As insect resistance to phosphine continues to climb across both on-farm and bulk storage, growers face a constant challenge: how to slow resistance while protecting grain quality and market access. Recent GRDC-supported resistance monitoring shows a slight dip in strong phosphine resistance this year, but the overall national trend remains upward, with about 30 per cent of pests now showing strong resistance—well above the 10–15 per cent level considered manageable.
The monitoring program covers five key pests—lesser grain borer, red flour beetle, rice weevil, saw-toothed grain beetle, and rusty grain beetle—and confirms that resistance patterns vary across regions, but the broader problem is industry-wide. Encouragingly, no resistance has yet been detected to newer products such as sulfuryl fluoride or spinosad, though one suspected case of spinosad resistance in lesser grain borer is being investigated.
Research leader Dr Manoj Nayak says the latest findings offer some good news but warns that the long-term outlook is still serious. “A small dip is welcome, but we must not relax. Phosphine resistance is still at unacceptably high levels.”
Phosphine Best Management Practices
At the 2025 Australian Grain Storage and Protection Conference in Melbourne, GRDC launched the new Phosphine Best Management Practices guide, designed to help growers and bulk handlers preserve phosphine’s effectiveness.
The guide stresses that phosphine should only ever be used in gas-tight, sealed silos that pass the AS2628 pressure test, and that pressure testing must be carried out before every fumigation, with leaks repaired immediately. Correct application at full label rates and exposure times is critical to avoid under-dosing, which fuels resistance.
The guide also emphasises moderation in use. A single parcel of grain should not be fumigated with phosphine more than three times, and growers are encouraged to rotate treatments by incorporating alternatives such as spinosad to break resistance cycles. Finally, good hygiene and aeration cooling are presented as cornerstones of best practice, reducing the need for frequent fumigation in the first place.
Resistance management must become part of everyday storage practice, not just a reaction when problems occur. Using sealed silos, testing pressure before every fumigation, applying phosphine correctly, and rotating with alternatives like spinosad are all essential steps if phosphine is to remain effective. Small victories are possible, but the battle is unrelenting—and every grower has a role to play.
Original article:
Fumigant resistance: small victories in an unrelenting battle
Read the full article on GRDC GroundCover here.