Fumigation for the control of insect pests in storage

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Rolando Tiongson

INTRODUCTION

The earliest use of fumigants dates back to about 1000 years B.C. when "sulphur fumes" were used to disinfect ancient homes. However, it was only in 1854 that the use of carbon disulphude and hydrogen cyanide for disinfestation was acknowledged. The advent of these chemicals gave rise to the modern concept of fumigation. To date, fumigation offers one of the most effective control measures against varieties of stored product pests.

Fumigation is generally defined as a chemical which at a required temperature and pressure, can exist in the gaseous state in sufficient concentration to be lethal to a given pest organism. Fumigants may be applied as gas (hydrogen cyamide, methyl bromide), as a solid which will evelve as a gas (aluminum phosphide which evolves hydrogen phosphide, calcium cyanide which evolves hydrogen cyanide) or liquid (ethylene dibromide, carbon tetrachloride, ethylene dicholire and carbon disulphide).

Fumigation as a control technique may be described simply as the establishment of an athmosphere containing a lethal gas in the environment of an insect of a concentration high enough and an exposure period long enough to kill the insect.

This briefly discusses some important principles governing the use and handling of fumigants. Properties of some commonly used fumigants are also presented. More emphasis is given on phosphine, being the fumigant now widely used worldwide especially in the developing countries.

CONCENTRATION AND TIME RELATIONSHIP

The effectiveness of fumigation largely depends on the quantity of fumigant absorbed by an insect from its gaseous environment over a certain period of time. Therefore, it is clearly understandable that the dose absorbed by an insect is a function of concentration and time of exposure. As an illustration, 33.2 milligrams per liter of methyl bromide must be maintained for 5 hours in order to kill 99 percent of larvae of Tenebroides mauritanicus (c). Monro and Bond 1961). The product 33.2 milligrams per liter x 5 hours = 166 milligrams per liter per hour is known on the concentration x time product needed to kill 99 percent of this insect (table 1) It can be abbreviated and referred as Ct(or C.T.) product.

If the-dose is a function of concentration and time, it follows that both concentration and time can be varied within a certain limit, provided that the combined effect remains the same. Hence, the same effect may be produced by a low concentration for a long time or a high concentration for a short time.

FACTORS AFFECTING THE TOXICITY OF FUMIGANTS

1. Temperature

Temperature greatly affects the toxicity of fumigants to insect. At a temperature below 15°C, stored product insects are not as active as at higher temperatures and tend to absorb less fumigant over a given period of time. Chemical reactivity and rates of diffusion of fumigants are also reduced by lowering the temperature. From 10°C to 35°C, the concentration of a fumigation required to kill a given stage of an insect species decreases with the rise in temperature.

Table 1 Required concentration x time (cf) products to obtain 99% mortality of Tenebroides mauritanicus (Monro, 1969)

2. Adsorption/Desorption

Adsorption is the most important physical factor modifying the penetration of a fumigant. Physical sorption of the fumigant by the material containing the insect is reduced when the temperature increases and proportionately highed concentrations of fumigant are needed to kill the insects.

As the temperature is lowered, the amount of gas physically adsorbed increases and it is necessary to add more fumigant in order to sustain concentrations free to act on the insects. Also, at low temperatures, diffusion of the gas into porous commodities is slower and there is a corresponding decrease in the rate of desorption afterwards.

Chemical reaction of the fumigant with some of the fumigated materials increases as the temperature is raised. If the residue formed is significant, it is advisable to conduct the treatment at as low temperature as permissible.

3. Relative Humidity

Variation in response at certain humidities have been observed not only between different species subjected to different fumigants, but also between stages of the same species exposed to a single fumigant.

4. Carbon Dioxide

Cotton et al (1939) Jones. (1938) and Bond et al (1978) have demonstrated that the addition of CO2 to some of the fumigants may increase or accolerate the toxic effect of the gas by simulating the respiratory movements and opening of the spiracle of the insect. However, excessive amount may tend to anaesthesize the insects thereby enterfering the action of the fumigants.

5. Narcosis

At high concentrations phosphine acts as a narcotic and the tolerance of insect is greatly increased. In adults of a susceptible strain of T. castaneum a seventy fold increase in tolerance at the LD99 level occurs when they are exposed to a concentration of 50 mg/ compared with that of 0.5 mg/l. Although narcosis is symptomatic of a high concentration it does not appear to be the neehanesm to resistance.

TYPES OF FUMIGATION ENCLOSURE

The choice of fumigant and its method of application are largely determined by the situation and type of enclosures in which the treatment is to be undertaken. Fumigation enclosures for grain may be clasified as follows:

1. Sealed fixed enclosures

• fumigation chambers
• silo bins
• storage sheds or godown

2. Sealed movable enclosures

• road and rail vehicle
• barges and ship
• freight container

3. Temporary enclosures

• under gas proof sheet
• spot fumigation

Table 2 provides an outline of the types of situations where fumigation can be applied and the appropriate process of fumigation.

PROPERTIES OF SOME COMMONLY USED FUMIGANTS AND THEIR MODE OF APPLICATION

A. Phospine (PH3).

Phosphine is now probably the most widely used fumigant in the world at present. It has reached this position because it has most of the properties desirable in a fumigant and is commercially available in easy-toapply solid formulation.

Properties

Phosphine is a highly toxic colorless gas. It has a boiling point of 87.4°C and a molecular weight of 34.04. It is highly flammable and slightly soluble in water. The safety limit for prolonged exposure is 0.3 ppm in air or 0.4 milligram/cubic meter of air. Phosphine is slightly heavier than air (20%)

Mode of Action

Aluminum phosphide is available in a number of different forms including tablets, pellets, blanket and sacket. All contain a mixture of aluminum phosphide and other ingredients. On contact with water vapour in the air these preparations liberate a gaseous mixture of phosphine and other gases which act to lower the combustibility of other mixtures and allow fine grinding of the aluminum phosphide which results in quicker and more complete decomposition of the preparations. Ammonium carbamate is present in some formulations and liberates ammonia and carbon dioxide which inhibit inflamation by diluting the hydrogen phosphide formed during subsequent hydrolysis. The evolution of ammonia performs the important dual of function acting as warming agent (because of the pungent odour) and once this is noticeable the decomposition of aluminum phosphide has already commenced releasing hydrogen phosphide with the parallel increase in the carbide like odour.

Manufacturers of phosphine tablets have claimed that there is a delay period of or more hour before phosphine is freely evolved. However, laboratory trials disproved such claims since the delay period has never been observed. Gas was evolved as soon as the tablets were exposed to humid air and evolution continued in almost linear rate.

Commencement of the process involving the evolution of hydrogen phosphide (phosphine) is accompanied by a change in external appearance of the tablets or pellets. The shiny gray-green surface turns matte rough and whitish and finally commence to bloom. Grey white dust appears on the surface, the structure becomes brittle, volume of the tablets and pellets increase finally resulting in the pile of grayish dust (composed of aluminum oxide hydrate, a constituent of clay) 5 times the original volume of the tablet or pellet. This process takes 3-4 days for complete decomposition of the formulation (although depending on the temperature and humidity once the tablets are exposed to the athmosphere, decomposition may take 48-72 hours or in the case of pellets 12-48 hours).

Most of the gas will be developed between the 4 th and 12th hour. Cook (1983) states that 3 gram tablets have a "peak off" or maximum release of phosphine in 19-30 hours. Even during the period of maximum gas release, the tablet remains cool since the heat generated by the hydrolys is of aluminum phosphide is used in the decomposition of the unstable carbonate.

Methods of Application

Aluminum phosphide tablets and pellets can be applied directly to a grain stream being loaded into a sealed silo bin or flat storage either by an automatic dispenser or by hand. Natural gas movement will then distribute the gas throughout the storage. Care should be taken to ensure that no tablets or pellets touch each other in order to avoid the risk of localized heating, explosion or spontaneous combustion.

In shallow grain bulk, tablets may be probed into the surface at intervals. Probes for this purpose may be obtained from the manufacturer's agent. For a small scale application a simple hollow tube of appropriate internal diameter may be used. As the probe or tube is withdrawn tablets may be placed singly at selected depths within the grain mass.

Alternatively, tablets or pellets can be placed on trays that will facilitate the removal of residues at the completion of fumigation and ventilation. With this method, precaution must be taken to avoid piling of tablets or pellets on the trays, i.e. it should be spread out in a single layer. This mode of application is particularly applicable when fumigating bag stacks under sheet covers. Likewise, it is extremely important that the sheeting materials to be used will provide sufficient gas tightness to ensure maximum retention of fumigant vapours and for safety reasons. Sheeting materials

Ventilation After Completion of Exposure period

There are the minimum ventilation periods for the following structures containing treated commodities.

1. With throughflow and forced drought (flashproof fan) operated 2 hours on and 2 hours off in period of 12-24 hours, depending on the size of the structure.
2. With throughflow and natural draught (wind) for structure of 300 tonnes or greater capacity for a period of 2-5 days; depending on size, for structure of less than 300 tonnes capacity, ventilation period is 5 days.

For well-sealed, plastic covered, bunker storages of not less than 1000 tonnes capacity, ventilation period is 2 hours, after removal of covering.

For Empty Buildings

1. With throughflow and forced draught (flash proof fan) operated 2 hours on and 2 hours off for a period of 1 to 2 days, depending on the size of structure.
2. With throughflow and natural draught (wind) for structures of 300 tonnes a greater capacity not less than 2 days depending on structure size, opening, prevailing wind speed; for structures of less than 300 tonnes capacity ventilation period is 5 days.

Withholding Period

Allow a period of 2 days after completion of ventilation before using the treated commodities for human consumption or for stock feed. Treated commodities may be safely transported after completion of the recommended ventilation period.

Criteria for Successful Phosphine Fumigation

Bank and Annis (1983) have proposed certain criteria that will result in more efficient phosphine fumigation. These are:

1. The grain bulk must be free of live insects at the completion of the exposure period by conventional sampling methods.
2. The average maximum concentration should not be less than 50% of the expected concentration based on the applied dosage and total gas volume in the system.
3. The average concentration should remain above the minimum effective concentration against insects at the end of the exposure period.
4. The ratio of minimum to maximum phosphine concentrations should not be less than 0.25 (1:4) after 25% of the total exposure period has been completed and remain higher than this level throughout the fumigation.

In most cases unsuccessful fumigation can be atributed to the following factors:

1. Excessive loss of fumigants
2. Inadequate dosage in localized regions
3. Excessive delay between application and the fumigant reaching some areas resulting in shorter exposure periods than expected
4. Excessive loss rate combined with slow dispersion

Safety Precautions

Exposure to phosphine is reported of cause depression of the central nervous system and impairment of the respiratory function. Inhalation of phosphine may produce the following symptoms:

The current Threshold Limit Value for phosphine in the air is 0.3 ppm. This is the concentration at which, it is believed, men may be repeatedly exposed for eight hours per day, five days per week without adverse effect on the general health.

A full face mask equipped with the correct phosphine-absorbing canister must be worn during fumigation. Also, gloves should be worn when the phosphine generating tablets or pellets are handled.

B. METHYL BROMIDE (CH3 Br)

The insecticidal value of methyl bromide was first reported by Le Goupil (1932) and it had since been used for the fumigation of a very wide range of commodities including cereal grains.

Properties

Methyl bromide is a colorless liquid with a boiling point of 3.5°C. Hence unless under pressure it exist as gas at normal temperature. At a concentration normally used for fumigation purposes, methyl bromide vapour is odorless. This is the reason why 2% chloropicrin is added as a lachrymatory warning agent. At high concen "rations methyl bromide is said to posses a musty or sticky sweet odour. Methyl bromide gas is 3.3 times heavier than air.

Formulations Available and Dosage Rates

Methyl bromide is packaged in cans, cylinders and ampules and is available as pure material or a mixture containing 2% chloropicrin (CC13 NO2) as a warning agent. For normal fumigation purposes cans or cylinders are used which contain liquid methyl bromide under pressure. Pack sizes range from cans containing 700 grams (1.5 Ibs) to 150 kg (220 Ibs) cylinders.

For almost all species of stored product insects, a dosage of 200 g/hr/m 3 (i.e. 200 mg/li) can be expected to produce a complete kill at temperatures greater than 10°C.

Under local Philippine conditions, methyl bromide is used for the treatment of bagged grains at the rate of 1-4.5 lb per 1000 cu ft.

Methods of Application

Because methyl bromide is heavier then air and due to its vapor density, the gas is introduced at the top of the fumigation enclosure via an appropriate pattern of distribution piping and outlet nozzles. In small fumigations, a single tube leading into the center of the top of the enclosure may be adequate. Methyl bromide may be introduced into the fumigation enclosure as a gas or as a liquid.

a) To introduce the fumigant as a gas, the can should be attached to the piercing attachment with the piercing tube uppermost. The liquid will progressively vaporize with the can and can be expelled into the fumigation enclosure. After the can has been connected to the fumigation enclosure and attached to the piercing attachment, it should be placed in warm water not exceeding 77 C. This is necessary because of the cooling effect vaporization. No attempt should be made to warm cans before they are attached to the piping via the piercing attachment.

b) To introduce the fumigant as a liquid, a shallow metal evaporating pan (not aluminum) should be Iorated within the fumigation enclosure under the outlet nozzle of the piping. The pipe should be sufficiently large to hold more than the full liquid dosage applied. In addition, the outlet nozzle should be loosely wrapped by towel or hessian cloth to prevent splashing of the liquid, i.e. Iiquid should be prevented from contacting the commodity. With this method the can should be attached to the piping via the piercing attachment and then inverted so that the liquid is expelled by the vapour pressure of methyl bromide within the can.

Ventilation Requirements

Following fumigation with methyl bromide, structures or commodities must be thoroughly ventilated. Methyl bromide is more difficult to ventilate from commodities than phosphine since it is more strongly sorbed. Desorption from commodities is the limiting factor in methyl bromide ventilation not the amount or velocity of air passing through a commodity. To ventilate methyl bromide it is necessary to provide a throughflow of air and where possible a fan of suitable capacity should be used to provide force drought.

Safety Precautions

Methyl bromide is highly toxic to all forms of animal and human life. It is very dangerous to inhale the vapour and to allow the liquid or concentrated vapour to come into contact with the skin. The effect of a single inhalation depends on the concentration and the period of exposure. The new Threshold Limit Value (TLV) for the methyl bromide was set at 5 ppm is 1981 from 20 ppm in 1964, this is the maximum limit that can be tolerated at continuous exposure 8 hr/day at 40 working hr/week., The effect of repeated exposure is cumulative. It is important to note that symptoms of poisoning are slow to appear and may take several days to a few weeks or months.

For protection against occasional exposure to low concentrations, use a full face gas-mask fitted with exhaust value and special methyl bromide filter or canister. Even if the operator is protected by a gas mask his total exposure to methyl bromide vapours should not exceed 15 minutes per day.

C. HYDROGEN CYANIDE (HCN)

Hydrogen cyanide has been used to control insect in stored grains and seed and may be generated in practice by the action of moisture on sodium or calcium cyanide, by dispensing of gaseous HCN from a cylinder or by release of HCN absorved into an inert material HCN was first used extensively in the late 1800's against scale insects on citrus trees in California.

In disinfestation of grains hydrogen cyanide is used in the form of pressurized liquid in gas cylinders or as a liquid absorbed into special cardboard disks. It may also be generated in situ from calcium cyanide which produces HCN on contact with water vapour or by adding acid to sodium cyanide.

Monro (1969) recommends a rate of 40/9 tonne with a 24 hour exposure for hydrogen cyanide alpplied with recirculation to bulk grain or 160 g/tonne of calcium cyanide applied to the grain stream with a 7 day holding period.

D. CARBON DISULPHIDE

Carbon disulphide is a liquid fumigant. It is sprinkled from a sprayer or watering can over the surface of bagged or bulk grain within an enclosure. The heavy vapour is then allowed to percolate through the commodity by gravity. Carbon disulphide is highly inflamable and explosive when mixed with air. It can be used in combination with nonflamable liquid, typically carbon tetrachloride to reduce this hazard.

Carbon disulphide should be applied only to small grain bulks (e.g. 10 tonnes) Application rates of 75/g/m 3 (60 ml/m3) and 150g/m³) should be used at greater than 21 C and at 16-21 C respectively (Willian et al 1980)

E. CHLOROPICRIN

Chloropicrin is a relatively non-volatile liquid fumigant often used in the treatment of localised infestations. The use of this fumigant is not so reliable and should not be used except under conditions where no other process is available. Chloropicrin is applied by probing pieces of rag soaked in the correct dosage of the fumigant into parts of the infestation so the the whole infestation region is treated. After application the grain surface above the infestation is sheeted. Where local regulations permit, chloropicrin may be used to fumigate bag stacks of grain, by sprinkling the liquid over the top of the stack and then sheeting it.

Chloropicrin applied as spot treatment should be used at a rate of 8 g/m³ (5 ml/m³) with an exposure period of 7 days.

F. DICHLORVOS

Dichlorvos is being used as a space fumigant by suspending inpregnated resin strips above the grain in sufficient quantity to produce a vapour concentration lethal to moths such as Ephestia cautella. Dichlorvos is also applied as a spray to grain and at least some of its action is that of a fumigant.

ACKNOWLEDGEMENT

Much of the information contained in this lecture material has been extracted from compilation on "Insect Pests of Stored Grain and Their control, Vol. 5, Grain Fumigation and Sealing Requirements" by R. L. Semple and from the Proceedings of the Australian Development Assistance Course on the Preservation of Stored Cereals edited by B. R. Champ and E. Highley.

DOSAGE RATES AND EXPOSURE TIMES FOR PHOSPHINE

(according to Degesch, GMBH, Frankfurt, West Germany)

The effective results obtained with any fumigation strategy depends on applied dosage, and the exposure time required to reach and maintain a lethal atmosphere. To a certain extent with Phostoxin and Magtoxin formulations generating phosphine gas, a correlation exists between concentration and time (Ct product), but the factors of concentration and time cannot be interchanged at extreme values with the time element being more important than applied dosage.

The dosage of Phostoxin or Magtoxin tablets, pellets, prepacks, plates and strips depends on;

• Type of storage (gastightness requirements)
• pests to be controlled
• prevailing temperature inside the commodity.

The following tables indicate the manufacturers recommendations regarding applied dosage:

NO:

• When applying pellets by using the automatic dispenser, the dosage represents one third of that applied manually in the grain stream because of the very uniform distribution. An automatic dispenser is now available for Phostoxin round tablets with a hopper capacity of 6 kg ( = 2000 tablets) and with a dispensing plate with a maximum of 20 holes, 1150 tons of grain can be treated.
• Silos made of wood or loosely constructed metal or not very gaslight and consequently even a considerable increase in dosage will not achieve 100% kill. Such containers need to be sealed or a polyethylene film can be used to completely envelop the container, which results in a considerable saving in applied dosage.

For resistant insects such as T. granarium and S. granarius, the highest recommended dosage for each storage situation should be used.

For more susceptible insects, such as. Tribolium spp., or any type of moth, the lower dosage should be sufficient.

For mite control, the highest recommended dosage for each respective storage situation should be increased by 20%

At temperatures of greater than 25°Crespective inside the commodity the dosages in the lower range of the respective storage situation can be used.

Table 2. Recommended dosages when using Degesch plates and strips*.

Notes on Table 2.

* Degesch plates and Strips are magnesium phosphide based formulations that do not contain ammonium carbamate, and therefore do not liberate ammonia (NH3) or carbon dioxide (CO2) on the evolution of phosphine upon exposure to atmospheric moisture. Fumigation of fruits and vegetables (1 plate per 2040 m³ or 1 strip per 320-640 m³) can therefore be carried out without any adverse phytotoxic effects.

The active ingredients are embedded in an inert, plastic matrix and fabricated in the form of a semi-rigid plate covered on both sides with a moisture-permeable paper, with each plate or strip being sealed in gaslight foil pouches.

Once removed from the foil, hydrogen phosphide is evolved after a delay period of around 112-1 hour. (unlike tablets and pellets, the plate or strip formulations are not dependent on temperature and moisture (RH) for evolution of PH3, which will occur at O°C and 10% RH.

Degesch plates weigh 206 9, are 280 x 170 x 5 mm in size, each tin contains 32 sealed plates (3 tins per wooden case), and evolve 33 9 of PH3 per plate.

Degesch strips weigh approximately 3.2 kg, are 4,480 x 170 x 5 mm in size, are composed of 16 plates, come sealed in tins (2 strips per tin, 3 tins per wooden case), and evolve 528 9 of PH3 per strip.

The advantages of plates and strips when compared to tablets and pellets are:

• easy and quick application to large volumes
• simple collection after fumigation
• conveniently disposable and no contamination
• conveniently disposable and no contamination of treated commodities.

EXPOSURE PERIODS:

The required exposure time for an effective fumigation using phosphine generating formulations is dependent on;

• temperature inside the commodity
• humidity
• types of commodity being treated
• pest species to be controlled

The determining factor in each case is the MINIMUM COMMODITY TEMPERATURE.

Under no circumstances should the recommended minimum exposure periods be reduced, and under normal operating conditions, extended exposures beyond those prescribed in Table 3, is considered advantageous in achieving more efficient control using phosphine-generating formulations.

Very dry grain or densely packed commodities (such as tobacco in casses or hogsheads) requires longer exposures. For the control of Sitophilus spp., exposures should be extended to 10-14 days, while for mite infested grain, the grain should the grain should be left under gas for up to 10 days.

The following table gives approximate guidelines for the necessary exposures needed for commodity disinfestation.

Table 3

Notes on Table 3:

A

• Phostoxin tablets, round tables and pellets are composed of 56% aluminium phosphide; 26% ammonium carbamate (approx); 3% edible parrafin; and the rest (18%) inert materials. The release of ammonia in the proprietary formulations is phytotoxin.
• Decomposition of tablets and pellets in air leaves approximately 1.5% AIP and 1.2% AIP residues in dust (calculated as PH3). Maximum concentrations of PH3 are realized after 48-72 hours with tablets, and 12-48 hours with pellets (because of their greater surface area).
• At 60% RH and 20°C, AIP containing products have released only 45% of their PH3 content in 24 hours.

B:

• Magtoxin tablets, round tablets and pellets are similar in composition (and therefore phytotoxic) except that aluminium is replaced by magnesium in the proprietary formulation.
• Decomposition is almost complete (99.9%) and therefore only a small amount of PH3 (0.1 %) remains in the dust residue.
• In both cases, the trace amounts of PH3 decomposes almost completely when the grain is moved. For grain and similar products, there is an established residue limit of 0.1 ml.m^-3 (0.1 ppm) expressed as PH3, by the FAD/WHO Codex Alimentarius Committee.
• At 60% RH, 20°C, almost 75% of the total PH3 content of Magtoxin formulations has been released in 24 hours. The maximum concentrations of PH3 is reached in 24-36 hours, therefore exposure times can be reduced. Under best gas conditions and depending on the pests to be controlled, the application can be realized in a minimum of 2 days.

C:

• Round tablets, because of their spheroid shape, have an extreme hardness, matching that of Phostoxin and Magtoxin pellets (crumbling and dusting due to abrasion is reduced to 0.3% minimum). This allows packaging in resealable flasks, available in sizes of 100 tablets (300 g. nett) and 500 tablets (1500 g. nett) for Phostoxin; and 70 tablets (210 g. nett) and 350 tablets (1050 g. nett) for Magtoxin formulations.

D:

• Degesch Plates and Strips do not contain ammonium carbonate and are therefore safe to use on fruits and vegetables.
• The release of PH3 is less dependent on temperature and relative humidity as for Phostoxin and Magtoxin formulations.
• Maximum gas readings are normally achieved within the first 24 hours, so the minimum exposure periods can be significantly reduced, if deemed necessary.

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