Section 8 - Chemical control methods insecticides and pesticides
Contents - Previous - Next
Insecticides in stored product pest
Methods of insecticide application for grain protection
Safety in insecticide usage
Related problems to use of insecticides
International code of conduct on the distribution and use of pesticides
FAO plant protection paper 21. "Recommended methods for measurement of pest resistance to pesticide"
Test method for determining resistance to insecticide in the rust red flour beetle, tribolium castaneum (HERBST)
Insecticides in stored product pest control
Romeo S. Rejesus
Despite the increase in food production due to technological advances in agriculture, enormous quantities of harvested food are wasted due to inadequate protection of stored products. According to are FAO estimate, the losses due to insect infestation is about 10 per cent or more in developing countries. The losses can be potentially greater now with the increasing attention being given to the establishment of national and international buffer stocks of foodstuff to guard against irregularities in production due to the unpredictable climatic conditions.
The popular outcry against the use of synthetic pesticides is agriculture cannot overshadow the necessity of their use nor can any speedy decline in their use be foreseen, so that protection of the consumers of treated produce and education of the users of the chemicals is imperative. Chemical pest control methods, if carried out intelligently and knowledgeably, can be both effective and safe. It is extremely important for users to have a knowledge of the classification, mode of action, properties, metabolism and residues of the pesticides, to enable them to make proper appraisal of the benefits and potential hazards of the pesticides. Thus, they shonld be able to choose insecticides judiciously and formulate efficient control measures in any particular set of circumstances.
II. CLASSIFICATION OF INSECTICIDES
Insecticides are classified accourding to their mammalian toxicity, chemical origin or composition, mode of entry, and formulation.
A-1. Mammalian Toxicity
Toxicological studies are conducted to determine the threshold limit of a chemical which an animal or human is capable of handling without significant biological effects. The usual beginning in any toxicological evaluation is the assessment of the acute toxicity, i.e. the effects of a single dosage of the chemical. The general technique is the determination of the LD50 (the dosage necessary to produce death or reproducible effect in 50% of the animal population tested). The compound is administered on a weight/weight basis (milligram or gram of compound per kgm of body weight of test animals) in a suitable solvent or suspension system. This is evaluated by acute tests, orally (AO) or dermally (AD); chronic oral tests (CO), Vapor toxicity tests (VA) and chronic vapors tests (VC) or inhalation tests (IT).
Insecticides can be classified according to their toxicity based on the LD50 values:
1. HighIy toxic
AO LD50 = 0-50 mg/kg
AD LD50 = 0-200 mg/kg
IT LD50 = 0-2000 ug/l
Danger, skull and crossbones and poison on label.
2. Moderately toxic
AO LD50 = 51-500 mg/kg
AD LD50 = 201-2000 mg/kg
IT LC50 = 2,001-20,000 ug/l
Warning on label.
3. Slightly toxic
AO LD50 = 501 -5000 mg/kg
AD LD50 = 2000-20,000 mg/kg
IT LC50 = more than 20,000 ug/l
4. Relatively non-toxic
AO LD50 = 5000 + mg/kg
AD LD50 = 20,000 + mg/kg
Generally, the insecticide used in stored product treatment is of low mammalian toxicity (Table 1) in a formulation that is likely to be effective against the species involved, persistent for the required period of time under given storage conditions and will not alter the flavor, color and odor of the stored commodity.
A-2. Chemical Origin of Composition
A-2.1. Inorganic compounds - The toxicity of these compounds (arsenicals, flourides) are usually associated with the concentrations of elements. They are highly toxic to man, domesticated animals, and plants. They accumulate in the soil.
A-2.2. Organic Insecticides - These groups are characterized by organic carbon bonding (i.e. c-c; c = c).
a. Botanicals - The toxic principles are extracted from plants such as pyrethrum from the flowers of Crysanthemum cinerariafolium and C. cocciniu. It has remarkable low toxicity to mammals but toxic to insects.
b. Synthetic insecticides - They are syntihesized in the laboratory and are classified into three main groups: the organochlorine, organophosphates, and carbamates.
b-1. Organochlorines - These are chlorine (C1) - containing compounds further subdivied into DDT type, Hexachloro-cyclohexane type and Cyclodicnes. Most of them are quite toxic to man and are not used on stored food commodities.
c. Organophosphates - This is a generic term for all pesticides containing phosphorus which can be an ester of phosphoric acid (P = 0) or phosphorothioate acid (P = S) and can be represented by this formula:
The formula implies that sulphur (S) or oxygen (O) is directly linked to phosphorus. R1 and R2 may be alkyl or aryl groups or amine radical whereas X is the acyl or leaving group which may be radical of an inorganic acid. This is a very large class of compounds which features a greatly varying activity despite the very uniform mechanism of action. The organophosphorus compounds, however, decompose rapidly and recent advances in understanding the mechanisms of selective toxicity of insecticides such as malathion have led to safer insecticides such as malathion, bromphos, pirimiphos methyl, chlorpyrifos methyl, etc.
d. Carbamates - These are esters of carbamic acid, HOC (O) NH2 and can be represented by this formula:
R1 can either be an aliphatic or aryl radical. This contains compounds of high to low mammalian toxicity such as carbaryl and have so far shown limited application in stored product pest control.
e. Insect Growth Regulators - These groups of compounds are synthetic analogues of naturally occuring hormones in insects, ecdysone and juvenile hormones. Ecdysone regulates metamorphosis by initiating the moulting process while juvenile hormones regulate growth and development under normal concentrations. These growth regulators (IGRs) also control other developmental processes in insects such as sexual maturation, colour differentiation and reproduction. Examples are difludenzuron and methoprene.
A-3. Mode of Entry
Insecticides can be divided into three main groups depending upon the way they penetrate into the body of the insect.
A-3.1. Contact insecticides - These insecticides are applied in such a manner that they come in contact with some part of the body of the insect; the compound is able to penetrate the exoskeleton and is transported to the tissues via the circulatory system. Most of the insecticides used in storage belong to this group. The inert insecticidal dusts which disrupt the thin epicuticle leading to the dessication and death of the insect are included in this group.
A-3.2. Stomach insecticides - These materials exert their toxic action only when they are consumed and absorbed through feeding on treated surfaces through the guts.
A-3.3. Systemic insecticides - These are translocated to the untreated parts of plants or animals in concentration that makes the final translocation site toxic to insects. These are not used in stored product pest protection.
A-3.4. Fumigants - These are insecticidal gases at normal temperatures penetrating through the tracheal system into body tissues and are used in enclosed spaces. Examples are phosphine and methyl bromide. Some contact insecticides like dichlorvos, vaporize partially in warm ambient conditions, thus having fumigant and contact actions.
A-4. Mode of Action
A-4.1. Physical insecticides - These materials such as the heavy mineral oils and inert dusts, characteristically exert a physical rather than a biochemical Mineral oils exert a purely asphyxiant effect and dusts affect loss of boby moisture by abrasion (aluminum oxide) or by absorbing moisture (charcoal).
A-4.2. Protoplasmic insecticides - The action of these compounds is associated with the cellular destruction of the midgut epithelium such as inorganic insecticides.
A-4.3. Respiratory poisons-These include the fumigants and those that block cellular respiration such as rotenone. Rotenone inhibits the catalytic action of cytochrome oxidase and other Fecontaining oxidase.
A-4.4. Cholinesterase (CHE) inhibitors - The OPs and carbamates, inactivate the cholinesterase, consequently causing sickness or if applied at correspondingly high dosages, death of the affected organisms.
The CHE are a group of esterases which are capable of hydrolyzing acetylcholine, a chemical transmitter of nerve impulse. Normally, acetylcholine is rapidly hydrolyzed to acetate and choline in insect or in vertebrates in the presence of cholinesterase. But, with an inhibitor (either OP or carbamates) there is an accumulation of acetylcholine, which will cause an excessive stimulation (twitching) and finally complete block of the system (paralysis). Depression of the cholinesterase of the central nervous system results in restlessness, discomfort, giddiness and anxiety, followed by a headache, sleeplessness, ataxia, tremors, ultimately resulting in coma, generalized spasms and disappearance of reflexes.
A-4.5. Neurotoxicant - These materials act on the central and peripheral nervous system by changing the required balance between input and events. There is an excessive stimulation which later will cause excitation, convulsion, paralysis and death - these are characteristic symptoms of nerve poisoning. To this group belong the organechlorine insecticides, nicotinoids and pyrethroids.
A-4.6. Enzyme inhibitors - These compounds (flourides, arsenates) inhibit enzymes necessary for normal metabolism. Flouroacetate blocks the tricarboxylic cycle by combining with acetyl CoA forming fluorocitrate which inhibits aconitase which convert citrate to succinate. The blockage leads to reduced energy production and O2 utilization resulting in respiratory disorders causing death. Arsenates and nitrophenols kill primarily by inhibiting respiratory enzymes which block the production of AIP (energy).
A-4.7. Insect growth regulators - There are two types of IGRs: the chitin synthesis inhibitors such as diflubenzuron and the antijuvenile hormone such as methoprene that disrupts the moulting process. Chitin which is essentially a polymer of N-acetyl glucosamine is a structural component of the insect cuticle essential for proper insect development. Diflubenzuron interferes with the larval cuticle deposition and disrupts the moulting process by inhibiting the synthesis of chitin. The function of hormone is to maintain the larval tissues at a moult. The presence of JH mimics such as methoprene in the insect when the natural hormone level is low would be expected to disturb normal morphogenesis.
Insects exposed experimentally to large concentrations of IGRs during the life cycle when they are not normally active, inhibit various developmental and morphological abnormalities including a juvenilizing effect.
A.5. Usage in Stored Grain
Insecticides used on or around grain may be classified based on usage:
III. PROPERTIES OF SOME INSECTICIDES
The selection of insecticides for treatment of edible commodities is based mainly on the toxicological data (low mammal fan toxicity), effectiveness and persistence under certain storage conditions and absence of side effects such as discoloration, flavor alternation and odor. FAO and WHO collect toxicological information and give advice on overall tolerance figure. However, the maximum levels of pesticide residue acceptable in grain consumption must be calculated by local authorities who have relevant information on the inert feeding habits and agricultural practices.
To qualify for selection as possible candidate material for use on or around grain, the insecticide must fulfill the following requirements (FAO, 1982):
Of the many insecticides reported to be toxic and/or effective against stored product pests, the number which have been cleared for application are stored grain and for which maximum residue limits are established is limited.
The common insecticides used in stored product pest control belong to four groups: the pyrethroids, the organophoshporus; the organochlorines, and the carbamates.
A. The Pyrethroids
The Pyrethroids are either isolated from plants or synthesized.
A.1. Natural Pyrethrins
Pyrethrins remain one of the more widely used materials in stored product pest control. The rapid knockdown effect, a wide spectrum of activity against insect pests, a general acceptance of their use associated with foodstuff and an established codex of tolerance have been the principal reasons for their use. Their use has not been intensive as their cost has been prohibitive.
The disadvantage of high cost, poor stability, inadequate toxicity to some species and lack of ovicidal and acaricidal action have been offset somewhat by the use of synergists, i.e. piperonyl butoxide, sesamin, piperonyl cyclonene, propyl isome, sesamex and sulfoxide. Synergists are usually present at ratios 1:3 or 1:10 Tinsecticide: synergist).
A.2. Synthetic Pyrethroids
The synthetic pyrethroids such as biores-methrin, deltamethrin, permethrin, fenvalerate and phenothrin are becoming acceptable over pyrethrins because of their high levels of activity against a wide range of pests and a cost advantage. Bioresmethirn appears to be the most commonly used. Several other synthetic pyrethroids showing promise against storage pests are under development.
There have been deficiencies as with pyrethrins. For example, resistance is known although not extensive, and breakdown to malodorous decomposition products has presented a problem where repeated applications have been made on the same surface. In addition, like pyrethrins, control of T. castaneum has been less than desirable. Pyrethroids are highly summarized with piperonyl butoxide.
A-2.1. Bioresmethrin - It is one of the most potent broad spectrum insecticides currently available and has a good knockdown preformance against insects. Bioresmethrin, at low concentrations, is an effective killing agent against most insect pests attacking households, industrial premises and food storage. Bioresmethrin is currently being used as:
a. household aerosols and sprays formulated in combination with pyrethrum, bioallethrin, tetramethrin and piperonyl butoxide;
b. an insecticide for the control of pests in food premises; and
c. in grain disinfestation and protection.
Bioresmethrin has an exceptionally high potency against some insect species, particularly Rhizopertha dominica and it has proved useful when applied at the rate of 1 mg/kg in conjunction with selected organophosphorus insecticides, enabling the amount of OP to be reduced considerably without loss of effectiveness. It has lower toxicity against Sitophilus granarious, Tribolium castaneum and moderately toxic against mites. The toxicity of bioresmethrin can be improved to a significant extent with piperonyl butoxide, the factor of synergism ranging from 2 to 9 fold. Bioresmethrin at 1 mg/kg plus fenitrothion at 12 mg/kg controls typical malation-resistant strains of S. oryzae, R. dominica, T. castaneum and Ephestia cautella.
A-2.2. Fenvalerate - It is an ester related and in many ways similar to pyrethroids. It is a highly active broad spectrum insecticide with adequate stability and relatively low mammalian toxicity. It has been shown to be effective at low doses against R. dominica and at higher doses against most species such as Sitophilus and Tribolium. Fenvalerate is an effective alternative to bioresmethrin. It combines well with OP and its potency is synergized by the addition of piperonyl butoxide. Deposits on grain are stable, though the bulk of the deposit is removed with bran or hulls. Those residues which carry through the white flour or milled rice remain substantially undiminished following cooking. Fenvalerate at 1 mg/kg along with fenitrothion at 12 mg/kg and piperonyl butoxide at 8 mg/kg control common field strains of S. oryzae and R. dominica and completedly prevent progent production in T. castaneum, T. confusum and E. cautella. The same combination of fenvalerate control typical malathionresistant strains of above -mentioned species.
Table 1. Acute oral LD50 (mg/kg body wt. rat) of insecticides used for storage pest control.
|2. Pirimiphos methyl***||2050||2000|
|3. Chlorpyrifos methyl*||1650-2100||3000|
|6. Dichlorvos* *||80||107|
|7. Fenitrothion* *||250-500||3000|
|14. Carbaryl *||400-850||4000|
|15. Pyrethrum * *||1500||1800|
|21. Permethrin *||4000||4000|
|22. Methoprene*||5000||relatively nontoxic|
|23. Piperonyl butoxidea||relatively nontoxic||relatively nontoxic|
a Used as synergist
B. Organophosphorous compounds
There are severe, organophosphrous compounds (OP) used in stored product pest control. The most common are malathion, dichlorvos, fenitrothion, pirimiphos methyl and chlorpyrifos methyl.
B-1. Malathion - Malathion is the only OP that has been widely used for over 20 years for the routine protection of stored products especially cereals in practically every country in the world. It is effective against the many destructive pests of stored products at 5 to 20 mg/kg. It is virtually ineffective against stored product moths and requires higher dosage to control non-resistant R. dominica.
Malathion is intensively used in developed exporting countries such as Australia, Argentina and USA. Whereas before, malathion (premium or deodorized grade) was the most important material for admixture with grain, increasing use of dichlorvos was evident. Due to development of insert resistance, there appeared to be a significant move away from malathion for disinfestation of storage facilities and a less marked but noticeable change to use alternative materials for treatment of bags.
The amount of the malathion deposit which penetrates the individual grains is relatively small and therefore most of the deposit is removed in the milling of wheat and rice. More than 95% of the deposit on the raw cereal grain is removed or destroyed before the cereal food reaches the consumer.
Malathion is used as admixture in dust or spray form; for building and fabric treatment; floor wash; and surface treatment of bags and grains and associated buildings and fabrics.
B-2. Dichlorvos - It is the most commonly used material next to malathion. It is an extremely effective stored product insecticide due to its high vapour pressure and is used as fumigant rather than as contact insecticide. Enclosed spaces such as warehouses, storage and grain bins allow build-up of air concentrations of vapours toxic to most flying and crawling stored product insects. The vapour will not penetrate into grain masses, other commodities or the fabric of buildings. The principal uses fordichlorvos are in aerosol-dispensing units which could be programmed for automatic daily release, usually at dusks; space sprays or fogs; slow release formulations in which dichlorvos is dissolved in solid strips (or beads) of polyvinyl chloride plastic suspended in the free space of storages; surface application of concentrates on wooden floors; surface treatment for protection of bagged commodities; and direct application to grain either as a surface treatment for moth control or for admixture with grain as disinfestation treatment alone or mixed with residual protectants.
Dichlorvos has the advantage over other residual insecticides in that it is considerably more active against immature stages of pests that develop within individual grains. Whereas malathion, for example, will kill only first instar larvae of S. oryae, dichlorvos will give a significant kill of all larva, stages except final instar larvae and the pupla stage. Resistance from has R. dominica, been detected in low levels.
B-3. Fenitrothion - It is a broad spectrum insecticide with a much lower acute mammalian toxicity than many similar insecticides. It is widely used for the control of pests of many crops and principally as a residual spray in houses for the contro, of mosquitoes; pests of forest trees and for the management of locust swarms.
Fenitrothion has been used for a considerable time for structure, treatment and for surface treatment of bag stocks particularly where malathion resistance was present. Fenitrothion is considerably more effective than malathion against Sitophil's spp. and Lepidoptera and of comparable effectiveness against Tribolium spp. but not fully effective against P. dominica. High potency and good stability mean that deposits in the region of 5-10 mg/kg are sufficient under most storage conditions to give complete protection for 9 to 12 months. When combined with pyrethrum or synthetic pyrethroids, the effectiveness of fenitrothion is increased and the dosage level can be reduced. Fenitrothion is more effective than malathion for conditions where it is generally applied in the form of a very dilute dust. There is minimal penetration into the grain so that the deposit is mostly removed in bran of wheat and husks of rice.
B-4. Pirimiphos-methyl - It is a fast-acting broad spectrum OP with both contact and fumigant action. It gives long lasting control of insect pests of inert surfaces such as wood, sack and masonry. It retains its biological activity when applied to stored agricultural commodities including raw grain, nuts, pulses, dates and cheese.
Pirimiphos-methyl has been used in many situations against stored product pests. The minimum effective dose against a wide range of inserts is lower than most other OP on use or under development as grain protectant. It is potent against beetles, weevils, moths and mites, but not sufficiently effective against some strains of R. dominica. It is useful against immature stages within the individual grains and it appears quite effective against many lindane-malathionresistant strains. In the Philippines, pirimiphos-methyl was found to be are effective protectant of corn grains against a variety of pests especially Sitophilus spp. for 6 months. Pirimiphos is more persistent in maize than in sorghum. Pirimiphos methyl-impregnated sacks is more effective than malathion for the control of storage pests of shelled corn.
B-5. chlorpyrifos-methyl - It is a broad spectrum organophosphorous insecticide of relatively low toxicity and moderate persistence. It shows reasonably good stability in stored products such as grain and dried fruits. In these products it controls a wide spectrum of beetles, weevils, moths and mites including several species which may have developed resistance to insecticides.
Chlorpyrifos-methyl is potent against all storage pests except resistant B. dominica. It is effective against moths which are not readily controlled by malation. Deposits on grains and sacks are stable under most storage conditions. Studies in the Philippines show that it is a grain protectant more potent than tetrachlorvinphos, pirimiphos-methyl, malathion, MIPC against sitophilus spp. for grain use. However, it was not as effective as tetrachlorvinphos against B. dominica. In general, the residual toxicity increased with higher concentration. Chlorpyrifos methyl was the most stable among the five OPs, evaluated in corn and sorghum.
B-6. Tetrachlorvinphos - This compound has been used in the field for control of non-storage pests for considerable time and it is only recently that it is being evaluated as grain protectant
Tetrachlorvinphos has a very low level of toxicity and is a suitable compound for the protection of foodstuff. It has been shown to be effective against many species of stored product pests, both the immature and adult stages. It was also found to be highly stable in dry grain and posed no odour problem. In the Philippines, it was found to be affective as preharvest spray in sorghum at 2% for controlling S. zeamais and Rhyzopertha in the field. It is more effective than malathion and pirimiphos-methyl and equally effective as chlorpyrifos ethyl against Rhizopertha. Tetrachlorvinphos deposits do not penetrate the individual grains to any extent and appear to be removed on husks and bran.
B-7. Metacrifos - The compound acts as a contact, vapour and stomach poison against all important arthropod pests of stored products. It is also highly effective against major malathion and lindaneresistant insects. It is one of the few compounds that is effective against R. dominica. It is particularly useful where grain temperature can be regulated and where aeration of the grain mass can take advantage of the high potency of methacrofos vapour. It is marketed under the trade name Dam Fine.
Metacrifos penetrates the individuals grains fairly rapidly and is therefore effective against larval stages within the grain. It is extremely potent at lower temperature and has a pronounced vapour action, but it degrades rapidly at high temperature and humidity.
B-8. Bromophos and others - Bromophos is used in limited scale as grain protectant and residual spray of warehouse facilities and bagged grain.
Other OPs that have been subjected to extensive study are diazinon, etrimphos, phoxim and iodafenphos. These compounds have been used as residual sprays but are currently registered as grain protectant
As insecticides, many of the organochlorines are cheap and have excellent insecticidal property against many insects. These insecticides are photostable, and are resistant to degradation both in the environment and in biological systems. However, their resistance to environmental degradation and their stability after entering biological systems have led to general contamination of the world ecosystem. Also, because of its low cost and effectiveness, its widespread use has resulted in high levels of resistance in many insects. In addition, many of these chemicals exert profoundly deleterious long-term effects in animals, effects that were not known until after a long time that these chemicals were put into extensive use. Many organochlorines, including DDT and several to the cyclodienes have been shown to induce the formation of tumors in laboratory animals fed with a low dietary dose. Aside from tumorogenicity, it has been shown to upset reproduction in birds, and mammals.
Organochlorines that have been used for postharvest treatment were DDT, lindane and methoxychlor. DDT was used in foodstuffs since its introduction in the early 1940s. When its persistence and tumorogenicity effects were uncovered, its use has been stopped. Methoxychlor is a DDT analog where the chlorine is substituted with methoxy (CH3O) groups which render it more rapidly degraded by sunlight but is slowly converted to methoxy-DDE. But unlike DDT, it does not ac - cumulate in fatty tissues. Although it has a very low mammalian toxicity (5000-7000 mg/kg rat) its ues is limited because of its high cost.
Lindane (gamma-BHC) replaced DDT which found a limited use in the immediate post-war years in postharvest treament. It has many of the desirable traits required especially for stored pest control; a wide spectrum of insecticidal activity in both beetles and moths and reasonable effectiveness against mites; stability under a wide range of conditions; a significant vapour pressure allowing some fumigant effect; repellency in some circumstances; and a comparatively low mammalian toxicity. The major uses for lindane apart from geed dressings have been in surface treatment of bagged stocks of grains, coffee and cacao beans and particularly, in treatment of warehouse and transport facilities.
It loses much of its effective residual life through evaporation and is very susceptible to dehydrochlorination when applied to alkaline surfaces.
Many of the properties of carbamates such as mode of action, lack of toxicity, lack of environmental persistence and lack of safety to beneficial insects are similar to organophosphates. However, the inhibition of carbamates is less permanent than with organophosphates. This means that at least for man, these insecticides are less dangerous than organophosphates as they have had a better safety record under practical use conditions.
For carbamates have been proven useful for storage pest control in the past, and the one that has carbaryl - has been largely replaced by the synthetic pyrethroids for controlling Rhyzopertha. Carbaryl, like other carbamates, is rapidly degraded into watersoluble, hydroxylated products which are easily conjugated to glucosides.
Methoprene - This compound is the most commonly advanced example of the class of IGRs with juvenile hormone activity. This is available under the trade name Altosid. Methoprene is effective as protectant at 5 mg/kg against Plodia interpunctelta, Lasioderma serricorne, R. dominica, Oryzaephilus surinamensis and mercator, and at 10 mg/kg against Ephestia cautella and T. castaneum. The juvenile hormone analogure is relatively non-toxic and has moderate stability under storage conditions but is rapidly decomposed by light. It is expensive and effective against few insect species.
BENGSTON, M. Grain Protectants. In "Pesticides and Humid Tropical Grain Storage Systems". ACIAR Proc. No. 14. pp. 277-290.
BUSVINE, J. 1966. Insects and Hygiene. 2nd Rev. Ed. 467 pp. London. Methuen & Co.
CHAMP, B.R. and C.E. DYTE. 1976. Report of the FAO Global Survey of Pesticide Susceptibility of Stored Grain Pests. FAO Plt. Prod. and Prot. Ser. 5. FAO, Rome. 297 pp.
MATSUMURA, F.1975. Toxicology of Insecticides. Plenum Press, N.Y. 503 pp.
MORALLO-REJESUS, B. 1983. Toxicology and Properties of Pesticides Used for Post-Harvest Grain Treatment. p. 171-180. In teter et al (eds.). "Maintaining Good Grain Quality". Proc. 6th Ann. Workshop on Grain Post-harvest Techn. Buncak, Bogor, Indonesia. May 3-6, 1983.
O'BRIEN, R. 1967. Insecticides. Action and Metabolism. Academic Press, N.Y. 1967.
QUEBRAL, F.C., B. MERCADO, B. MORALLO-rejesus and C. ADALLA. 1975. Pesticide Information Manual. 1st Rev. Ed. Pub. by DDC, UP at Los Banos. 98 pp.
SNELSON, J.T. Grain Protectants. ACTAR, Monograph No. 3, x + 448 p.
TROPICAL STORED PRODUCTS CENTRE. 1970. Food Storage Manual. Part l-lilt World Food Programme. 799 p.
Romeo S. Rejesus
In the commercial development of insecticides, one of the first steps that a manufacturer enplays is to chemically produce the compound in a form which is called the "technical grade material". This technical grade material or toxicant may be sold in solid form such as crystals, or powder, or in a liquid or gas forms.
Since it is not normally used in these forms for insect control, the technical grade material must be formulated. Formulation involves processing the technical grade material by any method that will improve its effectiveness, storage, handling, safety and ease of application
A. Formulation Process
This is usually accomplished by grinding the material to a powder or dissolving it in a petroleum solvent. The toxicant may then be diluted with other substances to make the desired formulation and is then known as the active ingredient, which is often abbrevated to "Ai"
The other substances which are added to the formulation are called adjuvants. An adjuvant is anything that enhances the physical properties of an active ingredient, are by itself it may have no killing properties. Some examples are xylene, talc, flur and bran.
The preparation and use of pesticide formulations also involve the use of various accessory agents such as dust diluents, solvents, emulsifiers, wetting and dispersing agents, stickers, deodorants, and masking agents. Accessory agents are given names that denote their specific action or enhancement of the formulation. For example, a "spreader" would help spread the pesticide over a surface.
B. Properties and Ingredients
The physical form in which insecticides are purchased may be either a dry or a liquid formulation. One of the most important components of a formulation is the carrier. This is the substance which carries the active ingredient of the target surface. Carriers also may be either dry or liquid according to the formulation.
B-1. Dry Formulations
The commonly used dry formulations are: dust, granules, baits, wettable powders and soluble powders.
Dusts (D). They are in a dry, powdered form and usually contain from one-half to ten percent se. Most of the material in this formulation is an inert clay diluent or carrier. Dusts do not always adhere well to plants or animals or structures; and they are extremely subject to drift by the wind, therefore posing a greater toxic hazard to the applicator and the environment than many other types of formulations. For these reasons, dusts are usually recommended only for localized application, home control programs and storage system. Dilute dusts are used mainly as grain protestants.
Granular (G) These are large particles, dry formulations that usually contain two to twenty percent active ingredient. Various types of inert clay pellets, peanut hulls and corn cobs are often used as the diluent or carrier. The toxicant is applied to, and adheres to these granules. Granular formulations are easy to apply and do not drift as readily as other formulations. Granules are used widely for spot and broadcast soil applications and are often applied at planting time to protect the roots from soil insects.
Baits (B). Baits contain a low percentage of active ingredient ranging from 1/4 to 5%. The toxicant is mixed with various carriers or attractants such as bran, orange pulp, corn cobs and sugars. The bait is placed or scattered where it will be consumed by the target pests. Baits are commonly used for subterranean soil pests such as ants, mole crickets and cutworms.
Wettable Powder (WP). The Al in a wettable powder usually ranges from 25 to 75%. The Ai is essentially a concentrated dust which has been finally ground to a powder mixed with a fine claylike diluent or carrier. While a WP is a dry formulation, it is mixed with water which acts as the secondary carrier. Most Ai of a WP formulation are immiscible with water. This incomptibility is overcome by adding a bipolar compound such as a wetting agent. The wetting agent ties the Ai and diluents with the water carrier to form a suspension. However, this WP suspension is unstable and it must be constantly agitated to prevent settling and ensure its desired effectiveness.
Soluble Powder (SP) - These are finely ground, highly concentrated powders containing 75 to 90% se. The powder is soluble in water and needs no wetting agent. Simple mixing of powder with wate forms the spray. The potentially hazardous concentrated material is packaged in a soluble bag, and the entire package is placed into spray tank with water.
B-2. Liquid Formulations
Most insecticides that are applied as liquid or spray use water as the carrier. The following liquid formulations will be discussed: emulsifiable concentrate, oils, solutions, fumigants, aerosols and ultra-low volume.
Emulsifiable concentrates (EC). - EC are available from 20% to 80% Al/gal. (2 to 8 pds Al/gal.). The al is dissolved in a petroleum solvent such as xylene and an emulsifier which allows the material to mix with water. The percentage of the emulsifying agent which is present in the insecticide is indicated by the inert ingredients. Emulsifers and wetting agents are bipolar in that one end is hydrophobic and other end is hydrophilic. EC when mixed with water, forms a milky colored emulsion. This is generally stable for a period of several hours without agitation and should only be mixed in quantities that will be used immediately. EC are used for treating storage structure or fabric, external bag surfaces and disinfecting transport facilities.
Oil. These are sprays in which oil inself is the se. The oil may be refined to reduce phytotoxity to plants and is usually mixed with an emulsifier so water may be used as the carrier. The percentage of oil in this type of formulation may range from 1 to 99%.
Fumigants. They are gaseous poisons which boil at room temperature. Application is generally limited to plants or products in tight enclosures or those that can be enclosed in gas-tight tents. Most are highly toxic and must be applied by trained, certified applicator.
Flowable Suspensions (F). - Flowable suspensions are an ingenious solution to a formulation problem. Earlier it was stated that some insecticides are soluble in neither oil nor water, but are soluble in one of the exotic solvents, making the formulation quite expensive. To handle the problem, the technical material is blended with one of the dust diluents and a small quantity of water, leaving the insecticide-diluent mixture finely ground but wet. This "wet blend" mixes well with water and can be sprayed with the same tanksettling characteristic as wettable powder.
Liquified Gas Aerosols. - Liquified gas aerosols or bombs are the common method for producing small amounts of aerosols for indoor use. The aerosol is formed by the release of a solution of insecticide in a liquified gas through a capillary tube with very small diameter.
Ultra-low-Volume (ULV). - ULV is both a formulation and an application technique. These are usually sold as technical grade materials. They are not further diluted before application by special spray equipment. The extremely fine spray is applied at rates as low as one-half pint to one-half gallon per acre.
VAN VALKENBURG, W. Pesticide formulations. Marcel Dekker, New York. 482 pp.
WARE, G.W. 1975. Pesticides: An autotutorial approach. W.H. Freeman and Company, San Francisco. 205 pp.
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