John Cameron and Cathy Nicoll report on the latest research findings and advice from the Australian Post-harvest Technical Conference, held recently in Canberra. The conference was supported in part by growers and the Federal Government through the GRDC.
"A NEW strain of one of our worst stored-grain insect pests - the lesser grain borer - has resistance to phosphine at 240 times that of susceptible strains and its incidence in storages on the eastern seaboard is on the increase.
Growers with unsealed storages will not only be unable to kill these insects, they are likely to be actively selecting for strongly-resistant strains if they persist in fumigating in unsealed storages," said University of Queensland researcher Paul Ebert speaking at the recent Post-harvest Technical Conference in Canberra.
But it's not all doom and gloom. Research by Pat Collins of the Queensland Department of Primary Industries has shown that "the highly-resistant strain can still be controlled, providing it is exposed to high dose rates of phosphine for extended periods, as can be achieved in sealed storages," Dr Ebert said.
Resistance to phosphine in tbe lesser grain borer (Rhyzopertha domillica) has been known in Australia since tbe 1980s. The resistance then was fairly moderate, with resistant populations needing around 30 times tbe dose of phosphine required to control susceptible populations. A single major gene controlled this 'weak' resistance. Such resistance is now very common in lesser grain borer populations in eastern Australia.
In 1997, entomologists detected a new higher-level resistance, with resistant populations some 240 times that of susceptible strains. Molecular studies by Dr Ebert revealed that this new resistance was due to the appearance of a second resistance gene. By itself, this second gene provides only weak protection for the insect, but when it occurs together with the first gene in the same insect, their effects multiply producing a very strong resistance.
The resistance equation - unsealed storages plus phosphine
The implications for growers are simple. High-level resistance can occur in insects due to a very simple genetic change. To slow or stop this from happening, widespread use of phosphine in unsealed storages must stop. Phosphine used in unsealed silos will select for insects that have resistance.
If insects that have moderate levels of resistance are allowed to survive, they can readily produce offspring that are highly resistant. By dosing properly in a sealed storage, this does not happen.
In contrast, the dose rates of phosphine used in a sealed storage will be high enough for long enough to completely kill all insects that are weaklyresistant. This will substantially slow the development of the super-bug. Selection of resistant insects is a numbers game. Stack the deck in your favour by ensuring all insects are killed and fumigate only in properly sealed storages.
The Post-harvest Technical Conference was supported in part by growers and the Federal Government through the GRDC.
Contact: Dr Paul Ebert 07 3365 2973
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