UNIVERSITY OF CALIFORNIA • COLLEGE OF AGRICULTURE 
 
 AGRICULTURAL EXPERIMENT STATION 
 
 BERKELEY, CALIFORNIA 
 
 CIRCULAR 363 
 October, 1945 
 
 SILK CULTURE IN CALIFORNIA 
 
 E. 0. ESSIG 1 
 
 Although sericulture, or the rearing of silkworms and the production of 
 raw silk, was advocated in California as early as 1855, actually the first silk 
 is believed to have been produced by Louis Prevost at San Jose, in 1860, from 
 silkworm eggs brought from China and mulberry trees grown from seed im- 
 ported from France. Because of the favorable climate, the ease with which 
 mulberry trees could be grown, the freedom from silkworm diseases and para- 
 sites, and the high quality of the silk produced in this state, California was 
 destined to become a great early testing laboratory for the production of silk. 
 So successful were these first attempts that, in 1864, the California State 
 Legislature was persuaded to encourage and support the infant industry by 
 direct financial aid. A bounty of $250 was offered for every plantation of 5,000 
 mulberry trees of two years' growth, and an additional sum of $300 for each 
 lot of 100,000 salable cocoons. This act created a boom in sericulture and 
 everything that pertained thereto, including the sale of silkworm eggs, mul- 
 berry trees, suitable land, and equipment. Indeed, development and produc- 
 tion increased to a point where there could hardly be a doubt about its future. 
 Increasing bounties paid by the state created a financial burden so great, 
 however, that a special session of the legislature was called, in 1867, to revoke 
 the act providing for financial aid. 
 
 EARLY DEVELOPMENT OF THE INDUSTRY 
 
 The California Silk Center Association of Los Angeles w^as formed early 
 in 1869 under the direction and enthusiasm of Louis Prevost, who had been 
 trained in the arts of sericulture in France. In June or July of the same year 
 this organization secured properties in Riverside County comprising the 
 Rubidoux Rancho, the Hartshorn Tract, and the Jurupa, consisting in all of 
 8,629 acres in the area now partly occupied by the city of Riverside. Unfor- 
 tunately, before developments were undertaken, Prevost died on August 16, 
 1869, and the project received a very serious setback. In the summer of 1870, 
 J. W. North, of Knoxville, Tennessee, organized the Southern California 
 Colony Association which took over the whole tract of land and laid out a town 
 which was first named Jurupa and later Riverside. Other associations were 
 also organized during the succeeding years in various parts of the state, but 
 none of them actually succeeded over any considerable length of time. 
 
 1 Professor of Entomology and Entomologist in the Experiment Station. 
 
 [1] 
 
2 California Experiment Station Circular 363 
 
 In 1872 various bills providing* premiums for silk culture and silk manu- 
 facture were introduced into the state legislature, but failed to pass. In spite 
 of this lack of support, silk production increased and the boom continued until 
 1872. Up to that time the industry had been carried by a wave of enthusiasm, 
 supported chiefly by the sale of silkworm eggs, land, mulberry trees, lumber 
 and equipment, and by the bonuses paid by the state. Orchardists and vine- 
 yardists were advised to border their roads and property lines with mulberry 
 trees in preparation for an even greater development in silk culture. Remnants 
 of these mulberry avenues still exist. The great Buena Vista Company near 
 Sonoma, owners of the largest vineyards in California at that time, had, in 
 addition to 500 acres of vines, 3,000 mulberry trees. 
 
 From 1872 to 1880, a considerable amount of good raw silk was produced in 
 the state. During this period, the supply of silkworm eggs was cut off, and large 
 profits were made in California in supplying" silkworm eggs to many parts of 
 the world as well as to home markets. However, this spurt of success was short- 
 lived and the silk industry slowly faded out. 
 
 As soon as the peak of expansion was reached and state aid was withdrawn, 
 the actual producer of raw silk was forced to recognize the fact that it was 
 impossible to compete with rapidly increasing imports of silk produced by 
 cheap labor in unirrigated lands in the Orient and in Southern Europe. About 
 this time, also, the United States entered actively into the manufacture of 
 silk goods. Compared with England in 1860, "the proportion of United States 
 manufacture was 13 per cent, in 1880 it reached 38 per cent, and in 1882 it 
 was 40 per cent of the entire consumption." Reeled silks were principally 
 manufactured at Paterson and Hoboken, New Jersey, and at Brooklyn and 
 New York City, New York, and the spun-silk industry flourished at South 
 Manchester and Hartford, Connecticut. In 1883 the United States ranked 
 third in importance in silk manufacture. The value of silk commodities, pro- 
 duced by various countries in that year, was as follows : 
 
 Millions of Millions of 
 
 dollars dollars 
 
 France 85 Russia 16 
 
 Germany 45 Australia 12 
 
 United States 35 Italy 12 
 
 Great Britain 25 Spain 5 
 
 Switzerland 18 Other countries 17 
 
 Growing demands for raw silk in the United States were supplied from foreign 
 countries and did not keep alive the struggling industry in California. It is 
 interesting to note that the value of raw silk imported into the United States, 
 amounting to $396,286,471 in 1925, reached the high peak of $427,126,383 in 
 1929 and slumped to $88,811,021 in 1938. 
 
 The California Silk Culture Association was founded in San Francisco, 
 November 18, 1880. Its officers consisted of a president and a vice-president ; 
 an executive committee comprising a recording secretary and corresponding 
 secretary ; a treasurer and board of directors ; and ten standing committees. 
 The object of the Association was "to promote silk culture in the State of 
 California by all practical means thereto." The officers were mostly business 
 men and women who were interested in the preservation of the silk industry 
 
Silk Culture in California 3 
 
 here. With little or no actual experience and without accurate knowledge of 
 any of the principles of sericulture, their services were more promotional than 
 scientific. Nevertheless, this organization was no doubt responsible for keeping 
 the industry alive for some years afterward. 
 
 To aid further the rapidly diminishing industry, a State Board of Silk Cul- 
 ture of California was provided for by the state legislature and approved 
 March 15, 1883. The membership included a president, three vice-presidents, 
 a secretary, treasurer, superintendent of filature, and six standing committees, 
 each consisting of at least three persons, "two of whom shall be members of 
 the Board and others selected from members of the California Silk Culture 
 Association." This board was organized in San Francisco on May 10, 1883, 
 with many of its officers selected from the membership of the California Silk 
 Culture Association. 
 
 Records seem to indicate that the California Silk Culture Association and 
 the State Board of Silk Culture of California were unable to maintain the silk- 
 culture industry in California ; both soon more or less ceased to function, and 
 on January 2, 1885, the Ladies Silk Culture Society of California was organ- 
 ized as the successor to the California Silk Culture Association. No evidence 
 exists that this organization really salvaged what was left of the industry in 
 the state, but from that time on there have been many other attempts to re- 
 juvenate it. 
 
 Prior to 1900 C." W. Woodworth, of the University of California, became 
 much interested in reviving the silk industry in California and attempted to 
 find plants other than the mulberry for feeding silkworms. These feeding 
 experiments did not come to any fruition, however, and there was then no 
 serious return to the commercial growing of silkworms in the state. 
 
 In 1917 Guy Wilkinson secured 800 acres of land at Wyandotte, Butte 
 County, and organized a silk company under the place name of Seriterre. He 
 secured financial support from a group of businessmen associated with the 
 San Francisco Chamber of Commerce and with other commercial organi- 
 zations of that city, and attempted to prove that silk could be produced 
 profitably in California if labor-saving machinery and up-to-date methods of 
 production were employed on a large scale. He secured the most desirable 
 mulberry varieties available and planted 175,000 trees in all. In his first efforts 
 Wilkinson was somewhat hampered by restrictions imposed by his financial 
 contributors. He carefully kept figures of the operation of the cocoonery, and 
 worked out a plan for the correct number of workers to handle a certain num- 
 ber of eggs and to care for all of the necessary operations, from the hatching of 
 the eggs to the production of the raw silk. Although Wilkinson quit in 1928, 
 he felt that he had then proved that silk could be produced commercially in 
 competition with foreign countries. 
 
 The American Silk Factories, Inc., was organized about this time and began 
 to test new methods for modern silk production. It bought out the San Diego 
 County Silk Company and began operations in 1928 at San Marcos, where 320 
 acres of land were prepared for mulberry trees. This company has continued 
 to operate, but its activities have been somewhat limited by war conditions. 
 It has used the best types of mulberry trees, an overhead system of irrigation, 
 the finest possible varieties of yellow Cevenne and white Chinese silkworms, 
 
4 California Experiment Station Circular 363 
 
 and modern equipment. The company plans very extensive expansions which, 
 however, have not yet been fully executed. 
 
 Thus, the actual raising of silkworms in California has been given a suffi- 
 cient number of trials to show, without doubt, that it can be successfully carried 
 on here. Sericulture has not been continued for many reasons, chiefly because 
 properly trained labor could not be secured here (or perhaps anywhere else 
 in the United States) at a sufficiently low wage to enable American growers 
 to compete with those of the Orient and Southern Europe. Silk production 
 in the Old World is a household industry, carried on almost entirely by women 
 and children. The worms require painstaking and regular attention because 
 they must be meticulously fed day and night and their rearing cages kept 
 scrupulously clean at all times. 
 
 Another difficulty in attempting to raise silkworms commercially in Cali- 
 fornia at present is the scarcity of mulberry trees suitable for establishing a 
 source of food for the caterpillars. True, some of the larger nurseries can 
 supply small quantities of the white mulberr}^, but none of the special varieties 
 which have proved to be most useful in feeding the caterpillars is now readily 
 available. Certain common varieties used for ornamental purposes and for 
 fruit may not be at all suitable for silkworms, especially if the foliage is hard 
 and tough. Silkworms may be reared temporarily on osage-orange, dried let- 
 tuce, and a few other plants, but these foods are poor substitutes for the mul- 
 berr}^. 
 
 This somewhat extensive historical sketch has been thought necessary in 
 order to inform those who are now interested in sericulture in California that 
 this is not a new venture. Many large, well-financed, and skillfully handled 
 organizations, employing trained sericulturists, have had a difficult time in 
 years past in attempting to establish a silk industry in this state. Those who 
 appear to have profited most from the varied attempts at sericulture in Cali- 
 fornia were the recipients of state bounties, promoters of real estate, mulberry 
 nurserymen, and those who have sold to the prospective producers the facilities 
 for embarking upon the venture. 
 
 REARING SILKWORMS 
 
 For those who wish to experiment with, or actually to begin, the production 
 of silk, a few directions may serve as a basis for operation. 
 
 Eggs. — The eggs (fig. 1) vary in color, but are usually purplish or slate- 
 colored. They may be purchased in the fall or winter months and held in 
 refrigeration for future hatchings at temperatures of from 40° to 50° F. They 
 may be incubated at temperatures of 80° to 85°, whenever mulberry leaves 
 become available for food. By means of frequent hatchings, the caterpillars 
 may be carried along in consecutive hatches in order to utilize continuously 
 the space allotted to the different stages. 
 
 To produce eggs for future use, the choicest cocoons are selected and care- 
 fully tended until the moths emerge in from 12 to 20 days. Such cocoons should 
 be placed on specially constructed racks, among suitable branches, or should 
 be given other supports that will permit the moths to cling and develop prop- 
 erly. The moths escape from the cocoons through a hole, at one end, made by 
 
Silk Culture in California 5 
 
 dissolving the silk with a liquid secreted through the mouth. The moths eat no 
 food during their brief existence. The female moths may be distinguished from 
 the males by their larger bodies. Care must be taken to see that mating occurs. 
 Each fertilized female should be confined over a sheet of paper under a metal 
 funnel while she is laying her eggs. These eggs may be kept for subsequent 
 hatchings. A well-developed female will produce from 300 to 700 eggs during 
 a period of 2 to 3 days, and will die 6 to 12 days later. 
 
 Eggs from one moth will ordinarily produce more than 2 pounds of fresh, 
 or green cocoons. One ounce contains about 40,000 eggs, or a sufficient number 
 to produce from 80 to 104 pounds of fresh cocoons. Silkworm eggs are gen- 
 erally referred to on the market as seeds, or grains. When laid they are more or 
 less stuck together by an enveloping liquid glue which hardens on exposure 
 to the air. This sticky substance may be removed by dipping the eggs in cold 
 
 Fig. 1. — Silkworm eggs, or seeds, glued to paper are often offered for sale. They are gray 
 or lead-colored. Those shown at the right are twice natural size ; those at the left are enlarged 
 20 times. 
 
 water. They should then be carefully dried and put in suitable containers of 
 1-ounce units. Small cambric bags are the most practicable. For future use, 
 eggs should be kept at a temperature of 40° F. 
 
 Eggs of desirable varieties of silkworms have usually been imported from 
 special sources in Asia and Southern Europe, but these supplies probably will 
 not be available until sericulture is reestablished in the old world. A small sup- 
 ply may be available from dealers in this country, but there is no ready means 
 of finding the supply or of determining the quantity and quality. A silk indus- 
 try of considerable importance has been developed in Brazil and, to some ex- 
 tent, in Mexico. Eggs might be secured from these countries through United 
 States governmental agencies. Eggs are usually handled for hatching in lots 
 of 1 ounce, the progeny of which may be reared on a tray 9 feet square. For 
 hatching, a temperature of 80° to 85° F is preferred. 
 
 Caterpillars. — In their development, the caterpillars, or worms, pass through 
 four molts, or shedding of the skin, which permit growth ; and through five 
 stages and instars. 
 
 First stage : This stage embraces the period from hatching to the first molt, 
 from 5 to 6 days. These newly born caterpillars are y 12 inch long and almost 
 black in color, remaining so until they shed their first skin. The optimum 
 temperature for them is 75° F. The molting period requires 22 hours. The tiny 
 caterpillars must be frequently and regularly fed on tender new leaves, 
 
6 California Experiment Station Circular 363 
 
 chopped fine, and must be carefully protected against drafts of cold air and 
 excessive moisture. They should be fed as often as they will eat clean, day 
 and night, at least 4 to 8 times in 24 hours, until they are ready to molt. They 
 must be kept in partial darkness by screens or by other protection from bright 
 sunlight. During this stage the caterpillars hatched from 1 ounce of eggs 
 will consume 6 pounds of mulberry leaves, and require 9 square feet of clean 
 tray space. The molting process is a very critical period in the life of the cater- 
 pillars ; during this period, when they shed their skin, they become quiescent 
 and must not be handled or moved, and should be kept in warm, somewhat 
 dark quarters. They usually molt early in the morning and, after shedding 
 the skin, the worms frequently eat it. After molting, the caterpillars eat greedi- 
 ly and must be kept supplied with fresh food. 
 
 Second stage : This stage is from the first to the second molt, 4 to 6 days 
 (9 to 12 days after hatching). The second molting period lasts 22 hours. The 
 temperature should be between 73° and 75° F. The color becomes whitish or 
 
 Fig. 2. — The full-grown silkworm is almost white with brown head and body markings. 
 
 (Natural size.) 
 
 pale yellowish. During this stage the caterpillars hatched from 1 ounce of 
 eggs will consume 13 pounds of leaves chopped somewhat coarser than for the 
 first stage. 
 
 Third stage : The third stage is from the second to the third molt, 4 to 5 
 days (13 to 17 days after hatching). The molting period lasts about 27 hours. 
 The temperature should be between 71° and 73° F. The larvae are still fairly 
 small and are much the same color as in the previous stage. During this period 
 the caterpillars hatched from 1 ounce of eggs will consume 50 to 60 pounds 
 of well-sorted coarsely chopped leaves. 
 
 Fourth stage : The fourth stage is from the third to the fourth molt, 5 to 
 7 days (18 to 24 days after hatching). The molting period lasts 41 hours. 
 Optimum temperature for development is between 68° and 71° F. The color 
 is the same as in the previous stage. The larvae are more than half grown, 2 
 inches long. The caterpillars from 1 ounce of eggs will consume 180 pounds 
 of coarsely chopped or whole leaves. Some experts feed leaves left on branches, 
 thus avoiding the necessity of removing the leaves, a method that has gained 
 widespread use in California. It is now claimed there are certain patented 
 machines that quickly and cheaply remove leaves and chop them as desired. 
 
 Fifth stage: This period extends from the fourth molt to full develop- 
 ment of larvae, 7 to 12 days (25 to 36 days after hatching). Optimum tem- 
 perature for development is between 68° and 71° F, or the same temperature 
 as for the previous stage. It will be noted that the temperature has been 
 gradually decreased as the caterpillars have developed. Color varies from 
 
Silk Culture in California 7 
 
 almost pure white to soft yellow shades. The worms become from 3 to 4 inches 
 long and have a short dorsal anal horn (fig. 2). During the last stage the 
 worms hatched from 1 ounce of eggs require about 1,200 pounds of leaves. They 
 should be fed every 4 hours, day and night, and will need 250 square feet of 
 space on the trays. During this stage, the maturing caterpillars produce from 
 600 to 675 pounds of excrement, which must be carefully removed daily. One 
 of the most remarkable features of the silkworms, which shows the domesticity 
 of the species, is their utter dependence upon the human race for care and 
 existence. If left alone in a small box, tray, or other container, the caterpillars 
 will not wander away, like other caterpillars even though starved. This de- 
 
 
 Fig. 3. — Various types of silkworm cocoons. A and B, Chinese races; C, Araya, 
 a Japanese race; D, Italian race. (Enlarged.) 
 
 pendence makes their care unusually easy. One important consideration in 
 their maintenance is to provide scrupulously clean, airy, temperate, and light 
 quarters. Some properly screened sunshine in the rearing rooms is very desir- 
 able, and under no conditions should the worms be kept in totally dark, or in 
 cold, damp, and dirty surroundings. The great accumulations of excrement 
 should be thoroughly removed daily. During this last period the caterpillar 
 spins its cocoon. First it seeks a suitable place for attachment. To supply this 
 need, clean special racks, or clean brush suitably or conveniently arranged 
 on trays are selected. The caterpillar then spins a considerable amount of loose 
 silk as a means of attaching and securing the cocoon. The silk is developed in 
 two large elongated glands which are four times the body length and occupy 
 much of the interior of the caterpillar. The spinning is done by means of a 
 small spinneret protruding from the lower lip, or labium. Although the spin- 
 neret has a single apical opening, the base is perforated by two tiny holes 
 through which the fluid silk is forced by means of a thread press. In the process 
 the two threads become somewhat twisted and cemented together, in order to 
 emerge through the common apical opening in the spinneret as a double, rib- 
 bonlike single strand. It has been found that raising the temperature one or 
 two degrees, during the time of spinning, hastens the process. Once started, the 
 caterpillars continue to spin without ceasing for 4 days and nights, or until 
 the cocoon is completed. The first thread is the largest in diameter and, as the 
 spinning progresses, it gradually decreases in size as the strength of the cater- 
 
8 California Experiment Station Circular 363 
 
 pillar and the supply of silk fluid diminishes. If, by any chance, the tempera- 
 ture drops below 68° at the time of cocoon spinning, the caterpillars may be 
 unable to form good-quality silk and if the temperature gets too low, they 
 will cease to spin altogether. A modern air-conditioned rearing room and co- 
 coonery could be regulated to make temperature and moisture conditions per- 
 fect for the sericulturist in almost any part of the state. 
 
 Cocoons. — The cocoons are white, tan, or yellow according to the variety or 
 strain of the silkworm. They are normally about 1% inches long and half as 
 wide (fig. 3). The silk which forms the cocoon is an unbroken thread, varying 
 in length from 750 to 2,400 feet, the length depending, of course, upon the 
 size of the cocoon and the fineness of the silk. Ten pounds of cocoons are 
 usually required for 1 pound of raw silk. According to the published testi- 
 mony, 1 acre devoted to silk production will yield 1,000 pounds of green 
 cocoons (equals 700 pounds dry silk) every month during the feeding season. 
 
 Fig. 4. — Adult female silkworm moth. Although their wings are well developed, the 
 adults are unable to fly. (Natural size.) 
 
 The number of generations of moths may be regulated by intervals of egg 
 hatching so that as many as 3 to 5 or more may be possible. The adult female 
 moth is shown in figure 4. According to all reports, California-reared silk- 
 worms not only produce unusually large cocoons, but the silk is of the highest 
 possible quality, offsetting to some degree, it is claimed, the differential in 
 costs of production. 
 
 As previously stated, the choicest cocoons should be set aside for the rearing 
 of moths for egg production. These requirements should be very small. The 
 cocoons to be offered for sale as green, or raw silk, must be safeguarded from 
 destruction by the emergence of the moths through a treatment that will kill 
 the pupae, or chrysalids, inside without injuring the quality of the silk. Various 
 treatments may be used, such as dipping the cocoons in boiling water for 10 
 minutes, heating to 150°-170° F for 20 minutes, fumigating with such gases as 
 hydrocyanic acid, chloropicrin, methyl bromide, carbon tetrachloride, or 
 ethylene dichloride, cooling to 40° or lower for 1 hour, and other methods. 
 
 FOOD PLANTS 
 
 The mulberry is the natural food of the Chinese silkworm. The leaves are 
 ordinarily eaten, but under stress of starvation the caterpillars may be fed for 
 some time on the bark of the younger twigs. Other plants, including osage- 
 orange, lettuce, and, possibly, also dandelion and black salsify may be substi- 
 
Silk Culture in California 9 
 
 tuted for short periods of time. Mulberry trees were first introduced into Cali- 
 fornia from France by Henry Hentsch, of San Francisco, who obtained seeds 
 in 1854 "which grew well." 
 
 White Mulberry. — During centuries of sericulture the white mulberry. 
 Morns alba L., has proven to be the best all-round species for the feeding of 
 the domestic silkworm. This tree is deciduous, hardy as to climatic require- 
 ments, and fairly drought resistant. It may grow into huge proportions and 
 attain a height of 80 feet if not controlled. The shining green leaves are 4 
 inches long, coarsely toothed, and often lobed. The fruits are whitish, pinkish, 
 or even dark purple and are highly prized as food. It is a native of China, but 
 has been employed in all parts of the world where sericulture is practiced. 
 According to many authorities the white mulberry and its varieties produce 
 the best quality of silk. It is easily propagated from seed, by layering, from 
 cuttings, and by grafting. Many varieties have been selected for special 
 purposes. 
 
 Moretti Mulberry. — This variety of the white mulberry was discovered by 
 Professor Moretti of Pavia, Lombardy, Italy, in 1815. It was this variety that 
 was first introduced into California in 1854 and used by Louis Prevost to 
 produce the first silk in California at San Jose in 1860 and was subsequently 
 distributed throughout the state. The leaf is ovate, sharp pointed, entire or 
 cordate at the base, shiny and somewhat wider than that of the true alba. 
 Prevost thought this would be "the very best variety for California" because 
 of its vigor, its large, thick, tender leaves, and its influence on the production 
 of a superior quality of silk. 
 
 Chinese Mulberry. — The Chinese mulberry (Morus multicaulis Perr) is 
 also known as the Philippine mulberry, the many-stalked mulberry, and the 
 Perrotet mulberry. It was first noted in Manila, P. I., by Perrotet who sent it 
 to France in 1821. The tree is rather small. The leaves are entire, serrate 
 toward the tips, and convex dorsally. They are dark green, rough, large, and 
 tender, and are especially suitable for the feeding of newly hatched cater- 
 pillars. The tree is a vigorous grower that suckers freely from the roots. It is 
 very easily propagated from these suckers and from cuttings and layerings. 
 It is more susceptible to severe cold than are the kinds referred to previously 
 and it can endure the winters along the Pacific Coast. It proved unsuitable 
 in the North Atlantic States where it was so greatly desired when first 
 introduced and bought for such high prices that a near panic was created in 
 1839 when it was found to be readily winter-killed there. It has been exten- 
 sively and successfully grown in California. 
 
 Propagation and Care of Mulberry Trees. — The proper culture of the mul- 
 berry is of great importance in silk culture. The trees may be easily propa- 
 gated from seeds, cuttings, suckers, layerings, and by grafting. Of these 
 methods, the use of seeds is likely to give poorest results, because of the great 
 variation in the characteristics of the seedlings so produced. Cuttings, espe- 
 cially when treated with modern rooting hormones, are easily and cheaply 
 grown. Suitable cuttings are 6 to 8 inches long, with the top cut just above a 
 bud ; they are planted deep enough in beds of sharp sand, or in a good sandy 
 loam soil to cover the tip V2 inch in order to protect it from the heat of the 
 sun. In the nursery the cuttings may be spaced 6 to 8 inches in rows 3 to 4 
 
10 California Experiment Station Circular 363 
 
 feet apart. After two years' growth they may be transplanted and spaced 3 
 to 4 feet, in rows 10 feet apart. Such plantings must be kept low and relatively 
 close-trimmed at all times to make harvesting as easy and rapid as possible. 
 Subsequent pruning is largely influenced by individual preference. The first 
 branches should be left within iy 2 feet of the ground, and at the next pruning 
 only two twigs for each branch should be allowed to grow; in February, their 
 twigs are headed back and, in turn, each twig is treated in the same manner 
 the following year. Thus a thick, bushy tree about 5 feet tall is formed which 
 may be easily maintained by judicious pruning during the dormant season, or 
 at the time when twigs and leaves are harvested for silkworm food. Such low- 
 growing trees permit all operations of pruning and harvesting from the 
 ground. 
 
 Soils. — Mulberry trees require a deep, rich, light loam soil, well drained 
 for the best results. It is true that they will grow on many types of somewhat 
 poor soils, but such orchards are likely to prove very unsatisfactory for profit- 
 able sericulture. Heavy, continuously wet soils and rocky, poor, dry soils 
 should be avoided. 
 
 Irrigation. — In California mulberry trees should be regularly irrigated in 
 order to produce continuous new growth suitable for feeding the young 
 worms. Well-watered orchards will produce enough foliage to carry a con- 
 tinuous succession of silkworm generations throughout the long summer and 
 fall periods ; this would not be possible on nonirrigated lands in regions farther 
 
 north. 
 
 OTHER FOOD PLANTS 
 
 A great deal of attention has been given throughout the ages in searching 
 for foods that might be substituted for mulberry leaves. So far, little of value 
 has been achieved. The American osage-orange, Madura pomifera Schneid., 
 has been utilized somewhat as a temporary expedient. However, it cannot be 
 continuously used as a food for more than one or two generations of worms. 
 Since it is native to North America, its association with sericulture has been 
 relatively short, and there has been little real reason for using it in place of 
 mulberry. 
 
 Lettuce has also been used as food over limited periods of time. Although 
 fresh lettuce has been accepted by the worms, experienced growers recommend 
 only wilted leaves and leaves free from outside moisture (not wet leaves) . 
 
 PESTS OF SILKWORMS 
 
 Among the common pests of silkworms that must be guarded against are 
 rats, mice, birds, and certain insects. The Argentine ant, which was first dis- 
 covered in California about 1902, is likely to be a very serious pest of the 
 silkworm here. This ant occurs abundantly throughout much of the state and 
 is a very general nuisance, particularly in the middle and southern coastal 
 areas, although it also occurs in the interior valleys. Although it is very gen- 
 erally attracted by sweets, it is also extremely fond of insects and is likely 
 to be destructive to the newly hatched caterpillars unless great care is taken 
 to keep it out of the breeding areas. With this insect in mind, pains should be 
 taken to provide suitable ant traps in the construction of the buildings. These 
 traps could be metal troughs placed just above the foundations and should be 
 
Silk Culture in California 11 
 
 filled with oil to prevent the entrance of ants into the buildings. The use of 
 sweetened poison baits reduces the numbers of this ant, but baits are not 
 effective in protecting a breeding room or a cocoonery from its ravages. 
 
 In many regions where silkworms have been propagated for many years, 
 certain tachina flies — somewhat resembling houseflies — lay their eggs on the 
 leaves of the mulberry trees. These eggs are subsequently eaten by the silk- 
 worms, along with the mulberry leaves, and hatch within the bodies of the 
 caterpillars as parasites; they not only prevent the silkworms from spinning 
 their cocoons but eventually kill them. No parasite of this type is known to have 
 attacked silkworms in California. 
 
 DISEASES AFFECTING SILKWORMS 
 
 Silkworms are subject to a number of very serious diseases that often do 
 great damage if they gain entrance into the cultures. Any attempt to start 
 a new silk industry in California should be safeguarded by severe restrictions 
 against the introduction of any of these infectious diseases. For this reason, 
 too, private individuals should refrain from purchasing eggs or silkworms 
 without a guarantee that the stock is free from such diseases. One of the 
 most essential requirements to the success of this ancient industry is strict 
 and constant cleanliness, as well as proper sanitation in every operation having 
 to do with the propagation and handling of the insect from egg to adult. Unless 
 strict state supervision by qualified officials is maintained to inspect and to 
 provide for the control or eradication of diseases it is doubtful if a silk indus- 
 try of any magnitude could be continuously maintained in any given area for 
 any great length of time. Among the serious diseases, the four discussed below 
 are the most important and should be guarded against. 
 
 Pebrine. — This is a contagious disease caused by a microsporidian parasite, 
 Nosema bombycis Nageli. It is usually transmitted from the parent female 
 moth through the eggs to the caterpillar. The disease may be detected in all 
 stages of the insect by a microscopic examination of the blood for the presence 
 of oval spores. Caterpillars infected with pebrine grow slowly and vary 
 greatly in size, though of the same age. Brown and black spots on the skin are 
 indications of the presence of the disease. Infected moths also may show 
 unexpanded antennae or wings. This very destructive disease has often done 
 great damage. Louis Pasteur became famous through his discoveries and re- 
 searches on the control of the organism. Methods of prevention consist in 
 using eggs from uninfected moths only. The appearance of the disease in any 
 establishment will necessitate the complete destruction of all forms of silk- 
 worm life therein and very drastic sterilization of all equipment used in the 
 care of caterpillars, cocoons, and moths. A microscopic examination of the 
 eggs or the females will prove the presence or absence of this disease. 
 
 Flacherie. — Although now suspected of being produced by a filtrable virus, 2 
 this ailment has generally been considered to be caused by the bacterium 
 Bacillus bombycis, which is most frequently associated with it. Bacillus bom- 
 bycis is the same organism to which Pasteur referred as being a "vibrion with 
 
 2 According to Dr. E. A. Steinhaus, eminent authority on the diseases of insects, who has 
 furnished much of the information contained herein on the diseases of silkworms, "the pri- 
 mary cause of both flacherie and gattine is a certain filtrable virus and the bacteria 
 associated with these diseases are secondary invaders." 
 
12 California Experiment Station Circular 363 
 
 a nucleus," or spore. There is probably a whole group of bacteria which, under 
 favorable conditions, may cause flacherie. This disease is associated with the 
 spoilage of eggs, with the feeding of wet or fermenting mulberry leaves, poor 
 ventilation, excessive heat and moisture, overcrowding, and general poor 
 health of the caterpillars. Some authorities report that susceptibility to this 
 disease is hereditary, or accidental, "referring to the fact that epidemics may 
 start from a few individuals which have been injured." In true flacherie there 
 is usually a very characteristic odor. The control measures consist in the de- 
 struction of all diseased worms and the disinfection of the premises. 
 
 Gattine. — According to the French authority, Paillot, the primary cause 
 of gattine "appears to be a filtrable virus as in the case of flacherie." It pre- 
 disposes the insect to the disastrous effects of the bacterium, Streptococcus 
 bombycis. However, it may show up any time after the first stage. The infected 
 caterpillars appear emaciated and somewhat transparent, and often wander 
 restlessly about, many of them being unable to spin their cocoons. Control 
 consists in the destruction of all infected caterpillars and the removal of 
 healthy ones to new and thoroughly sterilized quarters, where they should be 
 fed scantily for several hours. 
 
 Muscardine or Calcino. — Caterpillars infected with this disease frequently 
 turn rose-colored or reddish. According to Dr. Steinhaus, many authorities 
 believe that the red coloration so frequently seen is not due to fungus itself, 
 but rather to a red pigmented bacterium, Serratia marcescens, which prevails 
 when the insect is attacked by the fungus. After death the body is covered 
 with a white, chalklike efflorescence. This disease is caused by fungi, particu- 
 larly Beauveria bassiana and B. tenella. The presence of this fungus indicates 
 poor sanitary conditions. A recommended disinfectant is made by dissolving 
 1 pound of copper sulfate in 100 gallons of water. 
 
 Grasserie or Jaundice. — This disease of silkworms is caused by a filtrable 
 virus. The infected caterpillars are covered with yellowish spots, and just be- 
 fore death their skin becomes a dirty-yellow color. After death the corpse 
 rapidly disintegrates. A microscopic examination indicates that the body fluid 
 contains great numbers of minute crystal polyhedral bodies. The disease is 
 spread from feces of infected living worms and from other body fluids of the 
 dead, melting caterpillars. The virus survives for some time. Although egg 
 transmission is suspected, it has not yet been proved. Some varieties of cater- 
 pillars appear to be resistant. Although the disease may become epidemic, 
 how infection first takes place does not seem to be known. 
 
 NEW METHODS SUGGESTED FOR SERICULTURE IN CALIFORNIA 
 
 Those who have maintained that the rearing of silkworms has been success- 
 ful in California are undoubtedly right. In reviewing the literature on the 
 subject, however, it is a notable fact that very few real improvements have 
 been made during the past fifty years in the development of sericulture in 
 this slate, and there has been little to encourage the revival of the industry. 
 Among the factors that might enter into the modernizing of the industry are 
 those discussed below. 
 
 Neiv Races of Silkworms. — The development of hardier and more useful 
 univoltine races of silkworms is needed to extend the breeding season through- 
 
Silk Culture in California 13 
 
 out the spring, summer, and fall. Consideration should also be given to the 
 development of size and of greater silk-producing capacities of the worms, as 
 well as to their resistance to diseases. It is possible that certain races might be 
 desired for spinning very fine silk, and others for spinning coarser and more 
 durable types of silk. 
 
 Improved Mulberries. — In California there seems to have been only one 
 named selection of improved mulberries during the past fifty years. This may 
 have been simply a chance selection of a variety of Morns alba by Luther Bur- 
 bank which has been called the Burbank mulberry. Its adaptation to seri- 
 culture in California has not been thoroughly tested. The development of 
 varieties of mulberries that would leaf early, and perhaps of other varieties 
 that would hold their leaves late in the fail, thus lengthening the period of 
 feeding, should be possible. In fact, such possibilities appear to be extremely 
 promising for a climate like that of this state which has such a long spring, 
 summer, and fall growing season. 
 
 Sanitation. — Better methods for sterilizing and maintaining the general 
 good health of silkworms are possible under present knowledge of the physi- 
 ology of insects. The use of hormones and vitamins in the feeding of silkworms 
 is apparently an unexplored and promising field of experimentation. 
 
 Modern Equipment. — The development of certain modernized equipment 
 necessary for the handling of worms and the reeling of silk is possible since 
 some new inventions apparently are already available. 
 
 Irrigation. — The development of irrigation in California during the past 
 fifty years has been tremendous. Irrigated lands of all types of soil and in 
 many climatic zones are now available for the growing of mulberry trees. 
 Under irrigation the bearing period of the mulberry tree is very greatly 
 extended and an abundance of green, succulent vegetation can be produced 
 almost at will through a judicious application of water. Such conditions are 
 in sharp contrast to the early attempts to grow mulberry trees on nonirrigated 
 lands. 
 
 Air Conditioning. — Among the most important factors in maintaining 
 necessary conditions of temperature and moisture are climate and equipment. 
 Modern methods of refrigeration will take care of practically any such prob- 
 lem that might arise. It will make possible the production of silkworms in the 
 warm interior valleys with minimum cost of operation. Even along the coast 
 the supplying of a small amount of heat at certain periods of the day or year 
 may make a tremendous difference in the production of quantity and quality 
 of silk. 
 
 Refrigeration. — Quick freezing and refrigeration methods may also be 
 applied to the handling of food for the silkworms during certain critical 
 periods. So far as we know, no thorough investigations of these methods have 
 been made, but the quick freezing of greens used for human consumption 
 might give some helpful suggestions. 
 
 New Types of Food. — A great deal of experimental work is yet to be done 
 with the varieties of food which might be available for rearing silkworms. 
 Since lettuce and other substitutes have proved to be useful for short periods 
 of time, it is possible that a method might be developed, whereby early spring 
 broods could be fed on these plants until the mulberry leaves appear later in 
 
14 California Experiment Station Circular 363 
 
 the spring. Lettuce is an important winter crop, as well as a summer crop, in 
 California, and it would be available for emergency feeding at almost any 
 time. The use of hormones and vitamins also may be of importance in the devel- 
 opment of more vigorous worms and, therefore, of finer silks. 
 
 REFERENCES FOR FURTHER READING' 
 
 Banks, C. S. 
 
 1911. A manual of Philippine silk culture. 53 p. Bureau of Science, Manila, P. I. 
 
 Borah, Woodrow. 
 1943. Silkraising in colonial Mexico. 169 p. Univ. California Pubs, Ibero-Americana 
 20:1-169. 
 California Silk Culture Association. 
 
 1882. First annual report, constitution and by-laws, objects of the association. 
 
 1883. Second annual report. 
 
 Comstock, F. G. 
 
 1836. A practical treatise on the culture of silk, adapted to the soil and climate of the 
 United States, 108 p. F. G. Comstock, Hartford, Conn. 
 
 Dandolo, Count. 
 
 1925. The art of rearing silkworms. 365 p. [Translation by John Murray, London.] 
 
 EssiG, E. O. 
 
 1926. The silkworm and silk culture. In: A history of entomology. 1,029 p. The Mac- 
 millan Company, New York, N. Y. (See specifically p. 231-36.) 
 
 Ewer, W. B. 
 
 1882. The California silk growers' manual. 20 p. California Silk Culture Association. 
 
 Fletcher, Frank C. 
 
 1945. Sericulture, its successes and failures. Ward's Natural Science Bui. 18(5) : 73-75; 
 19(1):11-13. 
 
 HlTTELL, ELISE C. 
 
 1881. The California silk growers' instructor. 25 p. California Silk Culture Association. 
 
 Honda, I. 
 
 1909. The silk industry in Japan. 198 p. The Imperial Tokyo Serieultural Institute, 
 Tokyo, Japan. 
 
 Julien, Stanislas. 
 
 1838. Summary of the principal Chinese treatises upon the culture of the mulberry 
 and the rearing of silkworms. [Translated from the Chinese by P. Force.] 198 p. 
 P. Force, Washington, D. C. 
 
 Keagle, D. L. 
 
 1938. Silk culture in the San Joaquin Valley. California Cultivator 85 : 421, 433. 
 
 Kelly, H. A. 
 
 1903. The culture of the mulberry silkworm. IT. S. Div. Ent. Bui. 39 (n.s.) : 1-32. 
 1903. Silkworm culture. U. S. Dept. Agr. Farmers' Bui. 165:1-32. (Same as: U. S. 
 Div. Ent. Bui. 39 n.s.) 
 
 Kurosawa, K. 
 
 1907. Report on an experiment in rearing silkworms. California State Commr. Hort. 
 Second Bien. Rept. 1905-06:261-70. 
 
 Minns, Susan. 
 
 1929. Book of the silkworm. A plea for the cultivation of silk and the silkworm in the 
 United States. 75 p. National Americana Society, New York, N. Y. 
 
 3 Published accounts on silk culture are very limited in number. Excepting the works of 
 Dandolo, Fletcher, Paillot, and Pelletier, which are in the author's library, the references 
 listed here are all on file in the University of California Library, Berkeley, and indicate the 
 miscellaneous character of the publications available on this subject. 
 
Silk Culture in California 15 
 
 Pacific Eural Press. 
 
 1871-1928. [Miscellaneous articles.] Pacific Eural Press 1:296 (1871) ; 3:121 (1872) ; 
 11:274-75 (1876) ; 18:305 (1879) ; 116:219 (1928). 
 Paillot, Andre. 
 
 1930. Traite des maladies du ver a soi. 279 p. G. Doin & C le , Paris. 
 Pelletier, J. B. 
 
 1945. "Silk raising in California" postwar "A billion dollar industry." California Digest 
 3:14-19. 
 Pelletier, J. B., S. L. Heisinger, and E. S. Bennett. 
 
 1945. Eeport of the Assembly Interim Committee of the Legislature to investigate the 
 study of the possibilities of silk production and industry in California. Califor- 
 nia. Legislature. Assembly. The Journal of the Assembly, Jan. 17, 1945. (See 
 specifically p. 15-37.) 
 Prevost, Louis. 
 
 1867. California silk grower's manual. 246 p. H. H. Bancroft, San Francisco, Calif. 
 EiENzi, Louise. 
 
 1886. General instructions for rearing silkworms, with a treatise on securing healthy 
 silkworm eggs. Also a sketch of the habits and structure of the silkworm. 21 p. 
 State Board of Silk Culture. (Pamphlets on California Agriculture, v, 3, no. 7.) 
 Riley, C. V. 
 
 1879. The silkworm; being a brief manual of instructions for the production of silk. 
 
 U. S. Dept. Agr. Spec. Sept. 11:1-31. 
 1885. The present status and future prospects of silk culture in the United States. 
 Amer. Assoc. Adv. Sci. Proc. 35:1-516. 
 State Board of Silk Culture. 
 
 1884. First annual report for the year 1883. 
 
 1885. Second annual report for the year 1884. 
 U. S. Dept. of Commerce and Labor. 
 
 1905a. Sericulture in Italy. U. S. Daily Consular Eept. 2253:1-28. May 9. 
 
 1905b. Sericulture in China and Japan. U. S. Daily Consular Eept. 2254:1-27. May 10. 
 U. S. Dept. of State. 
 
 1899. Sericulture and silk reeling from the cocoons by machinery. U. S. Special Con- 
 sular Eept. 15(11) : 131-52. 
 Wickson, E. J. 
 
 1881-1905. [Miscellaneous notes and pamphlets on silk culture.] Univ. California 
 Library, Berkeley, California. 
 Woodworth, C. W. 
 
 1904. Silk culture. California Agr. Exp. Sta. Cir. 12:1-6. 
 
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