The food of the caterpillars is preferably the foliage of the white mulberry tree (Morus alba); but several other species are employed, notably the Japanese mulberry (M. japonica), the alpine mulberry (M. alpina), and M. multi caulis and M. morelti. The osage orange (q.v.) is occasionally used as a substitute, but is said to produce an inferior grade of silk. The black mulberry (M. nigra) is objectionable because it is slow in growth. The caterpillars also thrive upon the leaves of lettuce and some other plants.
In the cultivation of the silkworm the first requisite is an ample supply of foliage. This is usually obtained by growing the mulberry trees far enough apart to give each ample room to develop its branches in all directions, the distance varying from 15 to 40 feet depending upon the variety, method of training, etc. Fre quently the trees are treated much the same as in pollarding (q.v.), except that the branches are cut while small. The eggs having been kept over winter at a temperature of less than 50 F. in a dry circulating atmosphere, are placed in a room or an incubator in which the temperature is raised gradually to about 73°. In about 10 days the larvae emerge and are cov ered with sheets of perforated paper sprinkled with chopped mulberry leaves, which should be renewed about nine times during the first 24 hours, each time by placing a fresh paper above the soiled one, which should be burned. Paper with larger perforations is necessary as the worms grow. As they approach maturity their appetities become voracious. When ready to spin they should be supplied with brush, straw or other material upon which to form their cocoons and the temperature at this time should be kept about 75 and the humidity close to 65°. At all times scrupulous cleanliness is essential, as also is abundant fresh air. When spinning is completed, no sound is heard inside the cocoon. The cocoons are then sorted according to quality, size and color into nine grades for manufacture, a separate lot being selected for breeding. The sexes are readily
recognized, the males being smaller and their cocoons more pointed ends than those of the females. After grading they are heated to destroy the insects within, whose emergence, if left alive, would break the thread many times. The manufacturing process is then begun.
Where a uniform and favorable temperature is not maintained, where humidity and purity of the air are impaired, and still more, where there is deviation from the strictest cleanliness, silk growers have often experienced serious losses of their caterpillars at all stages of their growth. Purifying the breeding and feeding quarters after each ((crop,° and again before the introduction of a new one, are essential. The walls are whitewashed, the trays sterilized in various ways, and the apartments fumigated with burning sulphur. No detail that will ensure cleanliness throughout the feeding period must be neglected, else the worms may suffer from the diseases which follow in the wake of neglect. Four of these, called in France pebrine, muscardine, gattine and flacherie, are due to in fectious organisms. For a fifth disease, Bras serie, no ascribable cause has been discovered. Pebrine devastated France to such an extent that by 1847 eggs were all imported from Italy. By 1865 the only safe source of egg-supply was Japan. Pasteur showed that rational methods of cleanliness in the breeding and feeding were the only remedy; the result is that France ex ports over 300,000 ounces of eggs annually. See SILK AND SILK INDUSTRY.
Kelly, 'Culture of Mul berry Bulletin 39, Division of Entomology, United States Department of Agriculture (Washington 1903) ; Oliver, G. W., 'Mulberry and Other Silkworm Food Plants' (Washington 1907) ; Verson and Quajat, filugello e l'arte sericola) (Padua 1896) ; Villon, 'La Soie' (Paris 1890).