The silkworm is the larva or caterpillar of the domestic silk moth. It is an economically important insect, being a primary producer of silk. A silkworm's preferred food is white mulberry leaves.
Types of Silkworms
Mulberry silkworms can be categorized into three different but connected groups or types. The major groups of silkworms are the univoltine ("uni-"=one, "voltine"=brood frequency), and bivoltine categories.
1. he univoltine breed is generally linked with the geographical area within greater Europe. The eggs of this type hibernate during winter due to the cold climate, and cross-fertilize only by spring, generating silk only once annually.
2. The second type is called bivoltine and is normally found in China, Japan, and Korea. The breeding process of this type takes place twice annually, a feat made possible through the slightly warmer climates and the resulting two life cycles.
3. The polyvoltine type of mulberry silkworm can only be found in the tropics. The eggs are laid by female moths and hatch within nine to 12 days, so the resulting type can have up to eight separate life cycles throughout the year.
Silkworms were first domesticated in China over 5,000 years ago. Since then, the silk production capacity of the species has increased nearly tenfold. The silkworm is one of the few organisms wherein the principles of genetics and breeding were applied to harvest maximum output. It is second only to maize in exploiting the principles of heterosis and cross-breeding.
Silkworm breeding is aimed at the overall improvement of silkworm from a commercial point of view. The major objectives are improving fecundity (the egg-laying capacity of a breed), the health of larvae, the quantity of cocoon and silk production, and disease resistance.
Healthy larvae lead to a healthy cocoon crop. Health is dependent on factors such as better pupation rate, fewer dead larvae in the montage, shorter larval duration (shorter larval duration lessens the chance of infection), and bluish-tinged fifth-instar larvae (which are healthier than the reddish-brown ones).
The quantity of cocoon and silk produced is directly related to the pupation rate and larval weight. Healthier larvae have greater pupation rates and cocoon weights. The quality of cocoon and silk depends on several factors including genetics.
The cocoon is made of a thread of raw silk from 300 to about 900 m (1,000 to 3,000 ft) long. It has two elements: fiber and sericin. The fiber is very fine and lustrous, about 0.0004 inches in diameter, and makes up between 75 and 90%. Sericin, the gum secreted by the silkworm to glue the fiber into a cocoon, comprises about 10-25% of silk. Other elements include fats, salts, and wax. About 2,000 to 3,000 cocoons are required to make a pound of silk (0.4 kg). At least 70 million pounds of raw silk are produced each year, requiring nearly 10 billion cocoons.
The first stage of silk production is the laying of silkworm eggs, in a controlled environment like, for example, an aluminum box, which is then examined to ensure they are free from disease. The female deposits 300 to 400 eggs at a time. 100 moths would deposit about 40,000 eggs, each about the size of a pinhead.
The female dies almost immediately after depositing the eggs and the male lives only a short time after. The adult possesses rudimentary mouthparts and does not eat during the short period of its mature existence. The tiny eggs of the silkworm moth are incubated (about 10 days) until they hatch into larvae (caterpillars). At this point, the larva is about a quarter of an inch long.
Once hatched, the larvae are placed under a fine layer of gauze and fed huge amounts of chopped mulberry leaves during which time they shed their skin four times. The larvae may also feed on Osage orange or lettuce. Larvae fed on mulberry leaves produce the finest silk. The larva will eat 50,000 times its initial weight in plant material. For about six weeks the silkworm eats almost continually. They are covered with tiny black hairs.
When the color of their heads turns darker, it indicates they are about to molt. After molting, the instar phase of the silkworms emerges white, naked, and with little horns on their backs. They molt four times and their bodies become slightly yellow and the skin becomes tighter. After growing to its maximum size of about 3 inches at around 6 weeks, it stops eating, changes color, and is about 10,000 times heavier than when it hatched. The silkworm is now ready to spin a silk cocoon.
Spinning the Cocoon
The silkworm attaches itself to a compartmented frame, twig, tree, or shrub in a rearing house to spin a silk cocoon over a 3 to an 8-day period (pupating). Silkworms possess a pair of specially modified salivary glands called sericteries, which are used to produce fibroin – a clear, viscous, proteinaceous fluid that is forced through openings called spinnerets on the mouthpart of the larva. Liquid secretions from the two large glands in the insect emerge from the spinneret, a single exit tube in the head.
The diameter of the spinneret determines the thickness of the silk thread, which is produced as a long, continuous filament. The secretions harden on exposure to the air and form twin filaments composed of fibroin, a protein material. The second pair of glands secretes a gummy binding fluid called sericin which bonds the two filaments together. Steadily over the next four days, the silkworm rotates its body in a figure-8 movement some 300,000 times, constructing a cocoon and producing about a kilometer of silk filament.
Reeling the Filament
If the animal survives after spinning its cocoon and through the pupal phase of its lifecycle It releases proteolytic enzymes to make a hole in the cocoon, so it can emerge as an adult moth. These enzymes are destructive to the silk and can cause the silk fibers to break down over a mile in length to segments of random length which seriously reduces the value of the silk threads. To prevent this, silkworm cocoons are boiled. The heat kills the silkworms and the water makes the cocoons easier to unravel.
The silk is then unbound from the cocoon by softening the sericin and then delicately and carefully unwinding, or 'reeling' the filaments. Reeling may be achieved manually or automatically. The cocoon is brushed to locate the end of the fiber. It is threaded through a porcelain eyelet, and the fiber is reeled onto a wheel. As each filament is nearly finished being reeled, a new fiber is twisted onto it, thereby forming one long, continuous thread. The sericin protects the silk fiber during this process.
Degumming and Dyeing
To achieve the distinctive softness and shine of silk, the remaining sericin must be removed from the yarn by soaking it in warm soapy water. Degumming decreases the weight of the yarn by as much as 25%. After degumming, the silk yarn is a creamy white color. It may next be dyed as yarn, or after the yarn has been woven into fabric. The bundles of threads are soaked in the dye pots several times to achieve the proper color tone and quality.
The silk industry makes a distinction between pure-dye silk and what is called weighted silk. In the pure-dye process, the silk is colored with dye and may be finished with water-soluble substances such as starch, glue, sugar, or gelatin. To produce weighted silk, metallic substances are added to the fabric during the dying process.
This is done to increase the weight lost during degumming and to add body to the fabric. If weighting is not executed properly, it can decrease the longevity of the fabric, so pure-dye silk is considered the superior product. After dyeing, silk fabric may be finished by additional processes, such as bleaching, embossing, steaming, or stiffening.
Types of Silk
Raw silk is twisted into a strand sufficiently strong for weaving or knitting. This process of creating the silk yarn is called “throwing,” and prevents the thread from splitting into its constituent fibers. Four different types of silk thread may be produced from this procedure: crepe, tram, thrown singles, and organzine.
Crepe is made by twisting individual threads of raw silk, doubling two or more of these together, and then twisting them again. It is usually used for weaving crinkly fabrics.
Tram is made by twisting two or more threads in only one direction. It is usually used for weft or filling.
Thrown singles are individual threads that are twisted in only one direction. It is usually used for sheer fabrics.
Organzine is a thread made by giving the raw silk a preliminary twist in one direction and then twisting two of these threads together in the opposite direction. It is usually used for the warp threads of materials.
Broken or waste filaments and damaged cocoons are retained, treated to remove the sericin, and combed. This is then processed into yarn, marketed as spun silk, which is inferior in character to the reeled product and much cheaper.
Beauveria bassiana, a fungus, destroys the entire silkworm body. This fungus usually appears when silkworms are raised under cold conditions with high humidity. This disease is not passed on to the eggs from moths, as the infected silkworms cannot survive to the moth stage. This fungus can spread to other insects.
Grasserie, also known as nuclear polyhedrosis, milky disease, or hanging disease, is caused by infection with the Bombyx mori nuclear polyhedrosis virus. If grasserie is observed in the chawki stage (the first stages of silkworm rearing), then the chawki larvae must have been infected while hatching. Infected eggs can be disinfected by cleaning their surfaces prior to hatching. Infections can occur because of improper hygiene in the chawki rearing house. This disease develops faster in early instar rearing.
Pebrine is a disease caused by a parasitic microsporidian, Nosema bombycis Nageli. Diseased larvae show slow growth, undersized, pale, and flaccid bodies, and poor appetite. Tiny black spots appear on the larval integument. Additionally, dead larvae remain rubbery and do not undergo putrefaction after death.
N. bombycis kills 100% of silkworms hatched from infected eggs. This disease can be carried over from worms to moths, then eggs and worms again. This microsporidium comes from the food the silkworms eat. If silkworms get this disease in their worm stage, no visible symptoms occur. However, mother moths pass the disease to the eggs, and 100% of worms hatching from the diseased eggs will die in their worm stage. To prevent this disease, it is extremely important to rule out all eggs from infected moths by checking the moth's body fluid under a microscope.
Even though silk has a small percentage of the global textile market – approx. less than 0.2% - its production base is spread over 60 countries in the world. While the major producers are in Asia (90% of mulberry production and almost 100% of non-mulberry silk), sericulture industries have been lately established in Brazil, Bulgaria, Egypt, and Madagascar as well.
China is the world’s single biggest producer and chief supplier of silk to the world markets. India is the world’s second-largest producer. 10% of world silk is produced altogether by Brazil, North Korea, Thailand, Uzbekistan, and Vietnam.
Supply and demand of raw silk
The five largest fresh cocoon-producing countries are (in brackets average production of the last 4 years in tons per year is reported): China (500,000), India (126,000), Uzbekistan (20,200), Brazil (14,000), and Vietnam (13,000).
Countries with more than 300 tons of fresh cocoons per year are Thailand, North and South Korea, Japan, Iran, Tajikistan, Pakistan, and Indonesia.
Altogether approximately 35 to 40 counties are involved in sericulture. World production of raw silk is an average of 80,000 tons per year, about 70% of which is produced in China.
Raw Silk is the 701st most traded product and the 228th most complex product according to the Product Complexity Index (PCI).
The top exporters of Raw Silk are: China ($315M – 88%), Italy ($18.3M – 5.1%), Malaysia ($12M – 3.3%), Romania ($7.09M – 2%) and Germany ($3.32M – 0.9%).
The top importers are India ($157M – 44%), Romania ($84.7M – 23%), Italy ($39.2M – 11%), Japan ($20.8M – 5.7%) and South Korea ($15.7M – 4.4%).