Providing practical information to help you protect your farm from biosecurity risks

Phosphine-resistant grain insects eliminated

February 13, 2012

Red rust flour beetleThe application of commonly recommended management principles has eradicated a strain of grain-storage insects with strong phosphine resistance located on a Western Australian farm.

The achievement – involving more than three years of treatment and monitoring followed by a final check of the site – was made by Department of Agriculture and Food, WA (DAFWA) staff with support from the GRDC.

It is believed to be the first time that elimination of on-farm grain-storage insects with strong phosphine resistance has been scientifically documented and confirmed in Australia and possibly the world.

The strongly phosphine-resistant red-rust flour beetles (RFB) were detected on a farm near Wubin, in the Dalwallinu shire, through monitoring by DAFWA as part of a program funded by the GRDC and the Cooperative Research Centre for National Plant Biosecurity.

DAFWA grain storage specialist Chris Newman says the resistant population was confirmed as eliminated last year after recommended management practices were applied and silos were sealed. “The achievement demonstrates that applying standard management principles works, and by applying them growers will control insects before they have a chance to develop resistance in the first place.”

Mr Newman says the case of strong resistance was one of only a handful that had developed in WA within the past five years.
He says the resistant strain of RFB at Wubin was believed to have developed independently on the farm over an 11-year period after the farmer used incorrect phosphine-dosing practices to treat grain held in poorly sealed silos.

Mr Newman says many strongly resistant strains of insects could be eradicated with label-rate phosphine fumigation, provided the storage facility was gas-tight.

“DAFWA staff took immediate and rigorous action, involving hygiene treatments to remove resistant insect populations in and around silos, and correct phosphine fumigation to eliminate resistance from within the grain bulk.

“They visited the farm at least twice a year to check on silo hygiene and to ensure fumigations were done correctly.”

DAFWA senior research officer Rob Emery said: “Our project is very applied in that it is focused on determining resistance by whatever means possible, and then doing something about it,” Mr Emery says. “Early detection of resistance is one of the strengths of the inspection process we have in WA – we can track the rise of weak resistance and make the grower aware of an emerging problem.

“When strong resistance has been detected we initiate a survey of the area to ensure it hasn’t spread.”

Mr Emery says monitoring for resistance in stored-grain insects in WA is continuing.

He says strong phosphine resistance remains minimal in WA, unlike the eastern states. “WA farmers have worked really hard for years to minimise phosphine resistance and as a result there have only been a few cases of strong resistance reported. This gives our grain a unique edge, as it can be marketed as free from pests and contact chemicals.”

Mr Emery says research has shown that strong phosphine resistance develops when the frequency of weak resistance approaches 80 per cent of strains tested.

“The frequency of weak resistance across all species in WA recently reached 45 per cent,” he says. “However, 73 per cent of RFB are weakly resistant and therefore this species is at the greatest risk of becoming strongly resistant.”

Hygiene treatments used at the Wubin farm to eradicate the resistant RFB population included:

• pressure washing inside empty silos, storage surrounds, handling equipment and machinery

• the application of contact chemical insecticide to kill insects in grain residues at the base of silos and the removal of waste for burial

• treating clean, empty silos with diatomaceous earth protectant to prevent reinfestation.

Eradication of the insects, using phosphine, was ensured by:

• silo maintenance – including rubber seal replacement on removable hatches and permanent sealing of other gaps with flexible waterproof sealant

• silo pressure testing to ensure an extended fumigation period, verified by gas monitoring.

Reproduced with the permission of the Grains Research and Development Corporation

Image: Peggy Greb, USDA Agricultural Research Service, Bugwood.org