The term of sericulture, stricto sensu, is to be limited to the production techniques of the cocoons by a caterpillar, the silkworm. In a broader sense, it includes the stages of stifling and reeling to obtain the raw silk fleets, which is how raw silk is marketed. Other interventions are necessary before obtaining the silk fabrics. It is the ennoblement of silks.
I would limit myself to the restricted direction of sericulture. Since the silkworm is fed with mulberry tree leaves, the culture of the mulberry tree is part of sericulture. The breeding of the silkworms and the culture of the mulberry tree is a saga which shaped the world.
Tsunami in a cup of tea: splashed by the fall of a cocoon in her cup of tea, Princess Si-Ling-Chi discovered the secret of silk while trying to remove it. She drew only one very long and solid thread. With this observation in 2602 BC, the emperor put his wife in charge of producing this fabulous thread. Si-Ling-chi collected these 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, sentencing to death any person trying to reveal its secrets. It was only in the form of silk fabrics that this material was known. These fabulous fabrics became the basis of commercial exchanges with the same value as currency, opening trade routes (139 BC, Han's empire) towards areas west of China. It was the first sign of globalisation. These roads supported commercial, cultural and even religious exchanges. The exceptional character of those goods attracted heteroclite populations, diversifying the exchanges (gold, money, horses, new food products or alfalfa). The Silk Routes crossed the deserts, from oasis to oasis, or the mountains. In the East, the silk route started from Chiang' an (Xi' an) and, to the West, passed either north of the Caspian Sea and then north of the Black Sea, or south of the Caspian Sea towards Baghdad and then to Antioche. However, this road was not the one the silkworm traveled. The question of the origin of this fiber remains unknown.
In the 2rd century BC, Chinese emigrants introduced sericulture in Korea, but it did not endure there. The Silk Routes were not the dissemination roads of sericulture. On the contrary, the Routes protected its secret, because the merchants wished to keep the monopoly on the exchanges. In the 5th century AD, sericulture reached India, where it will endure. Gradually it propagated to most countries of Asia: India, Korea, Japan, Cambodia, Vietnam, Thailand and others.
Such is the stage where the first act of sericulture was held. Let us see who are the actors.
The cocoon fallen into the cup came from a caterpillar from Bombyx mandarina, a wild species present in nature. The current Bombyx of the mulberry tree, Bombyx mori, is its domesticated form, a species in which the caterpillar does not move and the adult does not fly. Consequently, the breeding of great numbers of worms and the hybridization of various lines are possible.
The worms are reared on open trays covered with the mulberry tree leaves, their only food. Over the centuries several hundreds of lines were selected, the lines most adapted to the various rearing area. The monovoltine (1 generation per year) and bivoltine (2 generations per year ) lines are bred in the temperate zones and give the best silks, whereas the polyvoltine lines (several generations per year) are bred in the tropical and subtropical areas and give silks of lesser quality.
Cocoon market (Indupur, India) (© 2005 / B. Mauchamp)
Stocked cocoons before their reeling in the textile mill (© 2005 / B. Mauchamp)
The worm is fed for about 36 days. It then produces its 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, with a length that can exceed a kilometer. Selections are made to have cocoons with a longer thread (1.5km). The adult leaves the cocoon by breaking the thread, which is prevented by killing the pupa through heat stifling. The pupa is killed and dehydrated - the cocoons will be preserved until reeling to draw the thread. In a breeding stock, all the worms have the same age. The sericulturist plans his interventions to the day. All the worms dribble their silk at the same time. The cocoons are thus collected all together.
If the worm of B. mori constitutes more than 95% of the reared worms, other wild species produce a silk of a different quality, such as tussah, tasar, eri and muga silks.
The most used species, Morus alba, is from China. It currently counts several hundred varieties, adapted to the areas and soils where they are cultivated. It is a tree easy to multiply: sowing, propagation by cutting and multiplication in vitro.
The worms are fed by bringing the leaves on the trays, imposing an expensive gathering work. In addition to the selection of the varieties, the efforts were made in relation to the control of the trees. Breeding with branches consists in cutting and giving whole branches to last-age worms, without thinning out the leaves. For first-age worms, the leaves are cut in thin straps, which remains the only way of nourishing. The use of an artificial food containing powder of mulberry leaves did not give satisfaction. The quality of the food has a direct incidence on the quality of silk.
To raise one ounce of seeds, nearly 40,000 eggs, one needs a total of 1,200kg of leaves over 36 days on a final surface of 60m². With one ounce, one obtains 60kg of cocoons, which amounts to 5kg of raw silk.
The breedings farms are either units of family size (small units), or units of the industrial type.
Knowledge progress supported the industrialization by reconsidering some techniques. The first progress is the separation of the activities of producing the cocoons and of producing the adults for the eggs. Two separate businesses, grainor and sericulturist, each with its own know-how. The grainor ensures the production of healthy eggs, giving at least 98% of hatching to a date scheduled by the sericulturist. This allowed, following studies of Pasteur in France, to eradicate the pébrine, a fatal worm disease. The grainor also produces hybrids, giving cocoons of a much higher quality than that provided by each parent. The grainor provides certified eggs on the local market, but can even export them abroad. The eggs, incubated at the same time, give homogeneous breeding stocks, facilitating the control of the interventions. The exchanges of know-how between Europe and Asia, modernize sericulture. Recent techniques are directly adopted in the countries where sericulture is established and also profit the family breeding farms.
At the end of the 19th century, Japan, India and China were the principal producers. More than 95% of the world silk is produced in Asia. The recent statistics place China as first producer (with more than 70% of the overall production).
|Country||Fresh cocoons (tons)||Raw silk (tons)|
|China||512,700||(2005) 584 220||58,600||(2005) 87,800|
Productions of fresh cocoons and raw silk
The market and consumption do not always reflect the sericultural activity directly. However, this activity is impacted by the labour cost and risk in the long term to diminish, since the industrialization induces rural depopulation. In the medium term, one is likely to see the Chinese and Indian productions decrease in favor of new production poles (Africa, South America…). Its future is subordinated to grants, the buyer's fee of the fresh cocoons remaining low (2 to 3 $/kg). The world demand is currently in light increase.
Which future for sericulture? The example of Europe would leave us to think that it is compromised because, although consuming worked silk, Europe is not producing any more cocoons. The probable increase in the demand of the emerging countries supposes an increase in the production, which is in contradiction with their industrialization. Will these countries become like Japan, France or Italy? Such an evolution implies a displacement of the production zones. Currently, silk is almost exclusively used as a textile fiber (eg: for the saree at the beginning of the article). This activity will be maintained in the future, but other outlets for silk are found without involving a notable increase in the sericultural activities: they use silk coming from cocoons of lower quality of the thread, which does not answer the criteria of a textile use.
The true future of sericulture lays on a new destiny of the silkworm: making him make proteins other than that which is silk by gene transfer in the genome of the silkworm. Japan and China are very strongly implied in this new strategic planning since France, although having developed this technology in 2000, was disengaged of these programs (2010)!.
Silk is synthesized in silk-producing glands. Silk, of a proteinic nature, is the expression of known genes and sequences. To obtain other proteins than silk from the cells of silk-producing glands consists in introducing, into the genome of the silkworm, genes of the other required protein and make sure that it does not function in these glands. It is a genetic modification of the silkworm. At the time of the transformation, one will ensure that the introduced gene is transmitted in its progeny, thus obtaining a new line in a direct way. This way,, a recombining protein of medical interest is mixed with silk and impregnates the thread. It is then easy to purify it and obtain a pure product free from any cellular contaminants. The strategic planning of transformation for a gene can be used for any kind of genes.
Another application of the trransgenesis having a strong impact on the future of sericulture in India is obtaining lines resistant to the viral diseases due to the Baculovirus. We saw that the lines giving the best silks cannot be bred in India because of their strong sensitivity to the viruses. By transgenesis, with our Indian colleagues, we introduced into the genome of the silkworm a genetic construction which, while being expressed in all the cells, blocks the replication of the viruses by inhibiting the structural proteins. The line obtained is then resistant to the virus. The hybrids obtained by crossing normal lines with the resistant line acquire this property. It is an enormous progress for the development of sericulture in tropical and subtropical areas.
To produce proteins of medical interest or silks particular is outstanding.
Sericulture, therefore the silkworm, was the origin of many major changes in the world, for example:
Sericulture, although practiced of more than 5,000 years, has certainly a role to play for the 5,000 years to come.
Dr Bernard Mauchamp
Agricultural Engineer, INRA Research Director