insects, suggesting that exclusion of phosphine is itself a resistance mechanism.³

This is a second layer operating before the molecular one, in the same architectural sense described for bed bugs elsewhere in this journal. Reduced uptake lowers the internal concentration; the DLD variant then reduces the damage that concentration causes.

7.1 The lipid finding

Phosphine resistant strains of both T. castaneum and R. dominica carry a higher amount of lipids than susceptible strains. In T. castaneum, significant variance ratios between resistant and susceptible strains ran from 1.13 to 53.10-fold for glycerolipids and from 1.05 to 20.00-fold for phospholipids.³

Phosphatidic acids are considered one of the main sources of triglyceride composition, allowing more energy sources, which can provide more energy to resistant insects to resist phosphine.³ Both rph1 and rph2 contribute to phosphine toxicity by damaging fatty acids.³

7.2 Metabolic suppression

Work on the rice weevil using global proteome analysis and mitochondrial DNA sequencing compared moderately resistant, strongly resistant and susceptible strains. The moderately resistant strain showed altered expression of genes encoding metabolic enzymes in catabolic pathways that minimise metabolic burden, while the strongly resistant strain showed changes in the mitochondrial respiratory chain itself.⁸

Two different routes to the same outcome, and both consistent with the framing in §3.1: if the poison targets active respiration, suppressing or reorganising respiration is the way out.

8. The variant unobserved in eukaryotes

One finding deserves separate attention for what it says about selection intensity.

Researchers compared the genetics of phosphine resistance in strains of R. dominica and T. castaneum from India and Australia, two countries with similar pest species but widely differing pest management practices. Phosphine resistance in India was found to be characterised by a dihydrolipoamide dehydrogenase variant that is otherwise unobserved in eukaryotes.⁴

The comparison with Australia is the analytically useful part. Two countries, similar pests, different management practice, different resistance genetics. That is close to a natural experiment in how management regimes shape evolutionary outcomes.

9. Inadequate treatment as the cause

The most important management finding in this literature is also one of the oldest.

A 1983 paper addressed resistance to hydrogen phosphide in stored product species associated with repeated inadequate treatments.⁵

9.1 Why fumigation is unusually prone to this

Fumigation efficacy depends on maintaining a lethal gas concentration for a sufficient period in a sealed space. Both variables fail easily and neither failure is visible.

A structure that does not hold gas loses concentration continuously. A fumigation ended early because a facility needs the space back delivers a shorter exposure than the label assumes. In both cases the operation looks complete. The insects that survive are those best able to survive a sub-lethal dose, which is the definition of a selection event.

9.2 The implication

Resistance here is not primarily a story about the chemistry being inherently vulnerable. It is a story about application discipline. Sealing quality and exposure period are the variables under human control, and they are the ones that determine whether a fumigation kills a population or breeds a better one.

That is an uncomfortable finding for the industry because it locates the cause in operational practice rather than in the product.

10. What the Oklahoma sequence shows

A single jurisdiction provides a useful longitudinal illustration.

Research from the late 1980s revealed low frequencies of resistance to various residual grain protectant insecticides and to phosphine in grain insect species collected in Oklahoma. Later work used the same previously established discriminating dose bioassays for phosphine.²

Subsequent studies examined phosphine resistance in T. castaneum and R. dominica from stored wheat in Oklahoma,² and in Cryptolestes ferrugineus collected from grain storage facilities in the same state.⁵

10.1 Why the methodological continuity matters

Using the same discriminating dose bioassay across decades is what makes the comparison meaningful. A change in reported resistance could otherwise reflect a change in method. Holding the assay constant means the trajectory is real.

This is also a reproach to jurisdictions without baseline data, which is most of them. Oklahoma can describe a trend because somebody tested in the 1980s. A place that has never tested cannot say whether its position is deteriorating, stable, or already lost.

11. Management under a single-chemistry regime

The literature's own prescription is that effective management requires integrated strategies combining monitoring, optimised fumigation, and alternative treatments.² Each term carries weight.

Monitoring means resistance testing, not insect counting. Discriminating dose bioassays exist and have been applied consistently for decades.² Counting beetles tells you there is a population. It does not tell you whether the next fumigation will work.

Optimised fumigation means sealing and exposure. Given §9, this is the highest leverage intervention available and it requires no new chemistry. It requires gas-tight structures, verified concentration monitoring, and the discipline to hold exposure for the full period.

Alternative treatments are constrained. The alternatives have serious shortcomings that restrict their use.² Work on the rice weevil found both resistant strains showed higher susceptibility to ethyl formate mediated inhibition of cytochrome c oxidase than wild type,⁸ which is an interesting lead precisely because it indicates a point where resistant insects are more vulnerable rather than less.

Synergists are a research direction. Laboratory studies have revealed treatments that synergistically enhance phosphine toxicity, which may be used to suppress resistance development and enhance efficacy.²

11.1 The negative cross-resistance opportunity

The ethyl formate finding⁸ is worth flagging as the most hopeful result in this literature. Where a resistance mechanism creates a new vulnerability, the resistant population can be targeted specifically. That is a rare and valuable situation and we would expect it to receive more attention than it has.

12. The prairie position

Several things follow for Manitoba and the Canadian grain system.

12.1 The species is ours

Cryptolestes ferrugineus, the flat or rusty grain beetle, appears in the phosphine resistance literature as one of the major grain pests in which strong resistance has been selected,⁶ and has been the subject of dedicated resistance surveys in grain storage facilities.⁵ It is also a principal stored grain pest of the Canadian prairie, and it is notable for tolerating the cold conditions that limit other stored product species here.

12.2 The exposure is real

Manitoba grain moves through on-farm storage, primary elevators, terminals and processing facilities, with fumigation used at multiple points. Every one of those is a potential selection event under §9, and the sealing quality of on-farm storage in particular varies enormously.

12.3 The data gap

We have found no published phosphine resistance survey of Manitoba or Canadian prairie stored product insect populations. Given that the global figure stands at roughly three quarters of surveyed populations,¹ that the relevant species is established here,⁶ and that the discriminating dose methodology is well established and has been applied consistently for decades,² the absence is striking.

This is the third such gap identified in this journal, alongside bed bug kdr genotyping and rodent VKORC1 status. The pattern is consistent: the methods exist, the questions are tractable, and nobody has run them here.

13. Limitations and open questions

The meta-analysis figure has a sampling bias. We set this out in §4.1 rather than burying it. Populations are tested for reasons, and control failure is a common reason, so 72.96 per cent¹ is a figure for tested populations and not for world grain storage as a whole.

Resistance is not a single state. The literature distinguishes weak and strong phenotypes with different genetic bases.¹ A population recorded as resistant may be controllable with optimised fumigation or may not, and the summary figure does not separate these.

Some sourcing is secondary. Several primary papers, including Mills 1983, Schlipalius and colleagues, and the Oklahoma series, are cited here through review and reference lists rather than read in full.²⁵ We have attributed findings to the primary authors where the reviews identify them.

The prairie argument is inference. That C. ferrugineus appears in the resistance literature⁶ and is a prairie pest does not establish that Manitoba populations are resistant. It establishes that the question is worth asking and has not been asked.

We perform fumigation, and have a direct interest in this subject. Fumigation, including of stored grain, is a service this company provides, so whether phosphine still works is a question about our own work. This paper is nevertheless a review of the resistance literature rather than operational guidance, and anyone conducting fumigation should work from the label and from current provincial requirements. An earlier version of this paragraph stated that we did not perform fumigation, which was wrong.

14. Conclusion

Phosphine protects the world's stored grain largely because everything else was withdrawn or restricted, leaving it as the only general use fumigant⁶ and the sole fumigant registered for routine stored grain protection.⁷ It works by killing actively respiring organisms while sparing metabolically dormant grain,⁷ which is an elegant selectivity and also the reason the escape route is metabolic.

A synthesis of half a century of testing across 46 papers, 13 species and 980 populations found that 72.96 per cent were resistant.¹ The mechanism is well resolved: two interacting loci,⁴ with rph2 encoding a DLD variant that reduces reactive oxygen species production under exposure¹ and expresses even in heterozygotes,¹ layered over reduced uptake of the gas itself.³ In India, selection has been strong enough to sustain a DLD variant otherwise unobserved in eukaryotes.⁴

The cause was identified in 1983 and has not changed: resistance associated with repeated inadequate treatments.⁵ Leaky structures and shortened exposures deliver sub-lethal doses, and sub-lethal doses select. The variables that matter are sealing quality and exposure discipline, both entirely within human control, neither requiring any new chemistry.

For the Canadian prairie the position is that the relevant species is here, the exposure is routine, the testing methodology has existed for forty years, and nobody appears to have published a resistance survey. Oklahoma can describe a trajectory because somebody tested in the 1980s. Manitoba cannot say anything at all, which is a choice that was made by default and can be unmade cheaply.

References

1. Phosphine resistance among stored product insect pests: a global meta-analysis-based perspective. Source for the review of literature from the 1975 survey through 2021 covering 46 papers, 13 species and 980 populations with 72.96 per cent exhibiting resistance; the intensified use following methyl bromide phase-out; the single point mutation in the DLD gene reducing reactive oxygen species production; the rph2 strong and rph1 weak phenotypes; and the expression of the strong phenotype in homozygosity or heterozygosity. https://www.researchgate.net/publication/375745907_Phosphine_resistance_among_stored_product_insect_pests_A_global_meta-analysis-based_perspective
2. Nayak, M.K., Daglish, G.J., Phillips, T.W. & Ebert, P.R. Resistance to the Fumigant Phosphine and Its Management in Insect Pests of Stored Products: A Global Perspective. Annual Review of Entomology. doi:10.1146/annurev-ento-011019-025047. Source for phosphine as the most widely used fumigant given a lack of better alternatives, the mechanisms of action spanning metabolic disruption, oxidative stress and neurotoxicity, the mitochondrial DLD mediation of resistance, the rph1 and rph2 loci, the synergist research direction, the integrated management prescription, and the Oklahoma discriminating dose bioassay series including the late 1980s baseline. https://www.annualreviews.org/content/journals/10.1146/annurev-ento-011019-025047
3. Role of Lipids in Phosphine Resistant Stored-Grain Insect Pests Tribolium castaneum and Rhyzopertha dominica. Source for reduced phosphine absorption by resistant insects as an exclusion mechanism, the description of DLD as a flavin-dependent oxidoreductase with a reactive disulfide and FAD cofactor involved in mitochondrial electron transfer, the elevated lipid content of resistant strains with glycerolipid variance ratios of 1.13 to 53.10-fold and phospholipid ratios of 1.05 to 20.00-fold, and the contribution of rph1 and rph2 to fatty acid damage. https://pmc.ncbi.nlm.nih.gov/articles/PMC9503450/
4. Kaur, R., Subbarayalu, M., Jagadeesan, R., Daglish, G.J., Nayak, M.K. et al. (2015). Phosphine resistance in India is characterised by a dihydrolipoamide dehydrogenase variant that is otherwise unobserved in eukaryotes. Heredity, 115, 188–194. Source for the synergistic interaction of rph1 and rph2 producing strong resistance, and for the India and Australia comparison across countries with similar pest species but differing management practices. https://www.nature.com/articles/hdy201524
5. Reference list of the Annual Review of Entomology global perspective, identifying primary sources including Mills, K.A. (1983) on resistance to hydrogen phosphide associated with repeated inadequate treatments; Mau, Y.S. et al. (2012) PLOS ONE 7:e34027 and 7:e31541 on the rph2 and rph1 genes in Rhyzopertha dominica; and Konemann, C.E. et al. (2017) Journal of Economic Entomology 110:1377–83 on phosphine resistance in Cryptolestes ferrugineus from Oklahoma grain storage facilities. https://www.annualreviews.org/content/journals/10.1146/annurev-ento-011019-025047
6. Alzahrani, S.M. & Ebert, P.R. (2019). Attenuation of radiation toxicity by the phosphine resistance factor dihydrolipoamide dehydrogenase (DLD). Scientific Reports. doi:10.1038/s41598-019-42678-w. Source for phosphine as the only general use fumigant following residue and environmental restrictions on sulfuryl fluoride and methyl bromide, the properties of low cost, ease of use, absence of residue and preservation of seed viability, the list of major resistant grain pests including Cryptolestes ferrugineus, and the identity of the DLD gene as dld-1 in C. elegans and rph2 in pest insects. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6478721/
7. Systems biology analysis using a genome-scale metabolic model shows that phosphine triggers global metabolic suppression in a resistant strain of C. elegans. Source for phosphine as the sole fumigant registered for routine protection of stored grain, and for its action as a metabolic toxin killing actively aerobically respiring organisms while sparing metabolically dormant grain. https://www.biorxiv.org/content/10.1101/144386.full.pdf
8. Minimization of energy transduction confers resistance to phosphine in the rice weevil, Sitophilus oryzae. Source for the proteome and mitochondrial DNA comparison of moderately resistant, strongly resistant and susceptible strains, the differing adaptations in catabolic pathways versus the mitochondrial respiratory chain, and the finding that both resistant strains showed higher susceptibility to ethyl formate mediated inhibition of cytochrome c oxidase than wild type. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6787191/