The art of silo fumigation

“Remote monitoring is changing the fumigation world. Precision fumigation is not cheap to achieve and in most cases monitoring is made only at the beginning and at the end of a fumigation. Fumigators hesitate to send operators to take readings every day in a faraway silo. Wireless sensors offer new capabilities with continuous information upload. They allow the observation of each treatment in real time, allowing corrective actions and evaluation of each application. The cognitive software offers treatment prediction and gives recommendation to users based on numerous monitored parameters. In the next years all fumigations will be monitored online.”

Vasilis Sotiroudas
Pest Management Expert
vasilis@centaur.ag

Large volumes of bulk grain have always been a challenge. A challenge to inspect, to maintain the quality, to locate the hot spots, to aerate, to fumigate, to preserve. Silos, ship holds, bunkers and flat warehouses shall offer safe storage but very often offer “blind” storage as it is really difficult to measure the conditions in the bulk mass. To successfully fumigate the large volumes, one needs to combine science with art!

Fumigation Plan
Everything good always starts with a plan. In the US and a few other countries, developing a fumigation plan and sharing it with the local authorities, is part of the legislation. In some other countries, the fumigation plan is developed as part of the Good Practices. In most countries the plan only exists in the mind of the chief fumigator. And this is not good…

Looking at the American standard plan {2}, it includes the exact location to be treated, the responsible people and the authorities involved. It requires details of the structure to be fumigated, like the type of walls, the adjacent buildings and even a drawing. The monitoring for safety depends on the risk assessment and continuous electronic monitoring is asked for the dangerous cases. The type of commodity must be stated, the volume and the quantity.
The times of preparation, treatment and aeration must be declared. The dosage and type of fumigant are declared and correlated with the label. The sealing is described. All doors must be locked, carry a label and have extra locks to avoid the chance of personnel entering with their own keys.

The aeration process must be described and even the deactivation of used fumigant. A release procedure and signing are also required.

Resistance management
Most countries in the world have issues with insects showing some level of resistance to phosphine. If resistance is detected, then a higher dosage is required in combination with longer exposure. The fumigator needs to search for resistance by collecting insects and putting them in the test. There are five tests that are commonly used [1]:

• The molecular test using PCR and molecular markers
• The FAO test using 30ppm for 20 hours.
• The Nayak et al 2013 modified test using 6 hours of exposure.
• The Dose response test using 3 days at a range of concentrations.
• The Detia Degesh Kit using 3000ppm for 9-12 minutes
The last test can be performed on site by fumigators.

Fumigation protocol
There are very few phosphine fumigation protocols internationally accepted.
1. According to the Australian GRDC standard [3] the concentration of phosphine must remain above 200ppm for 10 days or above 300ppm for 7 days.
2. According to Coresta Guide No2 [4] the concentration of phosphine must remain above 200ppm for 4 days when the product temperature is >20C or above 300ppm for 6 days when the product temperature (T) is 16C>T>20C. The Coresta is actually focusing on the insect Lasioderma sericorne of Tobacco but recent research showed that it has excellent results on most stored product insects.
3. The COFP standard (by the USDA) [5] requires the concentration of phosphine to remain above 300ppm for 6 days when the product temperature (T) is 16C>T>20C, or above 300ppm for 4 days when the product temperature is >20C.

The common thing between all protocols is that the fumigator needs to measure the concentration continuously as well as the product temperature. The general idea is that Fumigation is not recommended below 16C and the minimum duration is 4 days plus the time to reach the concentration. So no phosphine fumigation can last less than 5 days.

Silo tightness
If a silo/bunker/warehouse/container/stack is not airtight, the phosphine gas will escape. When the gas escapes, we face two serious problems: it may threaten the life of people and animals nearby and the treatment may fail. For both reasons we need airtight assets.

To determine the level of tightness there is an Australian standard for pressure testing issued by GRDC and revised in 2014. The bottom-line of the standard is that the “Half-life Pressure Test” time must be over 3 minutes. How do we measure that? We increase the pressure in the asset to 25mmHg and measure the time needed to drop to 12mmHg. This is the “Half-life Time” and it must be longer than 3 minutes.

If we cannot reach this level of tightness, we must be prepared to deal with leakages. We must be prepared to add gas and also to deal with the risks of escaping gas.

Recirculation
Most professionals know that phosphine (molecular weight: 33,9 g/mol) is a bit heavier than air (molecular weight: 28.9 g/mol). That means the phosphine gas would go down pulled by gravity. But this is not happening. The reason is that the phosphine gas is moving together with the air. In the grain we usually have air movement caused by the differences of the temperature during the day.

Unless we use recirculation of phosphine (j-system) we cannot guarantee that phosphine will reach all areas of grain mass and that it will create an equilibrium. What happens without recirculation is shown on the next graph and it allows insect survival.
Recirculation is mandatory for all grain masses larger than 100 metric tons. The ideal recirculation should last during all the treatment as long as it does not increase gas leakage. When the recirculation is stopped, the result is loss of equilibrium as shown in photo 4.

Measuring
Remote monitoring is changing the fumigation world. Precision fumigation is not cheap to achieve and in most cases monitoring is made only at the beginning and at the end of a fumigation. Fumigators hesitate to send operators to take readings every day in a faraway silo. Wireless sensors offer new capabilities with continuous information upload. They allow the observation of each treatment in real time, allowing corrective actions and evaluation of each application. The cognitive software offers treatment prediction and gives recommendation to users based on numerous monitored parameters. In the next years all fumigations will be monitored online.

Inspect
At the end of each fumigation, a thorough inspection must be made in search for surviving insects. Such a finding will indicate a gap in the process and should lead to corrective and preventive actions. There are several aspects in a fumigation that may lead to a failure like leakage, low dosage, low humidity, low temperature, wrong readings, short duration etc. By finding alive insects we realize a failure.

If we don’t find alive insects are we sure that the fumigation was successful? Probably not. Adult insects are about 15% of the total insect population (data on Tribolium), meaning that we search for alive adults while the 85% of the population (larvae, pupae and eggs) may also be alive and almost impossible to detect. When a treated volume shows infestation a month or two later, it is a clear indication that the fumigation failed.

Frequency of applications
Depending on the label of phosphine and the country legislation, a second phosphine fumigation on the same lot, in the same year, may not be allowed. Here is another serious reason for improving the art of fumigation.

Are non-chemical fumigation methods successful in silos?
Several silo treatments like: oxygen, controlled atmosphere, CO2, N2 are considered very successful and are available in many places around the world. Academic research has been conducted by several institutes leading to the conclusion that non-chemical methods can be equally successful. Precision monitoring is the key to success.

People safety
The right approach starts with a Risk Assessment for each treatment. This can be part of the Fumigation Plan as described on the first paragraph. Personal monitors, real-time monitoring, visual and sound alarms shall be involved in every fumigation. Life is very valuable to put it at risk and modern technology gives us many tools to use. There is no excuse for risking lives.

It’s all about precision
The fumigator must evolve. The art of modern fumigation is not in using traditional secret methods. The art is acquiring information and using it to succeed. Insects evolve, we should too.

Sources
1. Presentation to IMFO group, Prof. C. Athanassiou, 2017
2. Fumigation Plan, Pestcon Systems Inc., 2016
3. Fumigating with Phosphine, GRDC Australia, 2013
4. Phosphine Fumigation Parameters, Coresta Guide No2, 2013
5. Fumigation Handbook, USDA, 2016
6. Pressure testing sealable silos, GRDC, 2014
7. Centaur sensors and platform, centaur.ag, 2019

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