· Capability of ensuring fumigant disinfestation of bulk grain in tall narrow structures, or Jeep tanks and ships' holds, to an appropriate quarantine standard.· Current methods for gas application, distribution to uniform concentrations throughout the fumigation enclosure, and ventilation, have proven (on numerous occassions) to be inadequate for hulk grain, or for bagged grain in extremely large stacks when using methyl bromide.
Successful fumigation requires (simplistically); the application of the correct amount of fumigant (the applied dosage), for the correct length of time (the exposure period; which must make allowance for the time required to approach uniformity of gas distribution, if fumigant gases are being redistributed passively by convection currents), in an enclosure of sufficient gas-tightness (retention of sufficiently uniform and lethal concentrations throughout the entire enclosure under fumigation).
Previous criteria (BANKS and ANNIS, 1984) for successful phosphine fumigation require no greater than a 25% difference in minimum to maximum gas concentrations throughout the enclosure, after 25% of the proposed exposure period has elapsed, and remain less than that value for the remainder of the exposure period. This specific criterion is a measure of the evenness of gas distribution, and how rapidly gas dispersion occurs from applied formulations, either applied non-uniformly on the surface of the grain, uniformly mixed throughout the grain, probed to certain depths, or distributed by air-flow methods.
The distribution process should not be too slow as to allow phosphine concentrations to decay to a small fraction (below the minimum effective concentration) of the applied dosage, before the completion of the exposure period. The average concentration of phosphine achieved should not be less than 50% of the expected theoretical concentration, based on the applied dosage and the total gas volume in the system being fumigated.
Phosphine is generally applied as tablets or sachets of aluminum phosphide. The tablets or sachets are put in with the grain, and react with moisture in the atmosphere within the grain mass to produce phosphine gas. Using this application method, the gas reaches a peak concentration after about a day and a half and then begins to rapidly decay especially in leaky structures. This traditional fumigation method is quite effective in controlling larvae and adult insects, but does not readily control the other two stages of the insect life cycle -the egg and pupal stages. This means that after fumigation, the adults and larvae are controlled, hut some of the eggs and pupae may in fact survive the treatment. When Produce Inspectors see larvae or adult insects in either bagged or bulk grain, it is obvious that it is infested. Therefore, if the larvae and adults have been killed, the grain may appear to be free of infestation and traded with 'insect-free' status, even though the eggs and pupae are still present and alive. Eggs and pupae are very difficult to detect in grain. Since there is no residual fumigant effect to protect the grain, eggs and pupae can continue their life cycle by developing into larvae and adults, and the grain becomes "reinfested" again. The worst consequence of this lack of complete control is that insects with higher phosphine resistance are bred, meaning treatment failures will become more and more common place.
Additionally, ineffective treatments of this kind have long-term effects on supplier's reputation. Buyers give preference to grain suppliers who are consistently able to deliver good quality insect free produce. Grain which soon becomes infested after delivery can do permanent damage to a grain trader's reputation.
The tolerance of the egg and pupal stages against phosphine is considerably greater than that of either the adult or larval stage. Clearly, there are two approaches for solving the problem, according to WINKS (1986);
· apply a high dosage sufficient to kill either eggs or pupae irrespective of whether development (into larvae or adults) continues, or;· retain the fumigant long enough for eggs to hatch and pupae to moult into adults, and maintain a concentration at that time sufficient to kill these more susceptible stages.
In both approaches, the period of exposure (time as a dosage factor) is important in determining the level of mortality achieved, but is of far greater significance in the second approach, as time becomes the limiting factor.
As indicated by WINKS (1986), many grain handling authorities would like to shorten phosphine fumigation periods. This is clearly very difficult to do when dealing with stages as tolerant as early pupae or, for that matter, young eggs. When time for development is obviously so important, fast release formulations, such as those based on magnesium phosphide, would seem to he of little value unless some attempt is made to increase application rates high enough to cope with young pupae or eggs and, at the same time, improve the rate of distribution.
It is therefore apparent that there are major advantages in prolonged exposure periods. This allows time for insect development to proceed to more susceptible stages which are subsequently controlled at a much lower dosage. This approach also allows sufficient time for the phosphide formulation to completely decompose, and time for the gas to be distributed via natural convection currents. However, it is implicit in the use of longer exposure periods that the standard of gastightness is sufficient to permit the retention of an adequate concentration, for the required time.
Sealing storages, particularly for small complex bin arrangements, can be too expensive and difficult an option for many grain handlers.
15.2.1. SGRL/CSIRO SIROFLO® System1
1 Enquiries for use of SIROFLO should he directed to:Mr. Kevin Smith
Business Manager
CSIRO Division of Entomology
GPO 1700
Canberra ACT 2601
AUSTRALIA
A cost-effective option is to install SIROFLO® SIROFLO® extends the duration of exposure to the gas beyond the usual length of time in most unsealed storage facilities, thus making more effective use of the phosphine gas. The system continuously maintains the gas level until all eggs and pupae have passed out of the tolerant stage and are killed. The result is that the grain will truly be 'insect-free'.
The exposure time can he controlled and even extended to provide control of phosphine-resistant insects. Using the SIROFLO® system means that the grain handler can expect and get complete elimination of an infestation. SIROFLO® can he used either as a single fumigation treatment, or as a long term means of protecting grain from reinfestation. It can also be used for disinfesting parcels of grain as part of a throughput facility. The system is quite simple. A low concentration of phosphine gas from cylinders is mixed with an air stream. The air gas mixture is pumped into the base of a storage facility. The airstream produces a slight positive pressure differential that causes the gas to disperse more evenly through the entire grain mass.
One of main advantages of this technique is that it works on storage bins which are not fully gastight, so sealing and other modification costs can he reduced. If SIROFLO is used as soon as the silo is tilled, the system can eradicate any insects imported with the grain, thus making management of grain stocks a much simpler operation.
Fig 15.1. Schematic representation of changes in tolerance to phosphine of the different stages of stored product insects with time (Source: WINKS, 1986)
Fig 15.2. Figure
GAS DISTRIBUTION SYSTEMS IN BULK GRAIN
SIROFLO® is a method of applying phosphine, based on the dilution of a low concentration of phosphine into an air stream that is introduced into a storage. The method currently relies on the use of a 2% gas mixture of phosphine in carbon dioxide, but a controllable on-site generator is also being developed. Both the concentration and the exposure time can be varied easily before and during a fumigation to cope with the many factors that influence the outcome. Unlike recirculation systems, which have both positive and negative pressures and thus require high standards of gastightness, SIROFLO® is a flow-through process and in this way a positive pressure is maintained throughout the enclosure. Although it offsets most of the factors that give rise to gas loss, there are still certain minimum requirements. The greatest interests in the SIROFLO® process is as a grain protection method. Grain can be stored for long periods using the process without using residual grain protectants. Moreover, the cost of protection are substantially less than the cost that are associated with the current grain protectants. (SOURCE: Winks, 1990)
15.2.2. INTRANSIT SHIPBOARD FUMIGATION
The fumigation operation in ships holds or barges is a very specialised technique, and accordingly, the British Pest Control Association have a separate module in their operators training programme to cover the needs of in-ship fumigation. (CHAKRABARTI, 1994)
Most of the original development work for in-transit, ship-board fumigation of grain was carried out in the USA and this work forms the basis for much of the current methodology. The depth of grain in a ships hold can be between 15-20 metres. Under these conditions it is difficult for phosphine to penetrate to the bottom by dosing at the surface. In a tanker hold of 17.7m depth, applying a dose of about 1 g/m3 by probing tablets to 4.3m resulted in sufficient gas penetrating to 9m depth but concentrations remained low at 17m depth even after 24 days (Redlinger et al, 1982). However, the distribution of phosphine was improved progressively (1) when tablets were applied in layers during loading instead of surface dosing (Redlinger et al., 1979), or (b) by the use of perforated plastic tubing of about 10 cm diameter. Long lengths of tubing are laid at the bottom of the holds before loading and connected to vertical pipes run down the sides which opened above the surface of the grain when the hold was filled. Part of the dose of the aluminium phosphide preparation was mixed with grain and poured into the vertical tube, the rest of the dose was distributed on the grain and was pushed just under the surface the method, known as the "J System", uses a tan to re-circulate phosphine through the grain via the pre-positioned plastic tubing. Rapid and uniform distribution of phosphine was achieved by this process (Leesch et al., 1986) which is now widely used for pre-planned, in-transit fumigation with phosphine.
Even against susceptible insects, if a voyage is less than 5 days or if the grain temperature is lower than 10°C, in-transit fumigation with aluminium phosphide preparations is not currently recommended. If the grain is very cold or very dry, solid formulations may not react completely, and if the residues are not removed before discharge they may cause potential hazards to workers during unloading.
The method introduced by the French company, Desinsectisation Moderne (DM), now being used for some in-ship fumigations in the UK, uses patented "Fumisleeves" which are made of find mesh woven nylon with a lay-flat width of 11 cm and of length 6 or 8 m with one end closed. For dosing, these are slid over sections of 50 mm diameter UPVC piping which are pushed up to 6 m into the grain. Phosphine tablets or pellets are dropped into the sleeve through the UPVC pipe by means of a plastic funnel while the pipe is being gradually withdrawn. The tablets are held at different depths by the collapsed sleeve. Probing is done in a matrix according to the dosage necessary for the bulk. Excess lengths of the sleeves are tied in a knot and are left on the surface of the grain as markers. The technique is suitable for use in deep floor stores and in smaller ships holds. On ships, mechanically operated "MacGregor" hatch covers with gasket linings in good condition can provide a good gas tight seal for fumigation. At the end of the treatment, the sleeves are pulled out and the residues are disposed of, leaving no residues behind in the grain.
15.2.3. THE PHYTO-EXPLO® SYSTEM
Another method of dosing developed by "Désinsectisation Moderne" for use in deeper grain bulks, is the Phyto-Explo® System, a patented method for the introduction and distribution of phosphine or methyl bromide in bulk cereals, without the need for a pre-positioned piping system.
In this process an expandable, corrugated shaft of about 63 mm diameter, wholly or partially perforated, is slid over a metal pipe and fitted to metal probe which is introduced into the grain using a pneumatic hammer. When it reaches the desired depth, the probe is withdrawn leaving the shaft expanded in position. Aluminium phosphide tablets or pellets are put in nylon sleeves and introduced into the shaft. If the seals is adequate, phosphine generated in the shaft will spread evenly throughout the hulk. A further refinement has been the use of fans to draw phosphine generated on the surface into the grain mass to speed gas distribution. This system can also he adopted for fumigations using methyl bromide in ships' holds.
For tall silos, tan-assisted circulation of the gas through a single shaft with partial perforation is essential to achieve even distribution due to the strong upward movement of air frequently experienced with this type of silo. In this situation the phosphine fumigant is introduced into the head space rather than into the bulk. In this way. the spent phosphide residues can he retrieved easily at the end of the treatment or if necessary, a fumigation can he extended by further introduction of formulation into the head space. However, if the structure is not sufficiently gastight the use of a tan will cause excessive leakage and/or dilution of gas concentrations.
Figure 15.4 - TRADITIONAL FUMIGATION OF CARGO IN SHIPS HOLD USING METHYL BROMIDE
METHYL BROMIDE APPLIED ON SURFACE TRADITIONAL FUMIGATION OF CARGO IN SHIPS HOLD USING PHOSPHINE
Figure 15.5 PRINCIPLE OF PHYTO EXPLO®
Figure 15.8
