Grain growers in the southern cropping region are urged to ensure grain stored on-farm after harvest is properly fumigated to avoid resistance developing in stored grain insects.
Grain storage experts supported by the Grains Research and Development Corporation (GRDC) say it is imperative that silos meet the latest gas-tight sealing standards and that fumigant application rates and fumigation periods are sufficient.
Phosphine resistance has increased in the past 10 years because of failed fumigation practices. With more than 80 per cent of Australian grain treated with phosphine, the nation’s grains industry is highly reliant on the continued effectiveness of this fumigant as the cheapest and an efficient pest management method that allows the export of clean and "residue-free" grain.
“It is therefore critically important that growers implement best practice when storing grain, particularly in relation to fumigation,” said Peter Botta, grain storage specialist for southern New South Wales, Victoria, South Australia and Tasmania.
“Australian growers are fortunate to have off-the-shelf access to phosphine. It is such a unique and effective product when used properly, but if we don’t use it according to the label we are not only increasing the potential for insect resistance but we are risking our future access to this chemistry.”
Mr Botta said insecticide dichlorvos was now out of reach of most grain growers and was more suited to bulk handler use, underlining the importance of phosphine to the industry and the need to respect is availability through proper use.
He said that in order to kill grain pests, such as the lesser grain borer and rust-red flour beetle, at all stages of their life cycle (egg, larva, pupa, adult), phosphine gas needs to reach, and be maintained at, a concentration possible only in a gas-tight storage. Other key considerations and recommended practices include:
- The total time required for effective grain fumigation ranges from 10-17 days, accounting for the minimum exposure period, ventilation and withholding period. This highlights the importance of monitoring grain regularly and at least 17 days before out-loading to allow sufficient time to fumigate if required.
- When determining how much phosphine to apply, it is important to treat the entire storage volume, regardless of how much grain is contained inside. For example, a 100 tonne silo full of grain requires 200 phosphine tablets. If that same 100 tonne silo is only half full of grain, it still requires 200 phosphine tablets for effective fumigation.
- Arrange the tablets where as much surface area as possible is exposed to air, so the gas can disperse freely throughout the grain stack. Spread phosphine tablets evenly across trays before hanging or placing them in the head space of a gas-tight, sealed silo.
- Hang bag chains in the head space or roll out flat on the top of the grain so air can freely pass around them as the gas dissipates. Bottom-application facilities must have a passive or active air circulation system to carry the phosphine gas out of the confined space as it evolves. Without air movement, phosphine can reach explosive levels if left to evolve in a confined space.
- To control pests at all life stages and prevent resistance, phosphine gas concentration needs to reach 300 parts per million (ppm) for seven days (when grain is above 25C) or 200ppm for 10 days (between 15-25C). Insect activity is slower in cooler grain temperatures so require longer exposure to the gas to receive a lethal dose.
- When fumigation is complete, 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, as per label instructions. The withholding period is two days after ventilation, therefore the total fumigation process requires seven to 10 days.
- Phosphine is a highly toxic gas with potentially fatal consequences if handled incorrectly. For safety, use cotton overalls buttoned at the neck and wrist, eye protection, elbow-length PVC gloves and a breathing respirator with combined dust and gas cartridge.
Mr Botta said some growers who had experienced phosphine resistance may have replaced it with an alternative chemical, such as ProFume® (which requires application by a trained product representative) but not changed to a gas-tight storage system.
“Regardless of the chemistries being applied, they must be used in gas-tight silos, otherwise potential exists for insect populations to build resistance.”
The GRDC is investing at all levels to help ensure high-quality grain storage on-farm and in bulk storage can be maintained. These include projects that are improving our understanding of the ecology of stored grain pests, developing molecular tools and tests to rapidly identify resistant insects and investigating new and alternative control treatments.
Meanwhile, a Stored Grain National Information Hotline has been set up to assist growers with any grain storage enquiries. The number to call is 1800 weevil (1800 933 845).
More information on phosphine resistance and stored grain fumigation is available via:
Peter Botta, PCB Consulting
0417 501 890
Sharon Watt, Porter Novelli
Caption: Phosphine is used to kill grain pests such as the lesser grain borer. Photo: GRDC.
GRDC Project Code: PAD00001
GRDC Project Code