The silk industry began in China, where the source of silk was kept a secret for more than 2,000 years. Attempts to take silkworm eggs out of the country were punishable by death. A few eggs were smuggled out of China about A. D. 555 and taken to Constantinople. Since that time commercial production has sprung up in some of the warmer countries, but the industry has been confined largely to China, Japan, India, and the Mediterranean region.

Sericulture has been attempted in the United States and interest in it is considerable. Silkworms can be raised and mulberry trees grown successfully here, but a tremendous amount of hand labor is involved and Americans must compete with the low labor costs in China, Japan, and India. Silk also must now compete in price with synthetic fibers, which can be produced at relatively low cost. Men in the Department of Agriculture conducted experiments with silk culture in 1884-91 and 1902-8. That work and many commercial undertakings in different parts of the country proved the impracticality of silk culture in the United States.

The silkworm is the larva, or caterpillar, of the moth Bombyx mori. Man has taken care of it so long that it has become thoroughly domesticated. The ashy-white moth has a fat body and a wing expanse of about 2 inches. It takes no food and seldom attempts to fly. After mating, the female deposits 300 to 400 round, yellow eggs, which soon become gray or lilac and paler as hatching time approaches.

At summer temperatures, the eggs hatch in 10 days. The larval stage requires 30 to 40 days, during which four molts occur. The baby worms are one-eighth inch long, and the full-grown caterpillar is fully 3 inches long. It is grayish or creamy in color and hairless. It has a hump behind the head and a spinelike horn at the tail. When full-grown, the larvae become restless and, if they are given a suitable place, such as dried brushy plants, they soon begin to spin their cocoons. The operation takes about 3 days of constant motions of the head from side to side at the rate of about 65 a minute. The cocoon is formed from a secretion from two large glands that extend along the inside of the body and open through a common duct on the lower lip. As the clear viscous fluid is exposed to the air it hardens into the fine silk fiber. The filament forming a cocoon is continuous and ranges in length from 800 to 1,200 yards. The cocoons are oval and vary in color, according to strain or race, from white to a beautiful golden yellow.

The larva pupates within the cocoon. In about 2 weeks the moth escapes through an opening in the end of the cocoon. The cocoons from which the moths emerge are called pierced cocoons. They are of low value because they cannot be reeled, but they are carded and made into thread.

For rearing moths, the cocoons are usually strung on a thread and hung in a cool, dark place until the moths emerge. The males and females are then put on cheesecloth, where they mate and where the eggs are deposited and adhere lightly to the cloth.

The race of silkworms most commonly used produces only one generation of worms a year, but other races produce two and still others produce several. The eggs are held in cold storage until they are to be hatched. An ounce of eggs will produce 30,000 to 35,000 worms, which will yield 100 to 120 pounds of fresh cocoons. The cocoons produce 10 to 12 pounds of raw silk.

Mulberry leaves are used almost entirely as food for silkworms. The white mulberry, Morus alba, is the preferred species. Foliage of Osage-orange has been used as a substitute. Lettuce leaves are sometimes used when the larvae are small.

The rearing of silkworms is laborious. The larvae are kept in a rearing house on trays in constant shade at a temperature between 65 and 78 F. They are first fed on chopped mulberry leaves supplied about eight times a day. After 4 or 5 days, fresh leaves are put in a tray with bobbinet bottom. On it is placed the tray that contains the larvae. They soon crawl up onto the fresh food. As they grow, the larvae are transferred frequently to fresh leaves on clean trays. They consume a surprising quantity of leaves, which must always be dry. Wet leaves or other adverse conditions favor the development of certain diseases which often take a heavy toll of the silkworms.

For reeling silk, the cocoons are gathered about 8 days after spinning begins, and the pupae are killed, usually with heat, and thoroughly dried. They are assorted and are ready for reeling. Reeling also involves much hand work, although recently developed reels work largely automatically. The cement holding the fibers together is loosened by putting the cocoons into boiling water. After the loose strands have been removed by a revolving brush the cocoons are put in warm water and the filament from four or five of them is caught up and twisted into a thread which is wound on a reel. This raw silk is removed from the reel in 2-ounce hanks, which are weighed and baled.

Inspects do not feed entirely on plants w e are interested in growing. Many kinds feed on weeds. Certain species of this sort have been introduced with beneficial results into regions where some plant has become a serious nuisance.

The control of pricklypear by insects in Australia is an example of what can be accomplished in this way at low cost. About 1787 cactus plants were taken to Australia by Capt. Arthur Phillip for culturing cochineal insects for dye. Various species of cacti escaped later from gardens so that by 1925 some 20 different kinds were found growing wild. In the absence of natural enemies the pricklypears spread rapidly. By 1925 about 60 million acres were affected, half of it so densely covered as to make the land useless.

Australia established a Commonwealth Prickly Pear Board in 1920 and sent entomologists to America, the original home of these cacti, to study the insect enemies and methods of rearing and shipping them to Australia. This work was continued in North and South America in cooperation with American entomologists from 1920 to 1937, during which time more than a half million insects of 50 different species were dispatched to Australia. Several were successfully established, including cochineal insects, a large plant bug, a moth borer, and a spider mite. The insects checked the new growth of cactus and reduced the density of the plants so that some grass was returning. It was not until 1930, however, when 3 billion eggs of a moth, Cactoblastis cactorum, from Argentina had been released throughout the territory, that the hope of controlling the pest began to be realized. Seven years after the first introduction of this moth the last dense growth of pricklypear was destroyed and the land reclaimed and opened to settlement and livestock production.

The total cost was about 168,600, or a fraction of a penny an acre a modest figure as compared with 10 per acre for the much less satisfactory chemical and mechanical procedure previously used.

F. C. Bishopp, a native of Colorado, has been conducting or directing research in the Bureau of Entomology and Plant Quarantine on insects since 1904. Since 1941 he has been assistant chief of the Bureau.