Is low oxygen effective against insects in silos?

“Continuous monitoring and early detection of possible failures consist the basis for the establishment of an effective and reliable low oxygen application strategy in silos. In chambers a few sensors may do the work accurately, but when it comes to large bulks in silos the number of sensors should be drastically increased to minimize the chances of having oxygen nests in the treated area. Apart from monitoring of the percentage of oxygen alone, the utilization of a series of sensing devices can provide an “early warning” system.”

Prof. Christos Athanassiou
Laboratory of Entomology and Agricultural Zoology
Department of Agriculture, Crop Production and Rural Environment
University of Thessaly, Greece
athanassiou@uth.gr
Low Oxygen: Different applications for different commodities
Despite the fact that there are data available for the use of low oxygen in different types of facilities, this technique has become much more adaptable in the case of disinfestations of commodities. Low oxygen is a generic term, and its definition is much more complicated that the initial impression of this term indicates. Hence, low oxygen levels can be obtained by changing the pressure in a given area, which, indirectly, results in reduced percentages of the gases that occur in this area, including oxygen. At the same time, low oxygen can be also obtained by the increase of the percentage of other gases, which again results in reduced oxygen percentages in the target area. This last technique can be obtained with the use of different gases, such as carbon dioxide or nitrogen, with nitrogen being the most popular during the last couple of decades. The idea behind such an application is really simple, the increase of nitrogen causes a concomitant decrease of the other gases, especially oxygen, which is vital for the survival of different insects and various types of fungi and bacteria.

Silo treatment with nitrogen (low oxygen method)

Low oxygen can be used in packaged goods, as long as the packages are gas-tight, and there is no interaction with the external environment. This is usually achieved through flushing nitrogen or other gases inside the package before sealing. Moreover, atmospheric nitrogen can be taken from the air and applied in sealed chambers that contain different types of durable commodities, ranging from grains to dried fruit. In this effort, sensors (i.e. oxygen detectors) are essential to indicate the success of the application and the distribution of the gas throughout the target area. Finally, nitrogen can be also utilized with success in bulked grains, a scenario that is qualitatively different than the above, and probably the most demanding.

Controlled atmosphere mobile unit (blue container) treating metal silos with nitrogen (low oxygen method) in Greece.

Nitrogen in silos: A dynamic procedure
The application of nitrogen in silos has nothing to do with the application of nitrogen in chambers, for different obvious reasons. First, a silo needs to be sealed, and even if it is sealed, this sealing is definitely inferior that that in chambers. In this context, any application of low oxygen in silo needs a considerable amount of time to improve sealing, before starting the actual application itself. This can be done through pressure tests that may reveal a leaky structure on which the application of nitrogen is useless. Second, in a chamber the temperature level can be easily controlled, as elevated temperatures greatly improve the insecticidal effect of nitrogen. Conversely, in the case of a silo with 1,500 t of grain, the temperature level cannot be controlled and any application should be carried out at the conditions prevailing. This consists a complicated procedure, as the algorithms on which the application of nitrogen should be applied may change with the temporal change of temperature, providing different required exposure intervals each time. Third, a silo is very likely to have areas with “oxygen nests”, where the application is practically ineffective. Although the oxygen nest risk cannot be completely eliminated, sealing is the solution in this case as well. The presence of area with oxygen that is higher than 1 and often 3 %, can result in insect survival, that, despite stress, can lay eggs before death, causing additional quantitative losses.

Treating products in controlled atmosphere chambers (agrospecom.com)

Real world applications
Surprisingly, and despite the fact that the application of low oxygen has wide industrial applications in different types of food production facilities, the published data are disproportionally few, in comparison with other methods, such as heat, ozone or carbon dioxide. Relatively recently, in nitrogen chambers, Athanassiou et al. (2016) found that currants can be successfully treated with applications that are extremely short (less than one week), while the use of nitrogen resulted in the decrease of the presence of molds. When the same technique was applied or grains, it was found that there same efficacy levels could be obtained with a considerably higher exposure interval, that often exceeded three weeks (Navarro et al. 2012). In such an application, it is generally considered that some internal feeders, such as the lesser grain borer, Rhyzopertha dominica (F. ) (Coleoptera: Bostrychidae) or the rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae) are less susceptible than external feeders, such as the saw-toothed grain beetle, Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae). This is due to the fact that immature development of external feeders occurs outside of the kernel, resulting in increased susceptibility. Eggs and pupae have been found to be less susceptible than adults and larvae for both stored product beetles and moths (Athanassiou et al. 2016, Athanassiou and Arthur 2018).

Monitoring nitrogen treatments through an online platform (centaur.ag)

Continuous monitoring and early detection of possible failures consist the basis for the establishment of an effective and reliable low oxygen application strategy in silos. In chambers a few sensors may do the work accurately, but when it comes to large bulks in silos the number of sensors should be drastically increased to minimize the chances of having oxygen nests in the treated area. Apart from monitoring of the percentage of oxygen alone, the utilization of a series of sensing devices can provide an “early warning” system, that will provide alerts whenever the application is not going to be successful and provide the inferences necessary for taking corrective actions. Moreover, the inclusion of bioassays on a regular basis in industrial applications may help finding possible gaps in the treated area. In this bioassays, apart from parental mortality, progeny production should be also evaluated to indicate possible immature survival. Furthermore, the results from the bioassays should be further utilized as a means to “auto-correct” the application algorithm, through providing efficacy data that can be directly adaptable per application scenario. It is true that the algorithms that are currently in use require additional tuning for silo applications, mostly due to the fact that insect mortality data rely on laboratory bioassays, semi-field bioassays or applications in chambers, which are far more static than the dynamic nature of industrial silo treatments. In this regard, additional experimental work is required to shed light to the factors that affect low oxygen applications in silos, taking into account different biotic or abiotic conditions, such as the structure itself, the type of the commodity, the temperature and the target insect.

References
Athanassiou C. G., Chiou A., Rumbos C. I., Sotiroudas V., Sakka M., Nikolidaki E. K., Panagopoulou E. A., Kouvelas A., Katechaki E. and Karathanos V. T. (2016). Effect of nitrogen in combination with elevated temperatures on insects, microbes and organoleptic characteristics of stored currants. Journal of Pest Science 90: 557-567.

Athanassiou C. G. and Arthur F. H. (2018). Recent advances in stored product protection, Springer.

Navarro S., Athanassiou C.G., Varnava A., Vroom N., Yiassoumis D., Leandrou I. and Hadjioannou S. (2012). Control of stored grain insects using nitrogen in large concrete silos in Cyprus. Proceedings of the 9th International Conference of Controlled Atmospheres and Fumigation in Stored Products, Antalya, Tyrkey, 15-19 October 2012, ARBER Professional: 478-487.

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