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Sheets retaining fumigant vapours for a sufficient time when placed over infested materials may provide a safe and effective method of fumigation. The word "tarpaulin" has in the past been used to describe these sheets, but this term is considered to be too general and may lead to a misunderstanding of the properties required for satisfactory treatments. Materials spitable for this type of work have been called "gas-proof sheets" (UK, 1974). This is a more descriptive term, although, in fact, all the materials so far used have been shown to differ somewhat in their permeability to fumigants (Phillips and Nelson, 1957).
This technique has widened the usefulness of fumigants by making it possible to treat infested materials without moving them from their place of storage or diverting them from their usual trade channels. Under some conditions it is possible to retain the sheets over the stacks of goods after fumigation and aeration, so that continuous protection is provided against reinfestation, contamination by bird droppings, water leaks, dust and dirt.
The information given in this manual should be sufficient to enable the operator to carry out simple small-scale treatments under a wide variety of conditions. If large-scale operations are contemplated, it would be advisable to consult the British bulletin (UK, 1974), which discusses the subject at some length. Particular attention should be given to details of piping and fumigant discharge suitable for large stacks.
MATERIALS FOR SHEETS
In recent years, the development of new plastic materials has led to the introduction of types of sheeting which are well suited for fumigation covers. The sheets used for particular jobs vary according to the amount of rough handling and the weather likely to be encountered. There have been several critical studies on the diffusion of methyl bromide (Phillips and Nelson, 1957; Waack et al, 1955) and phosphine (Wainman et al, 1975; Kashi et al, 1977) through various materials which might be used for fumigation sheets.
Sheets made of polyethylene or polyvinyl chloride film are suitable for use on the ground or to cover stacks in protected situations. The types most often used for fumigation have a thickness of 0.1 mm (0.004 in). Sheets of this gauge weigh less than 100 9 per m (2 lb per 100 ft2 ) and are easily handled even in large sizes. When sheets are to be used several times a thickness of 0.15 mm is recommended. Sheets made of these films tear easily on sharp corners or projections. They are not recommended for use on high buildings or structures when there is any possibility of high winds which may tear them to shreds. In some countries where the sheeting material is inexpensive and easily obtainable, it is often more economical in terms of labour to discard low density sheets instead of folding them up for further use.
For use in large-scale operations outdoors in exposed situations, coated fabrics are more suitable. Types that have been successfully used are nylon or terylene fabrics coated on troth sides with neoprene, polyvinyl chloride, or butyl rubber, and cotton fabrics, coated on both sides with neoprene.
Coated cotton fabrics appear to be more permeable to methyl bromide than the others. It has been found that a smooth slippery surface makes handling easier, especially if both sides of the fabric are coated (Brown, 1959).
Manufacturers of coated nylon fumigation sheets supply them with ropes sewn in plain hems on all four sides to facilitate clamping large covers together. For most fumigation purposes, the sheets are purchased in rectangular shapes. In certain circumstances, the overlapping of sheets is inconvenient and in such cases tailored covers may be more practical. For example, for the fumigation of large pyramids of groundnuts in northern Nigeria special covers tailored to the shape of the pyramids are used (Figures 29 and 30).
FUMIGATION OF STACKS
The goods most often fumigated under sheets are cereals and other plant products in bags. Cereals in cartons, dried fruit in boxes and tobacco in boxes or hogsheads can also be treated in this way. The fumigation of buildings under sheets is discussed in the following sections.
An important consideration in this type of treatment is the most efficient utilization of a sheet of a given area; the dimensions of the stack should be such as to provide the maximum volume. Appendix 1 contains a table of maximum volumes together with a formula for calculating them, based on data supplied by Bower (1961).
Today, the principal fumigants used for this work are methyl bromide and phosphine. Ethylene oxide and HCN have also been used, but HCN does not penetrate as well as methyl bromide (Redlinger, 1957c).
For further information on the use of fumigants for plant product fumigation under gas-proof sheets, the following references may be consulted:
Ethylene oxide-carbon dioxide mixture: Plant products generally, Thompson and Turtle (1953); snails in imported cargoes, Richardson and Roth (1963 and 1965).
Hydrogen cyanide (HCN): Plant products generally, Thompson and Turtle (1953); rice, Redlinger (1957c).
Methyl bromide: Plant products generally, Brown (1959); Hall (1963); Harada (1962); Hayward (1963); Puzzi et al, (1966); cotton seed, Hingorani and Kapoor (1964).
Phosphine (generated from aluminium or magnesium phosphide): Plant products generally, McGregor and Davidson (1966); Puzzi et al (1966); Esin (1967); rice in sacks, Gogburn and Tilton (1963); grain sorghum in sacks, Hubert (1962); maize, Lochner (1964a), flour in polythene-clad sacks, Wainman et al (1975), shrink-wrapped pallets of bagged cornmeal, Leesch and Highland (1978).
Ethylene dibromide-methyl bromide mixture: Majumder and Muthu (1964).
Dosages
Recommended dosages for this type of fumigation are given in Schedule P.
Preliminary Precautions
In addition to the usual precautions taken before any treatment, which have already been discussed, it is essential in fumigation under sheets that, after treatment, the gas escaping from the stack does not endanger any persons working or living in the vicinity. The fumigations may be conducted inside sheds, warehouses and similar structures, which furnish protection from bad weather (Figure 31). Such an arrangement is satisfactory if there is no possibility that gas may diffuse into occupied rooms or quarters. Very often the work can be safely done in warehouse rooms in which the windows can be kept open.
Suitable warning signs indicating type of fumigant and date of application should be placed on all stacks while fumigation is in progress, whether indoors or outdoors. First aid kits and gas masks should be readily available.
Preparation of Stack
A first step is to make certain that there will be no leakage of fumigant downwards during treatment. There is no serious loss or danger if the stack is placed on firm ground or on a good cement floor. If the floor is not gasproof, another gas-proof sheet, or even rolls of tar paper overlapped and sealed with masking tape, may be placed on the floor and the stack built above it.
When using methyl bromide or other similar fumigant it is essential that special provision be made to prevent the fumigant from discharging directly into sacks near the gas outlet. A suitable space may be provided by lifting and moving some bags from the top layer. In small piles, four bags may be propped against each other to form a "dome". Under this is laid a pan or can to receive the liquid discharging from the end of the outlet tube (see Figure 20). In larger stacks, a long space two bags deep and two bags wide should be made by lifting adjoining bags to form a trench. For these larger volumes, the outlets of the gas lines should be either spray nozzles, which ensure volatilization of the fumigant before the liquid reaches the bags or the sheeting, or open tubes leading into pans or cans in the manner already described.
If electrical power and outlets are available, one or more 25- or 30- cm (10or 12 in) fans, the number varying according to the size of the stack, aid greatly in distributing the fumigant.
Placing Sheets
Sheets of standard sizes are easily placed on rectangular or square stacks. It is preferable to have sheets large enough to cover the individual stacks. At least one foot of sheeting should be left on all sides of the stack to provide room for sealing the edges to the floor.
If the sheets are not large enough to cover the stack, one or more must be joined by rolling the edges together. According to Brown (1959), a satisfactory seal for methyl bromide fumigation may be obtained by rolling together about 1 m (3 ft) of the edges of each of the adjoining sheets. "The leading edge of the first sheet is folded back three feet. The second sheet is laid to overlap the first so that the edges lie together then these edges are held together and rolled until all the overlap is taken up. When covering a large stack, joins may have to be made in two directions." When sheets on stacks exposed to the wind or on buildings are joined, clamping of the rolled junctions is necessary. This technique is described in the following section, "Fumigation of buildings under sheets".
Under some conditions, such as those encountered in west Africa with the large pyramidal stacks, the rolling together of sheets is impracticable. Sealing with sprayed plastic has been tried with success but the standard practice is to use tailored covers for these stacks.
The edges of the sheets are best sealed to the floor by "snakes", plastic tubes about 10 cm (4 in) in diameter, filled with sand or water, which gives an efficient seal on level ground or floors. The use of water is very convenient because the tubes may be drained and rolled up between treatments. In emergency, some of the bags of the material already fumigated may be used, but a seal is difficult to obtain with these. Chains weighing not less than 2 kg per m (1.5 lb per ft) may also be used, but care must be taken that these do not damage the fabric. Outdoors, earth may be dug and piled closely around the base of the stack. Rips and holes appearing in the fabric may be repaired with adhesive or masking tape.
Applying Gaseous Fumigants
When gaseous fumigants, especially those containing methyl bromide, are applied indoors, respirators must be worn (see Figure 17).
For large piles the fumigant is applied from cylinders under pressure. With cylinders of methyl bromide discharging into high stacks, additional pressure in the cylinders may be needed before the full dosage is dispensed. For many treatments with methyl bromide, 1 lb cans are convenient (see Figure 17). The special applicator for discharging the ethylene dibromide-methyl bromide mixture (Majumder et al, (1962) has been described in Chapter 7.
Acpivinq Aluminium Phosphide
Tablets, pellets, sachets or other formulations used for generating phosphine may be applied to the stack before it is covered with the sheets. With long stacks it may be safer or more convenient to apply them as the sheeting work proceeds. It is often convenient to place paper trays containing the proportional number of tablets or pellets in strategic positions throughout the stack. The sachets are obtainable in lots of ten, bound together by string. These strings may be draped over the stacks in evenly spaced positions before the fumigation sheets are laid on.
Under normal circumstances, when the generation of the phosphine takes place slowly, it is not necessary to wear a respirator. When tablets, pellets or sachets are directly handled, rubber gloves should be worn. After the edges of sheets are secured with sand snakes, tape or other suitable material, warning signs should be placed on all sides of the stack. If the stacks are properly sealed, workers need not vacate the premises but good crossventilation must be maintained during working hours and periodic tests with detector tubes or other suitable analyser should be made to ensure that no one is exposed to concentrations above the threshold limit value.
Fan Circulation
With gaseous fumigants, the fans should be operated for 15 minutes at the beginning of the treatment. Operation of the fans for a longer time will not greatly improve gas distribution and may tend to force the fumigant out, especially if the seal at the base of the sheets is poor.
Fan circulation is not recommended for phosphine treatments and should not be necessary if proper distribution is made of the aluminium phosphide before fumigation begins.
Leakaqe
When methyl bromide is being used, a careful check should be made with the leak detector around the entire base of the stack immediately after the fan is turned off. If the covering is made of more than one sheet, the rolled junctions should also be tested. Any serious leak can thus be found and corrected at once.
Aeration
During the process of airing, the operators must wear respirators while working on the stack or in its vicinity, until it is shown by an appropriate test that dangerous concentrations of the fumigant are not present. Aeration should be begun by quickly lifting several sheets of each corner of the stack. After an interval, more of the sheeting may be lifted. When it has been demonstrated by the use of the leak detector, or by other chemical tests, that high concentrations of fumigant have diffused away, the sheets may be carefully drawn off the stack.
Special precautions, such as opening all doors and windows and running exhaust fans, should be taken when airing a stack indoors. Careful checks should be made to determine that the indoor space is safe for human occupancy.
Paper Coverings
A specialized method of stack fumigation is the use of kraft paper for covering dried fruit. This technique is an inexpensive and simple means of effective fumigation and subsequent storage. It is particularly useful on farms .
The enclosure is made by framing stacks of fruit boxes with 2.5 x 10 cm (1 x 4 in) timber and covering it with paper. The paper used by Barnes and Reilly (1956) consisted of two layers of heavy kraft paper with an asphalt laminant reinforced with sisal fibres.
The shape and construction of the stacks are important to success. They should be built on 10 x 10 cm (4 x 4 in) foundations to protect the fruit from ground moisture and to facilitate gas circulation. The frame is covered by carrying the paper up one side, across the top, and down the other side, allowance being made for 30 or 45 cm (12 or 18 in) widths for sealing on the ground. The paper strips are lapped about 8 cm (3 in) and then battened with laths. Since the cover papers shrink after weathering, allowance should be made for this by spacing the frame boards 2 m (about 7 ft) apart to take the edges of the 2.5 m (8 ft) wide paper. The paper cover is sealed to the ground with sand or fine soil. The seal should cover the paper on the ground completely and should be 15 to 20 cm (6 to 8 in) high against the sides of the structure. Head room above the boxes is necessary for gas circulation; if the stacks are not under a roof, they should be peaked to allow rainwater to run off. Tears in the paper may be repaired with masking tape.
After fumigation, several months' storage is provided by the enclosure, and reinfestation by insects is prevented. This technique, or modifications of it, could be applied to a number of similar storage problems.
FUMIGATION OF BUILDINGS UNDER SHEETS
Some buildings can be rendered gas tight only by the costly and time consuming sealing of individual cracks and holes, and it is often much easier to cover the entire structure with sheets (Figure 32). An early successful application of this technique was the commercial control of dry wood termites in dwellings. Formerly, thick kraft paper was laid over the buildings, but this material has been largely replaced by the modern types of plastic sheets already described.
The effectiveness of this method of insect control was demonstrated during the campaign to eradicate the Khapra beetle in California, in the course of which many large mills, grain elevators and warehouses were completely covered and fumigated with methyl bromide. In one of the treatments, the volume fumigated under covers was 113 000 cm (4 million ft³) (Armitage, 1956). Rasmussen (1967) reported on a critical study of methyl bromide fumigation to control long-horn beetles in houses covered with gas-proof sheets during treatment.
The technique for fumigating large structures is basically the same as that already described for stack fumigation. There are a number of special requirements, however, brought about mainly by the fact that the covering sheets may be situated high above the ground and be exposed to winds of considerable velocity (Armitage, 1958).
Sheets. Although polyethylene sheeting may be used, the danger of its being torn by high winds, or even totally destroyed, has led to the use of the tougher, but much heavier, nylon or terylene fabrics coated with plastic. The weight of this material, best suited for covering buil dings, varies between 200 and 440 g/m (6 and 13 oz/yd2).