Fumigation

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Chamlong Chettanachitara

INTRODUCTION TO FUMIGATION

Fumigation is a treatment measure which is intended to eliminate 100 percent of pests infesting grains and other commodities as well with toxic chemicals or fumigants. The fumigants are generally preferred because they form lethal concentration within enclosures and inside commodities while other insecticides penetrate with difficulty or not at all. A pesticide that is effective as a fumigant, it must be volatile enough to produce a toxic concentration in a close space in a short period of time. So, fumigation is a method of insect control that can effectively eliminate pests from agricultural commodities providing, if follows established practices and procedure. A fumigation that is conducted improperly will not kill the pests. Such fumigations are meaningless from grain storage point of view end' they may be hazardous.

CONDITIONS OF FUMIGATIONS

There are two conditions of fumigations. One of them is vacuum fumigation which is usually done in very strong enclosures such as thick iron chambers or concrete rooms. Fumigations may be performed under different levels of vacuum according to kind of commodities and target pests. Living plants or perishable commodities cannot be fumigated under much vacuum conditions while grains and other dry produces are not effected. Another one is normal atmospheric pressure, NAP, fumigation. The NAP fumigations are performed in various temporary enclosures, for example a stack under tarpaulin, silo bin under values and vents closed, cargo container during the door locked, barge or ship holds under sealed conditions. Those NAP fumigations are popular among local dealers and exporters because they can be done commercially.

PRINCIPLES OF FUMIGATIONS

Basic knowledge that must be made understanding prior to start performing fumigations are as follows.

Sealed enclosures

During a fumigation, the enclosure must be completely sealed otherwise the fumigant used will leak causing the fumigation not to kill insects and may be harmful to any person. Stack fumigation under tarpaulin sheet should be performed on polished concrete floor. The tarp must be in good condition and thick enought to hold the fumigant. Sand snakes for anchoring the sheet to concrete floor should meet the standard of proper size which is usually 4 inches in diameter. The sand snakes must be over-lapping position on the tarp around the stack. For cargo container fumigation, the rubber sealing of its door must not be torned at any part and the rubber is not expired too. Barge fumigation, if the barge is all top opened, the barge is usually covered with tarp. The tarp should be sealed with very good masking tape, otherwise it longer not fixed to the barge hold edge. So far, other temporary enclosuires should have very good seal at all possible leaks or vents as mentioned before.

Dosage

Dosage is a given quantity of a fumigant to be applied. Methyl bromide may be applied at different dosages shuch as 2 Ibs (pounds) per one thousand cubic feet for common stored insects while optimum dosage for Kahpra beetle, Trogoderma granarium is 5 Ibs per one thousand cubic feet. For phosphine gas, the amount of fumigant given by number of pellets, tablets, sarchets, plates and strips which each can release a definite quantity of phosphine. In general, one pellet can release PH3 0.2 gram while one tablet releases 1 gram. However, good fumigater should follow recommended dosage schedule so as to apply the proper amount of fumigant at a time.

Dosage Schedules

To be effective, the fumigant must be applied in sufficient quantity. The correct amount depends on published fumigation schedules that are applicable to the infested commodity, pest, temperature, and certain other considerations. Failure to follow the specifications outlined in the schedules will result in faulty fumigation treatments.

Concentration

Concentration is the amount of a fumigant under fumigation period. The concentration of the fumigant can be measured by the thermal conductivity or Fumiscope, Kitagawa precision concentration detector tube and Toka concentration meter. The concentration under fumigation may be-varied according to leak and sorption. Even lethal concentration throughout the enclosure is needed for good fumigation. If not, insects are not kill completely and may cause insect resistant to the fumigant in not very long future.

Fumigation period

Fumigation period means the exposure period plus degas or aeration period. When the exposure period is ended the treated commodity should be left aerated for a certain time. Then the concentration inside the commodity will be detected to ensure that the fumigant left is not harmful to any person. If so, the fumigation period is over.

Sorption

Before a constant concentration of the fumigant can be maintained the sorptive capacity of the article under fumigation must be satisfied. It varied considerably with the article being treated. Sorption includes the terms adsorption, absorption and chemisorption.

Adsorption is concerned with the loss of fumigant due to adhesion of impingement of the gas molecules to the surface of the materials under treatment.

Absorption is concerned with loss of gas within the commodity due to capilary forces.

Chemisorption is a chemical reaction that takes place with a gas and commodity. The process is irreversible. When chemisorption occurs, residues are formed. These residues are considered harmful in edible products.

Desorption

Desorption is the result of absorbed and adsorbed gas molecules. Desorption time varies with the commodities and conditions, but it can be complete.

NAP FUMIGATION

Stack fumigation

Most of grains are stored short and long term by two methods They are bagged grain and bulk grain storage. In case of bagged grain, warehouse keepers are usually piling up them as stacks in the warehouse. Each stack may be as big as 10,000 tons. When insects are found infesting grain in the stack, fumigation must be performed to eliminate all of them.

Procedure and steps of stack fumigation are as follows:

1. Measure the dimension of the stack and calculate the volume of it. The volume is width x length x height.
2. Cover the stack with tarpaulin sheet. If the stack is very big, more than one sheet are needed. Sheets are connected by special clamps by rolling each edge together and cellamping.
3. Anchor the tarpaulin sheet to the floor by sand snakes.
4. Applied a fumigant according to dosage schedule. For phosphine gas tablets or any form of aluminium phosphide or magnesium phoside are applied before cornering the stack with tarpaulin sheet.
5. If Methyl bromide, detect leak at any suspected point by halide detecter lamp or an electronic detecter device. Leak should be prevented or mended.
6. Monitor the concentration of the fumigant.
7. When exposure period is ending, remove all sand snakes and then remove the tarpaulin sheet against wind direction.
8. May be one hour later, check the concentration of the fumigant inside the stack. To allow any person working, the concentration of the fumigant must be under harmful level or less than 5 ppm.

Barge fumigation

Barges are commercially benefit transportation for all commodities because they are able to carry much cargo at a time. When necessary to conduct a fumigation in a barge, it can be done more or less the same as stack fumigation as follows:

1. Load all bags in a barge up to maximum loading amount.
2. Form free space over the cargo by tarpaulin covering over metal arches. Seal tarpaulin sheet to the edge of barge opening with good quality masking tape.
3. Measure the dimension of the barge and calculate its volume.
4. Apply a fumigant according to dosage schedule.
5. For Methyl bromide, detect leak at every suspected point. If any, fix or mend it.
6. Monitor the concentration during fumigation period.
7. At the end of the fumigation, just remove masking tape and tarpaulin sheet.
8. Aerate the fumigant for one hour or more. For Methyl bromide, carefully detect the gas concentration at the bottom of the barge because Methyl bromide is 2.7 heavier than air.

Fumigation of bulk commodity in barge is quite different than the bagged one. A fumigant used must be proper because of its penetration power. Only phosphine gas can be used to eliminate all insects infecting. The method of application is simply conducted like this.

1. Apply all pellets or tablets of Aluminium phosphine into bulk cargo by probing equipments.
2. Cover the cargo with tarpaulin sheat. Then seal the sheet to the edge of the bage opening with good quality masking tape.
3. At the end of fumigation, remove masking tape and tarpaulin sheet. Then let the cargo aerated for 1-2 furs.

Silo bin Fumigation

Silo bin is a big bulk storage of grains. Because of the complications of the silo bin system, fumigaters must learn about valves and vents. All valves and vents must be completely sealed during fumigation. To use Phosphine gas as a fumigant, pellets or tablets of Aluminium phosphide must be applied during loading cargo into a bin or during transfering from one bin to another. Application of pellets or tablets can be done by both an automatic dispenser or manual dropping.

When Methyl bromide is required for fumigation in silo bin, it must be applied in a proper manner. It's very much necessary to run recirculation system of the bin to force Methyl bromide go through the bulk cargo and form even concentration. The recirculation machine should be continuously operated for 1-3 hours.

FUMIGANTS

There are more than ten fumigants considered as modern and effective gases. In Thailand, two popular fumigants are Methyl bromide and phosphine because of their effectiveness and easy application. However, we should know the criteria for acceptable fumigants. Because the mortality of the pest is the purpose of fumigant, so there are other factors for researchers to consider when developing a fumigant, including:

1. Degree of volatility to produce a toxic concentration of gas.
2. The fire and explosive hazard.
3. Residue.
4. Ease of detection.
5. Ease of desorption.
6. Compatibility with commodity.

However, good fumigants should have the following qualities:

1. Must by a substance or compound that is easily converted to gaseous state and maintained in that condition for the whole exposure period.
2. Must have a sufficiently low boiling point so it can easily be converted to gaseous state and remain in that state during fumigation.
3. Be insoluble in water, since those that are highly soluble in water are highly sorbed by treated commodities, and high sorption rates of a commodity are positively correlated with an excess amount of residue.
4. Must not leave an excess amount of toxic residue.
5. Must not cause external injury as burns, pitting or blackening.
6. Must not cause internal injury as discoloration of fruit or malfunction of plant systems.
7. Must not prevent uniform maturation and softening of fruits and vegetables or retard or excite growth of plant or affect seed germination.
8. Must not cause off flavor in fruits, vegetables or foodstuff.
9. Must not shorten shelf life of commodities.

Methyl Bromide-CH3 Br

Methyl bromide has become very popular because many plant species, seed, vegetable and some fruit are tolerant to concentrations needed to kill pests. It has become adaptable for use in treatments in temporary enclosures after the development of thermal conductivity units which can be used in the field for remote sensing of gas concentrations. Advantages and disadvantages of Methyl bromide are as follows.

Advantages:

Quick and deep penetration
Effective against all stages of insects and mites
High plant tolerance
Effective over wide range of temperature
Non-flamable and non-explosive
Readily metected

Disadvantages:

Diffuses laterally and downward rapidly, but slowly upward
Odorless
Requires volatilizing
Unfavorable response to low humidity
High sulfur content articles develop off odors Seed germination effected
Fruits and vegetable effected
High mammalian toxicity
Delayed poison symtomology
Lacks antidote

Properties-CH3 Br:
Specific gravity 1.732 (liquid) 3.27 (gas)
Boiling point 3.6°C
Vapor pressure 1420 mm. at 20°C
1 mg/liter (1 oz/1000 ft³) 252 ppm.
1 lb. = 262 cc.
Colorless

Phosphine-PH3

Another fumigant that has become promising for stored insects in Thailand is Phosphine gas which is the result of Aluminium phosphide or Magnesium phosphide and humidity in the atmostphere. Aluminium phosphide for pest control in stored products was first used by Dr. Werner Freyburg in the 30's. The Freyburg products are marketed under the trade name Detia and include Detia Gas-Ex-b, a preparation in paper bags consisting of 57 % aluminium phosphide and 43 % inert ingredients. After that Detia began producing a pellet and tablet preparation. Phosphine gas has become very popular among tobacco leaf pests control.

Chemical and Physical Properties:
Colorless gas
Does not enter into irreversible chemical reactions with commodities.
Reacts chemically with gold, silver and copper Is flammable and explosive in high concentration cat
26 g./cu.m.
Ordor is carbide or garlic
Boiling point 87.4°C
Molecular weight 34.04
One ppm. = 14 mg/liter

NECESSARY EQUIPMENT

When fumigation will be performed, fumigater must prepare necessary materials and equipment such as fumigants, sheet, masking tape, leak detection device and safety devices.

Halide Detecter Lamp.

The halide detector lamp is a device that has been widely used over many years in the refrigeration industry for detection of freon leaks from refrigeration equipment. The halide detector can be used for rough determination of concentrations of any of the halogen gases, thus its use with freon and more recently with methyl bromide and ethylene dibromide. The halide detector came into prominence with the use of temporary enclosures for on-the job determination of methyl bromide leaks. By detection of very small leaks with this instrument, temporary enclosure can be made more secure, and obtaining better effect of the available gas. Thus its use is operational as well as safety device. Its safety use is in the detection of leaks that could be harmful to humans and animals, particularly in rooms or other enclosures where there are offices and living or working areas.

The halide detector works and the principle that a flame in contact with a clean piece of copper will impart a green to blue color to the flame, if any of the organic halide vapors are present in the surrounding air. Heavy concentration will extinguish the flame.

Determining Methyl Bromide Concentrations with the Halide Detector:

Gas Mask

Gas mask consists of mask and canister that the two parts are connected by flexible rubber duct. The canister is a compact air purifying unit which is able to protect against certain harmful gases. During inhalation, the air-gas admixture is drawn through the canister to the facepiece or mask. It is purified in the canister and expelled from the facepiece through the exhalation valve. The valve is constructed to provide near normal voice communication.

Fumiscope

Fumiscope is a thermo-conductivity analysis-device used for remote sensing of gas concentrations which are under fumigation. The fumiscope consists of an electrical pump, a glass flow rate meter, a thermal conductivity cell and a scale. By activating the pump, air sample may be drawn into the unit through the conductivity cell and expelled from the unit. The flow rate can be adjusted to provide a constant flow of air. The conductivity cell compares the thermal conductivity of the mixture of air and gas to dry air. The difference is converted to electrical energy and displayed on the scale. So by using the fumiscope we can know the concentration of methyl bromide at various parts of the temporary enclosure.

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