Successful control of grain storage pests means killing all life stages of insects – egg, larva, pupa and adult. As eggs and pupa are dormant phases, time is required for those life stages to progress to an active state when they will take in the phosphine. Cold grain means a slower life cycle and longer fumigation; warm grain means a faster life cycle and shorter fumigation.

The standard fumigation exposure period is seven days when grain is over 25 degrees Celsius, or 10 days when grain is between 15°C and 25°C. If grain is below 15°C, phosphine is not effective.

In storages more than 300 tonnes, the phosphine label states 20-day exposure period for surface-only application. What this means is that if phosphine is placed in the headspace or base of the storage without any powered recirculation, it is classed as surface-only, requiring 20-day exposure. If air in the sealed storage is recirculated via a fan, then the standard seven or 10-day exposure period applies.

Fumigation recirculation boxes

Powered fumigation recirculation boxes for large silos aim to allow phosphine to be applied at ground level and distribute gas through the storage more evenly.

Recirculation can also be achieved safely and effectively by still applying the phosphine in the storage headspace and using a small fan plumbed between the headspace and base of the silo to recirculate gas.

A potential danger is if phosphine is placed in a recirculation box and the fan stops running during the first five days when the phosphine is typically liberating; concentration levels in the box can quickly reach explosive levels.

Essentials

Musts for fumigation recirculation boxes:

• be sealed;
• have an appropriate-sized fan (not too large);
• have an explosion panel that will let go before parts of the box become projectiles;
• have adequate space for phosphine to liberate without residue smothering unliberated tablets or being drawn into the recirculation airstream; and
• remain running and stay connected to the storage for the entire fumigation period.

Vent time

Venting is required to expel the phosphine so the grain is safe to handle, transport and consume. When the phosphine is removed after the exposure period, the label states a minimum of one day venting with an aeration fan, or at least five days without a fan just with the storage open.

Experience is indicating there are factors that influence the speed at which grain desorbs the phosphine, so venting may need to be extended.

• If the exposure period was lengthened, grain will have had more time to absorb more phosphine, so take longer to desorb.
• Cold grain has been found to take longer to desorb phosphine.
• Different grain types can absorb and desorb phosphine at different rates. Much of the research has only been done on wheat.
• Large storages can take longer to vent with just the lid open and no aeration.
• Some buyers stipulate venting to 0.3 parts per million tested via a spear sample at delivery, which may take longer to vent in order to comply.

A two-day withholding period then applies after venting.

Masks

The application of phosphine requires elbow-length PVC gloves and a full-face respirator with filters that remove inorganic gases.

The standard chemical masks used for handling paddock chemicals typically are not full-face and do not contain filters for inorganic gases. Start by choosing a mask that fits your face, then select filters for it that include removal of inorganic gasses.

Drager and 3M are two well-known brands that can supply a suitable masks and filters. Be aware that filters have varying life spans – from single use to resealable for multiple use based on exposure over time.

Gas-tight

Understanding that successful fumigation requires control of all life stages of insects, and that phosphine is only taken up during the active life stages, it makes sense that gas has to remain in the storage long enough for that to occur.

Gas-tight storage that meets Australian Standard 2628-2010 is a simple indication that gas concentration of at least 360 parts per million will be maintained for the required seven to 10 days. There are no substitutes for gas-tight storage for successful fumigation and, in addition to a pressure test, use of a fumigation meter is the surest way to be confident the 360 parts per million is being achieved.

Meters

Phosphine meters are highly recommended to aid in successful fumigation, venting and safety. There are two distinct types of meters for different purposes:

• high range – typically zero to 2000 parts per million used for measuring gas concentration during the fumigation; and
• low range – typically zero to 20 parts per million used to accurately measure low-level concentration for operator safety or for determining venting clearance.

No one meter can perform all purposes. While the high-range meters measure down to zero, the resolution accuracy at low level is not adequate for safety or clearance.

Always read and follow label directions and seek clarification on the process if unclear.

More information: Chris Warrick, 1800 WEEVIL, [email protected]; Stored Grain website.