Learn about how to control pests using insecticides!
The types of insecticide formulations are all lethal, effective in killing the insects sooner or later. Generally the insecticides are broadly classified on the basis of the mode of their entry into the insect body as stomach, contact and fumigant poisons. Modern classification however does not follow this basis since the present day synthetic insecticides enter the insect’s body in more than one way. To avoid this overlapping modern classification is now based on the chemical nature of insecticides and the major divisions are therefore inorganic and organic.
The organic insecticides are further divided into oils, botanicals and synthetics. Oils are petroleum products used primarily against fruit tree insects. Botanicals are of plant origin and include such insecticides as Rotenon, Pyrethrum etc. Among the synthetic group of organic insecticides, several chlorinated hydrocarbons, organophosphorus compounds etc. used in the past are now banned.
With regards the older classification, stomach poisons are materials which are ingested by the insects and which kill primarily by the action on or absorption from the digestive system. Usually they are limited to the control of chewing insects. Contact poisons are absorbed through the body wall and must come in direct contact with the insects to kill them. They are usually adopted for destroying sucking insects. Fumigant poisons enter the tracheal system of the insects in the form of gas. Insects such as stored grain pests and certain soil insects are killed by the fumigants.
1. Inorganic Insecticides:
Though inorganic insecticides have been largely replaced by organics, some of the forms are still used to control certain insects. Most of the inorganic insecticides are stomach poisons. Inorganic materials used in insect control are chiefly compounds of Arsenic, Fluorine and Sulphur.
Most of the arsenical and fluorine compounds have been banned for domestic and agricultural use in many countries.
The main compounds are:
1. Lead arsenate,
2. Calcium arsenate,
3. Paris green or aceto arsenate of copper,
4. White arsenic,
5. Zinc arsenate,
6. Sodium fluoride,
7. Barium fluosilicate and
8. Sodium fluoaluminate.
Sulphur Compounds:
Amongst the sulphur compounds finely ground sulphur has been widely applied as a dust for control of mites and other insects. Sulphur is boiled with freshly slaked or hydrated lime to produce a mixture known as liquid-lime sulphur one of the chief constituents of this mixture is calcium polysulphide. Liquid lime sulphur is used in the control of scale insects, aphids, etc. on fruit trees.
Another important inorganic insecticide is Bordeaux mixture which is prepared by mixing copper sulphate, lime and water in a definite proportion. This is chiefly used as an insect repellent against cucumber beetles and leaf hoppers.
There are many other inorganic compounds which are employed as insecticides. Sodium salinate applied in the soil acts as a systemic insecticide and is used primarily to control aphids on ornamental plants. Sometimes mercurous chloride (calomal) and thallous sulphate are also used as insecticides for controlling cockroaches and ants.
2. Organic Insecticides:
a. Oils:
These form one group of organic insecticides which in their natural stage are highly phytotoxic. However, when they are used in an emulsion they may under certain conditions be safely applied to plants. They vary greatly in the degree of refinement and in physical properties according to their intended use. Oils are employed in a number of ways; they may be used as solvents or carriers for insecticides.
Oils by themselves are insecticidal and there are different types of oils:
(a) Summer oils are highly refined and less phytotoxic. They are generally used to control mites and scale insects.
(b) Dormant oils are less refined and are also applied to control scale insects.
(c) Superior spray oils are chiefly meant for controlling Aphids.
(d) Emulsifiable oil containing 95-99% oil and emulsifier.
(e) Tank mix oils are made up of oil, emulsifier and water added separately to a spray tank. The last two are used for the protection of agricultural crops.
The use of oils has several advantages, initially it is cheap, have a good spreading capacity, are easy to mix and quite safe to animal’s life. Another important consideration is the fact that insects have not developed so far any resistance to oils. However, there are certain disadvantages with oils e.g. they have a low toxicity to insects. They are unstable and extremely phytotoxic.
b. Insecticides of Plant Origin:
These are insecticides derived from plants and majority of them are contact poisons. They have several uses in insect control.
The most important insecticides of plant origin are:
(i) Nicotine,
(ii) Pyrethrum,
(iii) Rotenoe,
(iv) Sabadilla,
(v) Ryania and
(vi) Neem, all of which serve as insect toxicants.
(i) Nicotine:
It comes chiefly from two plants, Nicotiana tabacum and Nicotiana rustica. They are extracted from the leaves and stems of these plants Nicotine is highly toxic to a great number of insects acting on the ganglia of the central nervous system possibly at the synapses. It is toxic when ingested, absorbed through the body wall or taken in through the trachea.
The most commonly used form today is nicotine sulphate at 40% content. Free nicotine solution is also used as a fumigant or greenhouse spray. Nicotine insecticides are used as 0.04% sprays 3% dust, fumigants and dips. Water or summer oil may be used as a diluent in sprays and hydrated lime, ash or sulphur for preparing dust. Nicotine sprays and dusts are safe to use on plants. They are compatible with most other insecticides.
(ii) Pyrethrum:
This derives its toxicity from 4-esters- pyrethrin 1, pyrethrin 2, sinerin 1 and sinerin 2. It is a plant product extracted from the flowers of Chrysanthemum cinerariaefolium. A synthetic compound recently developed known as allethrin has the same toxicity as pyrethrum.
It is prepared by extracting the toxins with kerosene, alcohol or ethylene dichloride from flowers of the Chrysanthemum. This concentrated form may be diluted with petroleum oil and used as sprays. Concentrated extracts as acetone, alcohol or a hydrocarbon solvent are available for spraying plants. These are diluted with water to form aqueous emulsion. Pyrethrum is harmless to warm blooded animals and man but is highly toxic to most insects.
It is used for controlling the houseflies and mosquitoes. It acts rapidly as a nerve poison and the affected insects fall quickly on to the ground. This is termed ‘knock down’ and the insects thus knocked down need not necessarily die, some may recover after a lapse of time. Often the toxicity of pyrethrum is enhanced by the addition of chemicals such as sulphoxide or piperonyl butoxide to the insecticide. Such chemicals which increase the toxicity of an insecticide are known as “Synergists”.
(iii) Rotenone:
This is prepared by grinding the dried roots and bark of several species viz. Derris, Lonchocarpus and Tephrosia. Rotenone may be extracted with a solvent like chloroform or carbon tetrachloride and a concentrated extract is then prepared. This may be diluted in sprays or mixed with a dust carrier and used as impregnated dusts. It is nontoxic to warm blooded animals and man. It is also non-phytotoxic. But it is poisonous to cold blooded animals. Alkaline carriers such as lime should not be used with it.
Rotenone sprays and dusts loose their toxicity rapidly on exposure to light and air. It is a selective insecticide and can kill only some insects i.e. rotenone is very specific in its toxic action to insects. It is believed to kill the insects by inhibiting the utilization of oxygen by body cells or depriving the tissues of oxygen through a depressing reaction on the nerve and muscles connected with tracheal breathing. They are used in combating cattle grubs, lice, aphids and beetles.
(iv) Sabadilla:
It is derived from the seeds of Schoenocaulon officinale. It is sold as a finely ground powder diluted with an inert filler. A 50% concentrate is manufactured. This should be diluted and applied as a dust at 10 to 20% sabadilla content. Sabadilla dust is strongly irritating to the nasal membranes causing sneezing and eye irritation. These effects are however transitory and soon disappear after exposure. Sabadilla is most useful as contact dust to control Hemipterous insects which are resistant to other insecticides. It is very specific in its action and is ineffective against many other common insects.
(v) Ryania:
It is obtained from the stems and roots of a plant called Ryania speciosa. This is used for the control of sugarcane borers and certain Lepidopterous insects. It acts both as a contact insecticide, as well as a stomach poison. It has residual effect and is relatively stable.
(vi) Neem (Azadirachta Indica A. Juss):
A lot of work has been done in the past few decades and several seminars and symposia were held to evaluate the insecticidal properties of various parts of the neem tree viz. leaves, flowers, fruit kernels in the powder or oil form to control insect pests infesting various crops and crop-products. It has been stated that the Neem is an important source of insecticide and all its parts viz., leaves, bark, flower, kernel and kernel oil are variously used as insect-repellents.
Since ancient times, its leaves are kept amongst woollen and silken clothes and books to protect them against insects. Farmers throughout the country are aware of the need to protect the grains from spoilage during storage and use of neem leaves in grain heaps.
The oil is also used as a disinfectant and emulsifying agent in insecticides. The maximum insecticidal activity has been found in neem kernel. Several workers have tried the neem leaves and kernel powder as grain protectant. Mixing of seeds with neem powder at 1.0 and 2.0% effectively protected from pest infestation. Neem oil at 0.5 and 1.0% provided the best surface protection when applied to pulses against the pulse beetles Callosobruchus chinensis and C. maculatus.
Repellent effect of extracts of neem leaves, flowers and kernels against rust red flour beetle Tribolium castaneum and Khapra beetle Trogoderma granarium has been evaluated and found that neem kernel extract was more repellent than the leaf or the flower. Adults of T. castaneum failed to reproduce when fed with flour treated with neem kernel extract. Neem oil was found effective in preventing adult emergence and prolonging the development from egg to adult stage of Rhizopertha dominica.
Chopped leaves of neem were found effective in controlling the Sitotroga cerealella in stored rice. In addition to stored grain pests, numerous other pests of agricultural importance have been controlled by neem products in the form of powder, seed cake or aqueous and solvent extracts.
Several commercial formulations have been evaluated against pest of tobacco, groundnut, black pepper, pulses, cotton, oil seeds, vegetable and other agricultural crops. Neem plant parts, also act as growth regulator, repellent and anti-feedant, thus help to control crop pest population without altering the quality and nutritive value. One of the most potent derivatives of Neem is Azadirachtin.
Powdered flowers and fruit kernel powder treated rice, caused mortality of Sitotroga adults, 59-64% after 7 days, 2-8% powder was used. Neem oil 1-2% was found most effective in reducing adult population of Sitophilus, Rhizopertha and Ephestia. The neem kernel powder when mixed with cowpea Vigna sinensis at 3%, caused 85-95% mortality of Callosobruchus.
c. Synthetic Insecticides:
(A) Chlorinated Hydrocarbons:
Some commonly used chlorinated hydrocarbon insecticides are described as follows:
i. Dichloro-Diphenyl Trichloroethane (DDT):
It is a very powerful insecticide, which exercises a prolonged lethal residual effect till covered by the dust or dechlorinated. It is a creamy white, crystalline powder having a faint unpleasant odour. It is waxy to touch, quite stable, resistant to oxidation and light. It is almost insoluble in water but soluble in organic solvents like castor oil, cotton seed oil, xylene, benzene, kerosene, dioxane, ether, acetone, chloroform, carbon tetrachloride, liquid paraffin etc.
It is not a repellent hence, insects do not avoid it but it acts as stomach as well as contact poison. The effect is gradual but sure and persists for a long time. It is non-phytotoxic except to cucurbits. It is very effective against most insects such as beetles, weevils, caterpillars, thrips, whiteflies, and leaf hoppers. For aphids, it is used in emulsion form. For a quicker action, the DDT is mixed with pyrethrum or other chemicals like BHC or malathion in sprays and smokes.
It is compatible with most insecticides and fungicides, except lime sulphur. Considering the effective insecticidal dose, it is comparatively less toxic to mammals (a human fatal dose taken orally is 30 gms). Insolubility in water renders it harmless to use on human skin in dust or aqueous emulsion but when dissolved in oils, its large doses may cause a number of diseases like convolution, cerebral depression, and necrosis of liver. Its inhalation is also harmful but due to its high toxicity for insects, large areas can be treated with small quantities of insecticides by ordinary hands or power sprayers.
It is never used in the pure form but compounds are made with certain bases to form dust or spray. The dose of DDT ranges between 0.5 to 20% in mineral oils, 0.1 to 5% of emulsified dispersible or wettable powder in water and 2 to 10% in dust mixed with talc, clay, chalk, sulphur, and other carriers. Most plants, except cucurbits do not suffer by treatment with DDT. The DDT analogue, DDD (dichlorodiphenyl dichloroethane) is less toxic to mammals and is used for flies and vegetable pests. The acute oral LD50 for male rats is 113 mg/kg. DDT is stored in the body fat of birds and mammals and excreted in the milk of mammals.
ii. Benzene Hexachloride (BHC) Gammexane:
It is a crystalline tasteless, perfectly stable powder of sand colour, having a camphor like odour, practically insoluble in water but soluble in oils and paraffins. It is available as a pure product or mixed with a carrier like lime, gypsum or chalk. It is perfectly compatible with sulphur and DDT but it decomposes, when mixed with alkalies like lime, Bordeaux mixture and nicotine etc.
It is five times toxic than DDT hence acts more quickly but being volatile, the residual effect is less lasting. It can serve both as stomach and contact poisons for most of the insects such as thrips, leafhoppers, aphids, woodlice, and earwigs but less effective against bollworms, citrus fly and is more or less ineffective against flour moths.
It can be used as a slow acting fumigant for stored products alone or in combination with carbon disulphide, carbon tetrachloride etc. It is less toxic to mammals when taken orally or absorbed to skin. A fatal oral dose for man is 30 gms (its gamma isomer is more toxic and 15 gms is fatal dose for man).
iii. Lindane:
Lindane is the coined name for the gamma isomer of BHC of a purity of not less than 99%. Using lindane in formulation, eliminates the disagreeable odour and tastes on some crops which result when they are treated with BHC. It exhibits strong stomach poison action, persistent contact toxicity and fumigant action, against a wide range of insects. It is non-phytotoxic at insecticidal concentrations. The technical BHC causes “tainting” of many crops but there is less risk of this with lindane.
It is stable to air, light, heat and carbon dioxide and is not attacked by strong acids but can be dehydrochlorinated by alkalies. It occurs as colourless crystals and is practically insoluble in water, slightly soluble in petroleum oils, soluble in acetone, aromatic and chlorinated solvents. The acute oral LD50 for rats is 88 mg/kg. It is effective against many soil insects e.g. beetle larvae and adults, fly-larvae, ants and many biting and sucking insects e.g. aphids, psyllides, whiteflies, capsids, midges, thrips etc. This insecticide is now restricted in use. Trade names- Lindane, Lintox etc.
iv. Aldrin:
It is a white crystalline solid, insoluble in water but is quite soluble in mineral oils and highly soluble in most organic solvents like acetone, benzene, xylene etc. It is stable to heat, alkaline and mild acids and can be mixed with alkaline soils, fertilizers and other insecticides like sulphur, Bordeaux mixture, calcium arsenate, nicotine, paris green, rotenone etc. It is broad spectrum, persistent, nonphytotoxic insecticide. It acts as a stomach as well as a contact poison.
Toxic effect on insects is cumulative and therefore, it is not advised to use on edible crops. It is used against all soil insects like termites beetles (both larvae and adults) fly larvae, cutworms, wireworms, crickets etc. Aldrin is dusted on soil at the rate of about 2.5 kg per acre. Aldrin can control a variety of soil insects at the depth of about 10-11 cm. It is used as 30% emulsifiable concentrate, 2.5-5% dust, 5 and 20% granules, 20-25% wettable powder, or liquid seed dressing. The treated seeds should not be used for human or animal consumption.
Insecticidal preparations containing aldrin, should be handled with care. Respirators should be worn by those applying this material and excessive skin contact should be avoided. Acute oral LD50 for rats is 67 mg/kg. It is absorbed through the skin.
v. Endosulfan:
It does not accumulate in milk or fat but has a high mammalian toxicity. In plants and animals both, it is metabolized to a sulphate. It is a brownish crystalline solid, insoluble in water but moderately soluble in organic solvents. Endosulfan is not phytotoxic.
It is a contact and stomach insecticide and acaricide. It has high toxicity to beetles, lepidopterous larvae and mites. It is also effective against sucking insects. It is extremely toxic to fish but is safe for bees. It is formulated as emulsifiable concentrate, wettable powder, dust and granules. Trade names- Thiodan, Endosel etc.
Other products viz. – dieldrin, endrin, chlordane, toxaphene and methoxychlor etc. were used in the past but currently banned.
(B) Organo Phosphorus Compounds:
These are extremely poisonous to man and involve special care, precautions, apparatus and protective clothings and rubber gloves for handling them. All systemic insecticides belong to this category.
Those still in use are given below:
(1) Malathion:
It is a non-poisonous, non-phytotoxic insecticide. The chemical is yellowish brown liquid slightly soluble in water and miscible with most of the organic solvents. It has a strong penetrating, unpleasant smell and is broken up into non-toxic ingredients when mixed with alkaline media. It can be used as stomach and contact poison, besides as a strong fumigant for mites and other insects which are not killed by other poisons.
It is non-toxic to man, 100 times less than parathion either orally or by skin absorption or by vapours. It can be used as contact poison for aphids, scale insects, mealy bugs, jassids and caterpillars in 5% dust or 25-50% emulsion for spraying. Two or three applications fortnightly will control white flies, mealy bugs and scale insects. Acute oral LD50 for rats is 2800 mg/kg. Trade names – Malatox, Cythion etc.
(2) Methyl Parathion:
It is a white crystalline substance in pure state, sparingly soluble in water and well soluble in most organic solvents. It has a low thermal stability. It hydrolyzes rapidly in alkaline medium. Due to its low stability, toxicity is lost soon from the treated surface.
In soil, micro-organisms decompose it within a week. It is a contact and stomach poison with mild fumigant action. It has penetrating action also. Within 3-4 days 80-90% of it decomposes. It can be used against aphids, mites, leafhoppers, whiteflies, scale insects etc. It is available as emulsifiable concentrate and dust. Trade names- Folidol, Dalf etc.
(3) Trichlorphon:
It is initially used for controlling house flies, acts as a contact as well as a stomach poison. It is a white crystalline powder, quite soluble in water, insoluble in petroleum oils, poorly soluble in carbon tetrachloride, diethyl ether, benzene and ethanol. It is stable at room temperature but is decomposed by water at higher temperatures and acid media to form dichlorvos.
Manufactured in various forms like wettable powder (50%) soluble powders (50 to 80%), granules (2 to 5%), emulsifiable concentrate (50%) and mixed with sugar crystals. Used for controlling ticks, mites, house flies and fly maggots and bugs. Trade names- Dipterex, Tugon etc.
(4) Diazinon:
It is a non-systemic insecticide with some acaricidal action. It may be phytotoxic at higher dosages. It is a colourless oil almost insoluble in water but is miscible with ethanol, acetone, xylene and is soluble in petroleum oils. It decomposes above 120°C and is susceptible to oxidation.
It is stable in alkaline media but is slowly hydrolyzed by water and dilute acids. It is compatible with most insecticides but should not be compounded with copper fungicides. The acute oral LD50 for male rats is 108 mg/kg. Effective against flies and fly-maggots, mites, thrips, aphids etc. Formulated as aerosol-solutions; 25% emulsifiable concentrate, 40 and 25% wettable powder, 4% dust and 5% granules. Trade name- Basudin.
(5) Dichlorvos:
This insecticide has combined actions of contact, stomach and fumigant. On account of its high vapour pressure, it has greater penetration and it gets readily decomposed. When applied as emulsifiable concentrate, it is effective against sucking and leaf- mining insects. It is non-phytotoxic and non-persistent. It is also used as a fumigant against mosquitoes and other dipterous insects. It has low dermal toxicity but it is toxic to bees. It is marketed in the form of an emulsifiable concentrate. Trade names- DDVP, Nuvan etc.
(6) Monocrotophos:
It is a crystalline solid with mild ester smell. It is miscible with water, soluble in acetone and ethyl alcohol, but insoluble in kerosene and diesel. It is corrosive to iron, steel and brass.
It is a fast acting stomach, contact and systemic insecticide. It can be used against a wide range of pests. It is absorbed by roots and then translocated to stem and leaves. Scorching of edges occur in some varieties of sorghum, apple and cherry. Monocrotophos is recommended for the control of aphids, white flies, leaf miners, thrips, shoot borers, boll worms, caterpillars, folige feeder-beetles and a number of other pests. It is formulated as emulsifiable concentrate. Trade names- Nuvacron, Corophos etc.
(7) Fenitrothion:
It is a brownish yellow liquid with faint odour. It is insoluble in water but soluble in most organic solvents. Fenitrothion is slowly hydrolyzed in the presence of alkali. Its thermal stability is also low and may explode at 100°C. Iron container encourages decomposition. It is a contact and stomach insecticide with penetrating property. It is a low toxic poison. It decomposes in plants quite actively. It is highly effective in controlling sucking pests like aphids, jassids, bugs as well as caterpillars and flies. Used in the form of 5% dustor 50% emulsifiable concentrate. Trade names- Folithion, Fenitox etc.
(C) Carbamate Insecticides:
Carbaryl:
It is a contact insecticide with slight systemic action. It is widely used against the insect pests of fruits, vegetables, cotton and other crops. It does not deteriorate in storage and has a long residual effect on the crops. It is comparatively safe for insect pollinators, parasites and predators. It is formulated as dust and emulsifiable concentrate. Trade names- Sevin, Carbaryl etc.
3. Systemic Insecticides:
These are the insecticides which are absorbed into the sap stream by the plant tissues on application to leaf or root system and are then transmitted to various parts in such quantities as to render its juices sufficiently toxic to certain insects specially sap suckers for several weeks. These are therefore termed as systemic insecticides.
Their use in food and fodder crops cannot be recommended but are extremely useful for cotton and ornamental plants. The systemic insecticides can be safely used against phytophagous insects but not against predators and parasites. They may be applied as sprays or granules to either soil or foliage. The concentration is determined with respect to specific pests.
It is important to note that a large number of organophosphorus compounds act as systemic insecticides. These, as a rule, are highly poisonous substances hence full protective clothing must be always worn. Although, majority of them are non-phytotoxic but many are harmful to bees, livestock, fish, game and wild animals. They are mostly used against sap sucking insects and phytophagous mites.
Following are some of the common systemic insecticides:
(1) Dimethoate:
This insecticide is used mainly against fruit flies and aphids. The pure compound is a white solid, only slightly soluble in water but soluble in most organic solvents except saturated hydrocarbons. It is stable in aqueous solution and to sunlight but it is readily hydrolyzed by aqueous alkali. It is incompatible with alkaline pesticides. The acute oral LD50 for rats is 250-265 mg/kg. It is effective against aphids, psyllids, some flies, sawflies, mites etc. It is marketed in the form of 20-40∼ emulsifiable concentrate, 20% wettable powder and 5% granules. Trade names- Rogor, Dimetox etc.
(2) Demeton-Methyl (Methyl Demeton):
It acts as a systemic as well as a contact insecticide and acaricide, with a fast killing ability and moderate persistence. It is a clear yellowish oily liquid insoluble in water and soluble in most organic solvents. The acute oral LD50 for male rats is 65 mg/kg. It is effectively used against aphids, mites on most crops, some flies and has only a limited effect on brassica aphids. It is used as emulsifiable concentrate. Trade name- Metasystox.
(3) Phorate:
It is a persistent insecticide used in granular and emulsifiable concentrate formulations for protection of seedlings from sap feeding and soil insects. It is a clear liquid only slightly soluble in water but miscible with carbon tetrachloride, dioxane, xylene, and vegetable oils. It is hydrolyzed by alkalies in the presence of moisture. The acute oral LD50 for male rat is 3.5 mg/kg, for females 1.6 mg/kg. Used in the form of granule. Trade names- Thimet, Foratox etc.
(4) Phosphamidon:
This insecticide is readily absorbed by the plants but with only a little contact action. It is non-tainting and non-phytotoxic except to some cherry varieties and sorghum varieties. It is a pale yellow oil miscible with water and is readily soluble in most organic solvents except saturated hydrocarbons. It is stable in neutral and acid media but is hydrolyzed by alkali. Compatible with all but highly alkaline pesticides. It corrodes iron, tinplate and aluminium and is packed in poly ethylene containers.
Acute oral LD50 for rats is 28.3 mg/kg. It is effectively used against sap feeding insects and leaf eating ones, particularly aphids, bugs, many caterpillars (but not Noctuidae) rice stem borers, thrips, beetles, grasshoppers, saw flies, fly larvae and mites. Used as 100% emulsifiable concentrate. Trade names- Dimecron, Daron etc.
(5) Quinalphos:
It is a white crystalline solid without any odour. It is sparingly soluble in water but readily soluble in most of the organic solvents. It gets readily hydrolized in alkaline medium. It is a strong contact and stomach insecticide and acaricide. It is effective against a wide range of insects, particularly sucking and caterpillar pests. Used in the form of 4% dust, 25% emulsifiable concentrate and 5% granule. Trade names: Quinatox, Desalux etc.
4. Fumigants:
These are respiratory poisons taken into the tracheae in gaseous form and exert their influence when absorbed into the tissues from tracheoles. The other fumigants are suffocating agents which simply clog the spiracles and slowly kill the insects by cutting oxygen supply. Such chemicals can be applied to plants in air tight chambers, green houses and nurseries and to plant products and grain houses, mills, granaries etc.
Time for fumigating granaries, mills etc. may be prolonged for 6 to 32 hours. It is extremely essential that no one must be allowed to enter the chamber in which operations are being carried out and provision must be made to open several doors, windows and shutters from outside without entering it.
(1) Carbon Disulphide:
This gas is highly inflammable and explosive when mixed with air, hence its use involves a great risk. Even a lightened cigarette or a spark from an electric switch is enough to ignite it. It is a yellowish or water white, heavy liquid with a disagreeable odour. Because of the fire and explosion hazards, the carbon disulphide should not be used in fumigation, if other fumigants can be obtained. It is very toxic to insects and plants.
For seed and grain fumigation carbon disulphide is applied to the top layer of grain, most effectively in the form of a coarse spray from a ground spray pump. As the vapour is heavier than air, it sinks down through the grains killing all form of insect life. The time of exposure is 36 to 60 hours.
(2) Nicotine:
It is used for fumigating green houses in humid conditions at dark hours. It can be obtained as such by heating nicotine to liberate the poisonous gas or paper soaked in liquid nicotine and dried may be burnt. Tobacco plants on roasting also emit the same gas. The acute oral LD50 to rats is 50-60 mg/kg. It can be used as contact insecticide with some ovicidal properties. It is effective against aphids, capsids, leaf miners, thrips, sawflies. Also used for fumigation of glass houses. Marketed as 95% alkaloid or as nictotine sulphate (40% alkaloid) also as 3-5% dust.
(3) Ethylene Dibromide:
It is very heavy colourless and odourless liquid which is used as a soil fumigant to control wireworms. It is sold in solution with petroleum oil and contains either 20 to 40% by weight. It is poisonous to man when inhaled as vapour or in contact with skin causing burning and irritation. It is inflammable so should be stored in light container in a cool place.
(4) Methyl Bromide:
It is a colourless gas which is heavier than air. Under pressure, it becomes a heavy colourless liquid. The gas is non- inflammable, almost odourless, very penetrating and poisonous to man. It can be used on a wide range of fruits and vegetables to destroy insects. It has been successfully used in fumigating cotton seeds to control the pink bollworms.
(5) Naphthalene:
It has been used as an insecticide for controlling the cloth moths. It is also used as a repellant to combat the carrot rust fly and as a greenhouse fumigant. It is phytotoxic so it should be used only by experienced persons. An unrefined or crude naphthalene has been found effective as a soil fumigant for controlling wire worms. It is sprinkled along the sides of fresh furrows from top to bottom as the field is ploughed. It is white crystalline compound with a penetrating but not unpleasant odour. The fumes are very irritating to the eyes.
(6) Aluminium Phosphide:
It is a yellow crystalline solid, stable when dry but in the presence of moist air phosphine gas is liberated which is inflammable in air. The phosphine gas is highly toxic and can be used for grain fumigation to control storage pests. Commercial product is a mixture of Aluminium phosphide and ammonium carbonate which is available as 3 gm tablet 20 mm diameter and 5 mm thick.
Like chlorinated hydro carbon insecticides, a number of organophosphorus compounds, systemic insecticides and fumigants which were used in the past, are not in use now.
5. Antifeedants:
The chemicals which possess the property of inhibiting the feeding of insects are termed as antifeedants or antifeeding agents. These chemicals neither kill nor repel the insect but only inhibit feeding by making the food distasteful. A number of carbamates have antifeedant properties, in addition to the insecticidal action.
Many compounds e.g. nicotine will temporarily inhibit the feeding activities of insects, but in the case of antifeeding agents this inhibition is more or less permanent and is the main cause of death. The fungicides- fentin acetate and fentin hydroxide have been shown to inhibit feeding in a wide range of plant feeders including Prodenia litura larvae and the adults and larvae of Leptinotarsa decemlineata.
It is relevant to note that these compounds also exhibit chemosterilant activity and insecticidal toxicity against a number of pests. Some of the common antifeedants used for controlling a wide variety of insect pests like beetles, weevils, Lepidopterous larvae etc. are Triazene, Brestan, ZIP (a complex compound of zinc salt), Baygon, Pyrethrum.
6. Attractants:
The attractants are the materials used to lure insects into traps and poisoned baits. The use of geraniol in sugar syrup as ant baits and fermenting syrup for trapping various moths are good examples. Other substances like eugenol, sugar, molasses, yeast extracts, milk, sweet fruits, amyl acetate and ethyl alcohol are the common insect attractants. The natural or synthetic pheromones mixed with poison baits have also been used as attractants for the control of insects.
The insects recognize the food and oviposition sites by olfaction and to a certain extent communicate with each other by the use of specific odours, the pheromones produced by a variety of glands and comprise a range of chemical types. The only pheromones which have been exploited on a large scale for insect control are the sex attractants which bring about mature sexes together for mating purposes.
The gypsy moth, Porthetria dispar is the only such species in which sex attractant was employed as a pest control method. The synthetic sex attractants i.e. pheromone analogue, named Gyplure was almost as active as Gyptol – the natural benzene extract of the abdomen of virgin females, but the Gyplure was much easier to synthesize. In field trials, quantities as small as 10-7 μg were able to attract males.
A great deal of success has been achieved, particularly with synthesized chemicals which are attractive to various species of fruitfly. One of the first compound to be found and used successfully to attract the Mediterranean fruitfly, Ceratitis capitata was Siglure. This fruitfly was also attracted and eliminated by another compound, Trimedlure mixed with insecticide. In a similar manner the Oriental fruitfly, Dacus dorsalis has been eliminated by methyl eugenol while a related compound eugenol (I part) mixed with geraniol (9 parts) has proved an effective lure for the Japanese beetle, Popillia japonica.
Trimedlure and methyl eugenol have been combined with a third compound Cue-lure, which is an attractant for the melonfly, Dacus cucurbitae. In this way three of the most important fruit fly pests are being brought under control by the use of attractants. Likewise, anethole attracts the adults of the codling moth, Cydia pomonella. Butyl sorbate has been used to monitor the numbers of Chafer beetles.
7. Repellents:
These are the chemicals which repel or drive away the insects although they do not kill them, thus they have the effect of keeping insects away from the food sources. Insects exhibit a phenomenon of negative chemotropism hence they keep away from articles smeared with repelling agents. Different types of repellents are needed for different types of insects because all insects cannot be repelled by the same chemical.
Repellents must be safe, economic and not too unpleasant to use, their efficiency depending on their ability to deter the greatest number of species from feeding for the longest period. The duration of protection afforded by a chemical depends to a great extent on where it is applied e.g. if placed on the skin, it is liable to be diluted by perspiration, to be oxidized, hydrolyzed or volatilized or simply removed by abrasion.
Thus, skin is usually protected for only a few hours but clothing may retain repellents for a few days or a week or more. As soon as repellents cease to be potent enough to offset the attractive qualities of host, insect pests will be able to return to their normal activities with consequent lack of protection.
Some of the important chemicals which drive away the insects are:
i. Bordeaux Mixture:
It is useful in driving away fleas, beetles, and leaf hoppers. It also destroys fungus and insect eggs. It is prepared with 2.5 kg of copper sulphate, 2.5 kg of hydrated lime and 250 litres of water. The copper sulphate may be dissolved in water in a glazed vessel and lime is diluted in another container. The inner water is then poured into copper sulphate and stirred vigorously. Finally the whole mixture may be diluted with the remaining quantity of water. The Bordeaux mixture renders sprayed plants distasteful and acts as a stomach poison for chewing insects, when mixed with 1/2 to 1 kg. of lead arsenate.
ii. Creosote or Coal Tar:
The creosote, coal tars and other derivatives of coal tar like naphthalene and carbolic acid etc. are successfully used as repellents to bugs, cloth moths, flies, beetles, and termites. These derivatives of coal tar can be mixed with soap and used as sprays.
A number of oils are used as repellents e.g. kerosene, cedarwood oil, fish-oil, castor oil, tar oil, pine oil, resin oil, citronella oil etc. are useful to protect the bodies of animals from insect attack. Similarly powdered alum, naphthalene, camphor, turpentine, calcium carbide, sulphur, tobacco dust, dry leaves of neem and powder of dry neem seeds can be used for dusting purposes. Benzyl and benzyl benzoate were found repelling to chigger and skin mites, dimethyl phthalate repels mosquitoes and mites and trichlorobenzene repells termites and ants.
The most widely used repellents these days are synergisted pyrethrins applied as sprays. In this case they act as repellents and not toxicant and their efficiency increased by the addition of compounds such as furfuryl alcohol (MGK 11). The compound furfuryl alcohol is also repellent to cockroaches and other household pests.
Precautionary Measures Needed while Using and Handling the Insecticides:
Before using any insecticide the operator must read carefully all the instructions given on the label of the packing which provides complete information about the nature of the poison, directions of their uses and warnings etc.
It is well known that most of the insecticides used now a days are poisonous to man and all other animal life including domestic animals.
The insecticide enters the body by numerous ways and vitally influences the various systems of the man and other animals e.g. poisoning is caused by absorption through skin, inhaling fumes, or dusts, or by ingestion through several sources:
(1) Hence care should be taken that no insecticide gets in the eyes, nose, mouth or skin.
(2) They must be stored in safe places where no children can reach.
(3) After the use of insecticides, the operator should wash hands etc.
(4) Hands should not be dipped in any preparation of the insecticide and all the equipments used in operation should be washed.
(5) Some of the insecticides may be absorbed through the skin. The preventive measures consist of using goggles, rubber gloves, washing of the contaminated cloths etc.
(6) The hazards of inhaling dusts, mists and gases may be overcome by using respirators.
(7) The cloths of the operator should be changed and the empty containers of the insecticides should be buried deep in the soil and away from the water sources.
(8) When treating with fumigants, avoid exposure to heavy concentration or inhalation of gas for longer duration and use gas masks.
(9) After the required exposure for fumigation, the room should be opened and entrance be allowed only after thorough airing.