Fumigation with Phosphine and Carbon Dioxide in Metal Grain Tanks for On-farm Use in Swaziland

J. agric. Engng Res. (2000) 76, 17}26 doi:10.1006/jaer.1999.0443, available online at http://www.idealibrary.com on

RESEARCH PAPERS

Fumigation with Phosphine and Carbon Dioxide in Metal Grain Tanks for On-farm Use in Swaziland J. R. Brice; P. Golob Food Security Department, Natural Resources Institute, Chatham Maritime, Kent ME4 4TB, UK; e-mail: [email protected] (Received 15 December 1997; accepted in revised form 27 May 1999)

Metal grain tanks, as constructed in Swaziland, were demonstrated to be extremely e!ective structures for fumigation on a small scale; phosphine gas concentrations were maintained well above the recommended minimum concentration of 150 p.p.m. throughout the minimum seven days exposure period. Current methods of sealing the tanks were shown to be inadequate but were improved by sealing the tank inlet and outlet using plastic sheeting and adhesive tape. Application rates of 3)33 g of phosphine per tonne of grain (as currently recommended in Swaziland) were more than adequate, provided the tanks were well sealed. &&Tiny Bags'' (produced by Degesch GmbH) were slightly slower in releasing phosphine when compared to conventional tablet formulations. However, the pattern of gas release and subsequent decay in concentrations were similar for &&Tiny Bags'' and tablets. As a result of the high degree of gas tightness which can be achieved using metal grain tanks, the recommended application rates for phosphine could be reduced. A system was devised for applying carbon dioxide to the base of the grain tanks using a probe inserted through the top of the tank. High initial gas concentrations were achieved but these fell quickly, necessitating frequent regassing of the tanks in order to maintain gas concentrations for the required 10 day exposure period. Given the proven gas tightness of the tanks, the most likely cause for the apparent loss in carbon dioxide gas would be adsorption by the maize.  2000 Silsoe Research Institute

platforms and frames, cribs, baskets, solid walled bins, metal storage bins, underground storage and bag storage. Of these store types, metal storage bins would appear to be the most promising with regards to potential gas tightness. Although not widely used by the smallscale farmer, considerable quantities of metal storage bins (of various designs) are used in Swaziland (Boxall et al., 1997), Guatemala (Breth, 1976), Korea (Tyler, 1978), and India (Bakshi & Bhatnagar, 1971) for on-farm storage. Approximately 160 000 t of maize are grown in the Kingdom of Swaziland each year [pers., com., Ministry of Agriculture and Co-operatives (MOAC)]. Much of this maize is stored, often for many months, on the farm in locally made corrugated steel grain tanks. Although these tanks are available in sizes from 0)35 to 9 t, most are approximately 0)8}1 t capacity. Insect control is usually achieved by the farmer through fumigation with phosphine applied in the form of aluminium phosphide tablets. Once the tank has been "lled with maize the tablets

1. Introduction Until recently, most grain fumigation in developing countries has been at the large-scale, centralized storage level by marketing boards and cooperatives. However, with increasing agricultural market liberalization and decentralization, many of these grain marketing and storage systems are breaking down, resulting in more grain being stored for longer periods by the farmer and trader. The need to fumigate grain at the small-scale level is therefore becoming more important. Given that much of the research into fumigation techniques has concentrated upon large-scale storage systems, there is now a very real need to develop methods and techniques which will be appropriate for much smaller scale, on-farm storage structures. Although many designs of stores may be found throughout the developing world, few lend themselves directly to the use of fumigation for pest control. Boxall et al. (1997) sub-divide those found in Africa into raised 0021-8634/00/050017#10 $35.00/0

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 2000 Silsoe Research Institute

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are placed on a sheet of paper on the top surface of the grain. The inlet and outlet are then sealed using plastic bags (usually old fertilizer bags) and the tank left for approximately 14 days, after which the tanks are opened and examined for live insects. Reports from the MOAC suggest that the absence of "eld failures indicates that resistance to phosphine may not have developed. However, there has been no detailed study to date to determine whether this is so. With the likely phasing-out of methyl bromide for fumigation purposes, increasing reliance is being placed on the fumigant phosphine. However, with the rising concerns over the development of insect resistance to phosphine (Taylor, 1989), it is imperative that all phosphine fumigations, no matter where they are performed, are successful and that the chances of insect survival are minimized. Primarily, due to the problem of possible insect resistance to phosphine, the use of alternative types of fumigants are being investigated by several researchers. One of the key alternatives is CO which is  generally more widely available than many of the other alternatives. A series of experiments were conducted at Malkerns Research Station, Matsapha, Swaziland to determine the e$cacy of the Swaziland method for fumigating grain on the farm, and the suitability of the metal grain tanks for fumigation purposes. Whilst most of the trials would examine the use of phosphine in the tanks, a separate trial was established to assess the practicalities of using CO instead. The information gained  would provide base-line data for the development of an on-farm fumigation system, suitable for use elsewhere in Africa.

2. Methods and materials Nine new tanks, each with a capacity of approximately 0)9 t (1)25 m high, 1)15 m diameter), were installed at the Government silos at Matsapha. The sides of the tanks were constructed from 24 gauge galvanized, corrugated sheets (thinner sheets were available on the market but these do not bend evenly and are not strong enough to support the grain; they are recommended for use on roofs). The sheets were curved using a special set of corrugated rolls in the workshop. The side of the each tank was constructed from two horizontal sheets, one above the other, with their vertical joints being staggered to maximize tank strength (the joints were on opposite sides of the tank). All joints were secured using steel rivets, one at each high and low point in the corrugations. Rivets were staggered to maximize strength; the rivets in the hollows were approximately 25 mm behind those on the ridges. The edges of the sheets were cleaned using

soldering spirits before covering the joints and rivets with solder to ensure a gas tight seal. The top, base and ancillary components (rims, lids, outlet tubes, etc.) of the tank were constructed from sheet galvanized steel. Joints between the top and sides, and the bottom and sides were formed by folding the top or bottom over the edges of the sides and soldering in place (Fig. 1). The tanks were placed on a layer of 10 evenly spaced cement blocks, with particular care being taken to ensure that they were level so that the loading on the base would be uniformly distributed across the whole area; failure to do this could have resulted in damage to the structure once loaded with grain. They were positioned under a shelter (galvanized roof on a pole structure) to provide shading. The tanks were modi"ed by inserting a short steel (Fig. 1) to allow access for gas sample lines. Three sample lines were used in each tank: one with an inlet at the base; one half-way up; and one at the top of the tank. The tanks were "lled with maize of approximately 11)5}12)5% moisture content to within 25 mm of the top of the tank. A total of "ve fumigations were performed, four using phosphine and one using carbon dioxide. (a) In the "rst fumigation (Trial 1), the tanks were sealed by covering the inlets and outlets with a sheet of polythene prior to "tting the metal covers. The seal was achieved by the close "t of the covers over the inlet and outlet. The objective was to assess the gas tightness of the nine tanks and to determine the e!ectiveness of the method of fumigation using phosphine currently employed in Swaziland. (b) The second trial (Trial 2) attempted to improve the degree of tank sealing. The inlet was covered with a circle of polythene, its edges being sealed with plastic tape to the metal rim around the opening. The tape was also applied around the joint between the rim and the top surface of the tank. The lid was then carefully "tted so that neither the plastic nor the tape were damaged. The metal cover was placed on the outlet and tape was applied between the cover and the outlet tube. This method of sealing was used for all subsequent trials. (c) At the completion of the second trial, i.e. after 7 days fumigation, three of the tanks were left sealed for a further 22 days to assess the maximum duration that phosphine gas concentrations in excess of 150 ppm could be maintained. (d) The "nal phosphine fumigation trial involved a comparison between the ability of conventional Phostoxin (Degesch GmbH, Germany) tablets (as used in the three previous trials) and &&Tiny Bags'' in creating phosphine gas. The &&Tiny Bags'' were supplied by Degesch (S Africa) speci"cally for these trials.

F U MIGA TI ON WI TH PHO S PHI NE AN D C A RB ON DIO XI DE I N ME TA L G RA I N TA NK S

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Fig. 1. Tank construction with gas sample lines in position

(e) In the "nal trial, carbon dioxide fumigation of six of the tanks was assessed. A system was devised to introduce CO into the base of the tank. 

normally 5 days, this was extended to 7 days due to the relatively low ambient temperatures at the time of the trials: Anon, 1985).

2.1. Phosphine fumigations

2.2. Carbon dioxide fumigations

Three aluminium phosphide tablets were added (equating to a dosage rate of 3)33 g of phosphine per tonne of grain) to each tank. They were placed on a piece of paper on the top surface of the grain to facilitate the removal of the tablet residues at the completion of the fumigation (Fig. 1). One &&Tiny Bag'', each with a net weight of 9 g and nominally producing 3 g of gas (equivalent to three tablets), was added to each of the three tanks in the fourth trial. Gas concentrations within the tanks were recorded once a day using a Bedfont EC80 thermal conductivity phosphine meter. Readings were averaged and plotted against time. It should be noted that the meter had an upper limit of 2000 p.p.m. * readings of 2000 p.p.m. on the following graphs of concentrations over time should be read as &&in excess of 2000 p.p.m.'' A phosphine fumigation was deemed as having succeeded if a minimum gas concentration of 150 p.p.m. had been maintained for 7 days. (Ambient temperatures at mid-day varied from 15 to 273C during the trials. Although the minimum exposure period for phosphine for tropical climates is

CO was supplied from a pressurized cylinder via  a conventional gas regulator (Sa$re model ArC GM-20l Argon}CO regulator, capable of delivering up to  20 l/min) and a length of #exible hose to a specially constructed probe (Fig. 2). The probe consisted of a 1)4 m length of 15 mm diameter copper tubing with a pipe "tting soldered to its upper end onto which the #exible tubing was attached. The lower end of the copper tubing was #attened and sealed with solder, immediately above which were drilled a number of 5 mm diameter holes (Fig. 2). The probe was inserted through the plastic sheet sealing the tank inlet and pushed through the grain until its lower end came into contact with the base of the tank. On opening the regulator, CO was introduced into the  bottom of the tank. The gas cylinder was placed on a platform scale to determine the quantity of gas added to the tank. The regulator was used to control the rate at which CO was added to the tank. During gassing, CO was   added as quickly as possible so as to maximize the quantity added before excessive losses were emitted from the tank inlet. Unfortunately, ambient temperatures fell

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Fig. 2. Carbon dioxide injection system

to 153C during the days that the tanks were initially "lled. Application rates had therefore to be reduced to avoid &&icing-up'' of the regulator under these conditions, thereby extending the time taken to "ll each tank. This &&icing-up'' problem occurred during the trial whenever ambient temperatures fell below approximately 203C or when the day was overcast. When CO was "rst introduced into the tanks, admis sion of gas was stopped when concentrations in the order of 65}75% were recorded using a Bedfont thermal conductivity CO meter positioned at the tank inlet. Sub sequent topping-up with gas was varied between the tanks as follows. E

E

Tanks 6 and 9 were purged with CO to maximize the  concentrations of CO in the tanks, thereby reducing  the frequency that CO had to be applied.  Tanks 7 and 8 were regularly topped-up with small quantities of CO to reduce the quantities of CO   required by minimizing gas loss during gassing. The main drawback of this method was that a higher frequency of gassing was required to maintain gas concentrations within the tanks. The end of gassing was

E

determined when the gas concentrations reached 65% CO at the top of the tank. Towards the end of gassing  tank 8 was purged with CO .  Tanks 3 and 4 used a similar gassing regime to tanks 7 and 8 although the frequency of gassing was reduced slightly.

Attempts were made to standardize the quantities of gas required to either purge or top-up the tanks: approximately 2 kg of gas was required to purge the tank; and 0)6}0)8 kg was required to top-up the tank. In practice, however, it was not always possible to introduce standard quantities of gas; for example, on the fourth day of gassing for tanks 3 and 4, it was only possible to add 0)3 kg of CO since the ambient temperature had fallen  to such an extent that it was very di$cult to add any gas without the regulator freezing up. On other occasions, it was necessary to use in excess of 2)0 kg if concentrations had fallen below 30% CO . Quantities of gas used are  listed in Table 1. Once gassing was completed, the probe was removed and the hole in the plastic covering the tank inlet sealed with a tape. Gas concentrations at the three sampling

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Table 1 Quantities of CO2 applied to each grain storage tank Quantity of CO2, kg Day no.

Tanks **topped-up++

Mixture

Tanks **purged++

Tank 7

Tank 8

Tank 3

Tank 4

Tank 6

Tank 9

1 2 3 4 5 6 7 8 9 10

1)2 0)7 * 0)8 * 0)6 0)8 * Vented

1)2 0)8 * 0)8 * 0)6 2)3 * Vented

1)3 * * 0)3 * 0)8 * Vented

1)2 * * 0)3 * 0)8 * Vented

1)2 * * 2)0 * * * 2)0 Vented

0)9 * * 2)0 * * * 2)1 * Vented

Total

4)1

5)7

2)4

2)3

5)2

5)0

locations were monitored each day. Readings were averaged and plotted against exposure period. A fumigation was deemed successful if gas concentration were maintained in excess of 30}35% for 10 days (Banks, 1978; Sharp & Banks, 1980).

3. Results 3.1. Phosphine fumigations Although the tanks were apparently identical, the rate of deterioration in phosphine gas concentrations varied considerably between tanks when sealed following current practices in Swaziland (Fig. 3). Gas concentrations in four of the tanks (tanks 2, 5, 6 and 7) fell below 150 ppm before the completion of the 7 day exposure period, i.e. these fumigations failed. Examination of the tanks did not indicate any obvious faults in their construction. Di!erences in the behaviour of the tanks were therefore assumed to be due to the e!ectiveness of the sealing of the tank inlets and outlets. Sealing of the tanks using plastic tape and polythene sheeting signi"cantly improved their gas tightness. Whilst all nine tanks were successfully fumigated, the gas concentrations in tanks 7 and 8 were found to fall more rapidly (Fig. 3). Examination of the sealing at the end of the trial indicated that, although the metal inlet covers had been carefully closed after sealing, the plastic had been damaged slightly due to the rough edges of the cover. Fumigation of tanks 1, 2 and 5 beyond the 7 day exposure period indicated that the tanks were extremely

e!ective fumigation chambers. Gas concentrations were monitored for a total of 27 days by which time they had fallen to approximately 250}350 p.p.m. (Fig. 4). The rate of reduction in concentrations fell to approximately 50}60 p.p.m. per day towards the end of the trial; similar patterns were shown for each tank. After 14 days (336 h) there was a rapid fall in ambient temperatures (from a mean mid-day temperature of 25}303C to approximately 133C) at which point there appeared to be a fall in gas concentrations of over 200 p.p.m. As ambient temperatures increased towards their original values, so the gas concentrations increased for a period prior to falling in-line with their original rates of decline (Fig. 4). A best"t exponential curve was "tted to the data beyond an exposure period of 168 h to model the fall in concentration C of phosphine in p.p.m.: C"3457 exp(!0)00398t) where t is the exposure period in h. Comparison of conventional tablets with the &&Tiny Bags'' indicated that the patterns of gas generation were similar in both cases (Fig. 5). The only discernible di!erence was that &&Tiny Bags'' appeared to release the gas slightly slower than the tablets but that the rates of deterioration in concentrations were similar. This was con"rmed by the statistical analysis of the gas concentrations at each sampling time. The only signi"cant di!erence between treatments was at 20 h when the gas concentrations produced by the tablets were signi"cantly greater than those produced by the &&Tiny Bags'' (1758 versus 1167 p.p.m.; standard error (sed) of 132; probability

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Fig. 3. Phosphine gas concentrations in the nine steel grain tanks using current and improved methods of sealing

P"0)011). No signi"cant patterns were found at any other sampling times.

3.2. Carbon dioxide fumigations The system developed for introducing CO into the  grain tank (Fig. 2) worked extremely well. The tank could be almost "lled with CO before signi"cant quantities  were lost to the atmosphere (indicated by the CO meter  at the tank inlet, through which air was exhausted from the tank during gassing). The quantities of gas used are listed in Table 1. Purging of the tanks (tanks 6 and 9) resulted in CO  concentrations approaching 100% (Fig. 6) after 70}90 h. Concentrations fell by up to 40% over the next 24 h, the rate of deterioration falling over subsequent days. Repurging was required after 72}96 h to ensure that concentrations remained above the 30% minimum. In tanks 3, 4, 7 and 8, concentrations of approximately 50}60% CO were established initially. Concentrations sub sequently fell by between 10 and 20% per 24 h over the

next 48 h. Topping-up of the gas was required every 48 h, i.e. twice as often as when the tanks were purged with CO .  On initial inspection of the results, CO consumption  appeared to be the highest when the tanks were purged with gas rather than regularly topped-up (Table 1). However, additional CO should have been applied to tanks  3, 4, 7 and 8 as concentrations had fallen below the minimum 30% for signi"cant periods (Fig. 6) * thereby reducing the di!erence between gas consumption "gures. Furthermore, tanks 3 and 4, and 7 and 8, were only fumigated for 9 and 8 days, respectively, whereas the purged tanks were fumigated for 9 and 10 days * more gas would be required if all tanks were fumigated for the full 10-day exposure period. It was therefore concluded that there is little, if any, di!erence in gas consumption whether the tanks were purged or topped-up.

4. Discussion Current application rates of aluminium phosphide tablets of approximately 3)33 tablets per tonne are

F U MIGA TI ON WI TH PHO S PHI NE AN D C A RB ON DIO XI DE I N ME TA L G RA I N TA NK S

Fig. 4. Phosphine concentrations in the three tanks with improved sealing monitored beyond the 7 day exposure period, showing an apparent fall in concentrations due to cold weather (between 310 and 350 hours)

su$cient. When used in well sealed tanks, extremely high gas concentrations can be achieved. Although the gas release from &&Tiny Bags'' was slightly slower than from conventional tablets, there was no apparent di!erences in peak gas concentrations or the rates of decay. The advantages of using the &&Tiny Bags'' over conventional tablets are primarily concerned with their packaging * small quantities of tablets, suitable for the fumigation of one or two grain tanks, are not commercially available. The result is that either farmers are forced to purchase non-resealable tubes of 30 tablets, in which case

Fig. 5. Comparison of phosphine gas concentrations achieved using **Tiny Bags++ versus tablets

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they use 3}6 tablets and then store the unused ones, or they are sold the required number of tablets loose. In the "rst case, the subsequent poor sealing results in gas leakage into the immediate environment, with the associated risks of exposure to the family and livestock. It is also likely that the remaining tablets have become useless by the time the next fumigation is required. In the second case, phosphine production, commencing soon after exposing the tablets to the atmosphere, results in obvious severe safety hazards to the farmer and those in the vicinity as he/she transports them to the farm. The advantage with the &&Tiny Bag'' is that each bag, su$cient for one tonne of grain, is individually sealed, and so the farmer may purchase as many or few of the bags as are required without compromising safety. Metal grain tanks, as constructed in Swaziland, were clearly shown to be capable of achieving extremely high levels of gas tightness. The potential for extended exposure periods was clearly demonstrated. However, these exposure periods can only be achieved if e$cient sealing methods are used * any gap, no matter how small, will dramatically reduce the exposure period. During the trials, plastic tape was used to seal the plastic sheeting over the inlet and outlet. Unfortunately, this tape will probably not be readily available to, or a!ordable by, the average farmer in Swaziland. Alternative methods of sealing the plastic sheeting to the metal tanks therefore need to be used. Given the high degree of gas-tightness achievable in metal grain tanks, it could be argued that application rates of aluminium phosphide tablets could be reduced from the relatively high 3)33 tablets per tonne of grain * possibly nearer to 1 tablet per tonne. However, this should only be recommended provided that the question of alternative methods of sealing (discussed previously) has been addressed. Although Harris (1970) indicated that mud or dung mixtures were not suitable for sealing large holes in the structure, they may be suitable for the small gaps around tank inlets and outlets. Qasim Chaudhry and Anwar (1988) used mud plaster to seal possible sites of gas leakage when they examined the suitability of metal grain tanks for fumigation purposes in Pakistan. Whilst gas concentrations were maintained in excess of 150 p.p.m. for only 5 days, such a duration would be su$cient under ambient temperatures higher than those encountered in Swaziland at the time of the trials (i.e. those typically found in developing countries). No information was provided as to the method of construction of the silos nor their quality, and so it would be di$cult to comment on the e!ectiveness of the mud sealing. They went on to examine the gas tightness of plastic grain bins * exposure periods in excess of 8 days were achieved but it is unlikely that farmers in Swaziland would be able to

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Fig. 6. Carbon dioxide concentrations in the six tanks with improved sealing: (a) frequent addition of CO2 (tank topping-up); (b) less frequent addition of CO2; (c) infrequent addition of CO2 (tank purging)

a!ord such structures (it should be noted that plastic water tanks, constructed in South Africa and apparently suitable for grain storage, were identi"ed in Swaziland). Although several other researchers have examined the question of sealing systems of stores, attention has almost solely been directed towards the large commercial-type stores. Expensive sealants and more complex store designs have been suggested, none of which are suitable for the small metal grain tanks used by farmers in Swaziland. Other improved methods of sealing could include tying the plastic tightly around the openings with string, using elasticated bands (such as old bicycle inner tubes), and the use of alternative materials such as natural plant materials or gums. However, none of these were tried during these trials. It is quite likely that as metal tanks age so the joints between the metal sheets will widen, reducing their gas tightness and therefore the e$cacy of fumigations. Tanks, which have been in use for several years, should be examined to determine whether their joints have opened

due to stressing/relaxation during store loading and unloading operations. Rusting of the base of the tanks (especially if they have not be supported o! the ground) is a particular problem with older tanks (Harris, 1970). Initial observations, followed by detailed fumigation trials, may be able to give some form of safe &&fumigable life'' for the tanks, after which, although they may be suitable for storage, they would be unsuitable for fumigation purposes. Given the high replacement costs, every e!ort must be taken to maximize the life of the tank (such as to ensure that tanks are raised o! the ground, ideally using evenly spaced block supports as used in the trials to reduce the degree of rusting around the base). Painting the base of the tank with a bitumen paint will help to extend the fumigable life of the tank. Careful handling (to avoid damaging the protective galvanized layer) during construction, transport, installation and day to day operations, will also slow the onset of rusting. Other factors, such as protection against wind, could help reduce rates of gas loss from the structures. Qasim

F U MIGA TI ON WI TH PHO S PHI NE AN D C A RB ON DIO XI DE I N ME TA L G RA I N TA NK S

Chaudhry et al. (1989) clearly illustrated that the rate of gas loss from a fumigated structure was increased when exposed to &&pu!s of wind''. This appears to be especially important when using thin polyethylene structures since they are particularly prone to wind-induced gas loss. This wind-induced gas loss is due to the &&chimney e!ect'' (Banks & Annis, 1984), and is proportional to the di!erence in gaseous densities inside and outside of the structure. Gas loss due to the chimney e!ect is also induced by changes in external temperature, for example, throughout the day. Therefore, careful selection of the location of the stores, where they are protected against both wind and extremes of temperature, should assist in reducing the rates of gas loss from storage structures. The suitability of other types of structure, for example those made completely from mud, for fumigation purposes varies between designs. Although Webley and Harris (1979) managed to maintain su$cient gas concentrations for four days in &&Banco'' stores in the Sahel, the rate of gas loss was demonstrated to be extremely high. This was despite a wall thickness of 650 mm and the presence of cement #oors (thus reducing the rate of gas loss into the ground). In smaller, farmer level stores, with wall thickness in the order of 100 mm or less, it is unlikely that successful fumigations can be achieved. Qasim Chaudhry and Anwar (1988) veri"ed this when they fumigated mud bins of various designs (plastered and unplastered, made from either mud or from mud blocks with cement mortar) of 0)43}4)05 m capacity * all gas was e!ectively lost by the second day, irrespective of the design. Although Wohlgemuth and Harnisch (1986) claimed to have successfully fumigated mud silos, examination of the gas concentrations achieved indicates that minimum concentrations of 150 p.p.m. were not obtained for 5 days in any of the stores. (NRI are examining the fumigability of mud silos in northern Ghana, and a paper is currently being written.) A method was devised by which CO was successfully  introduced into the grain tanks with concentrations of up to 100% being achieved. However, carbon dioxide concentrations in all the tanks fell faster than expected (based on the data obtained on the capability of these same grain tanks to retain phosphine). It is extremely unlikely that all six tanks were poorly sealed for this trial since the same methodology was used as for the phosphine trials. Given the similar molecular size of the two gases, the permeability of the plastic sheeting used to seal the inlet would be of the same order for both gases so this would not account for the high losses in CO concentra tion. The most likely explanation is that the maize itself absorbed large quantities of CO . Yamamoto and Mit suda (1980) found that maize absorbed approximately 170 ml of CO per kg of maize at 203C after only 3 h.  With 0)9 t of grain per tank and a density of CO of 

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1)839 g/l, this equates to approximately 0)3 kg of CO .  Compared with the quantities of CO used (from 2)3 to  5)7 kg depending on the treatment), the "gure of 0)3 kg would initially appear to be a relatively small proportion of the total CO used. However, this adsorption "gure  was for the "rst 3 h of exposure, and, given the duration of the trial, the excess CO required could, quite easily, be  accounted for by adsorption into the grain. Unfortunately, since Yamamoto and Mitsuda did not provide any information on subsequent rates of adsorption, it is not possible to develop this theory any further. Whilst CO has been successfully used for the fumiga tion of grain in bag stacks, extremely thorough methods of sealing are required. The stacks are built on a gasproof ground sheet, before being covered with a tailormade fumigation sheet (Annis & Graver, 1985, in Annis, 1989). These fumigation sheets are then sealed to the ground-sheet using PVC adhesive. The whole enclosure is then vacuum tested (to ensure that a su$ciently gastight enclose has been formed) before the CO is intro duced. This whole, relatively complicated procedure re#ects the di$culty in using CO gas as a fumigant * the  high concentration (compared to other fumigants such as phosphine) and the long exposure period combine to make CO suitable in only a few situations. Combined  with the cost and logistical di$culties (of purchasing and transporting cylinders of gas together with pipe-work and regulators), CO is not suitable for use in small metal  grain tanks as tested in Swaziland.

5. Conclusions (1) Current application rates of aluminium phosphide tablets of approximately 3)33 tablets per tonne of grain are su$cient. When used in well-sealed tanks, extremely high concentrations of phosphine can be achieved, indicating that application rates could be reduced in such cases. (2) The current method of sealing (namely, placing plastic sheeting over the openings and holding in place by the metal covers) is insu$cient with regard to maintaining the gas concentrations for a su$cient period of time. An alternative method, such as sealing the sheets with plastic tape, must be used to ensure that fumigations are to be successful. (3) Provided that they are in good condition, i.e. unperforated, and that care is taken to ensure that their inlets and outlets are well sealed, metal grain tanks are very well suited to phosphine fumigations * indicated by extremely high gas concentrations being maintained over long periods of time. (4) Although the gas release from &&Tiny Bags'' (Degesch GmbH) was slightly slower than from conventional

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tablets, there were no apparent di!erences in peak gas concentrations or the rates of decay. Given the bene"ts of the smaller pack size in that the farmer is able to purchase his/her exact requirements, &&Tiny Bags'' could provide an extremely useful alternative to tablets for the small-scale farmer. (5) Although a method was devised by which carbon dioxide was successfully introduced into metal grain tanks (with initial concentrations approaching 100%), subsequent rates of deterioration in concentrations (presumably due to absorption by the grain) necessitated frequent re-gassing of the tanks. Given the complexity of the method, it is extremely unlikely that such a method could be successfully adapted for on-farm use.

Acknowledgements This publication is an output from a project funded by the UK Department for International Development (DfID) for the bene"t of developing countries. The views are not necessarily those of DfID. We would like to thank the Director of the Ministry of Agriculture and Co-operatives, Mr Latuke, and his sta! for the assistance provided during the visit to Swaziland. In particular, we would like to thank Mr Mpanza for his continued assistance and advice throughout the visit, including allowing the use of the facilities and help of sta! within the Grain Storage Section at Malkerns Research Station. Our thanks also to Mr Adams of Swazi Oxygen who supplied the CO and the probe for injecting the gas into  the tanks (which he arranged to be specially made for the occasion) free of charge. Finally, we wish to thank Mr Moon and his colleagues at the British High Commission for the services and support provided during the visit. References Annis P C; Graver J van S (1985). Use of carbon dioxide and sealed storage to control insects in bagged grain and other commodities. In: Pesticides and Humid Tropical Grain Storage Systems (Champ BR; Highley E eds), Proceedings of an International Seminar, Manila, Philippines, 27}30 May, pp 313}321, ACIAR Proceedings No. 14 Annis P C (1989). Sealed storage of bag stacks: status of the technology. In: Fumigation and Controlled Atmosphere

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