by Dr Manjree Agarwal, Murdoch University, Western Australia

 

The grain industry worldwide is highly dependant on fumigants for controlling stored grain pests, which cause about 15-20 percent of losses every year.
The most commonly used fumigants are phosphine and methyl bromide, of which methyl bromide has been phased out after the Montreal protocol (a 1987 agreement to limit the use of ozone-depleting gases), and excessive usage of phosphine has resulted in a resistance problem with many stored grain pest species.

There is, therefore, a problem with currently used fumigants: there is no ideal fumigant as many are flammable and have high mammalian toxicity, and the choice of possible new fumigants for grain is very restricted. Even if an alternative fumigant were selected, the lengthy, time-consuming testing and registration procedures would present a formidable barrier to its adoption and usage.

Hence, there is an urgent need for a non-chemical method for effective, safe, and cheap control of stored grain pests, which does not have any registration requirements. Modified Atmosphere (MA) or Controlled Atmosphere (CA) is one of the answers to this problem.

MA involves the alteration of natural gases used in storage, such as carbon dioxide, oxygen and nitrogen, to create a lethal condition for pests. This has been practiced for centuries and has been promoted in recent years as a biorational substitute for chemical fumigants. Sometimes, in the use of controlled atmospheres, the cost of the gas may be a hindrance to adoption.

Carbon dioxide has been used as a viable alternative to phosphine for the control of insects attacking stored products; but CO2 is efficient only when concentrations higher than 40 percent are maintained for long periods, and so, in the long run, it becomes an expensive affair.

In that respect, N2 (nitrogen) for use in low O2 (oxygen) treatments is less expensive and can be generated from ambient air (which has close to 80 percent concentration of nitrogen), via use of membrane-adsorption technology.

Replacing oxygen with nitrogen not only helps in killing some of the stored grain pests but is also cheap, safe to both humans and the environment, with no registration requirement. It also maintains the natural quality of grain and pulses, including the colour, which is one of the important attributes for pulses and beans. This creates a win-win situation in all different scenarios.

Researchers from Murdoch University, Professor Yonglin Ren and Dr Manjree Agarwal, in collaboration with industry partners (especially grain bulk handlers and Plant Biosecurity CRC), explored the use of nitrogen as a complementary technology to the use of conventional fumigants.

The systematic laboratory scale trails were initially undertaken to find optimum dosage and treatment time. From this, developed efficacy data and optimal nitrogen operating levels were discovered, for the control of key pests found in Australian grain storage facilities. A key finding was that an atmosphere comprising 99 percent nitrogen was required for a minimum of 14 days to provide control of all stages of grain insects in storage.

Thereafter, trials were undertaken on medium scale wheat storage facilities to evaluate the cost-effectiveness and suitability of nitrogen generator and gas application methods. From these small and medium scale trails, "Controlled Atmosphere" offers
a completely chemical-free treatment for disinfestation and protection of stored grain.

Insect pests can also be controlled by reducing oxygen to very low levels within a silo. The levels required are obtained by purging the silos with nitrogen, generated through a nitrogen generator placed on a mobile truck or trailer.

Although sound in principal and efficacy, there were major applied research challenges to implementation of this technique on an industrial scale. These included the need for a practical and relatively cheap source of inert gas, the application of the gas
to the storages, and the requirement for a well-sealed system to avoid any leakage.

To overcome these challenges, we partnered with a nitrogen generator company, to develop a more economical nitrogen generator, which uses atmospheric air to generate nitrogen using membrane technology. This technology generated greater than 99 percent purity of nitrogen within two to four hours—enough to purge a farm silo of 75 to 200 tonnes capacity.

After a successful farm trial, the technology was used to treat a bulk handler"s grain terminal, with a capacity 2000 to 10,000 tonnes concrete vertical silo, which can be treated within four days, maintaining the required level of nitrogen for 14 days, which is the optimal condition to control all stages of stored grain pests.

The nitrogen treatment not only effectively controlled stored grain pests, but also maintained the quality of wheat, canola fatty acid content and the colour of pulses, chickpea and faba beans.

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