Although a large number of insects are injurious to crops and are to be eliminated or effectively kept under check, there are a number of insects which are of use to man in a number of ways and need protection.
There are many crops, particularly those of fruits and vegetables that cannot be successfully grown and harvested without the active and positive co-operation of insects. Some of these crops are apples, pears, oranges, peaches, prunes, cherries, plumps, raspberries, blackberries, cranberries, grape-fruits, lemons, and figs among the fruit crops; tomatoes melons, pumpkins, cucumbers, peas, beans, squash, egg-plant among the vegetables; and clover, alfalfa, soybean, cowpea, sweet clover and cotton among other crops.
These crops depend almost exclusively upon insects for the production of their fruits and seeds. This is so because of the phenomenon known as pollination, insects acting as pollinators. Because pollination goes on in nature in a rather inconspicuous manner without the farmer coming into the picture, he is generally unaware of these immensely valuable acts of his mute partners.
Such friendly insects have not received adequate investigational attention even from scientists because until recently it was not sufficiently realized that man can by his own intervention appreciably help this rather incidental but co-operative venture between insects and man.
Insect Pollinators:
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The scientific principal behind insects acting as pollinators is as follows. Although some plants can continue reproduction by asexual means, as by bulbs, tubers and cuttings, many plants have essentially the same type of sexual reproduction as in animals. The reproductive sexual organs of the so-called flowering plants with which mostly the farmers have to deal with are lodged in the flowers.
The principal part of the male organ is called the anther, which produces a large number of pollen grains, i.e., the male sex cells, and the principal female organ is the ovary containing the female sex cells and the stigma which receives the pollen.
In many plants these male and female organs are lodged, in the same flower; in others the flowers with male organs (male flowers) are separate from those having female organs (female flowers); in many species the two kinds of flowers are borne on the same plant, whereas in others the male and female flowers are borne on separate plants.
In some plants it is quite easy to comprehend that pollen from male flowers has to be carried by some agency to the stigma of female flowers. What is further interesting, however, is that even in many of those species which have both male and female organs in the same flower there is some kind of arrangement that sexual combination, i.e., fertilization, does not take place between its own pollen and ovary.
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There are often quite elaborate arrangements in favour of cross-fertilization or cross-pollination and against self-fertilization or self-pollination. In other words, there is a kind of code of sexual conduct even in plants.
Nature is a master geneticist exploiting the advantages of hybrid vigour often quite effectively. Insects have entered into indispensable partnership in this game with plants on the one hand and farmers on the other. The pollen of one flower is carried to the stigma of the other, mainly through 2 agencies, wind and insects. It is generally easy to make out wind-pollinated species from insect-pollinated species.
The former have inconspicuous inflorescence with small flowers; the latter have developed a variety of effective contrivances for attracting a particular species or group of insect species. The insects which are attracted to such flowers act as agents for the transference of pollen grain from the anthers of one flower to the stigma of the other, thus leading to cross-pollination.
In many plants the flowers produce nectar secreted by special glands situated in hidden niches at the base of the petals, and the petals are so modified and arranged that the insect seeking the nectar has to come in contact with anthers and stigma of the flower. In some the pollen grains are sticky, in others the stigmas are sticky; the insect body parts are also specially modified for such purposes.
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Some flowers have charming colours to attract insects; others tempt them by their perfumes. This is why some flowers have very spectacular petals but no perfume, and others have very sweet smell but inconspicuous flowers. Thus, the brilliant tinge and colour, aesthetic patterns and designs, enticing smell and aroma, and tasty and nutritious nectar of the flowering flora were originally evolved to seek the partnership not of man but of insects.
It is in comparatively recent times that man has begun to act as an effective agent of selection in the evolution of the garden flora. The positive correlation between the evolution of floral structure of certain plant species and the structural peculiarities of certain insects is so perfect that naturalists have been able to predict the presence of the one on the basis of observations on the other. It is difficult to narrate the various corresponding modifications in insects and plants specifically developed for ensuring cross-pollination.
But a few interesting examples are cited below:
i. Pollinator Bees:
Bees in general, and the honey bee in particular, are the best studied insect pollinators. The honey bee has exploited its role as nectar and pollen collector to the extent of developing an industry of its own. It has perfected not only the art of collecting nectar from flowers to convert and store it as honey but also the art of collecting pollen on a vast scale for providing proteinaceous food for the bee colony.
In nature there is pollen enough and to spare, and the value of the honey bee as an efficient pollinator is in no way reduced because it collects pollen on a large scale for its own use. The hairy body of the bee gets covered with pollen when it enters one flower after another, and then the bee collects this pollen from all over its body by means of a specialized brush on some of the sub-segment of the tarsus, which is the last segment of the hind-leg.
When this brush becomes fully laden, the pollen is transferred to a special pollen basket situated on the tibia which is the last but one segment of the hind-leg, and the bee carries this load of pollen in these baskets on the 2 hind-legs to its hive, where it is stored in special cells separately from honey. In this process, the cross-pollination necessary for seed setting in the flowers is also efficiently ensured.
The value of the honey bee for its pollinating activity can be gauged from the estimate that during a period in which the honey bee contributes honey worth Rs.5, its contribution to seed setting and fruit production is worth Rs.100. In other words, the value of the honey bee as a pollinator is 20 times as much as a producer of honey. The utilization of the honey bee colonies has become a commercial undertaking in many countries and 3 to 6 colonies an acre of a leguminous crop is considered to be optimum.
It is reported that in California 450,000 colonies were employed for pollination purposes during 1955. Yet the honey bee is not regarded as the most efficient pollinator. Other species of honey bees such as Apis florea and A. dorsata are also useful.
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There are a number of solitary bees belonging to the genus Nomia and Megachile which have begun to be exploited on a large commercial scale solely for purposes of pollination. Their habits have been carefully studied, and it has been found feasible for man to enter into a kind of symbiotic partnership with these bees. It is possible to persuade one of these species to come and rest in an artificial hive, which can be easily stored under controlled temperature conditions and taken to the field at the proper time for the progeny to emerge and pollinate the crop.
These genera are well represented in India also, but their study is yet in a very preliminary stage. There are also a number of other species of the bee family which do yeoman service in the production of fruit and seed. The carpenter bee (Xylocopas) in the plains, the bumble bee (Bombus) in the hills, and Anthophora in both are most spectacular of the flower-visiting insects. They are large-sized, long-tongued, and quite active.
One example of the value of these pollinators is afforded by the report that it was impossible to get seed from red clover in New Zealand until bumble bees were introduced from outside. The total value of insect-pollinated crops in the USA is reported to be more than 4.5 thousand million dollars annually.
ii. Other Insect Pollinators:
Besides the bee family there are many other insects, such as the wasps, some butterflies and moths, some diptera and beetles, which also carry out pollination; but the balance sheet of the good and harm that they do is not available and is rather difficult to work out. Hence it is not safe to call them friends of the farmer.
It is, however, desirable to mention in some detail the importance and activities of a chalcid wasp in the production of figs, for which this wasp is commonly known as the fig insect. The course of events has been well studied in the case of Smyrna fig. What is ordinarily known as the fig fruit is botanically an inflorescence enclosed within a fleshy pear- shaped receptacle, the inner surface of which is lined with innumerable small flowers; the receptacle opens to the outside through a very small orifice at the apical end.
The Smyrna fig produces only female flowers, whereas the corresponding male-producing plant is known as Capri fig. For proper development of Smyrna fig it is necessary that pollen from Capri fig be carried by some agency to the stigmas of Smyrna fig. The agent for this cross-pollination is a small chalcid wasp Blastophaga peenes. The female wasp enters through the apical orifice of both Smyrna fig and Capri fig; but it is able to lay eggs only in the Capri fig, and in this the progeny develops up to the adult stage.
The males are wingless, and fertilize the females within the fig itself. The fertilized females come out of the fig through the apical opening, with their body covered with Capri fig pollen grains. Subsequently, when these females enter Smyrna fig they inadvertently deposit the pollen grains on the stigmas, and this results in cross-pollination and proper fruit development. This absolute dependence of proper production of Smyrna fig on the activity of the fig insect was spectacularly demonstrated in USA.
The Smyrna fig produced there used to be of very inferior quality to that imported from Asia Minor, and it was found that this difference was due to the absence of the fig insect in USA so that fertilization of Smyrna flowers was not taking place and consequently the fig. fruit was not developing the proper flavour and quality. After the fig insect was introduced, production of normal figs was achieved.
A still more interesting example of symbiotic partnership is afforded by what is commonly known as yucca moth (Pronuiba yuccasella), which pollinates yucca flowers. The mother moth has a sharp-cutting ovipositor with which it makes an incision in the ovary of the yucca flower and lays an egg there.
Thereafter it shows a very extraordinary behaviour in climbing the stamen, scraping off a quantity of pollen grain, carrying it to the pistil in which it has laid the egg, and actually thrusting the pollen grain into the stigma, thus completely ensuring fertilization and development of the ovules.
The offspring of this moth eats a few of the seeds that result from the pollination carried out by the mother. This is a very interesting case of symbiosis between an insect and a plant founded on a well-calculated give-and-take basis.
Parasites and Predators:
This is another group of insects which silently do immense service to humanity. They are the enemies of our enemies — the pests of crops — and hence they are certainly confirmed friends of the farmer however unspectacular, inconspicuous, silent and inappreciable their activities may be. The two terms generally used for insect enemies of insect pests of crops are parasites and predators; but these are as difficult to define clearly as the terms ‘thief’ and ‘robber’, although in most cases there is no serious difficulty in deciding to which category a particular species belongs.
The Egg Parasite Trichogramma:
Many moths lay their eggs on the foliage of plants, and the caterpillars that hatch feed either on the leaves or by boring into the stem. There is a natural enemy of these insects which destroys a proportion of their eggs. It is Trichogramma, a minute chalcid wasp belonging to the order Hymenoptera.
The female wasp lays its eggs within the moth egg; the wasp egg hatches into a legless larva or ‘grub’, devours the contents of the host egg, and develops into adult, which comes out of the emptied host egg from where otherwise the larva of the moth would have emerged. The Trichogramma, adults fly about and mate, and again the females lay eggs in the moth eggs. The principle involved here is technically known as ‘biological control’ wherein one living organism is employed to check or control another.
The Trichogramma parasitizes the eggs of a large number of moth species, of which some are pests of crops, e.g., the sugarcane stem-borers, and some of stored grain, e.g., the rice moth (Corcyra cephalonica), or the paddy grain moth (Sitotroga cerealella). Moth pests of stored grain can be reared in huge numbers under indoor conditions where temperature and humidity can be maintained at the desired level. This makes it possible to rear Trichogramma also on a very large scale all the year round.
What is actually done is to rear indoors one of the moth species in thousands, collect the eggs in millions, paste them on semi-stiff cards, expose them to Trichogramma adults for parasitization (i.e., egg laying), hold the cards at suitable temperature and humidity, and then hang them at suitable places in the sugarcane field infested by stem-borers so that the Trichogramma adults emerging from the eggs may fly about, search out, and parasitize the eggs of sugarcane borer moths laid on sugarcane plants in the field.
In this way the good work which the natural population of Trichogramma may already be doing in the field can be considerably supplemented by the Trichogramma population bred under room conditions.
The one important point to remember is that this kind of biological control is feasible with Trichogramma only because it has a host which can be reared indoors; otherwise there would be no point in rearing it on sugarcane borers collected from the field, or in raising an acre of sugarcane for raising a population of borers and rearing Trichogramma on their eggs for saving just another acre of sugarcane elsewhere. The alternative approach of rearing the sugarcane borer itself on cheap artificial diet indoors is still in experimental stage.
Parasites of Aphids:
Aphids or plant lice are prolific insects which often densely cover the shoots and tender leaves of plants, suck the sap, and thus weaken the plants on which they are found. There are some parasites which exercise a certain degree of control on the natural abundance of these lice. The parasite which has done the most spectacular job in several countries including India is Aphelinus mali, which parasitizes the woolly aphid Eriosoma lanigerum, a serious pest of apple trees.
A native of USA, it has been successfully introduced into some 30 other countries in Europe, South America, Australia and Asia. It was imported into India sometime in the late thirties and it has been so successful that the woolly aphid has ceased to be a serious problem. The female parasite lays eggs in the abdomen of the host aphid. The eggs hatch in a few days, the larval period takes a few weeks, and the pupal period about a week or so; the whole life cycle takes about a month for its completion, depending on temperature.
All stages are passed within the body of the host aphid, which dies in due course, but by this time a sort of liquid oozing out from the aphid body fixes the aphid securely to the plant surface. After death the aphid turns black and becomes somewhat distended and hardened. The adult parasite cuts out a round hole in this hardened body wall to come out. This is an important example of an exotic parasite proving highly successful. When such successes are obtained in the field the repeated rearing and releases of parasites of insect pests become unnecessary.
An indigenous example is Trioxys indicus, a wasp belonging to the family Apidae of the Order Hymenoptera, which is parasitic on Aphis gossypii attacking brinjal and cotton. The parasite first appears in the field in November, and an increasingly large proportion of the aphids are subjected to its attack in the following months.
Another parasite belonging to the same group is Aphidius smithi, which is a natural enemy of the pea aphid Macrosiphum pisi in many parts of the country, and is believed to be an important factor in keeping this pest from becoming more serious. A remarkable control of the same pest was brought about in some parts of USA when this parasite was introduced and released there in 1959.
Larval Parasites:
The sugarcane borer has a natural enemy in Stenobracon deesae, a slender graceful-looking wasp which has a long sting or ovipositor. The female inserts this ovipositor into the holes made in the stem of sugarcane (as well as maize and jowar) by the caterpillars of the borer, stings the caterpillars, and lays eggs in them. The eggs hatch into tiny parasite grubs, and these feed on nearby full grown borer caterpillars by lacerating their integument and imbibing the liquid oozing out, grow in size, from pupae, and then become adult.
The sugarcane borer has another parasite. Apanteles flavipes, a small black insect. The female has a short ovipositor, and deposits its eggs in only young borer caterpillars. The eggs hatch and 3 well-defined larval stages are passed in succession before the grubs come out of the nearly dead host caterpillars, spin cocoons: and then emerge as adults. The parasite appears to tide over the cold winter of the northern plains as a second-stage grub inside the hibernating caterpillar of the maize and jowar stem borer, Chilo zonellus.
Bracon Greeni Lefroyi:
It is the best known parasite of the spotted boll- worm of cotton. The adult parasite is a small, brownish wasp with a prominent dark spot on the upper surface of the abdomen. The female wasp has a prominent ovipositor with which it probes and paralyses the host caterpillar and then lays its eggs on the caterpillar’s body. It is capable of laying up to 219 eggs, hatching in 3 to 4 days into tiny, creamy, legless grubs which lacerate the caterpillar’s body wall with their sharp mandibles and suck the body fluids that ooze out.
The grub period is shorter in summer than in winter, and there is a quiescent pupal stage before the adult, stage is reached. The emerging adults mate, and the females continue the racial job of host-finding and oviposition. Attempts have been made to utilize this species for biological control of spotted bollworms.
Pupal Parasite Trichospilus Pupivora:
A Pupal Parasite Trichospilus Pupivora Parasitizes he pupae of the black-headed caterpillar Nephantis serinopa, a serious pest of the coconut palm. The adult parasite is a yellow-brown wasp 1-2 mm long, with a small, nearly globular abdomen joined with the rest of the body by a short, narrow waist or petiole. The female wasp lays a varying number of minute eggs inside the host pupa by repeated puncturing with the ovipositor.
Each female may lay up to 236 eggs in 5 pupae or so and more than 1 parasite may lay eggs in the same pupa; however, up to 866 individuals have been recorded to emerge from 1 host pupa of Plusia peponis and up to 595 from Prodenia litura pupa. The eggs hatch into tiny grubs, which feed on the contents of the pupa, develop, and themselves become pupae in about a week from the time the eggs are deposited.
After 8 to 10 days in the pupal stage, the parasites become adults; thus, a little more than a fortnight is required for development from eggs to adults. It is believed that the adult parasites mate within the hollowed-out shell of the host pupa, and the males rarely come out of it; the females emerge from the remains of the host and search out fresh host pupae to carry on their appointed task. This parasite also attacks pupae of some other moths, such as Sylepta derogata and Prodenia litura.
It thrives best at moderate temperature (22°-25°C) and rather high humidity (92-94%); thus, it is active in coastal regions of Kerala from October to February but becomes rather scarce during the hot and dry months of March to May; for the same reason it is rather scarce in the drier coastal regions of Tamil Nadu.
This parasite has been employed in the biological control of the coconut pest Nephantis serinopa. The pupae of the pest are collected from the field and kept in cages for the emergence of the parasites, which are then liberated in areas of infestation. When the pupae of Nephantis are scarce, those of other Lepidoptera, such as Sylepta and Prodenia, are reared for use as host by the parasite.
The main points about the use of this parasite for biological control are:
(i) Each individual parasitizes on an average 2 or 3 pupae;
(ii) The female lays on an average 100 to 150 eggs;
(iii) The life-cycle can be completed in 16 to 17 days—22 generations were actually reared during 1 year at Calicut;
(iv) It can fly up to 5 km; and
(v) It can breed in a number of host species. Its main defect is that it is very susceptible to low humidity.
Parasite of Adult Flea Beetle:
Microctonus indicus is a parasite which attacks the adult stage of its host, Phyllotreta cruciferae, a beetle pest of various cruciferous vegetables. The female parasite inserts its ovipositor into the thin lower surface of the thorax of the beetle and deposits eggs in its body cavity. The eggs hatch into grubs, and of the 2 or 3 grubs in a host (each from 1 parasite egg) only 1 successfully completes its development up to the final larval instar owing to internecine struggle.
The fully grown larva issues out of the beetle by tearing a hole in the posterior part of its body; the quiescent or pupal stage is passed outside the host, and the adult parasite emerges from the pupa.
Predator Beetles:
The most useful group of predators or entomophagous (insect-eating) insects is that of the ladybirds or ladybeetles or ladybird beetles belonging to the family Coccinellidae of the insect order Coleoptera. The adults are medium to fairly large-sized insects, round or oval in shape; they have prominent, brightly spotted, hard, leathery forewings covering the abdomen.
The ladybirds are perhaps the best known of the beneficial insects; the sobriquets ‘ladybirds’ and ‘ladybeetles’ were probably given them as terms of endearment, possibly as recognition of their good work as destroyers of plant lice, scales and mealy bugs infesting various crops.
Coccinella Septempunctata:
It is a common ladybeetle found in many countries including India. The adults are fairly large, and the leathery forewings are brownish with 7 black spots on each. They are long-lived, and females lay up to a few hundred eggs each over a period of days.
The eggs are elongate and orange-yellow and are laid in clusters, each comprising a few eggs, on plants infested with plant lice, mealy bugs and scale insects. Small, dark, very agile grubs hatch out in about a week and actively search out and devour aphids and scale insects.
In about a month the grubs lose their activity and transform into dark-coloured pupae which can be seen sticking to leaves and stems of the plants on which the grubs once crawled about. Adult ladybeetles emerge from the pupae in about a week, and they too devour plant lice, mealy bugs and scale insects. Unlike the hymenopterous parasites, e.g., Trichogramma, Apanteles, Aphidius, Microctonus and Trichospilus, whose immature stages alone feed on other insects, both immature stages and adults of the coccinellids devour the hosts.
Another important ladybird beetle is Rodolia cardinalis, which saved the citrus groves of California, and later of many other parts of the world, from total destruction by the cottony cushion scale, Icerya purchasi. It was this beetle, popularly known as vedalia that first made people take cognizance of the possibilities of controlling insects biologically. After it was introduced into California from Australia in the last decade of the century, the control of the scale was so spectacular that this beetle was introduced in many other countries.
The scale was widespread in peninsular India, and the lady beetle was imported in 1928 from Australia. Large-scale rearing and releases of the predator were taken up in 1946, and by 1954 a very satisfactory control of the scale was obtained. The female beetle lays on an average 300 brick-red eggs, which hatch in about 4 days into dark little grubs; these moult thrice in about 9 days, then pass about 4 days in the pre-pupal stage and about 9 days in the pupal stage before emerging as adult beetles.
Thus, the life-cycle takes about 25 days. Both adult and grub stages of the beetle feed voraciously on eggs and young ones of the fluted scale, and it is estimated that on an average the beetle grub during its existence of about 9 days devours more than 200 eggs of the scale.
Insect Enemies of Weeds:
Some insects are useful to the farmers directly or indirectly in yet another way. These are the insects that feed on weeds and destroy them and thereby free arable land for cultivation or pasture. Weed control by insects came into its own after the splendid results achieved by the introduction of the moth Cactoblastis cactorum into Australia from Argentina for controlling the cacti (Opuntia spp.) which had threatened to destroy Australian agriculture (and livestock) through their fantastic usurpation of nearly 60 million acres of land.
In India many weeds pose a threat to arable and pasture lands. The cacti Opuntia vulgaris, Opuntia dilleni and Opuntia elatior had been introduced into India in the 18th century for culturing the cochineal insects. In due course the cochineal dye industry died out because of the advent of synthetics, but the cacti gradually became not only a nuisance but a real danger to agriculture. A mealy bug named Dactylopius indicus, which had been introduced perhaps inadvertently along with the cacti, was found to exercise a certain degree of control on Opuntia vulgaris.
In 1924-1926 another mealy bug, Dactylopius tomentosus, a native of Mexico already imported into Sri Lanka, was introduced in Tamil Nadu, and within a few years it cleared more than 100,000 acres of land encroached upon by Opuntia dilleni, it also brought about some control of Opuntia elatior.
The mealy bugs (Dactylopius spp.) are small sessile insects, encrusted in a white meal or powder (whence the name mealy bug); they suck the juices of the cacti by inserting their sharp mouthparts into the fleshy parts of the plants, which eventually wither away.
Conservation and Utilization of Insect Friends of Man:
Obviously, it does not stand in need of any serious pleading to state that every effort should be made to conserve the friendly fauna of the insect world, and that special precautions are called for to avoid as much as possible any harm to this section of the class Insecta. This consideration makes the task of pest-control scientists a very difficult and delicate affair. Selective killing of the insect pests and preserving the friendly insect when the two are rather inextricably mixed together is at present only an ideal to strive for.
This is particularly true in the case of chemical control of insect pests. All the same, scientific research during recent years has been revealing a number of promising avenues for realistic approaches to the ideal we have in view. Information on all these aspects is readily available in literature. Here only a few such ideas are recorded as do not seem to be quite common in literature or do not seem to be adequately appreciated.
Pollinators are different from parasites and predators of insect pests in certain respects. First the activities of pollinators are more restricted in time, i.e., to the flowering period of the crop. Hence, the general recommendation that as far as possible chemical control operation should not be undertaken during the flowering phase of an insect-pollinated crop. No such general statement is possible in the case of parasites and predators.
Secondly, the activities of pollinators are in favour of the other party—the flowering plants which they deal with—whereas the reverse is the case with parasites and predators whose activities are against the host and prey whom they parasitize and kill. The result is that flowers have evolved, and are evolving, various mechanisms of colour, design, scent, etc. to attract pollinators even from a distance, but the reverse is the case with parasite hosts and preys whose survival depends on their capacity to evade and hide.
The degree of specificity in insect-plant relationship as far as cross-pollination is concerned is of a low order. Generally speaking, a plant can be cross-pollinated by more than 1 species of insect, and an insect species can gather nectar from more than 1 species of plant. In this regard this relationship is more akin to that between a prey and a general predator than to that between a host and a specific parasite.
The implication of these seemingly insignificant differences is that whereas the efficiency of a specific parasite’s activities is highly density-dependent and is reduced with reduction in host population, the activities of pollinators are likely to be much less dependent on population density of flowers. However, since scientific investigation so far carried out on pollinators is very inadequate, our knowledge about them is meagre and imperfect.
Parasites and Predators:
Much work has been done on parasites and predators and on their conservation and use for biological control of insect pests and weeds; yet there are some very significant surprises in the development of this branch of pest control science.
(a) Inadequate Appreciation of Biological Control:
Paul De Bach, a noted authority on the subject has recently edited an exhaustive and useful treatise, ‘Biological Control of Insect Pests and Weeds’. His opening statement, which depicts the factual position of the moment, is – “The biological control of insects, mites and weeds has received great and enthusiastic acclaim during the past 70 years with highly successful practical results achieved in over 60 countries around the world. In spite of this, some few scientists have looked upon the method with incredulity.”
This certainly does not depict a very happy situation in a scientific field. On the theoretical side there exist thought provoking contributions of great scientific value on the basic phenomenon of insect parasitism, host parasite population oscillation, etc., but they have not been put to actual use in the practical field. On the other hand, in the field of practical biological control there exist a large number of classical and spectacular successes, and a much larger number of disappointing failures, but little effort has been made, if at all, to adequately analyse these successes and failures.
Under these conditions it is no wonder that De Bach was reminded of the following warning of an earlier scientist – Reasoning unwarranted by facts, and facts not correctly and sufficiently reasoned out, are equally worthless and dangerous for practical use.
(b) Inadequate Appreciation of the Role of Predators:
Insect predators were the first to attract the attention of growers and orchardists. The Chinese had started since very ancient time purchasing and utilizing the predaceous red ant, Oecophylla smaragdina, for the control of the pest in citrus gardens. This ant is very common in Indian mango gardens. A similar practice is reported to be common in the Middle East where predaceous ants are transported from mountain regions to the plains for control of date-palm pests.
Coccinellids have been known for their good work for centuries, and it was the introduction of the predator coccinellid vedalia for the control of the cottony-cushion scale that for the first time established biological control as a valid branch of pest control science. The first discovery of insect parasitism is credited to Aldrovandi, who in 1602 mistook the cocoons of the parasite Apanteles glomeratus on the larvae of the cabbage butterfly to be eggs of some insect, and it took another century before Vallisnieri in 1706 correctly interpreted this as a case of insect parasitism.
Today the situation is that parasites have received much more investigational attention than predators. In the field of practical utilization, too, it is reported that of 95 species of entomophagous insects imported and established in the USA 81 are parasites and 14 predators; and that two-thirds of the successful cases of biological control are due to parasites. Obviously, these lop-sided statistics are the result of inadequate appreciation of the role of predators as compared with that of parasites.
The reason appears to be that parasites remain associated with the pest, which is collected in parasitized stage, and often the parasites can be seen to emerge therefrom. On the other hand, the predator eats away the pest and is rarely observed in the actual process of eating or harming it; hence the predator does not attract enough attention. This is all the more true in the case of nocturnal predators.
The extent to which predators bring about reduction in the population of harmful species has to be appreciated only through circumstantial evidence, as was done in the case of the biotic theory of locust periodicity, and the tentative ideas thus developed have to be tested by specially planned critical experiments.
(c) Dosage of Parasites in Experiments on Biological Control:
Many experiments have been carried out in various parts of the world to control particular pests by releasing parasites specially bred for the purpose, but the success has not been spectacular. It is now being slowly realized that the use of indigenous parasites can have more chances of success on an inundative instead of inoculative basis, and that one of the major causes of frequent failures is insufficient number of parasites released.
Few efforts have been made to determine the number of parasites needed to bring about the desired degree of control at different population densities of the host. In other words, there does not exist adequate information on dosage requirements in the field of biological control, and it is not surprising that the results have been erratic or discouraging.
Planned experiments should be carried out to determine the number of parasites necessary to control a particular pest at its different population densities, and the information thus obtained should be utilized in determining the number of parasites to be released under any particular situation.
In fact, we should go on increasing these numbers as the host population goes down so that the efficiency of the parasite population to bring about effective control of the host is not impaired by lowering the host population density. With improved techniques for mass rearing of parasites, we should aim at making their efficiency independent of host densities by flooding the areas with parasites, just as the screw- worm was eradicated by flooding with sterilized males. The rearing of screw-worm on animal tissues, on a large industrialized scale has led to parasite rearing also on a commercial scale.
From the foregoing it will appear that there remains much to be improved in the methodology of experiments on biological control, and the gloomy picture that often emerges from inconsistent results recorded in literature is in most cases probably due to some defect or the other inherent in the experiment. For example, experiments with Trichogramma against borers of sugarcane are reported to have given inconsistent results in India; but actually these experiments have shown that Trichogramma releases resulted in significant increase in parasitization in certain areas but not in others.
Adequate attempt was not made to interpret these results against the ecological background of the areas; otherwise most probably the interpretation would have been that the performance of the parasite depended on the climatic conditions of the region where it was released.
It seems to be almost certain that if ecological aspects are properly kept in view the percentage of success in biological control projects can be significantly raised. The failures so far appear to be failures of those engaged in these operations rather than of the principals involved.
Utilization of parasites and predators for the control of insect pests and weeds has to be tackled in one or more of the following ways:
1. Introduction of Exotic Parasites and Predators:
This consists of importing into an ecological region all such parasites and predators as are not present in the region and are likely to be of use on prima facie basis. For this purpose there now exists a world-wide organization, the Commonwealth Institute of Biological Control, through which such insects can be obtained and tried. They can also be obtained directly on exchange basis.
2. Utilization of Indigenous Parasites and Predators:
This requires a study of the indigenous fauna to understand how the efficiency of each individual species can be increased.
This can be attempted in a number of ways:
(i) Conservation:
This requires such efforts as are necessary during various operations of crop husbandry in order to ensure that parasites and predators are not adversely affected. An example from mechanical control is that the egg-masses when collected may not be destroyed but kept in the field itself in a wire-gauze cage from which the minute parasites can fly out but not the various stages of the pest host. Similarly, there are various precautions possible in the case of chemical control. Such examples will be found under the sections dealing with integrated control.
(ii) Releasing the Parasite in Suitable Dosage:
The parasite releases may be- (a) on inundative basis, i.e., in such large numbers as may be expected to attack almost every individual of the pest population, or (b) on inoculative basis, i.e., in numbers just enough to overpower the pest. The success of such operations would naturally depend on the amount of care taken to ascertain the correct strain of the parasite, the optimum number to be released, the correct timing of releases, and the suitability of the habitat where release has to be made.
(iii) Release of Parasites along with Alternative Hosts:
In the case of parasites, particularly those that are rather specific, it is likely that the parasite may get a setback due to the low population of the host. In such a case it is worthwhile trying to increase the host population by providing an alternate host. In the case of Trichogramma it should be feasible to increase the host density by hanging in a field of sugarcane a few cards containing Corcyra eggs so that those parasites that are not able to find the borer eggs may lay their eggs in Corcyra eggs.
This was tried earlier with the primary host, i.e., the pest itself, but the farmers were reluctant to allow additional pest population merely to give a fillip to the parasite. There will be no objection to the use of eggs of an alternate host which is not a pest in the field; the larvae emerging from unparasitized eggs are bound to die without doing any harm.
(iv) Genetic Improvement:
Genetic improvement is as much possible in parasites and predators as in crops and livestock. One fear has been that the improved strains developed in the laboratory are likely to be competed out when released in nature. This is quite a sound argument, but we can overcome the difficulty once we decide to maintain the improved strains in the laboratory and use them on inundative basis in areas and against pests for which they have been specially developed and bred. We can easily maintain a number of strains of each useful species specially developed for various situations that are likely to arise.
These are only some of the ways in which parasites and predators can be fruitfully utilized.