Methyl Bromide Substitutes...

by Teresa Acklin
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   Under the terms of the Montreal Protocol, an international agreement that controls ozone-depleting substances, countries around the world will phase out the use of methyl bromide over the next several years. Canada and the United States plan to end its use by the year 2001, while some European countries will eliminate the chemical sooner.

   Grain handlers and flour millers have relied for decades on methyl bromide as a quick, efficient and relatively cheap way to destroy insects. With methyl bromide's eventual demise, the private and public sectors are working together, trying to develop acceptable, cost-effective substitutes. Following are two reports from field tests using fumigation methods that could replace methyl bromide.

...in a flour mill

   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,” said Linda Dunn, a senior policy analyst with Agriculture and Agri-Food Canada (A.A.F.C.). “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.”

   One 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). A.A.F.C. worked 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 consists 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.

Test Methods

   Liquid carbon dioxide vaporized to a gas was piped in to provide a final average air concentration of 4.3%, and magnesium phosphide strips were distributed on several floors. Phosphine gas levels recorded during the eight- 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 37.3°C. The average relative humidity was 18%, and the low was 13%; humidity is an important factor when using this type of fumigation because 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 personnel 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 April 14, 1996, approximately 14 tonnes of CO2 were spread through the building using hoses. Initially, six magnesium phosphide 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 30.2° to 40.3°C.

   The final preparation before fumigation involved the placement of pests. Adults, larvae and eggs were placed in several locations by three experimenters. In one test, conducted by Colin Demianyk of A.A.F.C.'s Cereal Research Centre in Winnipeg, pairs of vials of confused flour beetles, each containing either 10 adults or 10 eggs, were placed in 10 locations on each floor.

   “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,” Mr. Demianyk said.

   Control vials of test insects were exposed to a maximum temperature monitored at 28°C for several hours during setup. Controls were then kept at ambient humidity and 20.2°C throughout the test.

The Results

   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. All insects, test and control, were incubated at 30.2°C and 70% 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,” said Bernie McCarthy of PCO Services Inc., 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.”

   Mr. Demianyk noted that the combined heat/carbon dioxide/phosphine treatment killed more than 98% of confused flour beetle eggs and 100% of adults. Under traditional methyl bromide fumigations, a 95% 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,” Ms. Dunn concluded.

   The treatment combining heat, phosphine and carbon dioxide was first tested in 1993 at Purdue University, West Lafayette, Indiana, U.S., on an experimental mill. To date, 38 fumigation treatments, including 24 on flour mills, have been performed successfully 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%,” says Mr. Mueller, adding that research is under way to better manage humidity and corrosion.

   Successful fumigations have been conducted in modern, computerized food-processing plants as large as 114,000 cubic meters with no startup or corrosion problems. The tests have worked even with the relative humidity above 70% and extended periods of rain.

   “But this approach is 25% to 40% more expensive (than methyl bromide),” Mr. Mueller noted. “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.”

   This article is adapted from the “Methyl Bromide Alternatives Newsletter,” October 1996, published by the U.S. Department of Agriculture's Agricultural Research Service.

...in a grain storage facility

   As an alternative to pesticides and other chemical treatments, carbon dioxide, an approved fumigant under Canada's Pest Control Products Act, was tested to determine its effectiveness under field conditions.

   An elevated level of CO2 controls insects by causing them to expand their spiracles and respire faster in an attempt to expel excess CO2 and take in more oxygen. In a carbon-dioxide-rich atmosphere, the attempt is self-defeating since more CO2 is taken in, worsening the physiological imbalance within the insect's body. Water loss from insects is also greater when relative humidity decreases as a result of the high percentage of dry carbon dioxide gas in the intergranular air during treatment.

   The experimentation was conducted in the Canadian province of Manitoba during mid-1996 in summer weather conditions, with Manitoba Pool Elevators providing concrete facilities at Starbuck and a bolted-steel bin at Oakville. The concrete bin selected at Starbuck has a capacity of 280 cubic meters, or roughly 209 tonnes, while the bolted steel bin at Oakville is 704 cubic meters, or 525 tonnes. The majority of the detailed study was conducted in the concrete bin at Starbuck because of its better air-tightness relative to the bolted steel bin.

Bin Modifications

   The greatest challenge at Starbuck was maintaining a gas-tight seal at the bottom of the bin. The gas losses proved to be in the 25% range for a full bin of grain, a measurement determined by the amount of make-up gas required to maintain a consistent level of CO2 inside the bin.

   Initially, a gas impermeable plastic was used to seal the bottom gate, which provided an effective seal but was time-consuming to install. The plastic also created the problem of removal in filled bins, where it could become entangled in the horizontal unloading auger.

   To overcome this problem, a new gate design was implemented at Starbuck that proved to be gas-tight and was operated with an external lever. The new design reduced the CO2 make-up portion to 5%.

   The bin was further modified to allow for CO2 readings at various intervals throughout the 30-meter height of the bin (24-meter sidewall, 6-meter hopper bottom). A thermocouple was installed from the top of the bin's entrance cover to determine grain temperature, and several sample tubes were extended to measure gas levels at heights of 12, 18 and 24 meters from the bottom, down the centerline of the bin.

   In addition to measuring gas concentrations in the bin, safety precautions required monitoring gas levels outside the bottom of the bin, which is in an enclosed area, and in low-lying areas, including the elevator pit. Monitors were used to detect any accumulated leakage because CO2 is 1.5 times heavier than air and settles and pools in low-lying, non-vented areas.

   An electric vaporizer was designed to vaporize liquid CO2 from a six-tonne bulk storage tank located on the premises. A bulk tanker was used to refill the storage tank when the level became low.

   Typically, less than one tonne was used for each batch fumigation, which took four hours to attain a target gas level of 80% to 100% concentration at the top gas-sampling location. The target gas level was set to compensate for some leakage to achieve the 60% CO2 required to kill insects rapidly. Initially, the gas level in the bin was topped up manually to maintain the specified concentration of CO2.

   For efficacy testing of the fumigation against insect pests, adult rusty grain beetles and the red flour beetle were used. Fifty adults of each species were placed into the bottom portion of plastic pitfall traps and suspended from rope at heights of 0, 6, 12, 18, 24 and 30 meters, measured from the bottom of the empty bin. Control vials of insects were not subjected to the fumigation and were kept at 20°C, the same temperature as within the bin.

Testings Procedures

   Tests were conducted for durations of two, three, four and five days under various concentrations of CO2 within the bin. At 12 meters, concentrations ranged from 46% to 94%; at 18 meters, concentrations were 9.5% to 91%; and at 24 meters, concentrations were 7.7% to 89%.

   All insects were killed at heights of 0, 6, 12, 18 and 24 meters, except in the two-day test, in which one rusty grain beetle survived at each height of 18 and 24 meters. Mortality was lowest at 30 meters, with survival ranging from 72% to 98% for rusty grain beetles and 94% to 100% for red flour beetles; survival at this level was directly attributable to CO2 loss, which was most rapid and greatest at this position.

   In the unfumigated control groups, 100% of red flour beetles and 99% of rusty grain beetles survived.

   Further tests were conducted on the bin filled with wheat at 21°C. Six probes containing insects were inserted into the wheat at depths of 5, 2.4 and 1.2 meters from the surface, which corresponded to 25, 28 and 29 meters in bin height; frictional and compaction forces of the grain prevented insertion and retrieval of probes from deeper positions. Additional gas-sampling tubes were located at 29.7 meters and at the spout to better monitor CO2 levels at the height where insects had survived the empty-bin tests.

   A three-day fumigation was conducted with CO2 levels ranging from 2% to 100% measured at the 24 and 29.7 meter spout levels. Mortality for the three-day test ranged from 18% at the 1.2-meter depth to 84% at 5 meters for red flour beetles; similarly, mortality ranged from 14% to 90% for rusty grain beetles at those depths.

   A four-day test had CO2 levels of 0.5% to 100%, with mortality for red flour beetles ranging from 0% at 1.2 meters to 90% at 5 meters. Mortality for the rusty grain beetle ranged from 14% at 1.2 meters to 98% at 5 meters. The mortality of each control species during both three- and four-day tests ranged from 0% to 8%.

   The test results were generally very positive, with high mortality rates occurring during relatively short exposure periods when CO2 levels were kept consistent.

   The present design of the bin allowed for some CO2 to exit through the distributor spout, which was not sealed, on the top of the elevator. This occurred during times when staff was not present to monitor gas levels and manually top-up the gas concentration. As a result, an automated injection system has been designed to provide make-up gas and maintain a more constant concentration, and the sealing of the distributor spout will be addressed in a future permanent installation.

   Presently, it seems feasible to use CO2 to control insects by filling the bin with grain to a height of 24 meters. Filling only to that level eliminates the problematic low gas levels encountered above this height at the top of the bin.

   The benefits of CO2 fumigation include no toxic residue on the grain, safer handling than other fumigants and relatively short processing times. Furthermore, the cost of this procedure is very competitive with traditional methods of grain fumigation when all equipment and capital costs are considered; a week of CO2 application cost less than U.S.$0.40 per tonne.

   The pooling of industry and government resources for this project combined a wide range of expertise. Manitoba Pool personnel conducted bin sealing, modifications and injection of CO2. Praxair Canada Ltd. was responsible for providing the CO2, associated equipment, and technical support to the project, with further technical support from Digvir Jayas, Professor, Biosystems Engineering, University of Manitoba. Noel White and Colin Demianyk of the Stored Products Group, Cereal Research Centre, Agriculture and Agri-Food Canada, Winnipeg, provided field and biological support, including insects for efficacy tests. A.A.F.C. involvement was made possible through support of the Matching Investment Initiative.

   This article is based on a report by Chris Chekerda, an engineer with Praxair Canada Inc., Edmonton, Al-berta, in the Jan. 10 issue of “Forum on Stored Grain Products” newsletter, published by Agriculture and Agri-Food Canada.

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