Everything you need to learn about how to control pests in crops. 5 Ways to Control Pests in Crops:- 1. Cultural Control 2. Physical and Mechanical Control 3. Biological Control 4. Chemical Control 5. Legal Control.
Cultural Control:
By cultural control, it is meant those methods of planting, growing and harvesting a crop which will prevent or lessen insect damage. This method consists of a slight departure from the usual time of planting, sowing or harvesting the crops or changing the plan of crop rotation, weed control, disposal of the crop remnants after harvest. Planting good seeds and resistant varieties are very important in controlling some insect pests.
Since cultural methods are usually economical, they are specially useful against pests of low unit-value crops. Practices which reduce the chances of buildup of pest populations may hold them below the level which will cause economic damage. Such methods are particularly applicable to field crops and forests.
Knowledge of the life history or bionomics of a pest species is essential to the effective use of cultural control methods. The principle of the “weakest link” or most vulnerable part of the life cycle usually applies. The environment is changed by altering farming practices at the correct time so as to kill the pests or to slow down their multiplication. In this way, the method is aimed more at prevention than at cure. If the environment is unfavourable, the pest may not reach a population level which will cause serious damage.
Cultural controls are often used when chemical or biological methods have not yet been devised for an injurious species. Cleanup of the sources of infestation and changes in the planting or harvesting time are particularly important when no effective method of killing the pest is known.
However, these methods are also used in combination with other controls:
1. Control by Planting Pest Resistant Varieties:
Seeds of healthy plants preferably of a resistant variety only should be sown as far as possible. Certain varieties are naturally resistant or less susceptible to insect attack e.g. certain varieties of wild apples are immune to aphid attacks while most of the cultivated varieties are very susceptible. Hence, by interbreeding wild with cultivated variety, a strain can be evolved which yield good fruits and shall also will be immune to pests.
Similarly the Indian cotton Gossypium orboreum is much more resistant to the attack of jassids, whiteflies and the pink bollworms than the American cotton, G. hirsutum. Also the indigenous cotton is resistant to leaf roller but susceptible to pink boll worm and the foreign variety behaves in an opposite manners. Hence, by crossing the Indian species with the American species a new strain, immune to aphids, jassids and pink bollworms etc. can be developed.
2. Tolerance to Insect Infestation:
A variety may be infected by insects yet survive and show less injury than others because of its ability to replace injured parts such as leaves and rootlets.
3. Ploughing in Relation to Insect Control:
The plough if used at right time is a good tool for combating many insect pests; it disturbs or kills them, eradicates weeds upon which they might feed and breed, exposes them to natural enemies or to weather control and buries them so deeply that few adults can emerge. A thorough stirring of the soil before planting is an indirect method for controlling the corn root aphids, because, it breaks up ant colonies, kills many aphids and prevents the growth of weeds on which they live until corn roots are available.
4. Clean Cultivation:
Clean cultivation means the removal of weeds, plant residues, and other materials from the fields and growing only healthy crops. The destruction of crop residues is a very good preventive measure for controlling certain species of insects e.g. caterpillars or beetles. The elimination of remaining effectively stops further insect breeding by cutting of their food supply and shelter.
Many insects infesting crops will develop on weeds which may or may not be related to these crops botanically. Hence, weed control is a good practice for reducing insect infestation. Damage due to Leptocorisa is very serious when paddy fields are surrounded by weeds which provide a good shelter to the pests till the ears have been formed in them.
Similarly various species of red hairy caterpillar, Amsacta moorei, Amsacta collaris and other species of this insect are polyphagous and lay eggs on a number of weeds and on emergence the caterpillars feed on the weeds and then migrate to the nearby fields of any crop.
5. Crop Rotation:
Crop rotation is effective as preventive measures against insects that feed on relatively few plant species or where the insects are incapable of long distance migration but crop rotation is useless against a general feeder. If one and the same crop is grown every year the insect population is bound to increase due to abundance of food material. On the contrary, if crops are grown in rotation or alternate years e.g. a crop of one plant family followed by that of different family e.g. barley grown in spring may be followed by legumes or pulses and then wheat in winter and so on.
The crops of the same family e.g. cotton, and lady’s finger (okra) belonging to the same family Malvaceae must not follow same year. Mixed crops are sometimes useful as pests do not thrive in large number.
6. Flooding or Irrigation:
Where water is available, it is sometime possible to destroy insects by flood or irrigation of the field. Flooding rice fields destroys many sugarcane borers. Similarly other burrowing insects e.g. crickets, grasshoppers, beetles and bugs come out of the ground after flooding the fields. They either die their natural death or readily picked up by birds. Ploughing followed by heavy irrigation within 30 days destroys many pink boll worms. The sugarcane and wheat crops can be saved from the attack of termites by irrigation.
7. Fertilizing:
Nitrogenous fertilizers in some soils tend to increase the susceptibility of sorghum to damage by attack of some insects. As a result of proper manuring the plant growth is stimulated. The healthy and vigorous plants can easily resist the attack to which the weaklings would have surrendered.
8. Pruning and Thinning:
Some pests are normally carried from old crop to the new one. Pruning and thinning decrease the intensity of attack. Infested plant shoots and dead hearts must be immediately removed. Ratooning should be avoided and there should be plenty of intervals between harvesting a crop and sowing a new one.
9. Time of Planting and Harvesting:
Time of planting and harvesting has great influence on insect infestation e.g. early maturing cotton is not attacked by pink boll worm in areas where moth do not emerge early. Late planted wheat escapes oviposition by hessian flies in America because the plants do not emerge until most of the flies have disappeared as they are very short lived. Similarly early sowing of rice in the Punjab between the 3rd week of May and mid-June is helpful in protecting it from the attack of rice borer, Tryporyza incertulas so the time should be so regulated that the crop may not be worth the attack at the active period of the pests.
10. Closed Season:
An insect species limited in its feeding to one crop may be eradicated by not growing this crop for a year or two.
This has been done with success in isolated areas for eradicating the pink boll worm. Such a closed season consists in not growing cotton for a period of several months.
11. Destruction of Volunteer Crops:
Volunteer crops are those which grow from self-sown seeds, and stubble crops are those which sprout from roots or stubbles (stumps), may be a source of insect infestation and they should be destroyed by ploughing or other means.
12. Trap Crop:
A trap crop is a small planting often only a few rows, made somewhat earlier than the main planting for the purpose of diverting insects away from the main crop. The trap crop can either be harvested early or cut and used as fodder before a generation of the insect can be completed. Otherwise it will serve as a breeding ground for the pest which will then attack the main crop in large numbers. The plant species used for trap crop should be one that is very attractive to the insect. The preferred host plants can also be grown around the main crop and when the pest has appeared it can be cut and destroyed.
The other methods of cultural control are early cutting of crop, rate and depth of planting, sowing, separation of complimentary host plants, pasturing the heavy infected crops and drying out the soil etc.
Physical and Mechanical Control:
The protection of crops and foods from insect attack by physical and mechanical means is the simplest and most effective method. Such measures consist of destruction of insects by mechanical means, burning, trapping, protective screens or barriers, use of high or low temperature, soaking in water and drying.
1. Hand Picking:
When only a few plants are infected, certain large conspicuous species may be removed from the plants by hand and destroyed just by pressing the abdomen or dipping them in kerosenized water.
2. Mechanical Means:
A rotary blow has been found to destroy over 96% of white grubs as well as cut worms, army worms and other caterpillars. The cutting wheel is thirty two inches in diameter and fourteen inches wide and carries sixteen steel blades. It operates by power take off from a tractor at 150 rpm.
3. Trapping:
The insects may be turned into a tray by light and baits or the trap may be mobile and drawn across a field.
4. Light Traps:
The attraction of nocturnal insects to light is a common observation and the light traps have been invented and tested for killing crop pests. The attractiveness of an artificial light depends upon its candle power and colour. A yellow light will attract 60% fewer insects than a white light of the same candle power and brightness. The insects attracted by light may be destroyed as they hit electric grid or they may be drowned in water and oil after they have been turned into the trap.
5. Bait Traps:
Baits are a form of control whereby insects are attracted to a selected spot on which they can be easily removed. Bait traps have been found to be very effective in controlling the nocturnal insects like cockroaches, crickets, caterpillars and nocturnal moths.
6. Temperature Treatments:
(a) Low Temperature Control:
Low temperature extremes are fatal to insects of stored grains. Usually a temperature of 28°C or lower for at least several hours will kill most of the insects. Tribolium confusum can be killed in 24 h when exposed to subzero temperatures. Low temperatures are utilized for the control of insects in mills, warehouses and similar establishments.
(b) High Temperature Control:
The Tribolium confusum cannot complete its development at 41°C and temperature of 44°C or above kills the adults. High temperatures are also fatal to other species and can be used to kill them. Heating cotton seeds to a temperature of 65.5°C for 30 seconds will kill all pink boll worms in the seeds. Temperature of 48.8-51.6°C in all plants of a flour mill for 10-12 h will kill all insects exposed to such temperature.
Exposing the infested grains to the sun in summer also kill all the adults of the stored grain insects.
7. Physical Barriers:
The protection of crops from insects by barrier is possible in case of crawling and migratory insects. Treated paper barriers and dust barriers are also used. Trenches are also used to stop army worms. Similarly water channels can be used for checking the migration of crawling insects.
8. Burning:
Although destroying insects by burning their shelters is sometime practicable, burning pastures and woodland to destroy insects should not be recommended. It has been found that such practices reduce soil fertility and increases soil erosion. Furthermore, it fails to destroy injurious species but does eradicate many beneficial forms.
Both cutting away dead hearts and burning them in time save valuable crops from destruction. Burning the field refuse and stubbles, are the other useful ways of pest control.
9. Sieving and Winnowing:
It is possible only for domestic purposes.
10. Flooding:
It is well known that a large number of insects breed and dwell in the soil, which may be destroyed by flooding the fields with water. There are several other simple methods through which a number of pest infestation can be controlled but the mechanical method of insect pest control involves a lot of labour like shaking the trees and killing the insects either in the larval or adult stages.
Biological Control:
The term biological control means control of insects by several biological agencies such as parasites, predators and pathogens. The introduction of sterile males in a normal population and use of sex attractants have been reported to bring about effective biological control. De Bach (1964, 1974), Reay (1969), Huffaker (1974), etc. have given exhaustive account on Biological control-
The theoretical basis of the biological control defends on the existence of a natural enemy.
The effective natural enemy can be deduced to have the following characteristics:
(i) High searching ability,
(ii) High degree of host specificity or preference,
(iii) Good reproductive capacity relative to the host and
(iv) Good adaptation to a wide range of environmental conditions.
The most essential characteristic is high searching ability. It should be borne in mind that a really effective enemy may be scarce in its native home because it regulates the host population at low level. There usually is one best enemy for each species in a given habitat and one frequently is sufficient for complete biological control, often, however, a second or third enemy species may add to host population regulation and may in fact be necessary to achieve satisfactory biological control.
The best enemy species may differ for different host habitats. Hence there is generally no single best natural enemy extending throughout the range of a pest species. With these qualities the introduced natural enemy by itself or in conjunction with other mortality factors is expected to prevent outbreak of the pest species or atleast to drop down major population fluctuations.
The essential principle of biological control is to maintain a state of biological equilibrium of a living organism. This biological equilibrium is normally maintained by a set of forces called environmental resistance.
Often the biological equilibrium of a living organism is disturbed either due to environmental or biotic factors and consequently a particular organism multiplies and spreads in a unlimited manner so as to assume the status of a pest. The inherent ability of an organism to survive and to reproduce within a given time and under optimal environmental conditions is known as biotic potential.
Biotic potential is a force opposed to environmental resistance. It takes into account the number of young produced per female in each reproduction, the number of reproduction in a given time and the sex ratio of the species.
Advantages of Biological Control:
Only a limited number of introduced pests have been successfully exterminated with chemicals. It would seem wiser in many instances to attempt a biological control programme which might reduce pest populations below the level which causes economic damage. Biological control has the tremendous advantage that, if successful, it becomes self-sustaining and integrated into the normal environment of the control area.
If pest populations are reduced to the point where economic damage is negligible, the control programme is a success. Biological controls tend to be particularly useful on low unit-value crops where complete control may not be required or where chemicals are not recommended. Pests of field and forage crops, forests, and range may be economically controlled by biological methods; chemical controls against such pests may be impractical.
A biological control destined to be fully effective will be easily and quickly established. If an imported parasite or predator is not established within three years of careful releases under good conditions for its development, the programme may be justifiably discontinued.
There are 4 main types of biological control viz.:
(i) Introduction of parasites,
(ii) Predators,
(iii) Pathogenic organisms such as bacteria, viruses and fungi etc. and
(iv) Removal of fertile males and creating sterility in males by gamma radiation and introducing these sterile males in a normal population. In addition to these 4 main types, there is a fifth type of biological control where the indigenous parasite of the locality is being utilized for this purpose.
Parasitic insects develop as larvae on or in a single host individual from eggs generally laid on, in or near the host and usually consume all or most of the host body, killing the host and then pupate, either within or outside the host. The free living adult parasite emerges from the pupa and starts the next generation a new by actively searching for host in which to oviposit.
They tend to attack only one host stage i.e. eggs, larvae, or pupae, although there is also some overlapping in certain cases, adult insects do not serve as hosts. Very often life cycles are commonly short, ranging from 10 days to 2 weeks or so in mid-summer but correspondingly longer in cold weather. In general, they all have great potential rates of increase.
Hymenoptera:
This is the dominant order among all entomophagous insects. Over two thirds of the cases of successful biological control of pest’s species have been achieved by hymenopterous parasites. The most important and numerous hymenopterous parasites occur in the major groups commonly known as chalcids, braconids, ichneumonids and proctotrupeds.
(i) The Chalcids:
The chalcids attack species in nearly all orders of insects, the preferred ones being Coleoptera, Diptera, Homoptera and Lepidoptera which also happen to include the bulk of our chief crop pests. The immature stages of chalcids predominantly develops in host’s eggs or larvae or pupae and some develop in adults of Homoptera. The encyrtids and aphelinids include many that specialize in attacking scale insects, mealy bugs, white flies, and related forms and have been so outstandingly successful in biological control.
(ii) The Braconids:
This is a major large family of parasitic wasps. They have been used in biological control. Only an occasional rare species is hyperparasitic hence the family is nearly entirely beneficial. Parasitism may be external or internal, solitary or gregarious. Their host preferences are mainly lepidopterous and coleopterous larvae but also commonly Diptera, including the damaging fruit flies and Homoptera, particularly aphids.
(iii) The Ichneumonids:
The Ichneumonidae comprise about 20 per cent of all parasitic insects. Most are primary parasites, hence the family is predominantly beneficial. Some of the largest and most conspicuous parasite species occur in this family, Ichneumonidae probably parasitize wood and stem boring larvae in the Lepidoptera, Coleoptera or Hymenoptera but many attack lepidopterous pupae too. Some species have a very wide host range.
Most of the Tryphoninae are solitary parasites of sawflies. The Ophioninae are predominantly internal parasites of lepidopterous larvae although a few develops in the grubs of root feeding scarabs in the soil.
(iv) The Serphoids:
This contains several families, the most important of which are the Platygasteridae and the Scelionidae. All are parasitic on immature stages of other insects and they are predominantly primary internal parasites.
In addition to tachinids which comprise the most important dipterous family in biological control, this order contains several parasitic families including – Cyrtidae, Nemestrinidae, Pipunculidae and Conopidae etc. However, tachinids constitute the major parasitic family both in numbers of species and in economic importance and have been the only group utilized extensively in biological control.
The Florida red scale, Chrysomphalus ficus invaded Israel and spread throughout the Citrus growing areas and soon become a major citrus pest. An indigenous ectoparasite, Aphytis chrysomphali was the only parasite commonly recorded from the scale in Israel. The attacks of this species were limited mainly to male scale pupae and its effect on the scale population was negligible.
Another species of Aphytis viz. Aphytis holoxanthus and the endoparasitic Pteroptrix smithi were introduced into Israel from Hong Kong during 1956-57 against the Florida red scale; and complete biological control was achieved throughout the coastal region, which is the main citrus growing area of Israel, during the first 2 or 3 years following the introduction of the parasite, and the Florida red scale was virtually eliminated from that area as a pest.
Similarly biological control of Olive scale was achieved by introducing two additional species of parasites viz. Coccophagoides utilis and Anthemus inconspicuus from Pakistan to California. The biological control of California red scale was achieved by introduction of Aphytis lingnanensis from South China to California.
A great deal of success has been achieved in New Zealand and other parts of the Australia by using Pteromalus puparum imported from United States to control populations of the small white butterfly, Pieris rapae.
Although various Aphidius species have been used for aphid control, the best known Braconid, is Macrocentrus ancylivorus. It has been reared on a large scale in the United States using the larvae of the Strawberry leaf roller as the host and liberated for the control of Oriental fruit moth, Grapholitha molesta in peach orchards.
Finally, mention should be made here of the spectacular eradication of the Citrus black fly, Aleurocanthus woglumi from Cuba and various islands in West Indies by Eretmocerus serius imported from Malaya. Control was achieved so rapidly that when Aleurocanthus woglumi become an important pest in Mexico some years later, Eretmocerus serius was first choice as control agent.
Unfortunately, it was a dismal failure and further investigations indicated that climatic conditions were unfavourable. The pest was eventually brought-under control by Amitus hesperidum and Prospaltella species imported from India.
One of the important factors governing parasite introduction for biological control is the geographical location of the area under experimentation and the conditions met within that area and presence of an indigenous fauna of a peculiar and restricted character. Introduced parasites meet with relatively little competition from indigenous forms.
Limitation of the area to be covered by parasitic colonization, resulting in the presence of only few main crops and therefore facilitating adequate organization and centralization of control experiments. Other factors which hamper an efficient programme of biological control are the phenomena of multi- parasitism and hyperparasitism, there is ample evidence to show that the efficacy of introduced parasites has been enormously reduced by these two factors.
Certain other considerations which have a direct bearing on parasitic introduction are suitable adaptation of artificial measures of control so as to put the destruction of more parasites than hosts, proper recognition of the time factor in biological control experiments and proper evaluation of the capacity of a parasite to discover its host.
Utilization of Indigenous Parasites:
Biological control is also brought about through the agencies of indigenous parasites of a locality, such operations consist either of conserving or increasing the number of parasite’s in a given area or of attempting to colonise such parasites by artificial culture methods under laboratory experimental condition in an area of a country where they did not previously exist.
One of the important features of this type of biological control is the alteration of the host parasite ratio in a given locality. Example of such a biological control is Trichogramma minutum, a chalcid egg parasite on notable pests such as Pyrausta nubilalis and the sugarcane borer, Diatraea saccharalis.
Predator insects differ from parasitic ones in that the larvae or nymphs, as the case may be, require several to many prey individuals to attain maturity. The adults generally deposit their eggs near the prey population and after hatching the active mobile immature search out and consume prey individuals. This need of larvae or nymphs to search is an important distinction from parasitic forms whose larvae develop on a single host individual and thus have the advantage of not having to discover additional hosts.
Adults of many species are also predatory. There may be one or several generations to one of the prey. Larvae and nymphs as well as corresponding adults may be predaceous or only one stage may exhibit the habit.
i. Coleoptera:
The main predaceous families of the Coleoptera are:
Coccinellidae, Staphylinidae, Histeridae, Lampyridae, Cleridae, Cantharidae, Meloidae, Cicindelidae, Carabidae, Dytiscidae and Gyrinidae. The Coccinellidae and the Carabidae are of most important in biological control of agricultural pests.
ii. Neuroptera:
Most of the species of the order Neuroptera are predatory. The most important families in biological control have been the Chrysopidae and Hemerobiidae which attack many agricultural pests including scale insects, mealy bugs, aphids, white flies, mites etc.
iii. Hymenoptera:
About one quarter of the families of this order are strictly predaceous. Most of the predatory forms tend to be social and live in colonies. The colonial vespid, wasps provision their paper nests with the bodies of caterpillars and other soft bodied insects. Species of Polistes have been used in several biological control projects.
iv. Diptera:
A few families are entirely predaceous and many others contain predatory species. The more common or important economic families include the Syrphidae, Asilidae, Cecidomyiidae, Anthomyiidae, Calliphoridae and Sarcophagidae. Of the latter four, many are predaceous in the egg masses of grasshoppers.
v. Odonata:
The dragon flies and damselflies are predaceous both as nymphs and as adults. The nymphs are all aquatic and feed on a wide range of aquatic insects.
Biological control has been very successful in certain cases where injurious insects have become established in land, they did not previously inhabit and have developed into persistent pests. A classical examples of such a biological control is that of the cottony cushion scale, Icerya purchasi in California, by a predatory coccinellid beetle, Rodolia cardinalis introduced from Australia.
Introduction of this beetle in California resulted in almost complete and permanent destruction of Icerya, within less than 5 years. This species was so completely destroyed that the survival become numerically insignificant. Subsequently the predator achieved complete biological control of the cottony cushion scale in 25 additional countries and a substantial degree of control in four other countries.
Many outstanding successes in the biological control of serious coccid pests have since been recorded. Some notable cases include the complete biological control of the Citrophilus mealy bug, Pseudococcus fragilis in California, the coconut scale – Aspidiotus destructor in the island of Fiji, Maritius and Principe, Green’s mealy bug, Pseudococcus citriculus in Israel; the red wax scale, Ceroplastes rubens in Japan; and Coffee mealy bug, Planococcus kenyae in Kenya.
A mention may be made of the vertebrate predators like fishes, frogs, reptiles, insectivorous birds and mammals etc. which are the regular feeders and deserve to be provided with suitable conditions to grow and breed profusely.
Pathogenic microorganisms attack insects and have life cycles more or less characteristic or similar microorganisms developing in other groups of animals. Insects are probably subject to as wide as variety of disease as are the vertebrates. Except for the fungi, disease organisms gain entry in the host via mouth or the digestive tract, i.e. the insect host must eat plant or other food contaminated with pathogen. In case of fungi, entrance is gained through the insect integument and free water or very high humidity is generally required.
Thus, fungi tend to be restricted to moist environments. However, fungi do have advantage of attacking sucking insects which because of the nature of their feeding on sap, tend to be fairly free of disease caused by microorganisms because they rarely ingest them. Virtually no insect disease organisms occur in mammals and none have been recorded from man. Thus they are safe to use in biological control even in large scale microbial spraying operations.
The most common disease of insects are caused by bacteria, fungi, viruses, and protozoa:
i. Bacteria:
Because it can be produced fairly easily on a large scale and has a wide spectrum of activity, Bacillus thuringiensis was of the first microorganisms to be incorporated into a commercial “microbial insecticide”. The most sensitive insects appear to be lepidopterous caterpillars, particularly those in which alkaline conditions prevail in the midgut. This is related to the fact that the spores (it is a crystal bearing spore former and can be conveniently stored and applied in the resistant state without loss of potency) can only enter the insect on ingestion and that the proteinaceous crystals of toxin is only liberated at around pH 9-10.
Those pathogenic bacteria which do not form spores have proved to be of little practical value in the field and indeed only two other groups of spore formers have been employed with any success i.e. Bacillus popilliae and Bacillus cereus. The former species produces a condition known as milky disease in the Japanese beetle, Popillia japonica and has been used extensively in North America for control of this pest and Chaffer beetles.
Although Bacillus cereus does not produce a crystalline toxin, it also requires closely defined mid gut conditions with an optimum around pH 7. This Bacillus is active against the codling moth and certain Hymenopterous pests.
ii. Viruses:
The classic example of the employment of viruses in pest control is the highly successful use of a polyhedrosis viruses against the pine sawfly, Neodiprion sertifer in Canada. A related pest Neodiprion swainei and the spruce sawfly, Diprion hercyniae are similarly controlled by a polyhedrosis virus.
Another example is the control of alfalfa caterpillar, Colias eurytheme by introduction of polyhedrosis virus. The cabbage looper, Trichoplusia ni and other brassica caterpillars have been controlled in a similar way. Only one other group of viruses, the granulosis type has been used against pests in field, in this case the Lepidopterans, Pieris brassicae and Pieris rapae. Further, for Nucleopolyhedroviruses, which are used for the control of insect pests.
iii. Fungi and Protozoa:
Pathogenic fungi have several properties which should make them ideal organisms for use in biological control. They produce highly resistant stages and are capable of prolific spore production once they became established. However, their great dependence on microclimatic conditions, especially humidity has seriously curtailed their use and only a few have been employed successfully of these – Entomophthora exitialis has produced promising results against the spotted alfalfa aphid, Therioaphis maculata.
Many protozoa appear to cause chronic rather than acute symptoms in their victims and the microsporidian, Thelohania hyphantriae takes upto four weeks to exert its full effect which has been shown to cause mortality in the fall webworm, Hyphantria cunea.
Genetic pest control implies the manipulation and use of genetic material in a manner injurious to pest insects. The control of insect pest populations by the release of sterile males has been demonstrated with at least five insect species. This spectacular technique has been termed autocidal control and involves using an insect species to bring about its own self-destruction.
It is accomplished by irradiating laboratory reared males of the species to an extent sufficient to disrupt the genetic function of the sperm nucleus but not appreciably interfere with the normal ability of the male to mate or of the sperm to penetrate the egg of the female. However, such fertilized eggs fail to develop so a wild female mated with a sterilized male produces no progeny.
If sterile males are released in large enough number in relation to the wild population they will fertilize more females than will the wild males. The advantage is cumulative in each generation hence eradication may be achieved within a few generations under ideal conditions. The major success thus far has been with the screw worm which was eradicated from the Island of Curacao, from Florida, and from Texas to California by massive annual release of sterile males.
A low infestation of the Oriental fruit fly was eradicated from Guam in 1963; the melon fly was eradicated from Rota Island in 1963 using a combination of a bait spray and sterile insect release, and the field cockchafer eradicated from a seventy four acre plot in Switzerland in 1962.
The boll weevil is reported to have been eradicated from a small isolated field by the release of chemically sterilized males. Chemosterilants have been under intensive investigation but are not as yet practical. They offer the advantage of treating the wild population directly and thus avoiding the mass rearing necessary with gamma radiation.
The ability of an insect to locate a mate and to copulate once it has been found is often directed by natural specific chemicals produced by the insect called pheromones. Natural chemical attractants and mating stimulants have been shown to occur in many pest insects. They are often effective in incredibly small concentrations; one caged virgin female of the introduced pine sawfly attracted over 11000 males.
Some have been chemically identified and even synthesized. Its high specificity for particular target insects however makes this approach highly desirable. Such phenomena gave rise to the intriguing idea of using these natural or artificial pheromones to attract and trap insects, to lure them to contact poisons, chemosterilants or pathogens and to mask the location of females by saturating the environment with synthetic sex pheromones. Such techniques could offer highly specific control methods with little or no ecological side effects.
An example of possibility is furnished by the eradication of the Oriental fruit fly on Rota I’sland where the artificial pheromone or attractant viz. methyl eugenol lured the male flies to a poisoned coated surface. A similar but more important achievement was the eradication of the mediterranean fruit fly from about one million acres in Florida in 1956-57.
It appears that adequate control from pheromones or attractants probably can only result if they are so highly effective as to be able to reduce the pest population to low level in the treated area and are used over a large area so that immigration of the pest from the outside would not negate the effort. Pheromone traps are commonly used for controlling various moths Heliothis, Spodoptera, Leucinodes etc.
Chemical Control:
Recent discoveries of new synthetic insecticides have sparked exciting advances and major breakthroughs in the control of insect enemies. Chemicals have subdued pests that once caused wide spread crop destruction, death of domestic animals and epidemics of insect borne human diseases. The modern insecticides are both effective and reliable. The whole world is resorting to them more and more for the solution of many insect problems.
The chemicals which kill the insects by their chemical action are termed as insecticides. They are used for the protection of men, domestic animals, crops, agricultural products from the attack of insects when other methods fail to control the pests. Insecticides are seldom used in full strength but are formulated in a variety of ways to make them easier for application.
Formulations of Insecticides:
Following are the common formulations of insecticides viz.:
(1) Dusts,
(2) Granular formulations,
(3) Insecticide-fertilizer mixtures,
(4) Wettable powders,
(5) Solutions,
(6) Emulsifiable concentrates,
(7) Aerosoles,
(8) Fumigants and
(9) Miscellaneous formulations.
Whatever may be the formulations the poisonous chemical present in an insecticide must penetrate the vital organs and tissues of the insect and ultimately kill it.
(1) Dusts:
Insecticidal dusts are those powders which are used dry and mixed with or impregnated with certain organic materials or pulverised minerals (powders) such as talc, pyrophyllite, bentonite etc. These minerals are called carriers or vehicles since they carry the insecticide. Dusts are blown to deposit on plants by dusting machinery or blowers.
Ground to a fine size, most dust will pass through a 325 mesh screen and range in size from 1 to 40 μ. The finished dust may be 0.1 to 25% active material. In dust form, in general, the toxicity of an insecticide increases as the particle size decreases.
(2) Granular Formulations:
These are similar to dusts except for larger particle size. The range of particle size in a granular product is designated by a two figures mesh classification e.g. 30/60 means that virtually all the insecticide granules will pass through a standard 30-mesh sieve while a negligible quantity will pass through a standard 60 mesh sieve.
Some of the common granular formulations are 16/30, 20/40, 24/48 and 30/60. Granular insecticides are generally used as dressings on or in the soil and may be applied with fertilizer spreaders or special granule applicators.
(3) Insecticide Fertilizer Mixtures:
Insecticide fertilizer mixtures may be formulated by adding granular insecticides to commercial fertilizers or by spraying insecticides directly on to the fertilizer. Such mixtures are applied at the regular fertilizing time to provide both plant nutrients and control of soil insects.
(4) Wettable Powders:
These are similar to dusts but they are meant to be diluted and suspended in water and used as spray. To make an insecticidal dust act in this manner a dispersing and wetting agent is added to the formulation. They are more concentrated than dust as they may contain as high as 75% toxicant.
(5) Solutions:
Many of the modern synthetic insecticides are insoluble in water but soluble in organic solvents. These soluble insecticides in solution form are used directly for insect control. They are however seldom used on plants because of their phytotoxic reaction.
(6) Emulsifiable Concentrates:
The most common and versatile formulation is the emulsifiable concentrate. This formulation consists of an insecticide, a solvent (for the insecticide) and an emulsifying agent. Mixing the concentrate with water forms an emulsion. The solvent used may evaporate quickly after spraying leaving a deposit of toxicant after the water has evaporated.
The use of an emulsifying agent serves several purposes; it makes possible the diluting of a water insoluble chemical with water, it reduces the surface tension of the spray thus allowing it to spread and wet the treated surface more effectively, helps the spray and make a better contact with the insect cuticle.
Generally oil soluble emulsifying substance is used. Normally emulsions are unstable and break up into their component parts. This action is termed breaking. For spraying on plants a quick breaking mixture is preferred. Since this results in heavier deposits of toxicant.
(7) Insecticidal Aerosols:
Aerosols are minute particles suspended in air or fog or smoke. The diameter of these particles range from 0.1 to 50 µ. The dispersion of insecticide into aerosol form may be accomplished by burning, vapourising with heat, atomizing mechanically or releasing through a small hole an insecticide that has been dissolved in a liquified gas. The last method of aerosol preparation gives the popular aerosol bomb.
(8) Fumigants:
Insecticides used in the gaseous form are known as fumigants. Fumigants are most often formulated as liquids under pressure and are held in tanks. When the liquid is released in open air it changes back to a gas, quite often fumigants are a mixture of two or more gases.
(9) Miscellaneous Formulations:
There are certain insecticides which are special formulations meant for specific uses. Often insecticides are stuffed in large pills and capsules and introduced into the stomach of animal. Insecticides may be mixed in shampoos, intended for use on house pets waxes for use on floors may contain an insecticide. Poison baits consist of toxicants combined with food stuff attractive to the insect pests. All these types of insecticides are special formulations designed for special purposes.
Synergists:
Some chemicals have the property of greatly increasing toxicity of certain insecticides. When the increased toxicity is markedly greater than the sum of the two used separately, it is termed a synergistic action. Most synergists have been used with pyrethrum or allethrin etc. Synergistic action is important because it provides a means for a more effective insecticide and it reduces the cost of control.
Legal Control:
Legal control is the lawful regulation of areas to eradicate, prevent or control infestation or reduce the damage caused by insects. This mainly involves the use of quarantines and pest control procedures. The central and state officials often work with local community or districts as in mosquito or locust control projects.
No attention was paid to the insect pests being carried from one place to another with the transportation of plants or seeds till it was noticed that the insects introduced into the foreign countries thrive best and increased at a very high rate in their new homes e.g. when American grapes were brought to France, Phylloxera caused such a heavy damage to French vine yards that the necessity of checking this pest from being carried to various parts of the world was seriously felt.
Hence, legislation had to form laws for this purpose. India and western countries like USA have passed a number of Acts to regulate the pest and to transport plant and their products. Under these Acts all imported goods should be carefully inspected to be insect free or should be fumigated and sterilized before allowing them to disembark. The export and import laws of the country now enforce the compulsory fumigation of food stuffs before and after embarkation.
The purpose of plant quarantine is to prevent by legal restrictions the entrance and establishment of a plant disease or insect pest in an area where the disease or pest does not exist. The area to be protected by this means may be a country, a state or it may be a smaller unit such as a part of the state.
There are four methods of plant quarantine viz.:
1. Inspection at the point of destination,
2. Inspection and certification at the point of origin,
3. Complete embargoes and
4. Controlled introduction of plants.
1. Inspection at the Point of Destination:
This inspection is necessary because many insect pests can be discovered and their accidental introduction thereby eliminated. Often it is physically not possible to examine all materials likely to carry insect pests with sufficient care to detect them. Also because some insects and plant diseases have inconspicuous stages they are extremely difficult to locate.
2. Inspection at the Point of Origin:
Most countries and many states follow this plan i.e. inspection and certification at the point of origin. In this case materials are allowed to enter the state or the country as the case may be, provided they bear a certificate issued by the plant quarantine officer which mentions that the materials have been inspected and found to be free from insect infestation or plant disease infection. Again here also it is not possible to separate physically all the materials likely to carry pests.
3. Embargoes:
By this we mean the exclusion of all plant materials or commodities which are classed as hosts of the insects. This is also not possible in practice. Because the area where the pest exists cannot always be definitely determined and the material listed in embargo may not include all of those which might carry or harbour the pests.
4. Controlled Introduction:
This consists of allowing the importation of only a very limited amount of material for propagating purposes, obtaining it from pest free or disease free areas and from plants believed to be insect or disease free, inspecting the shipment very carefully then planting in an isolated place for a preliminary period. After obtaining healthy stock in this way, it can be released for multiplication and distribution.
Enforcement of Plant Quarantine:
Foreign Plant Quarantine:
Plant quarantine inspectors are stationed by the Government at all of the principal ports of entry. Materials which must be watched are those which can carry the pests or diseases. They not only include the food plants of the insects but any article on which any stage of it might be found. Such as cotton used for packing, gift might contain seed infested with the pink boll worm and straw which could be infested. The central and state Governments have enforced several laws for this purpose, from time to time.
The Government of India have enforced several Acts like sea customs Act of 1876 to stop the entry of Mexican cotton boll weevil, destruction Insect Pests Act of 1914, Poison Act of 1919, Drug Act of 1940 etc. for preventing the entry of infested seeds and not allowing any material without proper inspection and certification that the material is free from all sorts of insect infestation as well as fungal, viral and bacterial diseases. As a rule, all the food stuffs are fumigated.