In this article we will discuss about the economic importance of insects.
A. Beneficial Insects:
Insects which produce honey, wax, lac, dyes and silk are commercially beneficial. Some insects are very helpful in destroying injurious insects.
1. Commercial Products:
Apis, the honeybees produce millions of tons of honey every year, it also gives bees wax from its combs.
Benefits of bees are cosmopolitan, not only in producing honey and wax, but also in bringing about cross-pollination of many fruits and flowers without which these plants could not exist. Tachardia, the lac insect secretes commercial lac produced from integumentary glands as a protective covering by females, shellac is made from lac in India.
Dactylopius, the cochineal insect of Mexico is found on cacti, dried bodies of females of this scale insect are used for making cochineal dyes. Bombyx and Eupterote are silk moths, they are reared in India, China, Japan and Europe, their larvae called silk worms spin cocoon of raw silk, the silk fibre is reeled off and used for making silk.
In Asiatic countries over 25 million kilograms of silk are produced annually. Dried elytra of two beetles, Lytta and Mylabris are used for making cantharidin, a powerful aphrodisiac.
The larvae of two flies, Lucilla and Phormia are used in healing such wounds of bones which do not respond to medicines, the larvae are put in wounds of bones and bone marrow, they clear away suppurating and dead tissues, prevent bacterial growth and excrete allantoin which heals the wounds.
2. Useful Predaceous Insects:
Some insects are predaceous, they feed upon and destroy a large number of injurious insects. Stagomantis, a mantis is voracious, it feeds on flies, grasshoppers and caterpillars, some of which are injurious to crops. The larvae and adults of Chilomenes, a lady-bird beetle, feed on aphids which infect cotton plants.
Novius, a lady-bird bettle, destroys scale worms which are pests of orange and lemon trees. Epicauta is a blister beetle, it deposits eggs where locusts occur, the larvae on hatching enter egg capsules of locusts and eat up masses of eggs. Calasoma, a ground beetle preys upon many kinds of lepidopterous larvae which destroy cereals and cotton.
3. Beneficial Parasitic Insects:
Some insects parasitise injurious insects, they usually lay eggs in the bodies of larvae and adults of harmful insects; the young on hatching from eggs finally kill their hosts. The larvae of Tachina and related flies are parasites of injurious lepidopterous larvae, such as army-worms which are injurious to cereals.
Larvae of hymenopteran flies and carnivorous wasps devour aphids in large numbers. Chalcids and ichneumon flies are parasitic, laying eggs in cocoon and larvae of phytophagous Lepidoptera. Apanteles, a hymenopteran fly lays eggs in army-worms and boll worms, the parasitic larvae gnaw their way through the skin of the host.
Some insects are scavengers, they eat up dead animal and vegetable matter, thus, they prevent decay. Some ants and larvae of some flies can devour entire animal carcasses.
B. Injurious Insects:
Compared with beneficial insects the number of injurious insects is very large.
1. Disease Transmitting Insects:
Many types of mosquitoes, flies, fleas, lice and bugs transmit diseases to man and domestic animals, they have been described earlier in insects and diseases.
2. Household Insects:
Human food is spoiled by cockroaches, ants, flies and weevils. Tinea, Teniola and Trichophaga are clothes moths, they lay eggs on warm clothes, the larvae on hatching eat and destroy clothes, they also feed on furs, carpets and dry fruits. Anthrenus is a carpet beetle, it is a scavenger eating decaying animal matter, but its larvae destroy carpets and preserved biological specimens.
Tenebrio is the mealworm beetle, its larvae are mealworms, they eat meal, flour and stored grains, such as rice. Lepisma, the silver fish and Liposcelis, the book louse live in and destroy books and old manuscripts. Termites, the white ants cause untold destruction of books, carpets, furniture and wood-work of buildings.
3. Injurious to Domestic Animals:
Glossina, the tsetse fly transmits Trypanosoma brucei which causes nagana in horses. Tabanus and Stomoxys, the blood sucking flies inject Trypanosoma evansi into horses and cattle which causes surra in India.
The larvae of Hypoderma, the warble fly bore below the skin of oxen and make holes for breathing, then they pass through the gullet and again pierce the skin on the sides of the spine to form swellings, they not only injure the hide but also reduce the meat and milk supply.
Gasterophilus, the bot-fly lays eggs on hair of horse, the larvae enter the stomach in large numbers. Melophagus, the sheep tick and Hippobosca, the forest fly of cattle and horses suek blood of their hosts and often cause haemorrhage. Menopon, the chicken louse sucks blood and causes destruction of fowls.
4. Injurious to Crops:
Many insects damage forest trees, growing farm crops, fruits and stored grain, the damage they cause annually runs into millions of rupees.
The number of such insects is innumerable, they are mostly Lepidoptera, Coleoptera, Diptera and Hemiptera. Euproctis, the brown tail moth and Lymantria, the gipsy moth are serious pests of shade and foliage trees, their larvae are a menace and destroy forest trees. Myetiola, the Hessian fly is a small sized midge, its larvae damage wheat plants.
The larvae of two Lepidoptera Chilo in India, and Diatraea in America bore into stems of sugar-cane and cause a great deal of damage. Pyrilla, a hemipteran sugar-cane leaf hopper sucks the juice of sugar-cane, both as adult and nymph, causing great loss of sugar.
Pyrausta is a moth found all over the world, but specially abundant in the tropics, its larvae known as corn borers are notorious for boring into stems and fruits of corn (maize). Nephotettix, the Indian rice leaf-hopper and Leptocorisa, the oriental pest of rice and millet are Hemiptera, they attack rice in very large number eating the leaves and ears.
The larvae of Schoenobius, a moth bore into the stems of rice plants in India, they kill the plants. Nymphs and adults of Hieroglyphus, an orthopteran eat up the growing shoots of rice plants, thus, preventing formation of grain.
Dysdercus, the Indian cotton bug, Oxycarenus, the Egyptian cotton bug, and Anthonomous the cotton-boll weevil are very injurious to cotton, they stain and destroy cotton- bolls, Aphis, a hemipteran is a serious cotton pest in India, the pests often attack cotton plants in large numbers causing the plants to wilt and die.
The larvae of two Lepidoptera, Agrotis and Gnorimoschema are potato cut-worms in India, the former feeds on potato leaves and cuts off the stems, while the larvae of the latter eat the potatoes in the field and stores, larvae also attack tobacco and tomatoes. Larvae of Agrotis are also destructive to peas, cabbage, tobacco, ground nuts, wheat and cauliflowers.
The larvae of some Coleoptera are called wire-worms, such as Agriotis and Limonius, they are root-feeders and are extremely destuctive to cereals, root crops and grasses. Many insects and their larvae destroy vegetables in India.
Siphocoryne is an aphis which feeds on cabbage leaves; Anasa, the squash bug is destructive to cucurbitaceous plants; Earias the spotted bollworm destroys ladyfingers; Aulacophora, the red beetle feeds on pumpkins; the larvae of Bruchus, a beetle bore into pods of beans and peas killing the seed.
Many insects attack fruit trees, they damage roots, trunks, stems, leaves, inflorescence and fruit. Drosicha, a mealy bug causes destruction of mangoes, plums, papaya, jack fruit, pears and citrus fruits in India. The nymphs and adults of Ideocerus, a mango leaf hopper attack the inflorescence and suck the sap, thus, they cause tremendous damage by preventing formation of mango fruit.
The laryae of Contarinia fly feed on young pears which soon decay. Psylla, an apple bug, lays eggs on apple and pear tree, the nymphs on hatching damage the blossom and shoots; the larvae of Anthonomus, a beetle also destroy apple blossoms and prevent formation of the fruit. Nysius, a bug is very destructive to several kinds of fruit trees.
Many moths, caterpillars and beetle cause a great deal of damage to stored grains: two beetles Tenebrio and Tribolium have similar habits and are commonly found in stores and granaries, the former is found in all stages in meal, flour and stored goods, its larvae are known as meal worms. Tribolium eats stored wheat and grain. Calandra, a weevil bores through grains of rice and other stored grain in India.