Insecticides are applied in various forms, depending upon the type of pest, the situation under which they are to be applied and the hazards involved. Apart from the physical properties of these formulations, a number of factors, both inherent in the pest and those of the environment, also influence the effectiveness of chemicals.
Factor # 1. Physical Properties of Formulations:
The insecticides are available as dusts, wettable powders, emulsifiable concentrates, water soluble concentrates, granules, solutions, soluble powders; low volume concentrates (LVC), aerosols, mists, fumigants, etc. For common use, the insecticides are rarely, if ever, marketed in pure form. They are always marketed in a reduced concentration which can be further diluted at the time of application.
(i) Dusts:
Dusts are fine powders mixed with various types of carriers, i.e., walnut-shell flour or pulverized minerals such as talc, pyrophyllite, bentonite, diatomite, etc. The finished product is generally available in the concentration varying from 0.1 to 25 per cent of the active material. In general, the toxicity of insecticides increases as the particle size decreases.
When used as dusts, a number of properties affect the efficiency of the insecticides. In the powdered insecticides, the particle shape varies. It may be spherical, polyhedral, tubular, irregular, and if ground further, the shape changes and becomes more spherical and, therefore, desirable because when various shapes of particles are present in a particular preparation, they may segregate during application and act in different ways.
Particle size is also important in stomach poisons. It is believed that 10 micro-metres (win) is the right size, as it gives a sufficiently large surface area and provides good adherence to the foliage or other parts dusted. If the particle size is too small, the dust settles down very slowly and has a tendency to drift at the time of application. The particle density determines the carrying power, segregation of the dust components and settling of the suspension of particles. If the particles are too dense, they do not spread easily.
The particle size, shape, density and chemical composition of the mixture determine the flowability of the dust. A good dust should have a higher feed-rate through the dusting equipment. Dusts with fibrous or needle-shaped particles have a slow feed-rate, whereas those with spherical particles have a faster rate.
These properties also determine the sticking quality of the dust, even though dusting is done when the plants are wet with dew. If the particles have a high absorptive capacity, they might have a tendency to form aggregations which will not be desirable. If the particle size is too small, again there will be a tendency for the dust to granulate.
When a dust is applied with a machine, an electrical charge is produced by friction among the particles, and between the particles and the dusting equipment. A high charge will make the dust to stick to the machine and not have a good flow. The magnitude of the charge depends chiefly on the composition of the dust, material of which the equipment is made, amount of agitation of the particles, temperature and humidity at the time of application.
(ii) Sprays:
Spray fluids are prepared by dissolving insecticides in water or oil or by the dilution of wettable powders (WP), emulsifiable concentrates (EC) and water soluble concentrates (WSC) in water. In wettable powders, it is not desirable that the solid particles should settle down too rapidly. Therefore, some dispersing agent or deflocculator is added to the powder to keep its particles in suspension. The common materials used are gelatin, glue, gum, etc.
Insecticides are also available in the form of wettable powders which have to be mixed with water at the time of application and diluted to the desired concentration. The particle size in such cases must be less than 10 µm in diameter and in some preparations; a particle size up to 1 µm is also available.
Emulsions are made by the dispersion of oil droplets in water, there being a protective film of emulsifier around the oil droplets. The size of droplet is important in many ways and it is governed by the kind and amount of the emulsifier used. If the size is large, the emulsion is unstable and the oil tends to rise and float on the surface, when allowed to stand for some time.
If the droplet size is very small, the emulsion becomes very stable and the oil does not rise even on standing for hours. However, in insecticides it is desirable to have an emulsion that is sufficiently stable in the spray tank but on leaving the nozzle or upon contact with the surface being treated, the film of the emulsifier should break and the oil droplet should stick to the surface or to the insect pests.
In case the film does not break, the oil droplet drains off. The effectiveness of a spray is influenced by a number of other factors as well like the leaf surface, the prevailing temperature, the insect species involved, the rate of break-down of the chemical, etc.
On spraying, a surface becomes wet when the liquid comes into direct and persistent contact with the surface of a solid, there being no layer of air in between. Immediately after wetting, the liquid forms a persistent liquid-solid inter-surface, solely by surface tension and, thus, the spread of the liquid is obtained.
The surface of the foliage, in some cases, is wetted more readily than that in others, depending upon the waxy layer on the cuticle. A high-quality spreader is used in the preparation of an insecticide when maximum coverage is desired in the field.
Where it is desirable to have the residual effect, the spreader is replaced by an adhesive such as protein or a similar colloid, which is absorbed on the particle of the insecticide and provides adhesion to a solid surface. Other adhesives used are hydrophilic inorganic compounds, molasses, dextrin, sugar, starch, wheat flour, gum, linseed oil, fish oil, etc.
Where solid particles are suspended in a spray, the size of the particle determines the efficiency of an insecticide. When the fluid is actually sprayed, the droplet-size varies according to the fineness of the spray. It is 400 µm or above in a coarse spray, 100-400 µm in a fine spray and 50-100 µm in a mist spray. An emulsifiable concentrate of an insecticide available in the market has to be diluted to the desired concentration by adding water.
The percentage of active material is written on the packing and the dilution is done as – The concentration of the active material in the product used is divided by the concentration required in the spray fluid. The figure obtained indicates how many parts of water are to be added to one part of the product, either by weight or by volume. Malathion which is available as 50 per cent emulsifiable concentrate is generally applied to a crop at the concentration of 0.05 per cent, i.e., one part of the insecticide is added to 1000 parts of water.
(iii) Granules:
The granular formulations are discrete aggregates of dust particles which disperse gradually and, thus, last longer and are very effective when placed at strategic sites, i.e., in the soil near the root-zone or in the whorl of leaves in maize or sorghum plants. The granule size is expressed as 30/60 or similar figures for other sizes. This figure indicates that all the granules pass through a 30-mesh sieve (30 meshes per linear inch or 2.5 cm) and only a negligible number of particles will pass through a 60-mesh sieve.
(iv) Aerosols:
The aerosol ‘bombs’ (non-explosive) are produced for use in houses or stores for killing flying insects. An aerosol consists of minute particles of insecticides suspended in the air as smoke, fog or mist. The particle size varies from 0.1 to 50 µm. The dispersion of the insecticide is achieved by burning a dry insecticide material or an insecticide in a volatile oil-base, by atomizing mechanically a liquid insecticide, or by pushing with force an insecticide through an orifice 0.03-0.06 cm in diameter, with the help of a liquid convertible into gas.
In the latter case, the liquid evaporates and the insecticide particles are dispersed in the air as a fine mist. These ‘bombs’ are very useful in the kitchen, pantry, living- rooms, bedrooms, cellars, etc., and before use the doors and windows are closed. If an aerosol is opened for 6 seconds in a room of 30 m3, mosquitoes, house-flies, sand flies, moths, etc. are killed and, if released for 15 seconds, fleas, wasps, hornets, etc., are also killed.
The particle size is important for obtaining the maximum effectiveness. If it is too small, the particles are deflected from the flying insects, and if too large their dispersion is poor and they settle down too rapidly. A particle size of 10-15 µm is the optimum.
Factor # 2. Penetration of Insecticide through Cuticle:
Apart from its entry through spiracles and the alimentary canal, the insecticide is also absorbed directly through the cuticle. The penetration and effectiveness of an insecticide depend upon the quality of the cuticle and chemical composition of the insecticide.
This is so, because most of the insecticides are lipophilic and are absorbed by wax in the epicuticle. However, some lipid-insoluble materials, such as arsenicals and flourides may also find their way through the cuticle. If the thin waxy layer of the epicuticle is dissolved in some organic solvent like ether, chloroform, xylene, etc., penetration becomes even faster.
The most vulnerable regions for action of contact insecticides are the head, neck, thorax or other intersegmental parts where the cuticle is thin and there are vital nerve-centres. Other such parts are antennae, bases of wings, trochanters, tarsi and intersegmental membranes.
The body characters that provide protection against contact insecticides are the hard elytra of beetles, hair-coating of hairy caterpillars, thick waxy secretions of mealy bugs, some aphids and coccids. All these mechanisms are normally meant to keep water away.
After penetrating the cuticle, a contact insecticide like pyrethrum, may directly affect the hypodermal cells, reach the haemolymph and destroy haemocytes. It may be deposited in various tissues as in the case of DDT or may be transported along the lipid sheaths of the nerves, starting from the nerve-endings, as in pyrethrum.
Whether the insecticide is lipophilic or not, the undissociated molecule has a much faster penetration than the dissociated ionic component. The type of carrier used also determines to a great extent the rate of penetration. An oil solution or an oil-in-water emulsion has better penetration than an aqueous suspension. Water, because of its high surface tension, is unable to enter the tracheae, whereas mineral oils or aqueous solutions of the wetting agent having reduced surface tension can enter the breathing tubes.
Factor # 3. Specific Susceptibility to Insecticides:
Different species of insects have a wide range of susceptibility to insecticides. An insecticide can act only if it hits the vital part of an insect. Lepidopteran tissue-borers or weevils that feed while hiding inside plant tissues, naturally escape the topical application. The habitat and the behaviour of the species play an important role. A very mobile insect like the cockroach or another having a strong sense of smell will stay away from a treated area.
Different developmental stages of a pest species are also affected differently by an insecticide. In the pupal stage, an insect neither respires very much nor feeds and is often protected by a cocoon or by an earthen cell and hence, it escapes being hit directly. In contrast, a larva, which feeds exposed, is very highly vulnerable to stomach as well as to contact insecticides.
Factor # 4. Weather Conditions:
The various components of weather influence the effectiveness of insecticides. If the wind is blowing at a speed of 8-10 km per hour, it will interfere with spraying. Dusting from an aircraft is best done early in the morning or in the evening when there is only a slight wind.
On the other hand, when the sun is up, the rising currents tend to carry the dust away from the crop. If aerosols are to be applied, wind velocity of 1.5-6.0 km per hour is useful and again cooler temperatures near the ground in the morning or after sunset are suitable.
A few meters higher up, the atmosphere is warm and the aerosol cloud has a tendency to rise and drift. In general, extremes of temperature are not favourable. Certain chemicals, when applied in hot weather, may injure the plants. The rate of metabolism is fast at high temperatures so the insects have an inherent capacity to resist the poison. If the weather is too cold, the oil sprays may cause injury to foliage.
If insecticides do not contain a good adhesive and the spray does not stick to the plants effectively, it may be washed off by rain. Dusts are more easily washed-off than sprays but if dusting is done when there is dew on the plants, the insecticide sticks quite well.
Factor # 5. Conditions in the Field:
The condition of the plant, the location of the pest, the time of application and the amount of insecticide applied, all determine the effectiveness of an operation. Some plants grow faster than others, still others grow faster in one season than in another, and the fruits grow much faster in the early stages than in the later stages.
In these cases if the entire surface is to be kept covered with an insecticide, naturally the frequency of application has to be changed accordingly. Even where it is not necessary to maintain a deposit, it is desirable to time the application properly. Spraying or dusting is to be done when the pest is in a vulnerable stage; if done too early, the maximum kill may not be obtained; if too late, much damage to the crop may have already been caused.
In general, crops should not be sprayed while in blossom, because the flowers in which seed is to set may be damaged or the bees and other insects so important for the pollination of crops, may be killed, thus causing reduction in yield instead of an expected increase. The structure of the leaf is also important; hairy leaves are more easily covered while those having smooth and waxy surface have to be sprayed with more sticky insecticides.
Since the pest is to be directly hit, the method or site of application of the poison would depend on its location. Insects feeding on the lower surface of the leaves have to be reached by the spray, those feeding on the roots have to be killed by mixing insecticides with the soil, and those rising up the trunks of the trees have to be intercepted with sticky or poison bands.
Various insecticides are effective for different durations. As a rule of thumb, the duration of effectiveness of dusts is for about one week and that of the sprays for about two weeks. The excessive use of insecticides is expensive and wasteful, and can be injurious to plants or non-target animals in the soil.
Even pond life can be affected by the run-off from treated fields. The selection of an insecticide and the frequency of its application should be such that the near maximum kill is obtained with the least quantity, so that the natural enemies of the pests can be spared as far as possible.
Factor # 6. Compatibility:
Since the farmer is interested in protecting the crop both from pests and diseases, the insecticides and fungicides or even some weedicides might have to be mixed together before spraying. Some chemicals are compatible whereas others are not; the former should not be mixed because they might become ineffective or damage the foliage.
The mixing should be done with the full knowledge of their compatibility or in consultation with an expert. Based on the compatibility of various compounds, a miscibility chart should always be consulted at the time of mixing the chemicals.
Factor # 7. Application of Pesticides:
Apart from precautions for spraying and dusting, there are many other points that should be considered for obtaining the best results. A given preparation of insecticide has instructions printed on the packing and these should be followed strictly. For example, an oil-base insecticide meant for use on walls, furniture, wooden structures and floors should not be sprayed on plants and again the one meant for use on dormant trees cannot be used on those having foliage.
Arsenicals (and chlorinated hydrocarbons) persist in the soil for many years and may accumulate in quantities sufficient to injure the plants under temperate conditions. The organophosphates are generally very toxic to man and domestic animals, but they have the advantage that they do not persist for long and are detoxified in the soil after 2 or 3 months. They are also generally destroyed when the food is cooked.
The old saying that “If little is good, more is better” does not apply in the case of insecticides. The right dose (quantity per hectare) or the right potency (concentration of an insecticide) should be applied at right intervals. To get the best results a preparation has to be applied in optimum quantity.
This is possible if the machine is correctly calibrated. It is particularly important with the power-operated sprayers and dusters. With the manually operated high-volume sprayers, generally 600-1000 litres of the fluid is required per hectare, whereas with the low-volume sprayers (carried on the back or on a trolley) only 125-200 litres is sufficient, hence the concentration is increased.
Since the rate of flow is important in power sprayers and power dusters, it should be measured by noticing the amount of the insecticide used on a known crop area. The rate of flow can then be increased or decreased by making adjustments in the apertures.
Certain safeguards are needed to protect the consumers of food (vegetable or animal origin) and other animals in the environment such as wildlife, domestic animals, honeybees, fish, etc. When applying insecticides inside buildings, we have to be sure that no food lies exposed or that the toys of children do not get excessive deposits. The unused insecticide should be kept out of reach of children or illiterate people and also away from food products. Empty containers should not be used for other purposes.
Strict precautions are to be taken at the time of application. All insecticides are poisons and, therefore, not to be swallowed. Many of them can also be absorbed through the skin in lethal quantities, or inhaled while applying them. Therefore, the operators should wear gloves, goggles, overalls, gum boots and gas masks, and should never dip their hands into the preparation. While spraying or dusting they should not work against the wind.
The left-over spraying fluid should not be dumped into a pond or a stream, because fish and other aquatic animals will die through contamination; nor should the equipment be washed there after use. The left-over fluid and the empty containers should be buried in soil, about half a metre deep at some isolated place and away from any water source.
It is important that the equipment after use is washed, otherwise there can be a mistake, particularly if it is used for the application of weedicides and then unknowingly, for applying insecticide or a fungicide. Even small residues of a weedicide like 2, 4-D can cause very serious damage to valuable crops like cotton and potato.
While fumigating, the operators using a particular gas should be thoroughly trained in its use. They should use gas-masks and protect their hands with gloves. The warehouse or the store to be fumigated should be made airtight by closing all cracks and crevices with mud, leaving open the main entrance which is closed after opening the outlet of container of the gas.
If the room to be fumigated is close to human dwellings, the persons should be asked to vacate them temporarily. After allowing the required exposure for fumigation, the room is opened, so that the gas may escape and only then should any person be allowed to enter it or to live nearby.
The dose to be used is generally expressed as litres/kilograms per 100 cubic metres space. This dose should be very carefully applied to plants in a greenhouse, because plants are sensitive to excessive doses. Moreover, the fumigation should be done at night owing to the possible injury to green plants in the presence of light. The duration is rarely more than one hour and the optimum temperature is 5-25°C.
Fruit trees like citrus, apple, walnut, etc. can also be fumigated in the field by enclosing them in canvas or nylon tents lined with polythene. The same procedure can be followed for fumigating open heaps of grain, a pile of furniture or a stack of timber.
The soil fumigants are very costly and, therefore, their application should be made with proper equipment which is generally tractor-driven or power-driven. If the fumigant is not applied at the right depth, at the right soil moisture, and is not properly packed afterwards, the gas can escape rendering the effort and money futile. For killing soil nematodes, D-D mixture, DBCP (Nemagon) and fensulfothion (Dasanit) are quite effective.