Making Silk in Lam Dong
Silk, most precious finery of the orient, is prominent in history and oriental mythology. Some say silk was invented so that women could go naked in clothes. A more whimsical tale credits a fourteen-year old Chinese empress with its discovery.
For centuries the wearing of silk was the exclusive right of the Chinese nobility and the knowledge of sericulture, the making of silk, was a zealously guarded secret. Even today, more than 4,000 years after its discovery, sericulture on a large scale is confined almost entirely to Asia. China is by far the world's largest silk producer, with an annual output estimated at 35,000 tons. Japan and India each produce about 8,000 tons. South Korea and Vietnam also contribute to the world's silk supply.
In Vietnam's Lam Dong province, silk production provides extra income for farmers who cultivate small plots of tea, coffee, pineapple, maize, cassava, pineapple and rice. Many are migrants who participated in a government program aimed at resettling people from overpopulated areas such as the Red River Delta. Much of Lam Dong's forests have been cleared for farming by families who received a hectare of land from the government. A visit to the Dambri waterfall, still surrounded by a remnant of jungle, gives one an idea of what the landscape must have been like even twenty years ago.
Silk production from egg to cocoon is a family affair in Lam Dong. Farmers like Dao Thi Kim Dung and Nguyen Ainh Thinh buy eggs of the silk moth, Bombyx mori, from a factory in Bao Loc, the district capital. One box containing 20 grams of eggs costs about 25,000 dong ($2.27) and can yield 40 kilograms of cocoons if all goes well.
Mrs. Dung and Mr. Thinh devote part of their land to mulberry plants, harvesting the leaves to feed the silkworms. Mulberry shoots are planted in dense rows and frequent harvests keep them from growing into trees. If harvested once a month, a field of one hectare can yield about 500 kilograms of leaves. In April and May, growing conditions are ideal and mulberry leaves can be harvested every three weeks.
Mr. Thinh and his family buy about four boxes of silkworm eggs every month and produce about 1.5 tons of cocoons annually. Their silkworm operation is large compared to Mrs. Dung's, whose is more typical, rearing only one or two boxes of eggs per month. Although Mr. Thinh's family has planted half of their hectare of land with mulberry, they still buy about 10 tons of leaves per year at about 1,000 to 1,500 dong per kilogram.
The tiny silkworm larvae that hatch from the factory-bred eggs feed on shredded mulberry leaves. They molt several times, increasing to seventy times their original size. Fully grown larvae are about as long and as thick as a human finger. The larger larvae are grown on round, woven bamboo trays and fed whole mulberry leaves two or three times a day. The silkworms are cycled onto fresh trays every day, and the uneaten mulberry stems and petioles left by the larvae serve as fertilizer. The bamboo trays are stacked in racks in a corner of the family house. The larvae grow faster at higher temperatures, but on the average, they take twenty-five to thirty days to develop from eggs to the cocoon-spinning stage.
When the caterpillars are ready to begin their metamorphosis into heavy-bodied brownish-white moths with short wings, they stop feeding and rear up on their stumpy legs as though searching for something. At this stage their human custodians distribute them onto vertical bamboo racks. Poorer farmers use trays of straw or dried maize stalks. Each silkworm winds itself into a cocoon, gyrating in an endless series of figure-eights, and binding a single interwoven strand of silk with a natural glue called sericin.
When the cocoons are completed, Mrs. Dung and Mr. Thinh collect and sort them, and sell them to the silk factory in Bao Loc. At the factory, the cocoons are heated to kill the silkworms inside and then soaked in hot water to soften the sericin so the silk filament can be untwisted from the cocoon and wound onto reels. Because single filaments are so fine, those from five to ten cocoons are wound together by drawing them through a porcelain guide and twisting them into a single fiber, glued together by the melted sericin. The reeled yarn made this way is called raw silk. The more sericin deposited on the filament, the lower the grade of the raw silk. Broken cocoons, partially joined ones, or those spun by two caterpillars together are inferior, and used much like cotton or wool to make spun silk.
Of all the natural fibers, silk has the greatest affinity for color, yielding shimmering, brilliantly-hued fabrics. Dyes are applied to raw or spun silk, or to woven fabrics after the sericin is removed by boiling. Stronger than any other natural fiber, the delicate look and feel of silk is deceptive. Silk cloth is lightweight, but warm in cool weather. Silk is comfortable in hot weather too, because it absorbs moisture up to 30 percent of its weight without feeling wet.
I had always imagined that silkworms were raised in factories. I discovered the real story of Vietnamese sericulture purely by accident while traveling from Ho Chi Minh City to Dalat, the capital of Lam Dong province, with agricultural scientists from the University of Agriculture and Forestry in Tu Duc. Our destination was a frontier area of the province that had recently been cleared and settled. This area was planted mostly with crops like maize and sweet potatoes and our mission was to assist farmers who were battling a pest called the sweet potato weevil. This insect can destroy a harvest of sweet potatoes by tunneling into the roots and rendering them unfit for either human or animal consumption. After meeting with farmers to discuss the problem, I suggested the possibility of using Beauveria bassiana, a fungus that attacks insects, to treat sweet potato cuttings as one way of lowering the weevil population.
On the way back from Dalat, I noticed woven bamboo racks leaning against the walls of many houses. After puzzling in vain over what their use might be, I finally asked my Vietnamese hosts what they were and why so many houses had them. The answer took me by surprise, "To provide a place for silkworms to anchor their cocoons."
"People rear silkworms at home?" I asked incredulously.
"Of course. That's how it's done all over Vietnam," they said. Several hours later we arrived in Bao Loc. With not much daylight left, my hosts were hurrying to get us to the Dambri waterfall. As we left the outskirts of Bao Loc, we passed the factory where silkworm eggs are grown. Unfortunately, it was shut, but I was still curious about homegrown silkworms. Enroute to the jungle, we passed through a hilly area planted almost exclusively with mulberry.
"Couldn't we stop for just a minute?" I asked. "I'd like to meet a silkworm farmer."
My ever-obliging hosts and I walked along the edge of a mulberry field to a modest house perched on the side of a steep hill. Tea bushes carpeted the hill below us. A woman was at home with her three small daughters and smilingly agreed to show us her silkworms. I saw the bamboo trays, stacked in columns and the vertical racks filled with newly-spun cocoons. A heap of mulberry leaves filled a corner, ready to be devoured by the fat, white eating machines. The mistress of the silkworms took my arm and guided me to a tray that she had pulled out and placed on a table under a bare light bulb. She took several of the larvae and put them on my palm. They were dead and had hardened in a strange way. "Many of the worms die of this mysterious disease, " she complained. "Sometimes I lose the whole lot." She didn't know what caused the disease, and received no help or advice from the egg factory.
Concerned, I asked if I could take the dead silkworms with me, explaining that we were agricultural scientists who worked with pests and that we might be able to identify the cause of the disease.
By the time we arrived in Dambri, the light was fading fast, and after climbing a long way down steep stone steps to the bottom of the cascade, we could hardly see it. Normally I would have been disappointed, but I felt satisfied I'd seen the Lam Dong jungle and felt the spray of the falls on my face. My mind was busy weaving images of silkworms, pondering their mysterious death and worrying about the income that farmers were losing because of it. That night in the hotel, I packed the silkworms in silica gel to dry them completely, and addressed them to Cornell University where they would be examined by an expert in identification of insect diseases.
The next day I went back to the mulberry hills to get an idea of how widespread the disease was. In the course of the morning we talked with several farmers including Mr. Thinh and Mrs. Dung. All of them had experienced outbreaks of the disease in their silkworms. Apart from my concerns about the infected Bombyx mori, the morning was the most fascinating and enjoyable of the entire trip.
A few weeks later, back at my home in Bogor, I received electronic mail from Cornell identifying the silkworm disease. I put out queries on the Internet for information about management of diseases in silkworms and received responses from Thailand, Australia and North America. I summarized what I'd learned in a letter to the Dean of the University of Agriculture Forestry in Tu Duc.
Since the Lam Dong trip, I've visited farmers in other parts of Vietnam who cultivate silkworms. In Tam Xa village near Hanoi, some farmers made me a present of mulberry leaves and two dozen silkworms, all ready to spin cocoons. Half of them, I was told, belonged to a special genetic strain that produces cocoons of yellow silk. I kept them stocked with leaves until I noticed them rearing up and circling their torsos in a figure-eight. All afternoon I watched them spinning their gossamer shrouds.
Now whenever I visit Hang Gai Street in Hanoi's Old Quarter, where silks are sold by scores of tiny shops, I have a special appreciation for the significance of a silk garment. My favorite item is the silk sleeping bag, an enormous piece of thin silk sewn into a sack, warm on a chilly night and feathery light and cool on a hot night, perfect for travelers to the tropics where elevation determines nighttime temperature. After seeing how it is produced, I take a particular delight at the feel of Vietnamese silk on my skin.
Published on 10/1/96