Economic Importance of Insects | Arthropods | Zoology

Of an estimated 5-10 million species of insects, probably not more than a fraction of 1 per cent interact, directly or indirectly, with humans. Insects have been associated with man’s interests in many ways. Their ability to both benefit and harm mankind makes this relationship a unique one.

They play a silent role in maintaining the balance of nature; some feed on other plants and animals, and some are eaten by others. Many of them are scavengers and convert the dead plant and animal tissues into humus and enrich the soil. Many species of insects live both as parasitoids and predators on insect pests of crops and help in suppressing their numbers.

They also play an important role in the pollination of various crops and thus increasing their yield. Some of them produce materials useful to man, i.e., silk, honey, wax, lac, etc. They also serve a useful purpose in being aesthetically beautiful and attractive. However, insects are also powerful competitors of man as they cause injury to crops and animals, and also act as vectors of many diseases.

1. Honey Bees:

There are about 20,000 kinds of bees, all belonging to the insect order Hymenoptera. Of them, three families of social bees, i.e. Bombidae, Meliponidae and Apidae, are honeys producing. Among these, Apidae is the main honey producing family. All the four species of honey bees, viz. Apis carnna Fabricius, Apis dorsata Fabricius, Apis florea Fabricius and Apis mellifera Linnaeus are found in India. Of these, the first three are indigenous, and the last has been introduced and acclimatized in India for the last about 50 years.

A. cerana makes parallel combs on trees, termitaries, hollows of rocks and all kinds of cavities. Its honey yield is 3.6-4.5 kg in hills and 1.3-2.2 kg in plains. A. dorsata makes large hives (15 cm long and 7.5 cm deep) that hang from high rocks and tall trees. Their colonies shift from place to place to avoid extremes of climate or in search of honey.

They make single vertical comb, are very industrious and produce about 36 kg honey per colony per year. They are most ferocious and do not spare their victim even inside water. A. florea is the smallest of all these bees. It makes a single vertical comb of the size of palm suspended from branches of bushes, hedges, buildings, caves, chimneys, empty cases, etc.

Its honey yield is poor, about 0.5 kg and the consistency of honey is thin. A. mellifera makes its nest in enclosed spaces in parallel combs and is endowed with all the good qualities of a hive bee. It has adapted itself well to modern methods of movable frame hives and, therefore, is the favourite of beekeeping industry the world over. Its honey yield averages 50-100 kg per colony per year.

The total global production of honey is about 1.54 million tonnes a year, gathered by about 60 million honey bee colonies. There are about 1 million A. mellifera and A. cerana colonies in India, maintained by about 2.76 lakh beekeepers. The present production of honey in India is estimated at about 90,000 tonnes, out of which about 25,000, tonnes is exported. In Punjab, 8500 tonnes of honey is produced by about 2,50,000 A. mellifera colonies maintained by about 25,000 beekeepers.

2. Silkworm:

The silkworms belong to the insect order Lepidoptera. While all Lepidopterans secrete silk, only some species weave their silk into cocoons to protect their vulnerable pupae from weather and predators. There are four species of silk worms in India, which can be exploited for commercial silk production.

These are mulberry silkworm, Bombyx mori (Linnaeus); tasar silkworm, Antheraea paphia (Linnaeus); muga silkworm, Antheraea assama Westwood and eri silk worm, Philosamia (= Attacus) ricini (Donovan). Among these, B. mori and P. ricini are domesticated, A. assama semi-domesticated and A. paphia wild.

Silk is the product of a pair of silk glands (salivary glands) of the larva. The secretion is produced in liquid form but on coming in contact with air, it solidifies into a fibre called silk. When mature, the 5th instar larva makes quick round movements of its head at the rate of 65 movements per minute while spinning its cocoon.

The weight in grams of 900 m long silk filament is called a ‘denier’ and the size of a normal cocoon is 1.8-3.0 deniers. A single cocoon weighs 1.8-2.0 g and its shell (without the enclosed pupa) about 0.45 g. About 2500 cocoons yield 0.45 kg of silk. The total production of raw silk from all the four species of silk worms in India is about 24,000 tonnes, of which about 90 per cent is mulberry silk.

3. Lac Insect:

The lac insect, Kerria (= Laccifer) lacca (Kerr)) belongs to the order Hemiptera. There are six genera of lac insects, of which five secrete lac and only one, i.e. Kerria secretes recoverable or commercial lac. The lac insects live in cavities or cells made in the resin or lac secreted by them on their host plant.

Two distinct strains of lac insects are known in India, i.e. kusumi and rangeeni. The kusumi strain is grown on kusum (Schleichera oleon) or on other hosts using kusumi brood while the rangeeni strain is grown on hosts other than kusum. Each of these strains gives two crops in a year.

The lac of rangeeni crop is collected while it is not fully mature. About 90 per cent of the production is contributed by rangeeni crop and only 10 per cent from the kusumi crop. Kusumi is considered superior because of the lighter colour of the resin. The annual production of lac in India is about 16495 tonnes.

4. Pollinators:

Many species of insects particularly the bees, butterflies, moths and thrips are the major pollinators of grain crops, cotton, fruits, vegetables and flowering plants. Some plants like many fruits particularly figs, peas, beans, tomatoes, many vegetables, sunflower, seasonal flowers, chrysanthemum and many ornamental plants would produce no fruits and seeds unless pollinated by insects.

It has been estimated that 50-70 per cent of the grain crops are pollinated by insects, a major portion of it by bees alone. For every Rs. 100 worth of honey collected, the bees make Rs. 2000 worth of seeds and fruits by pollinating the flowers.

5. Entomophagous Insects:

Entomophagous insects are those insects which feed on other insects destroying our crops and stored grains. Such entomophagous insects are either parasitoids (a parasitoid is usually much smaller than its host and a single individual usually does not kill the host) or predators (a predator is usually larger than its prey, kills its prey and requires more than one prey to complete its development).

The parasitic insects mainly belong to Hymenoptera and a few to Diptera. The insect predators are spread over a wide rang8e of orders including Coleoptera, Neuroptera, Diptera, Hemiptera, Hymenoptera, etc. It is due to the activities of entomophagous insects that many harmful insect pests have been kept under check under natural conditions.

6. Pests of Crops and Animals:

The greatest importance of insects lies in their being pests of crops and animals and as carriers of diseases. A large number of insects cause injury to economic plants by feeding on them externally and chewing their leaves or other parts; by sucking the cell sap with the help of piercing mouth parts and reducing the vitality of the plant; by disseminating various plant diseases and by living a subterranean life and damaging roots. The recent estimates put the total annual losses caused by insect pests to major field crops and the food grains in storage in India, to the tune of Rs. 900 billion.

At times, insects make it impossible for human beings to live in a locality by causing them untold misery. They annoy man by biting, cause him pain by injecting venoms, live on his body and on farm animals as ecto-and endo-parasites causing ill-health. They spread many pathogens by acting as their carriers or as intermediary hosts.

To mention only a few, the house fly spreads cholera and typhoid; mosquitoes transmit malaria, filaria and encephalitis; tse-tse fly transmits the parasite causing sleeping sickness; the rat flea transmits the bacillus of bubonic plague, etc. Thus, insects as a group of the animal kingdom clash directly with the interests of man for survival.