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9. Pest control using fumigants
9.1 Areas of application for fumigants
9.2
Fumigants
9.3 Measuring the gas concentration in the air
9.4
Safety regulations
9.5
First aid
9.6
Equipment
9.7
Further literature
The fumigants used in pest control are given separate attention in this book due to their properties and the special application techniques required. The main area of use of fumigation lies in the control of insects, their eggs, larvae and pupae in the stored produce. As the fumigants are highly toxic to mammals, treatment also has a useful side effect on rodents.
This also means, however, that fumigants are extremely toxic to humans and that therefore fumigations is only allowed to be carried out by well-trained staff
Correctly applied, fumigants are entirely successful. The tiny gas molecules easily penetrate large stacks right into the individual grains, reaching and killing all stages of development of the pests.
Fumigants do not have any long-term effect due to their high volatility.
9.1 Areas of application for fumigants
The most important areas of application for fumigants are the treatment of bag stacks in stores or bulk grain in silos. Additionally fumigants are used in sealed buildings, chambers and on ships, in gas-tight containers or wagons to disinfect produce.
When fumigating a bag stack, it is necessary to cover the stack with a gas-tight sheet and hermetically seal it, thus ensuring that the required concentration of gas is maintained for the entire exposure period.
Treatment of an entire warehouse can only be carried out when the structural conditions enable the store to be tightly sealed. Most stores do, however, have gaps or cracks in critical places, such as along the joint between the roof and the walls, making space fumigation impossible. There are only very few fumigable stores in the world. A fumigable store for village use has been presented in section 4.4.1.
Special fumigation chambers are excellently suited for the treatment of smaller amounts, but these are often not available on the spot. Stack fumigation therefore is the most practicable and convenient method in most cases. Particular attention is therefore given to this type of fumigation below.
Mainly two fumigants are used in pest control:
Phosphine (PH3) and Methyl Bromide (CH3Br).
Apart from this Hydrogen Cyanide (HCN) is applied especially in case of fumigating mills.
In the last years the use of fumigants has been more and more called in question. This refers especially to methyl bromide, but also to phosphine. The problems are mainly related to adverse environmental effects, the possibility of carcinogenicity and increasing development of resistance in target pests.
Methyl bromide is a potential ozone-depleting substance and in some countries it is classified as a product which causes with "reasonable suspicion" human cancer. The most disturbing problem with phosphine is the increasing resistance indifferent insect pest species, particularly in the Indian subcontinent. Resistance has been caused by poor fumigation practices, especially inadequate sealing. This limits the possibilities of application of phosphine severely.
Alternatives for phosphine and methyl bromide are extremely scarce. Carbonyl sulphide (COS) seems a rather promising alternative for methyl bromide. This naturally occurring gas can control all stages of stored product insects, with Rhyzopertha dominica being the easiest beetle species to control and Sitophilus oryzae the hardest. At a concentration of 25 mg / l and an exposure time of 24 hours it kills most species of storage insects. The flammability is within the range of methyl bromide and phosphine and it is said to be environmentally safer than both gases. Because of its good suitability it has been patented for fumigation purposes in Australia.
As far as phosphine is concerned, new methods of application may contribute to a reduction of the quantities required for treatment and improve its performance Among the technologies which have been developed, mixtures with CO?, combination with heat or the constant flow system which improves gas distribution seem promising. Due to the higher technical standard required the immediate application of these methods in developing countries is not feasible.
9.2.1 Phospine
9.2.1.1 Properties
9.2.1.2 Toxicity of Phosphine
9.2.1.3 Formulations and Forms of Packaging
Phosphine is available as Aluminium Phosphide (AIP) and as Magnesium Phosphide (Mg3P2). Magnesium phosphide liberates phosphine more completely and more rapidly at temperatures below 20°C than aluminium phosphide does. Since it does not generate ammonia as well - which may influence the taste of products with a high water content - it is applicable e.g. for fruits and vegetables and can replace Methylbromide in several cases. Both formulations are available in various forms and packs, as follows:
Tablets (round or flat): Each weigh 3 g and yield 1 g of PH3. They are sold in various sizes of packs.
Pellets: They weigh 0.6 g and yield 0.2 g of PH3. They are also sold in various sizes of packs.
Bags (only as aluminium phosphide): They contain 34 g of preparation and yield 11.3 g of PH3. They are sold individually, in belts (4 connected bags), bag chains (10 connected bags) or in bag blankets (with 100 bags).
Belts are designed to be probed into bulk commodities. Bag chains are well suited for stack and space fumigations, whereas blankets are ideal for large scale fumigations.
The bags are ready for use - never open them!
Plates (only as magnesium phosphide): They weigh 117 g and yield 33 g of PH3. They are sold individually or in strips containing 20 plates.
All phosphine formulations are ready-for-use in the forms described above.
9.2.1.4 Generation of Gas
Phosphine (PH3) is generated as a result of temperature and moisture (in the air) reacting with the solid aluminium or magnesium phosphide.
In the case of fumigation bags the generation of gas is slowed down because of the absorption of humidity by the bag itself This should be borne in mind when determining the duration of the exposure time (see section 9.2.1.5). Recent improvements led to a product with delayed decomposition packed in bags made of polyethylene fleece, which is permeable to water vapour and phosphine, but resistant to liquid water,
The generation of gas starts immediately when the container holding the fumigant is opened. Concentrations which are likely to be dangerous to humans, however, are not reached until at least one hour later. This period may be even longer if the temperature and the relative humidity are low.
The decomposition of the phosphide formulations is never complete. Only approximately 98% of the Phosphine is liberated during fumigation. The powdery residue still contains about 2% of unreacted aluminium phosphide (or 0.2% in the case of magnesium phosphide) and must be collected after fumigation. Tablets and pellets therefore should be placed on a sort of tray or piece of cartboard. The powder is disposed of by pouring it into water mixed with a detergent, thus fully liberating the gas. This should be done in the open air in order to avoid the inhalation of the gas!
The generation of phosphine is described by the following chemical reactions:
AIP + 3H2O => Al(OH)3 + PH3
Aluminium + Water Þ Residue +
Phosphine
phosphide
Mg3P2 + 6H2O => 3 Mg(OH)2 + 2PH3
Magnesium + Water Þ Residue +
Phosphine
phosphide
9.2.1.5 Factors Influencing Success of Fumigation
The success of fumigation depends mainly on the right dosage of the fumigant the strict adherence to the minimum exposure time and, in first place, on quality of sealing.
Recommended application rates
The recommended application rates for phosphine are as follows:
| Application | Tablets | Pellets | Bags |
| Fumigation | 3 - 6/t or | 15 - 30/t or | 1 bag/2 - 6 t or |
| under sheets | 2 - 4/ ml | 10 - 20/ m³ | 1 bag / 1.5 - 4 m³ |
| Fumigation of silos and air-tight containers | 2 - 5 / t | 10 - 25 / t | 1 bag /2 - 6 t |
The concentration of gas initially established first leads to the insects being narcotized before they are finally killed. The resulting reduction in their respiratory activity means that they take in less of the gas. Should the gas concentration drop rapidly as a result of insufficient sealing or damaged tarpaulins, the pests will reawaken after a certain period without having received a lethal dose. Good sealing is the most important element when fumigating as this will lead to excellent success.
Exposure Time
The minimum exposure time depends on the temperature, the relative humidity and the formulation used, and on whether there is any resistance against phosphine. The following minimum periods should be adhered to under all circumstances:
| Air temperature | Tablets | Pellets | Bags |
| 10 -15°C | 6 days | 5 days | 8 days |
| 16 - 25°C | 5 days | 4 days | 6days |
| over 25°C | 4 days | 4 days | 5 days |
With a relative humidity of below 60%: at least 6 days
In case of resistance: at least 3 days more in each case
When mites are present: 10 days
The lower the temperature and/or the relative humidity, the slower the chemical reaction to generate phosphine and the longer-the exposure times required will be. Fumigation is ineffective if the relative humidity is below 30% or if the temperature is below 5°C.
Under arid climate conditions the relative humidity under a sheet may be raised by placing bowls of water beneath the pallets or by sprinkling water underneath the pallets. However, under no circumstances must the fumigant come into direct contact with the water.
The following principle applies: the longer the gas is able to act, the better is the success. This, however, presupposes that the stored produce is perfectly sealed during the entire fumigation.
Sealing
The most important prerequisite for the success of fumigation is the quality of the fumigation sheet and the sealing in order to maintain the necessary concentration of gas for the entire exposure period.
- Fumigation sheets
A fumigation sheet has to meet specific
Many plastic materials do not fulfil these requirements as they are either not sufficiently gas-tight and resistant to mechanical damage or to heavy for handling. Therefore the following specifications should be made when fumigation sheets are purchased:
If the sheet is composed of several strips, they should be welded together and the edges of the sheet additionally reinforced to prevent them from tearing apart. Sheets with glued seams are not always able to withstand tropical weather conditions. Stitched seams cause gas loss due to the holes made by the needle on sewing.
If possible, the size of the sheet should be selected so as to enable fumigation of one stack with a single sheet. Standardized stack sizes are of considerable advantage.
- Care of fumigation sheets
Good storage and careful handling prevents damage and extends the life of fumigation sheets. They should be folded together neatly and stored on pallets. If the sheets are carelessly thrown in a heap in the corner of the store, rodents may use them as nesting sites and severely damage them.
When placing the sheets over the stacks, care should be taken to avoid any holes or tears. Do not drag sheets along the ground or over pallets, but carry them instead! Do not walk on the sheets when folding them up, as small stones will make holes in the sheets.
The sheets must be checked regularly. Any holes or tears must be repaired immediately. Small tears can be sealed using insulating tape on both sides of the sheet, and larger ones by sticking a piece of sheet material over them. A special adhesive may be required for this.
- Material for sealing the fumigation sheet to the floor
Even the best quality sheets are of no use if they are not well sealed to the floor. The sealing technique is described in section 9.2.1.7. The best-proven method is to use sand snakes, which has a number of benefits:
The following materials are required to make sand snakes:
Sand snakes should have a diameter of at least 10 cm and be 1 - 1.5 metres in length. Fill them with just enough sand to enable them to bend and to adapt to any uneven areas of floor. Never fill sand snakes tightly as they will get to rigid to fulfill their purpose. Sand snakes should be placed so that they overlap by at least,/+ of their length.
Stones, palettes, wooden beams or other similar materials are unsuited as they are not flexible enough and may damage the sheets. Never use bags filled with stored produce for sealing purposes as they may be infested and provide a starting point for reinfestation.
A further method of sealing is the use of paper and paste. A prerequirement for this type of application is a smooth and well cleaned floor. Mix a thick paste of water with wheat flour. Wallpaper paste is even better if it is obtainable. Spread a coat of paste in those areas where the sheet will be laid on the floor. Lay strips of paper (e.g. old newspapers) 15 - 20 cm wide on top of this coat and cover them as well with paste Place the sheet along the centre of the paper strips, coat it again with paste and place a further layer of paper on top of it. Finally give the upper layer of paper a further coat of paste. When the paste dries, you will have a lasting, gas-tight seal.
This method does not apply to the corners of stacks where folds form. Sand snakes have to be used here.
Application
It has already been mentioned that for safety reasons residue from tablets, pellets and bags must be collected after fumigation. While this is a simple matter with bags, there are difficulties involved in collecting the powdery residues from tablets and pellets. Tablets and pellets must therefore be placed on trays or pieces of cardboard and never simply distributed on the stacks. Egg-boxes provide ideal trays, as a single tablet can then be placed in each segment.
Place the trays/cardboards under the pallets or directly at the side of the stack before sealing. It is indicated to put the tablets or pellets needed for the fumigation of 500 t on six trays which should be evenly distributed.
As there is a danger of self-ignition with large concentrations of phosphine, tablets and pellets should not touch each other.
Chains of bags should be used in preference to single bags. These are then attached to the stack by wedging the first bag of the chain between two bags of grain in the stack.
At the beginning of the fumigation the concentration of phosphine in the sealed space is considerably higher than required. By and by it drops even when sealing is done according to high standards. This observation has led to the proposal to split the required amount of fumigant in three single dosages applied on days one, two and three of the fumigation process, respectively. In tests this method has given perfect results. The difficulty lies in the procedure which requires above-average organization capacities and perfect safety conditions for the persons doing the work. Under the circumstances prevailing in most stores, however, the preconditions for practising dosage splitting do not seem to be given.
Ventilation
At (be end of the fumigation process, the fumigant must be thoroughly removed from the stored produce and the store by means of extensive ventilation before the store can be released again for general access (see section 9.3). The minimum ventilation period for phosphine is three hours. Where aeration is reduced due to a lack of ventilation facilities, the period must be extended to at least six hours. If there is no gas detector available (see section 9.3), the ventilation period should be extended to 6 - 12 hours in order to avoid any risks.
9.2.1.6 Resistance to Phosphine
Correct execution of fumigation will lead to complete control of storage pests so that there is generally no possibility of resistance developing. Poor fumigation practices have, however, led to resistance against phosphine to alarming proportions world-wide, and the tendency is increasing. Resistance to phosphine was first discovered in countries in which space fumigation was performed in stores which were not gas-tight.
Today it is an undisputed fact that the development of resistance in storage pests is particularly favoured by poor sealing and the resulting loss of gas. When the gas concentration drops too rapidly the pests have the chance to survive and to reproduce.
The following measures should be taken
9.2.1.7 Fumigating a Stack of Bags with Phosphine
Fumigation work must only be performed by trained staff. For each fumigation, one person is responsible as head of the fumigation team from preparing the fumigation to the release of the store for general access. The head of the fumigation team is responsible for the success and safety of the fumigation. The fumigation of bag stacks can be divided into 5 steps:
The safety regulations (see section 9.4) and the instructions provided by the fumigant manufacturer must be followed during the entire fumigation process. The activities involved in the 5 steps are described in detail below.
Preparations
Example:
Length (L)= 6 m
Breadth (B) = 4 m
Height (H) = 3 m
- Calculate the volume of the stack:
L x B x H = 6m x 4 m x 3 m = 72 m³
- Calculate the number of tablets, pellets or bags in accordance with the recommended application rate, e.g. 2 tablets/m':
2 tablets / m³ x 72 m³ = 144 tablets
- Round the number up or down according to the size of the packs in order to use up all open tubes
(with 30 tablets tube use 5 tubes of 30 tablets = 150 tablets).
- Check the folded sheets for damage!
- Spread the fumigation sheet over the stack as follows:
- Place the folded sheet on the stack (1) !
- Unfold the sheet over the sides of the stack (2) !
- Pull the sheet over the stack so that at least 1/2 m is on the floor on all sides (3) !
- When covering the stack with more than one sheet:
- Roll the sheets together so that they overlap by at least 1/2 ml
- Keep the rolled part together with clips or with sand snakes on top and adhesive strips at the sides!
- Distribute a sufficient number of sand snakes around the stack!
Example: Stack circumference: 6 m + 4 m + 6 m + 4 m = 20 m
Required overall length of sand snakes: 1.5 x 20 m = 30 m
Application of the fumigant and sealing
- When using tablets or pellets:
In case that pallets are not available for any exceptional reason, place the trays/cardboards on the floor next to the stack.
- When using bag chains:
- Unfold the fumigation sheet smoothly over the stack (I)
- Place a sand snake as shown in (2)
- Fold the sheet over the edge of the stack (3, 4)
Controls during fumigation
Ventilation and release of the store
Cleaning up work
9.2.1.8 Fumigating Silos Using Phosphine
Silos can best be fumigated during filling. Care must be taken to seal all openings with kraft paper and paste, or with impermeable coverings. The fumigant is added to the produce on the conveyor belt at regular intervals or thrown into the silo from above through a hatch during filling. This is done in line with the quantity of loaded grain.
Example:
A silo with a capacity of 500 tons is to be filled completely. This is being carried out at a rate of 20 t/h. With a dosage of 3 tablets/t, 60 tablets would have to be added every hour. It would be practical to add 5 tablets every five minutes to ensure good distribution of the gas. If an automatic dispenser is available, it should be set at I tablet per minute. Automatic dispensers are available both for tablets and pellets.
As the filling of a silo takes a considerable time, the staff may be endangered by the generation of gas. Therefore masks must be Used when the fumigant is applied manually.
If a silo is not being filed completely, additional fumigant must be applied in line with the remaining silo capacity (volume in m').
The residues of the fumigant must be removed from the stored produce before it is forwarded:
9.2.1.9 Fumigation of Bulk Produce with Phosphine
Bulk grain can also be treated under a fumigation sheet. Fumigation should be performed in line with the instructions given in section 9.2.1.7.
If the height of the bulk produce is less than 2 metres, tablets, pellets (on trays) or sachets may be distributed onto the stored produce at regular intervals and collected again after fumigation Blankets (I blanket for each 100 - 300 t) are particularly well-suited for this purpose.
If the height of the pile is over 2 metres, then tablets, pellets or narrow bags must be probed into the produce at regular intervals. This requires good preparation and a good team, as it should take no longer than one hour from opening the first container with fumigant to the final seal being made.
The residue can only be taken out in such cases by means of mechanically cleaning the bulk produce when it is removed from the store (see section 9.2.1.8).
9.2.2 Methyl Bromide
9.2.2.1 Properties
9.2.2.2 Toxicity
It is absolutely essential when fumigating with methyl bromide that the work is only performed by well-trained, responsible staff incorrect application is extremely dangerous both for the user and for persons nearby.
The use of methyl bromide has been severely restricted in industrialized countries since the beginning of 1995, because of its ozone-depleting and carcinogenous potential. The prospects for the continued use of this fumigant appear generally poor. In the USA a general ban by the year 2010 is to be expected. But the discussions are going on and a further prolongation of the application up to the year 2010 or longer is possible. As quarantine and pre shipment fumigations seem to be the only fields of application in the next future, we have shortened the section on fumigation with methyl bromide in this edition of our manual.
9.2.2.3 Forms of Packaging
Methyl bromide is supplied in liquid steel cylinders of various sizes and in cans. The choice of the form of packaging by the user depends on the amount required.
9.2.2.4 Generation of Gas
Methyl bromide is gaseous at temperatures of above 4°C, but is kept in a liquid state under pressure in gas cylinders in a similar form to butane or propane. If the valve is opened, the methyl bromide is released and volatilizes in the air to become effective as a fumigant. It is important that sufficient space is available to enable the gas to disperse, as condensation of the methyl bromide will otherwise ensue.
9.2.2.5 Factors influencing the Success of Fumigation
Sealing, control measures during fumigation, ventilation, release of the store and cleaning up work are the same as for fumigation with phosphine (see section 9.2.1.7). The only differences are in the preparations (calculation of the required amount of gas) and in the application.
Recommended Application Rates
The recommended application rate in the fumigation of grains and grain legumes in stacks of bags under a fumigation sheet is 20 - 40 g m³.
It is essential that the recommended application rate is adhered to, as the success of treatment will be inadequate if the dosage is too low. This will further encourage the development of resistance. If the produce is properly covered and sealed, the recommended dosage will be perfectly adequate. If the amount of methyl bromide is too high, the maximum residue limit may be exceeded.
For effective fumigation of silos, devices for the recirculation of the gas are necessary, as it will otherwise settle at the bottom of the silo.
Exposure Time
Methyl bromide acts more rapidly than phosphine. The exposure time is generally 24 hours. Due to the danger of bromide residues being left in the fumigated produce, this period should not be exceeded.
Quality of Sealing
The same criteria for sealing apply as for phosphine described in section 9.2.1 5.
Application
Wear a full face mask with a new type AX breathing filter (brown coloured ring)! in contrast to working with phosphine, this mask must definitely be worn in any case on applying methyl bromide.
Methyl bromide is applied from steel cylinders using a polyethylene tube ending in a nozzle. Simple rubber tubes are not suitable for this purpose. Most convenient are tube systems with several outlets, as they enable application of the fumigant at more than one point at the same time. There should be one point of application for every 9 m² on the top surface area of the bag stack. Nozzles should be spaced about 3 m from each other and at least 1.5 - 2 m from the edges of the stack. In order to have approximately the same pressure on all nozzles, the endings of all branches of the system must be at equal distances from the cylinder.
Methyl bromide will settle on the bottom of the stack as it is heavier than air. Application must therefore be made to the top of the stack in order to allow the gas to penetrate the produce. In order to prevent any condensation of the gas (due to cooling as a result of the methyl bromide evaporation), air ducts or pits (one pit per nozzle) must be provided by restacking the bags in the top layers. This allows rapid air and heat exchange. The ducts and pits should be lined with sheets or empty bags in order to prevent the methyl bromide to drop onto the stored produce.
The dosage is controlled by observing the weight of the cylinder during the application. The cylinder must thus be placed on a set of scales.
For safety reasons the application must be finished within 10 - 15 minutes. This necessitates a perfect preparation and organisation before starting the operation.
Ventilation
The ventilation period for methyl bromide is at least 6 hours. In poorly ventilated rooms this period should be extended to at least 12 hours.
9.3 Measuring the gas concentration in the air
Before the store can be entered without danger, the gas concentration must be tested. For this purpose a gas detector is used, consisting of a bellow pump and an insertable glass tube:
The tubes are specific for certain types of gases. To take a reading, break open a tube at both ends and insert it into the pump in the prescribed direction indicated by an arrow. Then hold the detector in the air to be measured and draw it into the tube by pressing and releasing the suction rubber. The number of strokes needed to take a reading is stated in the instructions. The gas concentration can be directly seen by the change of colour in the calibrated tube:
The maximum admissible levels of gas in the air are 0.1 ppm (= 0.15 mg/m³) for phosphine and 5 ppm (= 20 mg/m³) for methyl bromide. Only if the values are below these limits the store can be released for access.
The presence of methyl bromide can also be tested using a halide lamp. This is particularly useful for tracing leaks in the sealing for fumigation. A halide lamp consists of a small burner which is run on commercially-available propane or butane gas. The flame produces an air current inside a fixed tube. Move the end of the tube along any potential areas of leakage (wearing a mask!). If methyl bromide is leaking out anywhere, the flame will take on a different colour (flame with a slight green coloured border => 50 ppm; flame changes more and more to an intensive green colour => 50 - 1 000 ppm; flame with a poor blue colour => more than 1 000 ppm). It becomes clear that the halide lamp can not be used to measure the maximum admissible level of gas. Be sure to follow exactly the manufacturer's instructions for the use of halide lamps.
