During the December 1995 meeting of the parties to the Montreal Protocol, Canada and the United States supported a phaseout of methyl bromide by the year 2001. Some countries, including the Netherlands, Denmark, Norway, and Sweden, will phase out the chemical sooner. In addition, by 1998, Canada, along with other developed countries, will implement a 25-percent reduction in methyl bromide production and use to their respective country's 1991 levels.

One of the primary uses of methyl bromide in Canada is for the fumigation of facilities such as oat and flour mills, warehouses, food-processing plants, and conveyances, such as shipping vessels. Chemical alternatives for structural fumigation in Canada are limited.

"Assessing chemical alternatives for this use is difficult, since it is commonly not only a structure that is fumigated, but also stored food," says Linda Dunn, a senior policy analyst with Agriculture and Agri-Food Canada (AAFC). "We needed to find innovative solutions to this problem, so industry and government teamed up to determine the feasibility of an alternative approach to methyl bromide space fumigation." This approach, developed by David Mueller of the U.S.-based Fumigation Service and Supply Inc., uses a combination of heat, phosphine, and carbon dioxide (CO₂).

AAFC teamed up with Canadian and U.S. pest control industries, the Canadian food-processing industry, Canadian and U.S. suppliers of CO₂ and magnesium phosphide, federal departments of health and environment, and the Ontario Ministry of Environment and Energy to test this alternative to methyl bromide for controlling insect infestations on a commercial scale.

The Quaker Oats Company of Canada, Ltd. donated its mill facility in Peterborough, Ontario for the test. The facility is composed of several joined buildings, parts of which are almost 100 years old, and is typical in some respects of many other Canadian milling and cereal-processing facilities.

"The combination of heat, phosphine, and CO₂ in just the right amounts successfully fumigated the building," Mueller says. Liquid carbon dioxide (vaporized to a gas) was piped in to provide a final average air concentration of 4.3 percent, and magnesium phosphide Fumi-Strips® were distributed on several floors.

Phosphine gas levels recorded during the 8- to 36-hour fumigation period ranged from a low of 10 parts per million to a high of 110 parts per million. The time-weighted average concentration was 36 parts per million at 24 hours and 38.2 parts per million at 36 hours.

The temperature of the building was raised to an average of 98.6 ^oF. The average relative humidity was 18 percent, and the low was 13 percent. Humidity is an important factor when using this type of fumigation. High humidity combined with high concentrations of phosphine may lead to corrosion.

To avoid any potential phosphine damage during the test, areas most likely to corrode were flooded with CO₂ to create small rooms of positive pressure and then sealed with tape. Several polished copper tubes were hung in the fumigated areas as a simple test of corrosiveness. No corrosion damage has been noticed in the facility.

The mill was cleaned and sealed. Quaker Oats employees were responsible for cleaning the facility and equipment during the shift preceding the fumigation. All food-processing equipment was taken apart, blown out, emptied, and cleaned. Equipment and elevator legs were left open to give easy access to the fumigant. Window sills and floors were cleaned of debris and dust, and windows, fire doors, and other entries were taped shut.

From April 12 to 14, 1996, approximately 14 tons of CO₂ were spread through the building using hoses. Initially, six magnesium phosphide Fumi-Strips® were placed on alternating floors. Because of the extremely low humidity within the facility, which led to a very slow release of the magnesium phosphide, 11 additional strips were used during the 36-hour test. The temperature was monitored and maintained at 86 to 104 ^oF.

The final preparation before fumigation involved the placement of pests. Adults, larvae, and eggs were placed in several locations by three different experimenters. In one test, conducted by Colin Demianyk of AAFC's Cereal Research Centre, in Winnipeg, pairs of vials of confused flour beetles (Tribolium confusum), each containing either 10 adults or 10 eggs, were placed in 10 locations on each floor. Demianyk notes, "The locations were chosen because they either seemed to be cool areas by windows or doors, or were potentially more difficult for the fumigant to reach, for instance, behind equipment."

Control vials of test insects were exposed to a maximum temperature monitored at 82 ^oF for several hours during setup. Controls were then kept at ambient humidity and 68 ^oF throughout the test.

After fumigation, control insects were brought back to the test building during the collection of the test insects, and then transported in hand luggage back to the Cereal Research Centre in Winnipeg. All insects, test and control, were incubated at 86 ^oF and 70 percent relative humidity within 30 hours after the completion of the test.

Adult pests were examined the next day for survival. They were then placed with vials of eggs to incubate for 30 days to determine if any eggs laid by adults during fumigation survived. No adults survived in a 900-insect sample.

"We learned a lot from this fumigation test," comments Bernie McCarthy of PCO Services Inc. (an S.C. Johnson Wax Company), project manager for the test. "The experience pointed out the importance of constant monitoring and adjustments to maintain the correct balance of heat, phosphine, carbon dioxide, time, and relative humidity."

Demianyk points out that a combined heat/carbon dioxide/ phosphine treatment killed more than 98 percent of confused flour beetle eggs and 100 percent of adults.

Under traditional methyl bromide fumigations, a 95-percent kill rate is considered successful. The combination fumigation method used at the Quaker Oats mill in Canada exceeded this rate, even under adverse conditions that included low ambient humidity, several leaks, and cold external temperatures.

"We believe that this commercially viable alternative fumigation method to methyl bromide in large facilities has the potential for extensive use in Canada's food industry," Dunn concludes.

U.S. Perspective

The treatment combining heat, phosphine, and carbon dioxide used earlier was first tested in 1993 at Purdue University on an experimental mill. To date, 38 fumigation treatments (24 on flour mills) have been successfully performed in the United States.

High humidity and high concentration levels can cause corrosion when fumigating with phosphine, but high humidity can be controlled by piping large volumes of carbon dioxide into the buildings. "This normally reduces relative humidity by about 10 percent," says Mueller. There are methods being researched to better manage humidity and corrosion.

Successful fumigations have been conducted in large (3,800,000 cubic feet), modern, computerized food-processing plants with no startup or corrosion problems. The tests have worked even with the relative humidity above 70 percent and extended periods of rain.

Mueller notes, "This approach is 25 to 40 percent more expensive. Although it is not a total answer to the methyl bromide problem, tests show it can be a successful alternative in flour mills and food-processing plants."