Phosphine – use it properly or lose it

Silos must meet the latest gas-tight standards if the industry is to reverse the increasing phosphine resistance among grain storage pests.

PHOTO: Deanna Lush

Pressure test your silo

  1. Time of testing: The best time to pressure test a silo is in the morning within an hour of sunrise or on a cool, overcast day. As silos warm up, air expansion can cause a pressure test to return a false reading.
  2. Check seals: Check the lid, access hatch, hopper and boot seals. Ensure the aeration fan is working properly. If there is no fan, install an air valve. Use a tubeless tyre valve to pressurise the silo with an air compressor then unscrew the centre of the valve to get higher air flow into the silo.
  3. Check oil levels: Before pressuring the silo, check oil levels are equal on both sides of the oil relief valve gauge. They can be bought and fitted if the silo does not have one, or a piece of clear tube connected to a second air valve fitted to the silo will suffice.
  4. Pressurise the silo: If there is an aeration fan, pressurise the silo by turning the fan on for a few seconds then seal the fan inlet. Stop the fan and close off the inlet immediately, as soon as oil levels are more than 25 millimetres apart or oil is bubbling. If there is no fan, use the tyre valve and air compressor to pressurise the silo. An air-operated venturi gun, such as Blovac, with connection fittings to the silo can be used.
  5. Time the half-life: If oil levels are further apart than 25mm, wait until the pressure drops and oil levels are 25mm apart. The time taken for oil to drop from 25mm to 12mm apart must be at least five minutes on new silos, or three minutes for older silos. If it is less than these times, there is a leak.
  6. Look for leaks: To find leaks, pressurise the silo again and use soapy water in a spray bottle to check for air leaks around all seals. Small pressure with a jack to a slide plate can help seal bottom outlets. Older silos may need more maintenance for a gas-tight seal. Repeat the pressure test once the leaks have been fixed.

View the full details of these steps in a fact sheet on Pressure testing sealable silos available at:

Phosphine is still an effective fumigant, but it must be used in a sealed, gas-tight storage or silo. 

That is the plain message from grain storage expert Peter Botta as growers gear-up to harvest this year’s crops.

Mr Botta emphasises the importance of phosphine and its lasting effectiveness: “It is inexpensive, easy and safe to use when done correctly, and can be accessed by growers,” he says.

“All recorded levels of resistance found on-farm can be controlled when using label rates in sealed, gas-tight storage,” he says.

“Resistance is a symptom of failed fumigation, which is typically caused when either the fumigation is done in an unsealed structure, the rate used is insufficient or the fumigation period is too short.

“Resistance management is all about using the correct structure or system.”

Mr Botta says some growers who have experienced phosphine resistance may have replaced it with another chemical, such as ProFume®, but not changed to a gas-tight storage system.

“There is the option to rotate ProFume® and use it as a break to incorporate different chemistries but the reason why phosphine is not working is because people are not using it in gas-tight silos,” he says.

“If that doesn’t change then the same thing will happen with ProFume®.”

Phosphine resistance has increased in the past 10 years because many grain silos fail to meet the gas-tight standard needed for fumigation.

Mr Botta also emphasises that there is a difference between sealed and gas-tight. A gas-tight silo is one that passes a five-minute half-life pressure test according to Australian Standard AS2628. To kill grain pests at all life cycle stages, gas concentration levels must remain at 300 parts per million for seven days or 200ppm for 10 days.

Research has found silos with just small leaks can have phosphine levels as low as 3ppm near the leak source and generally much reduced gas levels throughout the rest of the silo.

When fumigation has been completed, silos should be opened and ventilated using an aeration fan for one day. If there is no aeration fan, open the silo top and ventilate for five days. The minimum withholding period is two days after ventilation and the total fumigation time is seven to 10 days.

More information:

Peter Botta,
0417 501 890,

Pressure testing sealable silos fact sheet:


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