Western Australia Department of Agriculture (DAWA) Senior Entomologist, Rob Emery, has warned that 40 per cent of WA growers and 80 per cent in the eastern states are already experiencing weak resistance to phosphine.

Fumigation with phosphine is favoured throughout Australia and works by producing a gas which moves readily through stored grain, killing insects and leaving a chemical residue-free product. 

“Of even greater concern is the discovery of strong resistance in five per cent of grain storages in the eastern states,” Mr Emery said.

“This occurred when weak resistance was present and subsequent fumigations weren’t managed efficiently.”

Concern for increased occurrences of strong resistance led the Grains Research and Development Corporation (GRDC) to support a nation-wide campaign to retain the use of phosphine.

Trading of insect infested grain is prohibited by Australian regulations, compelling both the export grain industry and growers retaining grain on-farm for seed, feed or niche markets, to fumigate regularly to limit insect numbers.

Fumigation is the primary control method, with no viable alternatives currently available.

Co-operative Bulk Handling WA (CBH) and DAWA have run a project in WA titled ‘Phosure’, which featured awareness days for growers to help them implement stricter on-farm grain storage practices and utilise phosphine more efficiently.

DAWA Technical Officer Chris Newman explained that phosphine worked by the solid form reacting with moisture in the air to produce a toxic gas that penetrated the storage chamber, killing insects and breaking the breeding cycle.

Properly sealing the silo allowed the more robust stages in the lifecycle, the pupae and eggs, sufficient time in contact with the phosphine, whereas insufficient sealing allowed the more resistant insects to survive.

Easily removed on the lids and seal plates, seals should be changed every couple of years, or earlier if damaged, and then replaced with a good quality EPDM rubber.

Removing spilt grain at harvest, preforming monthly checks of the silo headspace and installing pitfall traps for early detection of insects, will constrain any problems. 

Growers have also been informed of the advantages of aeration fans in silos, which, when operated for a few hours each week, help inside temperatures remain below 20 degrees celsius. 

Keeping the grain cool had advantages for growers in harvest management, storage of high moisture grain and better retention of grain quality and ‘germability’. 

On-farm safety was also highlighted at the GRDC-supported field days, with growers told to use correct safety gear and dispose of the solid phosphine safely after fumigation. 

Mr Newman reminded growers that correctly calculating the amount of phosphine needed meant calculating storage space volume, not how much grain was stored. 

A rule of thumb was a 2000 bushel silo needed 100 tablets every time to eliminate all insects.