Finding a new way to fumigate

by Sebastien Hardy
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The island nation of Mauritius is located in the Indian Ocean off the southeast coast of Africa. For the nation’s 1.2 million inhabitants, wheat is imported and processed in the Les Moulins De La Concorde flour mill in the capital city of Port Louis.

Pest management at the mill has relied upon a proven combination of integrated pest management (IPM) and fumigation to manage stored product pests, which include red flour beetles, Indian meal moths and Mediterranean flour moths. However, as a signatory to the United Nations Montreal Protocol, Mauritius ceased using methyl bromide fumigant because it is an ozone-depleting substance.

Hardy Henry Services in Port Louis fumigated the mill in 2001. During the next few years, the mill used an IPM-only approach that involves scheduled cleanings, weekly inspections of pheromone traps and spot treatments with insecticides. It became increasingly difficult to achieve the desired results using only IPM, especially in areas that are inaccessible to inspection and treatment.

In 2005, the milling company, Hardy Henry Services and the Mauritian government began discussing sulfuryl fluoride as a possible alternative gas fumigant. Information about sulfuryl fluoride, which goes by the trade name of ProFume, as a potential alternative was obtained from the United Nations Environment Programme (UNEP) methyl bromide working group. Sulfuryl fluoride is an effective fumigant and not an ozone-depleting substance.

The United Nations Multilateral Fund (Montreal Protocol) offers special grants to help nations with limited resources evaluate suitable alternatives to methyl bromide. The Mauritius Ministry of the Environment obtained a grant that was implemented by GTZ (Windhoek), an organization based in Germany. Insects Limited Inc. of Westfield, Indiana, U.S., provided technical guidance. Product use and stewardship  training was provided by Dow Agro-Sciences LLC, Indianapolis, Indiana, U.S., manufacturer of the fumigant.

FUMIGATION PROCESS

Preparation for the fumigation involved a full day of classroom training. Dow AgroSciences requires training and a written examination as part of its product stewardship program. The training also included an introduction to the Fumiguide software program that allows the fumigator to calculate site dosage options, monitor the fumigation progress and provide documentation.

To alleviate the risk of exposure to the residents near the mill, the boundary areas were monitored regularly during the fumigation using two sulfuryl fluoride clearance devices which could measure concentrations to 1 ppm. A public relations outreach effort explained what was involved in the fumigation and why it was necessary.

Prior to the fumigation, the mill was sealed and six monitoring points were established, one in the basement and one on each of the five above-ground floors. All entrances and sides of the structure were posted with warning signs, doors were locked, guards were posted and the site perimeter was secured.

The Fumiguide software estimated that 7.4 cylinders of sulfuryl fluoride would be needed to fumigate the 10.464-million-cubic-meter space. The estimate took into account the fact that sulfuryl fluoride penetrates faster than methyl bromide, and that new windows had been installed in the mill. The halfloss time (HLT) — the time in which half of the released gas would be dissipated – was estimated to be 10 hours.

The fumigation was scheduled to take advantage of the mill shutdown over the New Year’s holiday. Fumigant was released at 6:30 p.m. on Dec. 31, 2007. Equilibrium was achieved two hours later. To better allow evaluation of how the fumigant would perform, six cylinders were initially released. An additional cylinder was released later to compensate for space taken up by silos on the top floor.

The fumigant exposure ended at 8:30 a.m. on Jan. 2, 2008. During the fumigation, the average temperature inside the mill was 30 degrees C (86 degrees F). Outside temperatures ranged from a low of 28 degrees C (82 degrees F) to a high of 35 C (95 degrees F). Winds ranged from 5 to 15 miles per hour. These overall favorable weather conditions along with excellent sealing of the structure resulted in an actual HLT of 12.9 hours, much better than the 10 hours estimated.

The concentration-multiplied-by-time dosage achieved was 855 gram-hours per cubic meter. The required dosage for insect control was achieved 22 hours after introduction. Largely due to the favorable HLT achieved, this dosage level  was 1.5 times what was targeted to get adequate control of the intended pests. This data will now serve as a benchmark for calculating a reduced amount of fumigant needed for future fumigations, or allowing for a shorter fumigation period using the same amount of fumigant.

While aeration of the mill could have begun earlier based on achieving the planned dosage, the decision was made to stick to the original aeration schedule. Aeration began at 8:30 a.m. on Jan. 2 and ended at 7:30 a.m. on Jan. 3. The mill was cleared for re-entry after sulfuryl fluoride clearance devices ensured that concentrations were at the required level of 1 ppm or below. While sulfuryl fluoride aerates faster than methyl bromide, aeration at the mill took more time than expected because the structure’s air handling system was not used. Instead, a passive aeration process was used – seals were removed and doors opened on the ground and top floors. The resulting chimney effect allowed the first three floors to be cleared for re-entry in two hours. But bins on the fourth and fifth floors required the use of fans and additional aeration. Using the mill’s air handling system will allow faster aeration in future fumigations.

No fumigant was detected along the site’s property boundaries during routine checks taken during the fumigation.

Excellent control of stored product insect pests was observed after the fumigation, and some areas of the mill had high concentrations of dead insects.

Overall, the mill shutdown time, dosage rate and results with sulfuryl fluoride were very similar to that of ethyl bromide fumigation.

The most noticeable difference between fumigation with methyl bromide and sulfuryl fluoride was the added level of professionalism provided by the Fumiguide program. The entire fumigation team was surprised how this technology gave direct readings to indicate whether more gas was needed and when the fumigation could be ended. The software also generates a detailed report of the fumigation, which is very reassuring to the mill.

Because the fumigation project met all of its expectations, the Mauritian government determined that sulfuryl fluoride is a technical and economical alternative to methyl bromide for stored product pest control. As a result, Pro-Fume gas fumigant is now registered by Mauritius for this use.

Sebastien Hardy is a licensed fumigator with more than 15 years of experience, and is the manager of Hardy Henry Services, a Rentokil Initial franchise in Port Louis, Mauritius. 

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