About 3,000 silkworms are boiled, steamed, cooked or gassed alive in their cocoons for just one pound of silk. The world annual production of silk is 339,384,000 pound. That’s about a trillion silkworms per year.

Silkworms are commercially exploited for about 5,000 years.
They are originally native of China and for about 3,000 years the exploitation methods were a secret known only to the Chinese. Along the years the know how of stealing from the silkworms was stolen from the Chinese and so since the 11th century everybody can learn how to violently murder to steal silk.
China is still the main producer of silk though there are over 40 countries that commercially produce it. India is the second largest producer followed by Japan, Brazil and Korea.

There are several species of commercially exploited silkworms, the most common one is Bombyx mori. After 5,000 years of selective breeding these moths have lost the ability to fly and are entirely dependent on humans. They are cultivated and raised under controlled conditions of environment and nutrition.

Life Cycle

The life cycle of silkworm includes four stages:
The moth who mates and produces the eggs, from the eggs will hatch the silkworms who will eat until they are big enough to start the pupa stage (or the chrysalis stage) which is the commercially relevant stage since then the silkworms secrete the viscous protein like substance which will become silk when it contacts with air.

After the mating the male moths are discarded as they are no longer useful for the industry. They are usually dumped into a basket and thrown outside the silk manufacturing centers to be eaten by birds who gather there looking for an easy snack. As for the females, after laying between 300 and 400 eggs, they become commercially useless as well. They are not left out to slowly die like the males but are crushed and inspected for diseases.

The Pupas

The eggs hatch after 20 days and for the next 25 days, the larvas who emerge at the size of eighth of an inch long will spend all of their time eating mulberry leaves only. They will eat 50,000 times their initial weight, growing to 10,000 times their hatching weight. During this period, the larvas molt four times, becoming much larger each time they shed their skin.

After 25 days the larvas stop eating. By this time, the larvas’ two silk glands are fully developed, and they begin to exude silk from both at the same time, as well as a sticky substance called sericin to bind the two threads together. That is when the farmers transfer them to specially-constructed frames where they start building their cocoons, there they are supposed to spend the 16 days of the chrysalis stage in which they would transform into moths. But in the silk industry the pupas disturb the farmers to steal the desired silk. If they were allowed to hatch they would have chewed their way out of the cocoon so the silk fibers would have been broken, the one long thread of silk would be cut into many small ones. Some chrysalises are kept aside to allow the moths to emerge, later to be used for breeding and have the same fate as their parents. But the rest “must” be killed when still in the cocoon without damaging it. Waiting too long will risk the pupas hatching and not waiting enough as said by the FAO: "If premature harvesting takes place, the silkworm will still be in its larval stage, weigh more, have fragile skin, and could likely be crushed, which would cause stains to the cocoon during handling and transportation."

Pupa Killing Methods

Soaking in hot water - To loosen the silk gum called Sericin, used by the larva to stick the fibers together, the cocoons are soaked in hot water boiling the pupas alive.
Hot and cold immersions - Putting the cocoons through a series of hot and cold immersions, to soften the sericin permitting the unwinding of the filament as one continuous thread.
Gassing – The pupas are gassed alive inside their cocoons.
Roasting – Roasting the cocoons in a hot oven kills the pupas without damaging the silk.
Cooking - First a small sample of cocoons is cooked to determine the correct cooking condition for the specific batch, then the entire batch is cooked alive.
Steam Stifling: Generally, in many tropical and sub-tropical countries, fresh cocoons are heaped into a basket and steamed in small reeling units.
In big reeling units with a boiler, steam chambers are used for stifling. After steaming, these cocoons may be easily cooked in an open pan, a method commonly used in India. It takes about 30 minutes for the pupa to die.
Drying - Exposure to excessive moisture within the cocoon causes putrefaction and mould, so drying is very useful to kill the pupa and to evaporate the moisture. This is done when the killing and the silk stealing occur in two different times or places.
Several options are available and recommended by the FAO to dry and stifle the cocoons:
Sun drying: No investment is required to kill pupas and dry cocoons in bright sunshine. Fresh cocoons are spread in thin layers on a mat or planks of wood and exposed to direct sunlight. The process takes two to three days.
Hot air-drying: A heater and a fan are operated simultaneously in a drying chamber, in which fresh cocoons are placed preferably in thin layers to maintain constant and uniform air current throughout the layers along with an efficient ventilator to drive out moisture avoiding condensation inside the chamber.

Types of Commercially Exploited Silkworms:

Mulberry: 75% of the silk is produced by the Bombyx mori which are also called mulberry silkworms since they solely feed on the leaves of mulberry plant. These silkworms are completely domesticated and reared indoors.

Eri: Also known as Endi or Errandi, is a multivoltine silk spun from open-ended cocoons. Eri silk is the product of the domesticated silkworm Philosamia ricini that feeds mainly on castor leaves. To produce this silk the pupas are not necessarily killed, the eri cocoons are spun only when they are open-mouthed. But even in this case many times the pupas are reared to be eaten or used as fertilizers.

Tasar: Tasar (Tussah) is copperish color, coarse silk mainly used for furnishings and interiors. Tasar silk is generated by the silkworm Antheraea mylitta which mainly thrives on the food plants Asan and Arjun. The rearing is conducted on the trees in the open.

Oak Tasar: A finer variety of tasar generated by the silkworm Antheraea proyeli J. that feeds on natural food plants of oak. The silkworm is originally from India but china is the major producer of this type of silk.

Muga: This golden yellow silk is reared in the Assam state of India. The silk is obtained from the semi-domesticated multivoltine silkworm Antheraea assamensis. These silkworms feed on the leaves of Som and Soalu plants and are reared on trees similar to that of tasar. The muga silk is considered as a high value silk and is used in products like sarees, mekhalas, chaddars, etc.

Other Uses

Oil: Silkworm chrysalis oil is used as edible oil, as health food and in cosmetics

Food: Silkworms and pupas are eaten by humans especially in east Asia and are used as food for “pet” reptiles and “pet” fish because of their high content of protein and low concentration of fats. The litter and Mulberry waste are used to feed cows, sheep and fish in factory farms.

Tea: Cordyceps sinensis is a fungus also known as caterpillar fungus which lives and feeds on the silkworm chrysalis. The funguses are inoculated into living chrysalises later to be blended into tea or other food materials. They are also used in oriental medicines

Medicine: Serrapeptase or Serratiopeptidase, an enzyme isolated from a microorganism in silkworms (naturally present in the silkworm intestine which allows the emerging moth to dissolve the cocoon) is widely used in clinical practice in Japan and for the last 25 years in Europe too.

Sericin: Sericin is the substance the pupas use to bind the silk threads when making their cocoon and is removed by humans when destroying it. It is used as an ingredient in various cosmetic products and in some countries also as remedies in human medicine.

Fertilizers: Silkworm chrysalis, Silkworm litter and Mulberry leaf waste are used as fertilizers and are relatively common in organic agriculture.

Male silk moth drops: Male silk moth hormones are separated and extracted. They contain various kinds of biological components such as unsaturated fatty acid, vitamins and considered to be an extremely potent male sexual stimulant and tonic in Asia. It is also believed that it resists fatigue and prevents dementia of old people.

Sutures: Silkworms are used in the making of sutures in two main ways: Some are prepared from the thread spun by the silkworm larva in the cocoon making process and then the thread may be twisted or braided. And some are made from what is referred as Silkworm Gut as the fluid secreted by the silkworms, when they are ready to come out of their cocoons, is pulled from the glands of mature silkworms before they are cocooned.

Silkworms used in labs: Silkworms have been used by researchers to study pheromones or sexual attractant substances since the female releases pheromones for the male moth to detect.

Empowering by Suppressing

As part of economic improvement programs for women in poor countries, the United Nations Development Programme, the USAID, and the World Bank are trying to revive traditional silk weaving, sericulture and silk production industries in countries like Bangladesh, Cambodia, Afghanistan, Madagascar, East Timor and parts of India by encouraging women to get into the industry.