(Dr. Girish Chandra)
Sericulture was first introduced into China by Hoshomin, the Queen of China. For a long time sericulture was considered to be a national secret by the Chinese and its industrial technique was not known in other countries. Later, it was introduced into Europe and Japan by smuggling the secrets from China through travelling monks. According to some sources, sericulture was introduced in India about 400 years ago and the industry flourished as an agro-industry till 1857, with an annual production of a million kg of silk fibre.
According to Chinese legend, the technique of silk production using Bombyx mori was invented at around 2,700 BC when prince Hoang-ti directed his wife Si-ling-chi to study the silkworm and explore the practicability of using its thread for textile. Si-ling-chi devised not only the technique of culturing silkworm but also the method of reeling the silk and making garments out of it. She was later crowned as “The Goddess of Silk Worm”. Subsequently sericulture spread from China to other countries and silk became a precious commodity, highly sought after in all countries. In 139 BC the world’s longest highway that stretched from Eastern China to the Mediterranean Sea was opened, which was called “Silk Route” due to trade in silk.
Five species of silk worms are reared in India:
1. Bombyx mori meridionalis, the Mulberry silk worm (Lepidoptera: Bombycidae), feeds on the leaves of mulberry (Morus alba) to produce the best quality silk fibre.
2. Antherea paphia or Antherea mylitta, the Tasar silk worm (Lepidoptera: Saturnidae), feeds on Terminalia tomentosa that occurs in the jungles of Bihar, Madhya Pradesh, UP and Orissa.
3. Antherea royeli and Antherea perniyi, the Oak tasar silk worms (Lepidoptera: Saturnidae), feed on oak trees and were introduced from foreign countries.
4. Antherea assama, the Muga silk worm (Lepidoptera: Saturnidae), is confined to the Brahmaputra Valley of India and produces the famous muga silk.
5. Phylosamia ricini, the Eri silkworm (Lepidoptera: Saturnidae), which feeds on castor (Ricinus communis) is raised in Assam, Madhya Pradesh, Rajasthan and Orissa commercially.
Four Indian species of mulberry, namely, Morus alba, M. indica, M. serrata & M. laevigata, are cultivated as main food plants of silkworm. Different systems of plantations for mulberry are practiced in India. In India where the temperature ranges from 16oC to 31oC, mulberry silkworm can be reared throughout the year. Karnataka, where the temperature ranges from 16-31 degree centigrade, provides ideal climatic conditions for rearing mulberry silkworm throughout the year, whereas in West Bengal, the multivoltine silk-worm rearing is practised even under adverse temperature conditions. In Jammu & Kashmir univoltine variety of silkworm is cultured only once a year during May-June.
Eggs of Bombyx mori are small and hard, about the size of a pin head and resembling poppy seeds. The egg stage lasts 10 days; the larval stage lasts the longest, 25-30 days and the pupal stage takes 10 days. The larvae are white, 4-5 cm long and moult four times during their growth. At the end of the larval duration, the silkworm produces silk from its mouth to constructs a cocoon in some hidden and secure place. Healthy moths are allowed to copulate for 4 hrs, after which the female is consigned to a dark plastic ‘cellule’ for egg laying. She lays about 400 eggs in 24 hours.
Considering the various factors, such as the place of origin, voltinism (number of generations in a year), the colour of cocoons, the larval markings, the colour, shape and size of cocoons, the silkworms are classified into different breeds. The multivoltine races are reared in West Bengal and Karnataka due to optimum temperature conditions. Bivoltine races diapause in winter and hence can have only two generations in a year. Univoltine and bivoltine races require more leaves than the multivoltine ones. However, the yield and quality of cocoons of the bivoltines are superior to those of multivoltines. The average annual yield of cocoons in India is as low as 150 kg under rain fed conditions and 400 kg under irrigated conditions The cocoons after cooking are reeled in hot water in different types of reeling machines. In India, 61 percent of the silk amounting to 1,320 tonnes is reeled on the country-type charkha.
NON-MULBERRY SILK WORMS
TASAR SILK WORM
Three species of Antherea are used for the extraction of tasar silk in India. They are Antherea mylitta, A. perniyi and A. royeli. Out of the total non-mulberry silk produced in India, about 400 tonnes is produced from Antherea mylitta in Madhya Pradesh, Orissa and Bihar. This silkworm feeds on Terminalia tomentosa and Terminalia arjuna found in the forests of central and north-eastern parts of India. The tasar silkworms is a wild species and hence cocoons are also collected by the tribal people from forests and silk is obtained. The first crop, usually called the seed crop is raised during May to July, whereas the commercial crop is raised during October-November.
The larvae are usually green in colour and moults four times before they complete their larval duration. However, yellow, blue and white larvae are also reported. At the end of the larval period, they spin a ring like structure around the twig and a long stalk from which the cocoon hangs. The cocoons are large and brown or yellow in colour. Moths emerge from the cocoons in June. To obtain silk, the cocoons are cooked in caustic potash and reeled to extract fibre and then spun to manufacture coarse thread.
The recent introduction of Antherea perniyi and A. royeli on oak trees in Manipur has opened up new opportunities for the production of superior quality tasar silk in India. The cocoons of Antherea perniyi can be easily reeled and fibre of superior quality can be obtained. Antherea royeli occurs in oak jungles of the sub-Himalayan region.
MUGA SILK WORM
The golden-yellow silk produced by Antherea assama is found only in the Brahmaputra Valley of India. This species of silkworm is semi-domesticated as the larvae which crawl down of trees at the end of their larval period are collected and allowed to spin cocoons in captivity. Antherea assama produces golden yellow silk that is of high quality which is expensive. The worms feed on Som (Marchilus bombycina) and Soalu (Litsaea polyantha) trees. At the end of the larval period, when the worms are ready to spin cocoons, they crawl down the tree in search of suitable places for making cocoons. To obtain silk, the cocoons are boiled in soap and soda solution and are reeled on a machine. The total production of muga silk in India is about 50 tonnes but there is plenty of scope for expansion of this industry.
A single Muga female moth lays 150-200 eggs after copulating with the male for 6-8 hrs. The larvae are yellowish with black markings on the body and have the habit of crawling down the trees in groups when all leaves are consumed on the trees and larvae have matured. If larvae have not matured and the leaves on the trees exhausted, they can be transferred to another tree. At the end of the larval period, when the worms are ready to spin the cocoons, they crawl down the tree in search of a suitable place for the construction of cocoons.
Spinning of Cocoons
The mature silkworms will come down from the trees to the base on ground in search of proper place for spinning of Cocoons. Therefore, bundles of semidried twigs are collected and the worms are placed on these twigs so that they make their cocoons.
ERI SILK WORM
The silk produced by Philosamia ricini is called Eri silk. It is grown in Assam and in the eastern parts of India. The heavy rainfall & humid atmosphere in these parts are conducive to eri culture. The food plants for Philosamia ricini is castor. This silk worm is multivoltine and reared indoors.
The eggs are white and hatch in ten days. The hatched larvae are mounted on castor leaves in the rearing-houses and are allowed to grow by feeding on leaves. The worms moult four times during the larval period of 30-32 days. Eri silkworm is generally hardy and not easily susceptible to diseases. At the end of larval period, the larvae crawl in search of suitable places to spin cocoons.
The cocoons of the eri silkworm cannot be reeled, as they are made up of several small fibres and hence the emergence of moths is allowed and the cocoons are spun like cotton to produce yarn. Approximately ninety tonnes of eri silk is produced in the country annually.
Recent efforts to rear tasar silkworms on oak plants in the sub-Himalayan range and in Manipur have contributed to the production of a significant quantity of quality tasar silk. It has also opened up new avenues for improving sericulture and also enhanced the employment potential in the tribal hilly areas.