Sericulture is an industry that is characterized by a two-step process, the cultivation of mulberry trees and the rearing of silkworms on mulberry leaves to produce cocoons. A cocoon is an oval- to football-shaped object made by a mature silkworm larva by spinning silk proteins; the silkworm larva develops into a pupa inside it. Silkworms are monophagous insects, feeding only on mulberry leaves (Moraceae, genus Morus). Because the mulberry leaves must be fresh, it is difficult to transport them over long distances or store them for long periods. This has resulted in the rearing of silkworms and cultivation of mulberry trees generally forming a single enterprise. Mulberry tree cultivation starts with the production of mulberry seedlings, followed by mulberry tree training, cultivation, harvesting, and insect pest control. Silkworm rearing includes preservation of silkworm eggs, management of rearing rooms, handling of rearing equipment, prevention of silkworm diseases, supplying mulberry leaves, and collecting mature larvae to transfer to the cocooning frame.
ORIGINS OF SERICULTURE
It is impossible to document when sericulture began. The silkworm, Bombyx mori, now has no wild populations; it is a completely domesticated insect. The oldest written record of sericulture is the Chinese silkworm topic Can-jing, which states that the queen of the Huang-Di empire started silkworm rearing. The Huang-Di era was around 2650 B.C . , but sericulture must have been carried out in China in even earlier times. From China, sericulture spread via the “Silk Road.” In the East, it was introduced into the Korean Peninsula and from there to Japan in about the 3rd century B.C. In the West, it spread to Central Asia and India and from India to Persia. Sericulture is thought to have reached Europe in 550 A.D., when silkworm eggs were presented to the Roman Emperor of the East. Silk was an important trade item along the Silk Road, where it was exchanged for its weight in gold. Commerce in silk along the Silk Road also made a major contribution to the exchange of Eastern and Western culture.
THE WORLD’S SERICULTURE INDUSTRY: PAST AND PRESENT
The state of cocoon production worldwide in 1997 compared to 1930 indicates that total global cocoon production, 617,910 tons in 1930 and 620,000 tons in 1997, was almost the same; the cocoon-producing countries, however, have changed considerably. In 1930, Japan ranked first, with a yield of 382,850tons, and accounted for 62% of global production. China (then the Republic of China), was second with 129,528 tons, 21% of the world total; Italy ranked third with 53,348 tons (8.6%) and the Soviet Union fourth with 15,300 tons (2.5%). At that time cocoons were produced almost everywhere in the world where the mulberry could be cultivated.
In contrast, in 1997 China had by far the greatest production, 423,000 tons, with 68.2% of global production. India ranked second with 127,000 tons (20.5%), followed by Uzbekistan, Brazil, Thailand, Vietnam, and North Korea. Cocoon production in Japan, which accounted for 62% of world production in 1930, dropped precipitously after World War II and today is 2500 tons, a mere 0.4% of the world total. The dramatic fall in cocoon production in Japan was the result of soaring labor and production costs and low cocoon prices compared with other agricultural products. Although an aging population of sericulture workers and a shortage of replacements were factors, the primary cause of the decline in recent years has been the development of large differences in cocoon prices between Japan and other cocoon-producing countries, such as China and Brazil.
CURRENT STATUS OF SERICULTURE IN DIFFERENT PARTS OF THE WORLD
From the 1980s to the 1990s the sericulture industry became concentrated in Asia. Brazil is the sole country outside Asia in which export-quality cocoons are produced. Although sericulture thrived in Europe during the first half of the 20th century, particularly in Italy and France, only a very small scale production remains in Eastern European countries, such as Bulgaria and Romania. The current status of sericulture in the principal cocoon-producing countries is briefly described below.
In China, the provinces of Szechuan, Jiangsu/Chekiang, and Guangdong are the three great sericulture regions. Cocoons of the wild Chinese oak silkworm, Antheraea pernyi, add to the production of silk from B. mori. About 50,000 tons a year of wild silkworm cocoons are produced by outdoor rearing in mountains and forests, chiefly in the northeast.
Sericulture suited to each of its regions is carried out nationwide in India, the second largest cocoon-producing country in the world, in which production has increased sharply in recent years. In the north of India, temperate-region sericulture is conducted with biv-oltine varieties of silkworm, and in the south it is being carried out with polyvoltine varieties or hybrids between polyvoltine and bivol-tine varieties. Many species of wild silkworm, including the Tassar silkworm (Antheraea mylytta) and the Muga silkworm (Antheraea assamensis), are used for sericulture in the northern regions of India. However, sericulture of wild silkworm species remains a manual industry from rearing to reeling, not like that of B. mori.
In Southeast Asia, traditional sericulture industries in Thailand, Vietnam, and Laos use tropical polyvoltine varieties suitable for clothing. In central Asia, Uzbekistan is the major sericulture country ranking third among cocoon-producing countries. There, univoltine silkworm varieties are used, and sericulture is conducted as a sideline industry to cotton growing. The Brazilian silk-reeling industry was started by Italian immigrants, and introduction of technology by Japanese silk-reeling companies has enabled the production of high-quality raw silk thread and silk, and Brazil is now a raw silk thread and silk exporter on par with China. In Japan, cocoon production amounts to less than 1%; however, Japan consumes about 25% of global production, ranking second to China.
MANAGEMENT OF SILKWORM EGGS
Management of silkworm eggs is one of the most important seri-cultural techniques. In tropical regions where the mulberry leaves are available all year, the management of silkworm eggs is not particularly important. Polyvoltine strains used in the tropics hatch year round and can be reared anytime. However, because univoltine and bivoltine silkworms are usually reared in temperate and subtropical regions, larval hatch must be coordinated with the season when mulberry leaves are available. Once the univoltine silkworms are reared, the larvae generally do not hatch from the eggs until early spring of the next year. Such eggs are called “hibernating eggs.” Artificial hibernation activates larvae to hatch by incubating hibernating eggs at about 10°C. Hydrochloric acid treatment can be used to induce hatching on demand.
Rearing silkworms consists of a series of tasks that includes harvesting and transporting mulberry leaves, supplying the mulberry leaf, cleaning the rearing beds, mounting the larvae so they can spin cocoons, and collecting and shipping the cocoons. The goal of silkworm rearing is to produce many high-quality cocoons while economizing on labor and material. The temperature range in which silkworms can be reared is 7—40°C, but practical rearing occurs in the 20-30°C range. Larval period rearing is roughly divided into the young silkworm period (first to third instar) and the grown silkworm period (fourth to fifth instar), and the fundamentals of rearing of each of them are different.
Young Silkworm Period
The growth rate is very high during the young silkworm period, especially during the first instar, but this period is characterized by higher susceptibility to bacterial pathogens and malnutrition. The young silkworm period is also physiologically characterized by a low quantity of food ingested, but a high rate of digestion. The amount of mulberry leaves ingested by young silkworms, that is, first to third instar, is only about 2% of the amount ingested during the entire larval period. The mulberry leaves provided in this period are finely minced.
The basic principles of young silkworm rearing are to provide relatively high temperatures and high humidity. A temperature range of 26-28°C is ideal, and as the instars proceed, temperature is gradually reduced. The ideal humidity is 75-90% and is lowered as the instars progress. In advanced sericulture countries, rearing of the young silkworms is generally carried out in cooperative rearing facilities.
Grown Silkworm Period
During the fourth instar silkworms ingest 10% of the total amount of mulberry leaves taken, and during the fifth instar they ingest approximately 88%. Because the fifth instar is the period when the silk proteins for cocoons are actively biosynthesized, an adequate supply of mulberry leaves must be provided. When silkworms are reared in temperate regions, at high temperatures of 30°C or more, disease or failure to spin silk can occur. When fifth instars are mature they start to spin. Silkworms discharge a great deal of fluid outside their bodies with the silk proteins, in addition to that from defecation and urination, during the cocoon production, or mounting, period.
REARING ON AN ARTIFICIAL DIET
In 1960, an artificial diet for silkworms was developed that could successfully sustain them from the first to the fifth instar (Fig. 1). The growth of the silkworms and the cocoon size obtained, however, were considerably inferior to those of silkworms reared on mulberry leaves and therefore this diet was inadequate for actual use in sericulture. Following many improvements the use of artificial diets during the young silkworm period has become widespread, and in
FIGURE 1 Silkworm larvae (fifth instar) reared on artificial diet.
1990 the rate of young silkworm rearing on artificial diets in Japan exceeded 40%. Japan is the only country in which artificial diets are used for cooperative young silkworm rearing.