CRC50150: Targeting mechanisms of phosphine resistance in stored grain pests

Effective control of insect pests is required for maintenance of grain quality as well as access to both domestic and overseas markets. However, the continued use of phosphine, the major disinfestation tool, is now threatened by the development of high-level resistance in a number of pest species. Minimising phosphine resistance and maintaining phosphine as the chemical of choice for controlling grain insects is a major challenge facing pest managers in the stored grain industry. Any strategy to combat resistance must be underpinned by accurate knowledge of resistance development in the field. This fundamental knowledge is needed to support scientifically based resistance management.

Research outcomes:

1. Resistance alleles have been demonstrated to provide useful information previously unobtainable, and can be used on historical collections in storage.
2. Resistance alleles are moving within local landscapes and are increasing on farms that do not use phosphine. This is inferred by observations of increasing resistance allele frequency between collections of R. dominica taken in 2006 and 2011.
3. Movement studies showed that changes in walking behaviour or flight initiation were minimal between resistant and susceptible R. dominca. We infer from this that dispersal of resistance alleles is likely to occur at a similar rate as dispersal of susceptible alleles within local landscapes.
4. We identified a specific and exploitable metabolic weakness in insects carrying resistance alleles at the rph2 locus. We found that mutations in the rph2 locus cause hypersensitivity to arsenical compounds. We predict that compounds that can mimic the reactivity of arsenic are likely to be potent phosphine synergists and are likely to be preferentially toxic to resistant insects.
5. We found that resistance is highly conserved globally by using genetic complementation between strongly resistant Indian and Australian strains of R. dominica and T. castaneum. It appears that the same two genes are responsible for resistance in their respective species worldwide. Since rph2 is known to be the same between species, it is now likely that it is also found in highly resistant strains of other species.
6. For T. castaneum, the candidate gene list for rph1 has narrowed to two likely candidates, of which one has mutations in multiple strains that are likely to affect gene function. However the gene list for R. dominica rph1 remains less clear, with a gene list that still stands at ~100 genes.

Research implications

Implication 1

The research has shown that resistance alleles are moving in local areas and that they do not appear to provide any impediment to local dispersal. The frequency of resistance alleles appears to be increasing across the local region studied (SE Queensland), even on farms that do not use phosphine.

Implication 2

The mechanism of resistance provided by the rph2 locus has an exploitable metabolic weakness in that it provides hypersensitivity to a class of compounds that might be developed for use as synergists or structural treatments for resistance management.

Implication 3

It was found that the genes responsible for resistance are highly conserved in R. dominica and T. castaneum across continents. The significance of this finding is that the mechanisms for phosphine resistance are highly likely to be the same across all major grain storage regions. This means that resistance management strategies that address the mechanisms of resistance directly should be effective in all regions where phosphine resistance occurs.

Acknowledgements

The authors would like to acknowledge the support of the Australian Government’s Cooperative Research Centres Program. We would also like to acknowledge the contributions of the University of Queensland which contributed to two PhD scholarships and an Honours scholarship that contributed to this project.