The development of phosphine resistance in stored-grain insects has, for the most part, been blamed on imperfect fumigation practices that allow survivors or carriers of resistance genes to survive and reproduce.
This makes each incidence of resistance the likely result of practices at a particular location, be it on-farm or a receival site, or possibly the transport of insects in grain between the two.
However, one of the major conclusions to come from ecological studies of several stored-grain insects, led by Queensland Department of Agriculture and Fisheries (DAF) entomologist Dr Greg Daglish, is that natural gene flow is also a likely contributor as it is for so many other agricultural pest threats. Gene flow is the transfer of hereditary traits from one population to another through interbreeding.
Dr Daglish has been collaborating with researchers from the University of Queensland (UQ) and the New South Wales and Western Australian governments on ecological research for the Plant Biosecurity Cooperative Research Centre (CRC), with funding from the GRDC.
When we first started investigating the ecology of grain pests almost 10 years ago, most people thought they were pretty much confined to the silos where they were found, Dr Daglish says. Maybe they could move 20 or 40 metres, to another silo, but that was about it. Now we know they can and do move much further afield.
The first studies trapped the lesser grain borer (Rhyzopertha dominica) and the rust-red flour beetle (Tribolium castaneum) at significant distances from grain storages more than five kilometres. In some cases the lesser grain borer was found up to 10km from the nearest grain storage, although there appeared to be no obvious alternative host plants for the insects.
Dr Daglish says it seems likely these species move to and from storages and other locations in the wider environment. The rice weevil (Sitophilus oryzae), however, has not been trapped at any great distance from storages.
When we first started investigating the ecology of grain pests almost 10 years ago, most people thought they were pretty much confined to the silos where they were found ... Now we know they can and do move much further afield.
More recent research has focused on the rusty grain beetle (Cryptolestes ferrugineus), which can develop a very strong resistance to phosphine. In one research project in Queensland, the rusty grain beetle infested clean grain bulks (50-litre bins filled with clean grain) placed adjacent to silos and 2km from the nearest storage, suggesting movement between the two.
However, it is the analysis of Australia-wide genetic sampling of this species that suggests natural gene flow may also be contributing to the spread of phosphine resistance.
Dr Alicia Toon (UQ) tested insects from 19 sites collected from farms across all grain-growing regions in late 2014. The first part of the analysis used markers from the mitochondrial DNA, which is inherited from the mother via the egg, for comparison.
This identified two distinct genetic lineages for the rusty grain beetle so distinctive that researchers initially thought they may have been two different species.
This is not just an academic point, Dr Daglish says. Species that are difficult to tell apart without DNA testing, but which potentially have different levels of resistance to phosphine would have considerably complicated identification and control strategies. Fortunately, the two lineages do mate, indicating that they are actually the same species, despite the differences.
In addition to the two lineages, research using 10 microsatellite markers identified four distinct genetic clusters: in WA; on South Australias Eyre Peninsula; in eastern SA and Tasmania; and in Queensland, NSW and Victoria.
The whole east coast is effectively one gene pool. And while there may be some movement between these four population groups, it is less likely, Dr Daglish says. There is also a clear restriction on rusty grain beetle gene flow between the eastern states and WA.
A second genetic analysis has been completed for the red flour beetle in a project led by UQs Professor Gimme Walter, with funding through the India-Australia Strategic Research Fund. UQ population geneticist Dr Graham McCulloch made the unexpected discovery of a single national gene pool for this species.
We thought at the very least there would be a difference between the east and west, Dr Daglish says.
It is only through the use of DNA markers that the researchers have been able to identify the potential flow of insect genes including for phosphine resistance.
It would not have been possible through traditional trapping techniques. Understanding that stored-grain insect resistance genes can move into new areas, not just arise as a result of local selection, is important. Actions in one place can have implications further away. If you want to manage resistance you have to take a very broad-scale view. You cant just look at your farm, Dr Daglish says.
More information: Dr Greg Daglish, email@example.com
GRDC Project Code NPB00013 Plant Biosecurity CRC Code 3039