Chemical control techniques

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General considerations

The chemical compounds, including both fumigants and contact insecticides, which are approved by FAD/WHO for use on food grains to control storage insects, are regularly reviewed. Their safety in use, for pest control operators and food-grain users, is carefully considered before approval is given and that approval may be withdrawn if new circumstances indicate the need for the exclusion of a particular compound. In general, the contact insecticides that are approved for use are compounds of relatively low mammalian toxicity, which are considered to be non-hazardous when applied at prescribed dilution rates for the purposes indicated. They are also relatively safer to handle than many of the pesticides quite commonly approved and widely used for preharvest pest control. Nevertheless, transportation and handling of the insecticide concentrates are hazardous. These hazards and the required precautions will be discussed further at the end of this section. At the outset, however, Tables 8.5 (see Table 8.5. Acute mammalian toxicities (LD50 - mg/kg body weight) for contact insecticides currently of use in stored-Brain insect control.) and 8.6 (see Table 8.6. Maximum residue limits (MRL) and acceptable daily intake levels (ADI) (mg/kg or ppm) recommended by FAD/WHO as at April 1992.) may be consulted for relevant data on mammalian toxicity, for a broad range of contact insecticides which are or have been used in the control of storage insects, and for the limits recommended by FAD/WHO for chemical residues, from contact insecticides and fumigants, in grain and grain products. It should be noted, in passing, that there is no acceptable residue for the insecticide DDT, which remains available in many developing countries but is no longer recommended for use in stored-grain pest control. Lindane, which is still of some use, is given a very low residue limit (0.5 ppm). This serves to preclude or discourage application to grains for export to countries which object to its use. It should also be noted that the limits for fumigant residues broadly represent the maximum levels that should be expected if fumigation treatments are properly done. This applies also to residues of inorganic bromide and these have recently been declared of no toxicological concern by the U.S.A's Environmental Protection Agency and by the British Government. The very low limit for phosphine in Table 8.6 reflects the generally negligible permanent residue which remains after the use of this fumigant.

A recent development affecting the distribution and use of pesticides has been the publication, by FAO, of a Code of Conduct (Anon., 1990). The purpose is to indicate responsibilities and establish voluntary standards of conduct for all public and private bodies engaged in or affecting pesticide distribution or pesticide use. It is particularly directed to countries where national regulatory legislation is lacking or inadequate. It draws attention to a comprehensive series of guidelines, also published by FAO, on regulatory practices, packaging and storage of pesticides, labelling of containers, disposal of waste pesticides and their containers, and other pertinent matters including especially the Prior Informed Consent (PIC) Procedure. This aims to protect the right of countries importing pesticides to be made fully aware of bans or restrictions placed on particular pesticides, elsewhere, so that informed decisions may be made on whether or not further importations of such pesticides should be permitted. Insecticides covered by the PIC procedure include, for example, DDT and lindane which have been mentioned already as compounds which remain in use in several developing countries although banned or severely restricted elsewhere.

Current usages for stored-grains pesticides, including fumigants and contact insecticides, constraints on their use and the ways in which chemical pest control may be integrated into storage systems are subjects which have received much attention. The following account may be usefully augmented by reference to various publications including, for example, Champ and Highley (Eds.) 1985.

 

The use of fumigants

Fumigants are toxic gases used to disinfest a commodity in an enclosure which, ideally, is completely gaslight. Fumigation enclosures should certainly be sufficiently gaslight for the gas to penetrate and remain in the commodity for long enough to kill all stages of the insects present in or amongst the grains. A gas or vapour that does not have the ability to penetrate the grain is not, strictly speaking, a true fumigant.

The purpose of a fumigation is thus to obtain a more-or-less immediate disinfestation of the commodity and the space enclosing it. Fumigation is the only chemical treatment that can achieve this effect and this relative immediacy of disinfestation, together with its completeness if done properly, are the main advantages of this particular chemical control technique. Its main disadvantages are that the treatment confers no residual protection against reinfestation, once the commodity is again exposed, and the fact that the most effective fumigants are all highly toxic to humans and other non-target organisms. The precautions required to ensure the safe use of fumigants are, necessarily, much more stringent than those required to ensure the safe use of most other insecticides.

The list of fumigants in Table 8.6 excludes those that are no longer widely approved for use on stored grain due to restrictions placed upon their use in some countries. Examples are carbon tetrachloride and ethylene dibromide, both of which are low-volatility fumigants with recently identified chronic user hazards. Another low-volatility halogenated hydrocarbon, ethylene dichloride, is not so clearly implicated but is less commonly available than it was formerly. Methyl bromide and phosphine are now the only fumigants commonly in use on a world-wide scale. The advantages and disadvantages of these two fumigants are summarised in Table 8.7. In addition, it should be noted that both phosphine and methyl bromide are currently regarded as gases with potential negative impact on the atmospheric environment. Constraints on their use are likely to increase and requirements for careful, responsible use, with more regular monitoring of application rates, are likely to be more strictly enforced.

Table 8.7. Phosphine and methyl bromide as fumigants: advantages (highlighted) and disadvantages.

Phosphine Methyl bromide
Easy to transport Refillable cylinders are expensive to transport
Easy to apply Difficult to apply, requiring special equipment and skill
Good penetration and distribution Distribution rather poor
Taint, residues and loss of viability in treated seeds are generally negligible Sorption occurs and may cause taint, bromide residues and loss of viability in treated seeds
Slow acting, particularly at low temperatures and humidities* Rapidly toxic and widely effective even at lower temperatures
Flammable: spontaneously explosive ignition can occur in some circumstances Non-flammable
High acute mammalian toxicity but low chronic toxicity Dangerous acute and chronic poison with delayed symptoms
Fairly easy to detect Very easy to detect
Rapidly lost by leakage unless fumigation space is well sealed and gas tight soon after application Needs very good seeing before application

* Not recommended for use at temperatures below 12C.
Source: Adapted from Pest Control for Food Security, FAO Plant Production and Protection Paper 63 (Prepared for FAO by ODNRI), FAO, Rome (1985).

The desirable properties of a grain fumigant, notably efficient penetration of the commodity, toxicity to target insects and lack of harmful residues, make it unlikely that new chemical compounds will become available as fumigants (Taylor, 1991). Carbon dioxide can be used as a conventional fumigant but low toxicity to insects and the consequent high degree of gastightness necessary for effective insect control makes it unlikely that this gas will find widespread use except in controlled atmosphere (CA) storage systems.

Detailed information on the properties and use of phosphine and methyl bromide as grain fumigants, including application procedures for fumigations in various types of fumigation chamber or under gas-proof sheets, is included in a separate FAO publication (Bond, 1984) and is not reproduced here. General guidance on dosage rates, however, is given in Tables 8.8a and 8.8b. It should be noted that, for phosphine, there are considerable differences in tolerance amongst the various insect pests of stored grain. The data in Table 8.8a (see Table 8.8a. Average concentrations of phosphine (mg/l) required to give 100 per cent mortality of all developmental stages of insects under experimental conditions.) are intended to illustrate this rather than to indicate practical dosages. A general dosage recommendation is given beneath the table. For methyl bromide, differences in the amount of gas sorbed by particular commodities are generally more important and these are taken into account by the schedules presented in Table 8.8b (see Table 8.8b. Dosage schedules for fumigation with methyl bromide where the enclosed volume is filled, e.g. stacks under gas-tight sheets.).

Notes: The dosage rate per tonne can be read directly from the table according to the commodity.

Recommended dosages are also given as g/m3 for situations where the volume, but not the weight, of the commodity is known. The volume dosages have been obtained by dividing the dosage per tonne by the stowage factor. These dosages are alternatives and should not be added together.

Where Trogoderma spp. are present, dosage should be increased by 50 percent.

If a 48hr exposure period is reduced to 24 hours the dosage rate should be increased by 50 per cent. If a 24hr exposure period is increased to 48 hours the dosage rate should be reduced by not more than 30 per cent.

Where stacks of less than 30m3 (approximately 20 tonnes) are treated under sheets, dosages should be calculated as if the volume were 30m3 (20 tonnes).

Adapted from: Pest Control for Food Security, FAO Plant Production and Protection Paper 63 (Prepared for FAO by ODNRI), FAO, Rome (1985)

Phosphine, because of its availability in solid formulations of metal phosphides which are relatively easy to apply, compared with the pressurised gas fumigant methyl bromide, has become the most popular and widely used fumigant in most tropical countries. Methyl bromide, which is in some ways more versatile, retains its place as the fumigant of choice wherever circumstances do not easily accommodate the protracted fumigation period, of several days duration, that is required for the effective use of phosphine.

The further prolongation of recommended exposure periods for phosphine, beyond the three day minimum that was formerly recommended for hot climates, followed from extensive investigations into the susceptibility of the developmental stages of storage beetles (Hole, et al. 1976). The pupal stage of grain weevils was found to be remarkably tolerant but other life stages were shown to be sufficiently susceptible to permit effective use of phosphine if the minimum exposure period were extended to 4 days, at favourable temperatures, to allow the tolerant pupae to pass into the more susceptible adult stage.

The growing frequency of resistance to phosphine in storage insects constitutes a problem, previously discussed, but does not generally invalidate the use of this fumigant which can still be expected to provide effective control of the major pest species when treatments are carried out using proven techniques (Taylor, 1991). Problems may arise where control measures against psocids are warranted. Considerable tolerance to phosphine, in all the life stages but especially the egg, has been demonstrated in the common species Liposcelis entomophilus (V. Pike, personal communication). The same investigator has shown that the currently available alternative fumigant, methyl bromide, should prove effective at normal dosage rates whereas effective phosphine treatment would require an extension of the exposure period beyond the normally practicable limits for sheeted-stack fumigation. Tolerance to phosphine in the egg stage has also been observed in other insects (Hole et al., 1976; Bell, 1976) but this does not generally persist throughout egg development as it appears to do in L. entomophilus. It is this persistent tolerance, throughout a 6-9 day developmental period (at 27C), which makes phosphine unreliable for psocid control. It may also explain the rapid and spectacular resurgence of psocid infestation, following phosphine fumigation and the elimination of susceptible predators and competitor species, in those grain storage situations where this phenomenon has been observed.

Practical constraints on the use of fumigants to treat stored grain include consideration of the chemical residues which they may leave in the treated grain and the effects which such residues, or the treatment itself, may have on grain quality. For seed grain this includes germinability and seedling viability. In this regard phosphine has considerable advantages and is certainly to be preferred over methyl bromide for seed treatment. It is also less commonly associated with problems due to persistent sorbed chemical residues. Problems can arise from the visible residues of the metal hydroxide which remain after the decomposition of tablet or pellet formulations. Moreover, these usually contain some undecomposed phosphide, which can also be found in spent satchels and other application packets. However, the hydroxide material itself is not harmful and the risk posed by undecomposed phosphine can be sufficiently minimised if recommended procedures are followed.

The available advisory literature on fumigation procedures relates mainly to relatively large-scale applications in warehouses and other storage complexes. Possible small-scale applications in tropical developing countries, at farm level or by urban traders, should not be disregarded. Such operations were, in the past, largely limited to the occasional use of low-volatility halogenated hydrocarbons: notably various mixtures of ethylene dichloride with carbon tetrachloride. Such formulations may still be available in some countries but their use is now generally discouraged because of recently identified long-term user-hazards. Methyl bromide and most other high-volatility fumigants are generally precluded by the much greater acute toxicity hazards and by the recognised need for special equipment and training for users. The advent of phosphine, however, increased the likelihood that fumigation treatments would be attempted by untrained people. The relative ease of handling the solid formulations of this fumigant, especially the familiar tablets and pellets, greatly facilitates their retail distribution, sometimes without the manufacturer's protective packaging, in any country where effective curbs on such distribution are not in place. Extension workers as well as opportunistic salesmen are sometimes at fault in this regard and, in consequence, phosphine treatments of small farm-level stocks of grain, or of larger quantities in traders' stores, may be carried out ineffectively and may in some instances be a serious hazard to the user or other people. Inefficient use of phosphine, as has been mentioned already, will also exacerbate the insect resistance problem. Proposed efforts to monitor phosphine use and to promote effective techniques should be extended to include small-scale applications and should give full attention to associated hazards. Where necessary, tighter controls on the sale and use of phosphine should be introduced and applied.

 

Developments in fumigant application techniques

(i) Store fumigation

The concept of fumigating the free space and entire contents of a store, rather than individual stacks of bagged grain, is not new and has been practiced regularly for many years, particularly in South Asia. This method of disinfestation has the potential advantage of controlling insects on the walls, floors and inner roofing surfaces, as well as in the grain, thus greatly reducing the immediate re-infestation pressure on the store contents.

Unfortunately, most whole store fumigation in the past has been carried out in buildings that were not designed specifically for this method of disinfestation. As a consequence, most were not capable of retaining fumigant gas sufficiently well to provide complete control of insects. There seems little doubt that whole store fumigation has encouraged the development of insects that are resistant to phosphine.

Recent investigational programmes have demonstrated that purpose-built storage buildings (Bisbrown, 1992) can serve effectively as fumigation chambers. In Sahelian West Africa, for example Senegal (Hayward, 1981), such stores already exist. However, there is little evidence that they are regularly used for that purpose.

Where existing storage buildings can be sealed to render them reasonably gas-tight, investigations have shown that effective fumigation can be achieved using a method of phased dosing with aluminium phosphide. The method involves application of fumigant in two portions, the second of these 24 or 48 hours after the initial application. Using this technique it is possible to prolong the period during which insects are exposed to a lethal concentration of fumigant, even in buildings in which some leakage of gas is taking place (Friendship et al., 1986).

(ii) Sheeted stack fumigation

In most developing countries, the commonest method of fumigating stored commodities is with bag stacks under sheets. The technology involved is relatively basic and good standard recommendations are available, including detailed advice on choosing suitable sheets (Friendship, 1989). Nevertheless, many fumigations of this type are carried out unsatisfactorily. Common reasons for treatment failure are the use of torn or perforated fumigation sheets, which allow fumigant to escape, or poor sealing of sheets at ground level which also allows excessive leakage. The most common method of sealing sheets at ground level is by means of tubular sandbags ('sandsnakes') which hold down the sheet in contact with the floor. Frequently, insufficent of these are provided to permit satisfactory sealing, or the sandsnakes are too small or too lightweight to effect a gas-tight seal. Proper sealing of sheets requires sandsnakes to overlap continuously around a stack, with at least two sandsnakes over the folded sheet corners. Latest experimentation suggests that for effective sealing of heavy-duty (and less flexible) sheets, such as those of laminated PVC, larger and heavier sandsnakes are necessary than those commonly used. The width of tubing used for the larger sandsnakes should be of the order of 150 to 200 mm. These, when filled, should provide a contact width on the floor of at least 100 mm. A disadvantage of this type of sandsnake is the increased weight, which is an important consideration for pest control teams with frequent operations or much travelling to do. It is therefore advisable to ensure that the heavier type of sandsnake is not too long, and is fabricated from strong material such as lightweight canvas. Where possible, sandsnakes should be provided for each individual store or store complex to avoid the need for further transportation.

(iii) Circulatory systems for phosphine

A recently introduced and patented technique known as 'Phyto-Explo Fumigation' enables bulk grain to be effectively treated in deep structures using phosphine. A shaft is driven into the grain, using compressed air, and is connected to a piping system which allows air circulation within the grain by means of a small pump. Fumigant is evolved from a phosphide formulation introduced into the headspace above the grain and gas is drawn down into the grain by the circulatory action of the pumping system. This technique permits effective distribution of fumigant in deep silos and in ships holds, rendering disinfestation possible without transferring the grain. The same technique can be used with methyl bromide enabling rapid treatment of silos not provided with a permanent circulatory system.

 

The use of contact insecticides

Currently acceptable compounds, and recommended rates for their application as dust formulations admixed with cereals or as liquid surface treatments, are given in Table 8.9. Compounds used for space treatments, and their recommended application rates, are given in Table 8.10. These two tables (and Tables 8.5 - 8.8) are reproduced from FAO Plant Production and Protection Paper 63 (Anon., 1985), a manual of pest control for food security reserve grain stocks prepared for FAO by the former Storage Department of TDRI (now NRI). This contains detailed information on application procedures and equipment for fumigants and contact insecticides.

Most reputable insecticide manufacturers also provide useful literature on application rates for their own products together with appropriate safety precautions which should be followed. Some also indicate suitable application equipment and there are many other publications, with or without commercial bias, which give comprehensive guidance on the various spray-pumps, mistblowers and fog generators that are available. The choice of a particular piece of equipment is generally less important than the care given to its use and maintenance. The best advice to give here is that the choice should be made, on the basis of information obtainable from accessible sources, with particular regard to cost, availability of spare parts, the user's own assessment of suitability for the purpose and the apparent robustness of the equipment.

The focus of attention in this bulletin is upon the differences between the various types of insect control treatments, i.e. the application techniques, with regard to pest control objectives and the constraints which limit effectiveness in particular circumstances.

Table 8.9. Recommended insecticide application rates.

Insecticide Dust admixture with cereals (ppm) Surface treatments (g/m)
Walls Bags
Malathion 8-12 1-2 1-2
Pirimiphos methyl 4-10 0.5 0.5
Fenitrothion 4-12 0.5 0.5-1
Chlorpyrifos methyl 4-10 0.5-1 0.5-1
Dichlorvos 2-20* 0.5  
Methacrifos 5-15 0.2 0.4*
Lindane 0.5    
Pyrethrin/piperonyl butoxide (1:5) 3 0.1  
Bioresmethrin (resmethrin) 2    
Phenothrin 5    
Permethrin 0.05-0.1 0.05-0.1  
Carbaryl 5-10 1-2  
Bendiocarb 0.1-0.2 -  
Dioxacarb 0.4-0.8 -  
Propoxur - 0.5 -

Notes: * Short persistence. '-' The insecticide is not normally used in that type of treatment.

Source: Pest Control for Food Security Plant Production and Protection Paper 63 (Prepared for FAO by ODNRI) FAO, Rome (1985).

(i) Grain admixture treatments

Admixture treatments depend upon reasonably uniform application of a suitable contact insecticide, or in some cases a mixture of insecticides, at an acceptable dosage level. Table 8.9 gives application rates in parts per million (ppm) of the active ingredient for a range of commonly used insecticides applied as dusts (dusting powders) on grain. Such formulations are generally recommended for small-scale treatments because dusting powders are fairly easily supplied, ready for use, in suitable small packs and are more easily applied. Liquid formulations can also be used, if suitable application equipment is available, and these are generally preferred for large-scale treatments. This is especially true in commercial grain storage, mainly because the admixture of dusts with grain alters the bulk density and may affect grading standards but also because spray applications are more easily automated and incorporated into grain conveying systems.

In either case the application rates specified in Table 8.9 apply and it is most important to understand that these rates are for the active ingredient (a.i.). It may help to think of these as a.i. dosage rates to distinguish them from the formulation application rates. These latter are more or less standard, at 50g or 100g of dusting powder per 100kg of grain or 1-2 litres of the dilute spray-mix per tonne of grain. For liquid treatments the most convenient carrier is water and the application rate is designed to allow effective treatment with minimum added water. A simple calculation shows that for grain treated at 1 litre/tonne with a water-based spray the moisture is increased by approximately 0.1%. The calculation of the required concentration of active ingredient in the spray-mix or dusting powder, to give the recommended dosage rate in ppm, weight-for-weight on the grain, is also quite straightforward. The simplest possible example is that an a.i. dosage of 10ppm on grain will require a 2% dusting powder applied at 50g/100kg grain or a 1% powder applied at 100g/100kg. Likewise, it will require an approximate 1% spray-mix for an application rate of 1 litre/ tonne.

The advantages of insecticide admixture treatments are that they are generally inexpensive and a single application of an effective insecticide, correctly formulated, will give control of existing insect infestation (including, eventually, any insect stages within the kernels) and will protect the grain against reinfestation for a substantial period. The duration of protection varies considerably between different insecticides and, more importantly, between different climatic conditions. In the tropics, relatively high grain temperatures may reduce performance to some extent, although the preferred insecticides will generally give good results for several months. High moisture content (above the recommended 'safe storage' level) in the treated grain will more seriously impair the performance of some insecticides: notably malathion and fenitrothion. However, it is usually possible, by good storage management, to delay the treatment of grain until it is sufficiently dry and this should be the objective (Daglish and Bengston, 1991).

Disadvantages of admixture treatments include the effect of admixed powders upon the bulk density of the grain, but in many situations this is unimportant, or the risk of overwetting the grain if water-based sprays are used carelessly or in unreliably automated spray-rigs. However, the practical problems of ensuring the availability of stable insecticide formulations, especially with ready-to-use dusting powders, have proved to be the major constraint on successful widespread use of the technique. Malathion in particular is very prone to instability if formulated as a dilute dusting powder (usually at 2% w/w) on an unsuitable carrier. Many of the more recently introduced insecticides, including for example pirimiphos-methyl ("Actellic") and the synthetic pyrethroids, appear less prone to this problem. However, effective quality control on dilute dusting powders, which are mostly formulated locally to avoid heavy transport costs, is essential to the success of insecticide admixture treatments recommended for use at the small farm level in developing countries. Formulations should be monitored for stability and to ensure that the nominal concentrations are initially correct. Distribution channels should also be controlled to ensure that retail packets are withdrawn before sale if they have been in stock for longer than the predicted shelf-life.

(ii) Insecticide deposits on bulk grain surfaces and bagstacks

Spraying the surface of a bulk of uninfested grain, in a bin or in flat bulk storage, can give quite good protection against reinfestation for a limited period, depending on the persistence of the insecticide used. Application rates would be similar to those indicated for other surface treatments in Table 8.9. For sustained protection the treatment would have to be repeated rather more frequently than is usually recommended. The decay of insecticidal effectiveness on exposed surfaces is generally faster than in a bulk treated by admixture and re-spraying at intervals of more than 1-2 weeks is likely to allow a limited build-up of infestation which, once established in grain below the surface, will be largely unaffected by retreatment. In practice, control of warehouse moths is often quite well achieved but control of beetle pests is generally less effective. Quite good control of the grain moth, 5. cerealella, is also likely since this insect is unable to penetrate far below the surface of a grain bulk.

An alternative treatment, for the same purposes, would be the use of a dusting powder applied to the surface and raked-in to a depth of 10-20cm. The application rate should be as indicated in Table 8.9, based on the estimated grain weight in the treated surface layer. Insecticidal sprays and dusting powders applied as surface treatments to protect fumigated bagstacks against reinfestation are also of limited effectiveness. Early work showed that layer-by-layer spray treatments, applied during the building of a bagstack and immediately prior to fumigation, could be reasonably effective. For example, malathion, applied at about 1g/m2 in a water dispersible powder formulation, gave complete protection against T. castaneum, in tropical conditions, for 1-2 weeks and a useful degree of control for 4-6 weeks (McFarlane, 1961). Respraying of exposed surfaces, at monthly intervals, was suggested for longer storage periods. However, in practice, the initial layer-by-layer treatment is rarely if ever used and bagstack spray treatments are generally limited to the exposed surfaces with re-application at monthly intervals or even less frequently. A broad concensus of opinion, based on observations in practical storage situations, regards these treatments as generally ineffective in tropical climates. Where they are used, even on a regular basis, the need for periodic refumigation to combat resurgent infestation is not avoided. There is considerable evidence that this resurgence is due to reinfestation at the stack surfaces although faulty initial fumigation may sometimes be partly to blame.

(iii) Insecticide deposits on the fabric of grain stores

The notional contribution made by fabric treatments to the sustained control of insect infestation in warehouses has rarely, if ever, been confirmed in practice. On the other hand, it is considered likely that they do contribute substantially to the build-up of insect resistance to pesticides.

Recent trials in grain storage warehouses in Java (Hodges et al., 1992) found no substantially significant differences in the resurgence of pest populations, following the fumigation of all bagstacks and an initial spray treatment of the warehouse fabric using fenitrothion at 1g/m2, between warehouses with routine, monthly respraying of walls, or walls and floors, and those with no respraying treatment.

There can be little doubt that properly applied surface treatments of walls and floors, using a recommended insecticide at the correct application rate (Table 8.9), will kill many, if not all, of the insects exposed to the insecticidal spray or to the residual deposit immediately after the treatment. However, although a persistent insecticidal effect can be found on some surfaces, for many days and even for many weeks, it is inevitably a declining effect. Actual efficacy, in terms of insect control, is unlikely to be very great. As noted by Hodges et al. (1992) many of the insects which enter a store in the tropics do so in flight and the proportion of these that will settle upon a sprayed surface for long enough to be killed by the diminishing insecticide deposit can hardly be very great.

The practical value of these treatments may be considerable when they are used as a supplement to physical cleaning, in an unloaded store, to kill insects which may remain on the fabric of the store even after reasonably thorough sweeping. Repetitive use, as an alternative to more effective measures to control infestation in the stored grain, are of little value and may, conversely, have negative effects in the long term by accelerating the development of resistance to the insecticides used.

(iv) Space treatments

This term is used to describe insecticidal treatments, by aerosols or vapours, intended to kill insects exposed to the treatment in the free space of a store or other enclosure to which the treatment is applied. They are thus quite distinct from true fumigations and cannot be expected to disinfest commodities within the enclosure.

Space treatments, to be effective, require reasonably good sealing of the enclosure which should certainly be made windtight. Complete gastightness is not essential.

Most of the insecticidal formulations that have been employed for space treatments leave a small residual deposit upon exposed surfaces and may have a slight persistent insecticidal effect. However, this is generally negligible unless the treatments are applied repetitively and frequently. Space treatments achieve most effect through their direct impact on insects in flight or trapped on exposed surfaces during the treatment. In general, and probably for this reason, they appear to be most effective against warehouse moths and some other insects (such as the beetle L. serricorne) that spend little or no time concealed within a commodity bulk or bagstack. For maximum effect, even against these more susceptible species, space treatments should be applied regularly and frequently: preferably daily at dusk when insects are generally most active in flight.

Aerosols containing pyrethrins, with or without a synergist, applied as thermal 'fogs' or mist-sprays ('cold fogs') were previously the treatment of choice in stored-grain pest control but cheaper alternatives are now more often used. Various synthetic contact insecticides can be used in aerosol formulations and one compound, dichlorvos, can be used effectively as a vapour treatment (McFarlane, 1970; Ashman et a/., 1974) in those countries where its use is approved.

Application rates for space treatments are given in Table 8.10 (see Table 8.10. Application rates for space treatments.). The mention given to lindane and pirimiphos methyl 'smoke' treatments relates to the use of the solid particle aerosols commonly referred to as smoke generators. Lindane 'smokes', in particular, have been shown to have a considerable residual effect, when applied frequently or at the higher dose indicated in Table 8.10, but the use of this insecticide in circumstances where its residues may accumulate in foodstuffs is not acceptable.

 

Safety precautions

Wherever toxic chemicals are used in pest control the maintenance of safety should have highest priority. This relates to those handling or applying the chemicals and also to those other persons or animals that may be affected, directly or indirectly, by the chemical treatments. FAO guidelines, previously mentioned, on many relevant asects of safety in use are available from that source.

Detailed information on safety procedures in the use of chemical pesticides is contained in other FAO publications; notably Bond (1984) on the use of fumigants and Anon. (1985) on both contact insecticides and fumigants. All those involved, in any way, in the promotion, planning or implementation of chemical control measures should be familiar with the recommended procedures and should ensure that all appropriate precautions are observed in the situations and circumstances for which they have responsibility.

In addition, the specific precautions recommended by pesticide manufacturers for the use of their products, which are or should be clearly drawn to the attention of the user on all product labels, should be observed by the user. Local sales agents should be required to ensure that hazardous materials are not retailed to users who may be unable to read or understand the accompanying information on application rates and safety precautions. The only exception that should be made to this is where another competent agency takes full responsibility for providing the necessary verbal instruction and practical training to potential users.


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