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Sericulture in Asia: yesterday, today, tomorrow

p. 17-21


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1August 2011

Introduction

2The term “sericulture” is strictly limited to the production techniques of the cocoons of a caterpillar, the silkworm. In a broader sense, it includes the stages of stifling and reeling to obtain raw silk fleets, which is how raw silk is marketed. Other interventions are necessary for obtaining silk fabric, the ennoblement of silks.

3I will limit myself to a restricted definition of sericulture. As the silkworm feeds on mulberry leaves, culture of the mulberry tree is part of sericulture. The breeding of silkworms and the culture of mulberry trees is a saga that shaped the world.

History: the sericulture of yesterday

4Tsunami in a cup of tea: splashed by the fall of a cocoon into her cup of tea, Princess Si-Ling-Chi discovered the secret of silk while trying to remove it. She drew out one very long, solid thread. After this observation in 2602 BC, the emperor put his wife in charge of producing this fabulous thread. Si-Ling-chi collected the worms and installed them in a closed room to feed them. With the cocoons obtained, she was able to reel the thread and to weave it. For centuries, this practice remained confined to the courts of the many Chinese dynasties, any person trying to reveal its secrets being sentenced to death, and the material was known only in the form of silk fabrics. These fabulous fabrics became the basis of commercial exchange, with the same value as currency, and trade routes were opened (139 BC, Han Empire) to areas west of China, in the first form of globalisation. The roads encouraged commercial, cultural and even religious exchanges. The exceptional character of the goods attracted heterogeneous populations, diversifying the exchanges (gold, money, horses, new food products, alfalfa). The “silk roads” crossed the deserts, from oasis to oasis, and the mountains. In the East, the silk road started from Chiang’ an (Xi’ an) and passed to the west either north of the Caspian Sea and then north of the Black Sea, or south of the Caspian Sea towards Baghdad and then to Antioch. This road was not, however, the one the silkworm travelled, and the origin of this fibre remains unknown.

5In the 2nd century BC, Chinese emigrants introduced sericulture into Korea, but it did not last there. The silk roads were not the channels for the dissemination of sericulture but, on the contrary, protected the secret, because the merchants wished to keep a monopoly on trade. In the 5th century AD, sericulture reached India, where it settled. Gradually, it spread to most of the countries of Asia, including India, Korea, Japan, Cambodia, Viet Nam and Thailand.

6This is the stage on which the first act of sericulture was played. Let us see who the actors are.

The silkworm

7The cocoon that fell into the cup came from a caterpillar, Bombyx mandarina, a wild species present in nature. The current Bombyx of the mulberry tree, Bombyx mori, is a domesticated form, a species in which the caterpillar does not move and the adult does not fly. Consequently, large numbers of worms and can be bred and various species can be hybridized.

8The worms are reared on open trays covered with mulberry leaves, their only food. Over the centuries, several hundred lineages have been selected that are best adapted to the various rearing areas. The monovoltine (one generation per year) and bivoltine (two generations per year) lineages are bred in temperate zones and give the best silks, whereas the polyvoltine lines (several generations per year) are bred in tropical and subtropical areas and give silks of poorer quality.

9The worm is fed for about 36 days. It then secretes silk to make the cocoon, inside of which it is transformed into a pupa, then into a butterfly. The cocoon consists of only one thread, which can be more than a kilometer long, and those with a longer thread (1.5 km) are selected. The adult leaves the cocoon by breaking the thread, and this is prevented by killing the pupa with heat. Once the pupa has been killed and dehydrated, the cocoons are preserved until the thread is reeled. Sericulturists plan their interventions to the day. All the worms in a breeding stock are the same age, and all secrete their silk at the same time, so that the cocoons can be collected all together.

10The B. mori worm constitutes more than 95% of all reared worms. Other, wild species produce silk of different quality, such as tussah, tasar, eri and muga silks.

The mulberry tree

11The most commonly used species, Morus alba, is found in China. There are currently several hundred varieties, adapted to the areas and soils in which they are cultivated. The tree is easy to multiply: by sowing, propagation by cutting and multiplication in vitro.

12The worms feed on leaves placed on the trays, which represents expensive work for gathering. In addition to the selection of varieties, efforts have been made to control the trees. Feeding branches consists of cutting and giving whole branches to last-age worms, without taking off the leaves. For first-age worms, the leaves are cut into thin strips, which is the only way of feeding them. Use of artificial food containing mulberry leaf powder was not satisfactory. The quality of the food has a direct effect on the quality of silk.

13To raise one ounce or nearly 40 000 eggs, a total of 1 200 kg of leaves are needed during 36 days over a final surface of 60 m2. One ounce produces 60 kg of cocoons, which produce 5 kg of raw silk.

14Breeding farms are either small family units or industrial-type units.

Sericulture today

15Progress in knowledge led to industrialization, with changes in some techniques. The first advance was separation of the production of cocoons from production of adults for eggs, resulting in two separate businesses, seeding and sericulture, each with its own know-how. The seeder ensures the production of healthy eggs, resulting in at least 98% hatching by a date scheduled by the sericulturist. Studies by Pasteur in France allowed eradication of pébrine, a fatal worm disease. The seeder also produces hybrids, resulting in cocoons of a much better quality than that provided by each parent. The seeder provides certified eggs to the local market and even exports them. When the eggs are incubated at the same time, the breeding stocks are homogeneous, facilitating interventions. Exchanges of knowhow between Europe and Asia modernize sericulture. Recent techniques are adopted in the countries where sericulture is established and also benefit family breeding farms.

16At the end of the 19th century, Japan, India and China were the principal producers. More than 95% of the world’s silk is produced in Asia. Recent statistics place China as the first producer (with more than 70% of the overall production).

Production of fresh cocoons and raw silk by country.

Image

Source : www.inserco.org

17The market and consumption do not always reflect sericultural activity directly. It is affected by labour costs and risks regression in the long term, as industrialization induces rural depopulation. In the medium term, Chinese and Indian production is likely to decrease in favour of new production centres (e.g. Africa, South America). Its future depends on grants, as the price of fresh cocoons remains low (US$ 2–3/kg). World demand is currently slightly increasing.

The sericulture of tomorrow

18Which is the future of sericulture? The example of Europe would indicate that it is compromised, because, although Europe consumes worked silk, it does not produce any cocoons. The probable increase in demand from emerging countries should lead to an increase in production, in contrast to their industrialization. Will these countries become like Japan, France and Italy? Any such evolution implies displacement of the production zones. Currently, silk is used almost exclusively as a textile fibre (e.g. for the sari at the beginning of this article). This activity will be maintained in the future, but other outlets for silk can be found without any great increase in sericultural activities, such as use of silk from cocoons with poorer-quality thread, which does not meet the criteria for textile use.

19The true future of sericulture lies in a new destiny for the silkworm: making proteins other than silk by gene transfer in the genome. Japan and China have become strongly implicated in this new strategy since France, which developed the technology in 2000, abandoned these programmes in 2010!

20Silk is synthesized in silk-producing glands. As it is proteinic, it is expressed by known genes and sequences. To obtain proteins other than silk from the cells of silk-producing glands, genes of other proteins are introduced into the genome of the silkworm, ensuring that it functions only in these glands. The silkworm is thus genetically modified. During the transformation, the introduced gene is transmitted to its progeny, thus directly obtaining a new lineage. In this way, a medically useful recombinant protein can be combined with silk, impregnating the thread. It can then easily be purified to obtain a product free of any cellular contaminants. The transformation strategy can be used for any kind of gene.

21Another application of transgenesis that will have a strong impact on the future of sericulture in India is the production of lineages resistant to the diseases due to baculoviruses. The lineages that give the best silks cannot be bred in India because of their high sensitivity to these viruses. With Indian colleagues, we introduced a genetic construction into the genome of silkworms, which is expressed in all cells and thus blocks replication of the viruses by inhibiting the structural proteins. The lineage obtained is thus resistant to the viruses. Hybrids obtained by crossing with the resistant line acquire this property. This represents enormous progress in the development of sericulture in tropical and subtropical areas.

22The production of medically useful proteins and special silks are under way.

Conclusion

23Sericulture, and therefore the silkworm, was the basis of many major upheavals, which changed the world, such as:

  • the silk roads, the first signs of globalisation;
  • study of the diseases of silkworms by Pasteur, leading to the advent of microbiology;
  • transgenesis of the silkworm in animal biotechnology.

24Sericulture, although practised fir more than 5 000 years, has a certain future in the 5 000 years to come.

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