Methods of insecticide application for grain protection
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Romeo S. Rejesus
The choice of insecticide concentration, type and frequency of application depend on a variety of factors such as the species of pest present, the type of storage facilities, length of storage, local insecticide regulations, etc. Therefore, only certain generalized recommendations could be made.
Tables 2 and are a suggested guide for insecticide application (FAO World Food Program 1970; Bengston 1970; Morallo-Rejesus 1976). Dosage rates may be revised depending upon the local regulations, especially on the admixture treatment for food grain. The recommendation only serves as a guide, but the person in-charge is the one who will assess the requirements of any specific case, in order for him to choose the insecticide, dosage and method of application which best meet the situation.
RESIDUAL (STRUCTURAL) TREATMENTS
A residual spray is usually applied to inside surfaces of warehouses, storage bins, transport vehicles or other structural surfaces. A good residual spray should not only kill the insects should a deposit on the treated surface to kill walking Insects. Application may be made during the cleaning of storage facilities before intake of new stocks or to fit in with fumigation or spraying of the stock in storage. It is important to make sure that corners, ledges, cracks and other places difficult to get at are treated. The effectiveness of the residual deposit will decrease with time. The effective life of the deposit depends on the insecticide used, the climatic conditions prevailing and the type of surface sprayed.
Examples are fenitrothion, malathion, synthetic pyrethroids, tetrachlorvinphos, and chlorpyrifos methyl. WP are generally more persistent than EC but are less easy to apply. For most surfaces it is preferable to use the dispersible WP formulations of insecticides especially for absorbent surface such as cement, brick, stone or white washed surfaces. EC may be used on non-absorbent surfaces like metal or painted wood. Insecticides are much more persistent on wood. Malathion is not very satisfactory on alkaline surfaces, e.g. whitewash, bare, concrete or cement, but it has been shown to be effective for over 20 weeks on plywood and fiber board and has remained active through 16 up to 52 weeks.
Frequency of treatment depends partly on the insecticide used and partly on the infestation to be controlled. For example, Lindane and malathion treatments should be repeated at least every three weeks in tropical climates.
Pyrethroid and carbaryl are more effective than OP against R. dominica, while OP are more effective against Sitophilus spp. Azamethifos has been found effective against malathion-resistant strains of stored product insects. Azamethifos applied on wooden, galvanized sheets and concrete surfaces at concentration ranging from 0.25 gm/mē or 0.5 gm/mē is stable for 32 weeks. It is also effective against resistant Oryzaephilus surinamensis.
Space spraying or fogging of the warehouse is used to control infestations of flying insects that are not controlled by residual treatments, and of flying pests migrating from outside. It has to be carried out frequently and at a time of day when the pests are most active which is generally at dusk. The insecticides used are those with knockdown action. Examples are pyrethrin sprays and aerosols with or without synergists, synthetic pyrethroids with or without synergists, lindane smoke or fog, dichlorvos aerosols and strips.
Aerosols may be dispensed from the household type canister, containing a mixture of liqulfied gas and insecticide. The internal pressure "blasts" the insecticide into aerosol-sized droplets as the mixtures leave the nozzles. The sophisticated machines which produce aerosol droplets of insecticides are available in more advanced countries and will not be discussed here. Slow release dichlorvos plastic strip hung inside the warehouse at a density of 1 strip/30 cu m. space is also recommended to kill flying moths.
Grain protectants are defined as pesticides which are incorporated directly into the grain mass to protect it against insect and mite attack. This is also known as admixture treatment. The insecticides used as grain protectants are of low mammalian toxicity and are generally safe to use and need only simple equipment for their application.
Grain protectants are usually applied as sprays directed into the grain stream during the movement of the grain or at the beginning of long-term storage. The most suitable equipment for spraying grain on a moving belt is a machine with an electrically operated pump, feeding in insecticide through a pressure regulating valve and a precision nozzle. Where a power supply is not available, a pressure retaining knapsack sprayer, equipped with a pressure regulating valve and precision nozzle, may be used. EC are normally used but the volume must not exceed 2.5 liters per 1000 kilos of grain to avoid any appreciable increase in moisture content. It is better applied through a precision apparatus. This is most efficient in bulk handling but could be practical in bag handling system. The use of small pumps with coarse spray nozzles and low pump pressure produce large spray droplets and gives good results while minimizing spray drift.
In the drip feed system, tiny quantities of the concentrate are dripped directly into the grain stream through microcapillary tubes.
For a small scale treatment of bulk or bagged grain dusts it is best to be admixed with simple mechanical aids (rotating drum, shovelling, etc.) but adequate mixing is difficult to achieve.
Fumigation is a widely used procedure particularly for the control of stored product insects because the fumigants diffuse and penetrate into places where other forms of control are impractical or impossible. A fumigant is defined as a chemical which, at a required temperature and pressure, can exist in a prescribed period of time. Fumigation is the process of applying the gas under appropriate conditions to control the target organisms.
Materials with suitable characteristics for fumigation are limited. These vary widely in chemical composition and properties. consequently, the types of formulations, methods of handling, methods of analysis, and the purposes for which they are used vary considerably. The choice of fumigant depends largely on its ability to give effective and economical control of insects without adversely affecting the commodity.
All of the fumigants used to control pest organisms are toxic to human beings. They also may have other adverse properties - they may be highly flammable or corrosive; they may produce offensive odors; they may be phytotoxic; or they have leave harmful residues in food materials. Usually, adverse effects can be eliminated by choosing the most suitable fumigant for the particular treatment in question and by applying the proper methods of handling and use. A few of the fumigants are known to have or are suspected of having the potential for producing longterm chronic effects on human health and some are listed as carcinogenic. Appropriate precautions should be taken to avoid exposure to all fumigants and additional measures should be taken to prevent any contact carcinogenic compounds.
In applying a fumigant to a commodity, it is particularly important to carry out the operation in such a way that the insects are controlled without damaging the commodity and without creating any hazard for personnel. Effective methods of detection and analysis of the fumigant are especially important. Where any possibility of chemical exposure of personnel exists the atmosphere should be monitored with appropriate equipment and threshold limit values (TLV) such as those recommended by the American Conference of Government Industrial Hygienists (1983-84) and should be strictly adhered to. Misuse or abuse of fumigants can lead to accidents that endanger human life or property, and consequently may give adverse publicity to the practice of applying chemicals to food commodities for insect control.
The most common fumigants that are extensively used throughout the world are methyl bromide and phosphine. According to Champ and Dyte (1976), methyl bromide fumigation in general was the most efficient pest control operation. The greatest single deficiency was its integration into pest control programmes. In many instances, methyl bromide fumigation was heavily relied upon that hygiene and stock management were neglected. As a result, reinfestation occurred immediately after fumigation was completed. Thus, frequent fumigation was necessary and the bromide residue was in excess of the permissible tolerance limit. Most usage involved is fumigation of bag stacks under gasproof sheets and in bulk with recirculation of air. It is used where short exposure periods only are practical and grain has a moisture content less than 11 %. FAO recommends 10 mg/kg body weight as acceptable daily intake.
Phosphine is a very efficient fumigant and its use complements that of methyl bromide. It is preferred in horizontal bulks of grain that can be probed with tablets and verticle storage where methyl bromide cannot be used. Exposure during fumigation must be of adequate duration and at minimum concentration of gas.
The other fumigants used occasionally are hydrogen cyanide, carbon disulfide, ethylene dibromide, chloropicrin, methallyl chloride and carbon tetrachloride. For cereals in international trade a tolerance of 0.1 ppm expressed as PH3 is recommended.
Surface sprays are treatments applied to the surface of bulk grain or to the outer surfaces of bags. Examples are pyrethrum synergized with piperonyl butoxide, dichlorvos, malathion, pirimiphos methyl; chlorpyrifos methyl and tetrachlorvinphos.
Treatment of bag stacks. There are two methods of treatments: layer by layer (Sandwich method) and external stack treatment. The former method is also known as "sandwich" method where sprays or dusts are applied to each layer of bags during construction of a bag stack.
The external stack treatment usually consists of a spray application to the four sides and the top surface of a bag stack. This method is used to prevent reinfestation. Spraying is usually made immediately following a fumigation, or prior to sheeting in order to minimize the risk of cross infestation. Insecticides most commonly used for this purpose are malathion, primiphos-methyl and fenitrothion at 1-2%. Treatment of bag stacks are only capable of reducing infestations.
Sacks may either be sprayed (50 ml/mē) or dipped in insecticide solution before filling with the grains. Treated sacks should be airdried after treatment. Malathion, pirimiphos-methyl, and chlorpyrifos methyl at 2-4% were found effective. At 2% malathion is only effective for 2-4 months while pirimiphos-methyi and chlorpyrifos methyl were effective for 4-6 months. At 4% these two compounds can control infestation for 9-12 months.
CHAMP, B.R. and C.E. DYTE. 1976. Report of the FAO Global Survey of Pesticide Susceptibility of Stored Grain Pests. FAO Plant Protection and Production Series, No. 5. Rome, FAO, 297 p.
MONROE, H.A.V. 1969. Manual of fumigation for insect control. 2nd Res. Ed. FAO Agric. Studies. No. 7.
MORALLO-REJESUS, B. 1981. Insecticidal evaluations on stored grain insects in the Philippines. In "Pests of Stored Products". Proc. BIOTROP Symp. on Pests of Stored Products, 1978. Bogor, Indonesia. pp. 175-182.
SNELSON, J.T. 1987. Grain Protectants. ACIAR Monograph No. 3.x + 448p.
Pesticides and growth regulators
Pesticides and growth regulators are used extensively in the world to improve the yield and/or quality of crops, including grains. In most countries, legal restrictions as to the use of pesticides are in effect and different kinds of action can be started against possible violators of the regulations.
The general philosophy behind the use of pesticides is laid down in the concept of Good Agricultural Practice (GAP), which includes the following statements:
As climatic conditions differ from one region to another, GAP is not identical over the world. It is implied that goods produced under GAP should move freely in international trade, even if it carries residues of pesticides not in use in the importing country. This is only possible if enough infomation is available as to the toxicity of the pesticide involved.
International bodies can help to make this information available to local authorities. International work on pesticides in coordinated by the FAO/WHO Codex Alimentarius Committee on Pesticide Residues (CCPR). The secretariat of the CCPR is located in the FAO headquarters in Rome.
The incidence of pesticide residues depends on the way the pesticide has been applied and on the nature of the pesticide. The treatment history of a lot of grain on the market is often unknown, even when the grain has been produced locally, and therefore only a few generalisations can be made.
Pesticides can be applied at several stages of the growth of the grain:
A special case is the treatment of grains intended for sowing: these seeds should be kept carefully apart from grain for consumption by men or animals involved in animal husbandry (cattle, pigs, poultry etc.)
Pesticides used for protection of grains during storage have a larger chance to be found in the grains than pesticides used in an earlier stage of the growth. It can also be anticipated that herbicides will occur less frequently in objectable concentrations than the other pesticides, as overdosage of a herbicide sooner leads to phytotoxicity than an overdosage of other pesticides.
A further indication as to the incidence of pesticide residues on grain can be obtained from the documents issured by the CCPR. In this body, the following pesticides are being discussed in connection to their use on grains:
|(A = acaricide; F = fungicide; GR = growth regulator; H = herbicide; I = insecticide; M = molluscicide; N = nematicide; R = rodenticide; S = synergist)||lindane||l/R|
|disulfoton||l/A||Inclusion of a
pesticide in a Codex list means that the pesticide
involved is used or found in commodities moving in
international trade. The list is therefore especially
useful when testing imported grains.
Pesticides and growth regulators belong to very diverse types of chemical compounds. It is convenient to divide them into groups having common caracteristics used in the analysis. Thus the pesticides mentioned above can be grouped as follws:
- electron-captive compounds (i.e. compounds giving a signal in the electroncapture detector (ECD) used in gas chromatography (GLC); most organochlorine compounds fulfill this requirement, as do most organobromine and iodine compounds;nitrogroups, aromatic rings and conjugated unsa turated systems also contribute to electron- captivity):aldrin/dieldrin, captafol, chlordane, chlorothalonil,
cypermethrin, DDT, deltamethrin, dichlofluanid, endosulfan, endrin, fenvalerate, flucythrinate, heptachlor, hexachlorobenzene, lindane, permethrin, phenothrin, propargite, pyrethrins, as well as the fumigants carbon disulphide, carbon tetrachloride, 1,2dibromoethane, 1,2-dichloroethane and methyl bromide.
The other pesticides mentioned in (ii), do not belong to any particular group and must therefore be determined separately.
A limited number of pesticides form metabolites of toxicological importance, e.g. thio-ethers which form the equally toxic sulphoxides and sulphones, and some organochlorine pesticides which form epoxides of toxicological importance such as dieldrin (epoxide of aldrin) and heptachlor-epoxide. In these cases it is mandatory that the metabolites are determined as well.
As very often the treatment history of a lot of grain is unknown, it is important to have methods available which cover as many pesticides as possible. These methods are "multi-residue methods" and will be discussed here in more detail, as they are most often the analyst's first choice.
The discussion will follow the same order as above.
Special methods (i.e. methods which cover only one pesticide) are necessary for the pesticides not covered by a multi-residue method, viz.:
As can be seen from the data presented above, pesticide residue analysis requires modern chromatographic equipment: at least two gas chromatographs, equipped with ECD, FPD, AFID and FID are necessary, as well a one HPLC-system equipped with UV-detection at 216 and 254 mm. A third gas chromatograph equipped with capillary columns is required for confirmatory purposes. A UV/VIS-spectrophotomter is also necessary for a number of analyses, but this instrument can often be shared with other departments, as it is seldom 100% in use for pesticide residue analysis. High purity gases, solvents and reagents must be available, and spare parts, service kits and maintenance manuals are indispensable. Clean-up of extracts can be carried by classical column chromatorgraphy, but gel permeation chromatography (GPC) is making rapid progress for this purpose. Automatic injection systems and electronic data processors are useful for improving labortaory efficiency, but they can only work to full satisfaction if the chromatography are up to standard.
Just as in agriculture, the general philosophy of pesticide residue analysis has been laid down in a document (Codex Alimentarius Commission, document CAC/PR-7-1984), describing Good Analytical Practice in pesticide residue laboratories. The document is especially useful when a new pesticide residue laboratory has to be set up or when an old laboratory must be up-dated. Special attention is drawn to the chapter on confirmation in the document, as confirmation is a crucial, but often negleted aspect of pesticide residue analysis.
Analytical Quality Assurance (AQA) is another important aspect of Good Analytical Practice.
A bibliograpohy with selected references to analytical methods for pesticides is also available through FAO (document CAC/PR 8-1984).
TABLE 1. The Geological History of the Insects General history indicated by continuous lines; spots are records of occurrences in Australia. Recent orders in bold face type.
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