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In recent years attention has been focussed on the nature and possible effects on human beings of insecticidal residues appearing in foodstuffs. World-wide interest in this problem is reflected in the fact that international organizations such as the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO) have set up special committees to investigate and report on the nature and significance of residues formed in foodstuffs as the result of the application of pesticides at different stages (as seed dressings, during growth, storage, transportation, etc.) prior to human consumption. These special committees review a number of pertinent factors involved in the use of each pesticide. Important factors, among others, are the toxicological significance of any residues formed and the average fraction of the total diet likely to be constituted by a food containing this residue. Through their Codex Alimentarius Committee these organizations undertake "to recommend international tolerances for pesticide residues in specific food products."
Such recommendations are not binding on Member Nations of these organizations but are intended to be used as guides when particular countries are formulating their own regulations for pesticide residue tolerances.
Fumigants may form residues when used on foodstuffs for insect control. In this manual the nature and significance of residues formed are discussed in Chapter 5 and under the heading of each particular fumigant in Chapter 6. No attempt has been made to list tolerances established by individual countries as these are altered from time to time. For current information it is necessary to consult the official publications on this subject.
A comprehensive review of fumigant residues has been given by Lindgren et al (1968).
Apart from the question of significant residues in foodstuffs, there is the problem of other effects which have a direct bearing either on the choice of the particular fumigant or on the decision as to whether fumigation is possible at all. The main types of reaction may be summarised as follows:
1. Nursery Stock and Living Plants
(a) Stimulation of growth
(b) Retardation of growth
(c) Temporary injury and subsequent recovery
(d) Permanent injury, usually followed by death
(a) Stimulation of germination
(b) Impairment or total loss of germination
(c) Poor growth of seedlings from germinated seeds
3. Fruit and Venetables
(a) Visible lesions
(b) Internal injury
(c) Shortening of storage life
(d) Delay of ripening
(e) Stimulation of storage disorders
4. Infesting Organisms
(b) Stimulation of growth or metamorphosis
(c) Delay in development
(d) Stimulation of symptoms of disease (so-called "diagnostic effect")
PHYSICAL AND CHEMICAL EFFECTS ON NONLIVING MATERIALS
1. Production of foul or unpleasant odours in furnishings or materials stored in premises.
2. Chemical effects that spoil certain products (for example, some fumigants render photographic films and papers unusable).
3. Reaction with lubricants followed by stoppage of machinery (clocks will often stop after fumigation with HCN).
4. Corrosive effects on metals (phosphine reacts with copper, particularly in humid conditions).
There should be a clear understanding of the difference between dosage and concentration.
The dosage is the amount of fumigant applied and is usually expressed as weight of the chemical per volume of space treated. In grain treatments, liquid-type fumigants are often used and the dosage may be expressed as volume of liquid (litres or gallons) to a given volume (amount of grain given as litres or bushels) or sometimes to a given weight (quintals, metric tonnes or tons).
From the moment that a given dosage enters the structure being fumigated, molecules of gas are progressively lost from the free space either by the process of sorption and solution described above or by actual leakage from the system, if this occurs. The concentration is the actual amount of fumigant present in the air space in any selected part of the fumigation system at any given time. The concentration is usually determined by taking samples from required points and analysing them. It may thus be said that the dosage is always known because it is a pre-determined quantity. Concentration has to be determined because it varies in time and position according to the many modifying factors encountered in fumigation work.
Three methods of expressing gas concentrations in air are in common use: weight per volume, parts by volume and percent by volume.
WEIGHT PER VOLUME
For practical designation of dosages, this is the most convenient method because both factors - the weight of the fumigant and the volume of the space - can be easily determined. In countries using the metric system, this is usually expressed in grammes per cubic metre (g/m³), whereas in countries using the British system of weights and measures, expression is usually in terms of pounds or ounces avoirdupois (avdp) per 1 000 cubic feet (lb/1 000 ft³ or oz/1 000ft ).
By a fortunate coincidence in units of measurement, grammes per cubic metro are, for all practical purposes, equal to ounces per thousand cubic feet. Thus, recommended dosages can readily be converted from one system to the other*. Conversion factors for the various units are given in Appendix 2.
In reports of laboratory experiments, dosages and concentrations are usually given in milligrammes per litre (mg/l), equivalent to grammes per cubic metre.
PARTS OR PERCENT BY VOLUME
Parts by volume and percent by volume will be discussed together because both modes of expression give the relative numbers of molecules of gas present in a given volume of air. The values for both modes have the same digits, but the decimal points are in different places (3 475 parts per million by volume of a gas is the same as 0.3475 percent by volume).
Parts per million of gases in air are used in human and mammalian toxicology and in applied industrial hygiene. Percent by volume is used in expressing the flammability and explosive limits of gases in air.
CALCULATIONS FOR CONVERSION OF CONCENTRATION VALUES
By means of simple calculations giving useful approximations, values may be converted from weight per volume to parts by volume and vice versa. These calculations take into account the molecular weight of the gas and the fact that, with all gases, the gramme molecular weight of the substance occupies 22.414 litres at 0°C and 760 millimetres pressure. (If precise values are needed for the other temperatures and pressures, corrections for absolute temperature and pressure may be made in the usual manner. )
A. To convert grammes per cubic metre (or milligrammes per litre or ounces per 1 000 cubic feet) into parts by volume.
1. Divide the given value by the molecular weight of the gas and multiply by 22.4; the resulting figure is the number of cubic centimetres (cm ) of gas per litre of air.
2. One thousand times the figure obtained is the value in parts per million by volume.
3. One tenth of the figure obtained in (1) is the percentage by volume.
Example. To convert 1g/m³ of PH3 (molecular weight 34 approximately)
|(1x22.4)/34||= .659 cm³per litre
= 659 parts per million by volume approximately
= .0659% by volume approximately
B. To convert parts per million (or percentage of volume) of gases to grammes per cubic metre (or milligrammes per litre or ounces per 1 000 cubic feet):
1. Divide the parts per million by 1 000, or multiply the percentage by ten to give the number of cubic centimetres of gas per litre of air.
2. Multiply this figure by the molecular weight of the gas in question and divide by 22.4.
Example. To convert 400 ppm of methyl bromide (molecular weight 94.95 = 95 approximately)
|400 ppm||= 0.04% of volume = 0.4cm(3)
= (0.4 x 95)/22.4
= 1.7g/m³ (or mg/l or oz per 1 000 ft³)
Comparative figures for weights and volumes at various levels have been calculated for the important gases, and these are given in the tables accompanying the subsequent discussion of each particular gas.
CONCENTRATION X TIME (c x t) PRODUCTS
Most fumigation treatments are recommended on the basis of a dosage given as the weight of chemical required for a certain space - expressed as grammes per cubic metre or pounds per 1 000 cubic feet or as volume of liquid applied to a certain weight of material - expressed as litres per quintet or gallons per 1 000 bushels. Usually, this designation of dosage is followed by a statement of the length of the treatment in hours and the temperature or range of temperature at which the schedule will apply. While such recommendations are usually based on treatments that have proved successful under certain conditions, they should also take into account the fact that certain factors may modify the concentrations left free to act against the insects. One important factor already mentioned is the effect of loads of different sizes (Figure 2). Another is the leakage from the structure undergoing treatment. What is really important is the amount of gas acting on the insects over a certain period of time. For instance, it is known (Bond and Monro, 1961) that in order to kill 99 percent of larvae of Tenebroides mauritanicus (L.) at 20°C, a concentration of 33.2 milligrammes per litre of methyl bromide must be maintained for 5 hours. The product 33.2 milligrammes per litre x 5 hours = 166 milligrammes per litre x hours is known as the concentration x time product needed to obtain 99 percent control of this insect (Figure 4). It can be abbreviated and referred to as the c x t product. In the literature it is often expressed numerically with the notation mg h/l (milligramme hours per litre) In this example it would be known as the lethal dose for 99 percent of the population, or the LD(99).
In order to apply this method of treatment designation to practical fumigations, it is necessary to make reasonably correct determinations of the fumigant concentrations required to kill the insects under certain specific conditions; important modifying conditions are temperature and humidity. One such determination is illustrated graphically in Figure 4. Note that in this figure the concentration curve tends to flatten out for short exposures at high concentrations and long exposures at low concentrations and at these extremes, which are not likely to be employed in practice, the constant value for the c x t product does not hold. To illustrate specifically the use of the data in Figure 4, Table 3 sets out the required concentration x time products to bring about 99 percent mortality of mauritanicus using methyl bromide at 20°C and 70 percent relative humidity for various exposures:
TABLE 3. - REQUIRED CONCENTRATION X TIME (c x t) PRODUCTS TO OBTAIN 99 PERCENT MORTALITY OF TENEBROlDES MAURITANICUS
|Concentration methyl bromide||Exposure||c x t product|
It must be emphasized again that before they are applied in practical use each product must be calculated for the different stages of an insect species at a certain temperature and humidity. Under practical conditions, variations in temperature are particularly important. In practice, several insect species or stages of a given insect may be treated and therefore the c x t product required is that which is effective against the most tolerant species or stage present in the system.
FIGURE 4. - Insect mortality and concentration x time products. A curve showing the relationship between concentration of methyl bromide and time of exposure against fourth instar larvae of Tenebroides mauritanicus for 99 percent mortality at 20°C.(Bond and Monro, 1961)
The value and possible application of the c x t product for the fumigation of insects has been investigated by a number of workers (see particularly Whitney and Walkden, 1961; Harein and Krause, 1964; Estes, 1965; Bell and Glanville, 1973; Bell, 1977a, 1978). The important modifying effects of temperature, humidity and the moisture content of the commodity are emphasized. Kenaga (1961) described the use of graphs to estimate the effective use of c x t products of eight different fumigants against Tribolium confusum Duv. under varying conditions of time and temperature. Heseltine and Royce (1960) showed how integrated c x t products of ethylene oxide and methyl bromide may be used in practice with the aid of specifically designed concentration indicators in the form of sachets.
The use of integrated c x t products is particularly useful in routine fumigations when the reaction of a particular species or groups of species has been carefully worked out under the range of conditions likely to be encountered. It has been used successfully in large-scale eradication campaigns (Armitage, 1955; Monro, 1958c).
Figure 5 and Table 4 show how an integrated c x t product of methyl bromide may be applied in dealing with a specific problem. In this instance a hypothetical situation is illustrated in simplified form to show how the method could be applied under more complex conditions with multiple gas sampling points. The target of the fumigation is an insect which requires for complete control, under the prevailing conditions of temperature and humidity, a c x t product of 190 gramme hours per cubic metre (9 h/m ), which is equivalent to 190 milligramme hours per litre (mg h/l).
Leakage from the 100 m³ structure and sorption by the commodity are two factors that in this instance influence the concentration of fumigant in the free space and thus within the commodity It is known that an initial dosage of 32 grammes per cubic metre (g/m ) may bring about the desired conditions for this load of commodity in a 12-hour exposure period if the concentration in the free space is maintained above 20 milligrammes per litre (mg/l) during the entire exposure. This nominal dose is introduced and concentration readings are made at regular intervals using a thermal conductivity analyser (see Chapter 4). Samples are taken from points in the free space and at the centre of the commodity then the data are plotted on a graph as shown in Fig. 5. At the beginning, particular attention is paid to the free space readings. After 2.5 hours it is clear that the free space concentration will fall below the stipulated 20 mg/l and 0.5 kg of fumigant are added to the system. Again, after a further 2.5 hours (total elapsed time 5 hours) another 0.5 kg are added to sustain tbe concentration. After 11.7 hours the desired c x t product of 190 9 h/m³ has been attained and the treatment is terminated by initiating aeration. The integrated c x t product obtained within the commodity, calculated from the concentration plot, is arrived at as shown in Table 4.
Recommendations based on the c x t product principle provide a sound means of ensuring that the treatment is adequate to control the insects.
FIGURE 5. - Chart of progress of a fumigation with methyl bromide designed to achieve a cumulative c x t product of 190 9 h/m3.
TABLE 4. - INTEGRATED CONCENTRATION X TIME PRODUCTS WITHIN THE INFESTED COMMODITY
Dosage schedules are, perhaps, best given in terms of weight of chemical required for a certain space for a specified period of time along with the c x t products necessary to achieve control. Thus by monitoring gas concentrations during treatment, an applicator can add gas, extend the exposure or make other changes necessary to ensure success. For plant quarantine work, recommendations based on the c x t principle are particularly valuable because they promote uniformity in standards and permit reliable certification of goods so treated. Schedules based on these concepts are in use in several countries, e.g. Plant Protection and Quarantine Treatment Manual (USDA, 1976). For other treatments of stored products, where sorption in the commodity is appreciable, schedules based on the c x t principle but given in terms of weight of fumigant per unit volume of space and per unit weight of goods for specified exposure times have been worked out for some commodities (Thompson, 1970).
While the c x t method is useful for most fumigants, it cannot be employed with phosphine. Although concentration and exposure time are still the main factors that determine toxicity of this fumigant, the length of the exposure time is of great importance. Phosphine is a slow acting poison that is absorbed slowly by some insects even at high concentrations (Bond et al, 1969). Therefore, high concentrations may not increase toxicity; in fact, they may cause insects to go into a protective narcosis, as described later in this chapter. In a phosphine fumigation certain minimum concentrations are required, and therefore gas analysis should be carried out to ensure the presence of sufficient gas. For most treatments the manufacturers' directions will provide adequate treatment if no excessive loss through leakage or sorption occurs and adequate periods are allowed under gas.
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