By Shafiya Hussein, Grains Biosecurity Officer, South Australia
South Australian grain growers rely heavily on phosphine to keep stored grain insect free. Domestic and international markets demand high-quality grain, free of insects, and phosphine is extensively used to protect against grain storage pests.
Phosphine is a universal phytosanitary treatment of grain silos, seeds, plants, dried fruit, and other products.
Due to the widespread and continued use of the treatment, producers can run the risk of pests developing resistance to phosphine. Grain growers can however, take certain steps to reduce the risk of this happening.
Phosphine treatment leaves no residue and is effective in targeting grain pests at all stages of their life cycle (egg, larva, pupa, adult). It is also popular due to its relatively low cost, easy application, ability to be used on other commodities, and no impact on seed germination or milling quality of grain. When applied appropriately phosphine penetrates the insect’s exoskeleton and interrupts their respiration, which ultimately kills them.
The conditions throughout summer in South Australia are ideal for infestations of grain storage pests. Grain stored for an extended period of time such as that to be used for livestock feed, or seed for the next season has an increased risk of pest infestations.
According to data from the Grains Research and Development Corporation (GRDC) funded National Resistance Monitoring for Insect Pests of Stored Grain project, 94.7% of insect populations tested were ‘weakly’ resistant (controlled by one gene, R1) to phosphine. This included pests such as lesser grain borer, flat grain beetle, rice weevil, flour beetle and saw-toothed grain beetle.
The high rates of ‘weak’ resistance indicates that there is a much higher likelihood of resistant genes in the overall population becoming more prevalent and occurring in different combinations, to potentially result in strongly resistant populations. These rates of resistance largely occur due to ongoing phosphine misuse by underdosing or having leaking fumigation structures and multiple fumigations on the same grain batch.
Concerningly, 2.63% of pest populations sampled from farms in South Australia in 2021 showed strong resistance to phosphine. In all samples analysed, lesser grain borer (Rhyzopertha dominica), showed strong resistance (due to two major genes R1 + R2).
Weak resistance can be controlled with current phosphine label rates. However, it is much harder to control strong resistance.
When treating stored grain you should ensure:
To help growers manage their use of phosphine and confidently prevent resistance, South Australia’s Grains Biosecurity Officer, Shafiya Hussein, in collaboration with Chris Warrick (grain storage specialist leading the GRDC project), will conduct a series of free workshops across the state in July 2022. The workshops will focus on how to pressure test grain storage structures and the recommended rates of phosphine to control and manage resistance.
Grain growers implementing a routine monitoring system of on-farm stored grain by sieving grain monthly, can help to detect pests early and maintain effective pest resistance control and management.
For more information about resistant pest sampling methods, testing institute, grain pests and diseases, biosecurity practices, and pressure testing workshops, please contact Shafiya Hussein at shafiya.hussein@sa.gov.au
The Grains Farm Biosecurity Program is an initiative to improve the management of, and preparedness for, biosecurity risks in the grains industry at the farm and industry levels. Visit the Grains Farm Biosecurity website for more information.