Silkworm Farming – Silk is an animal protein fiber produced by certain insects to build their cocoons and webs. Many different types of silk are produced by a huge variety of different types of insects other than moth caterpillars. Yet none of these have been exploited for commercial purposes, though there has been basic research into the structures of such silks. Silk is most commonly produced by larvae, and thus largely limited to insects with complete metamorphosis.
The commercial process of silk making is highly complex and labor intensive. The following will provide basic information on how silk is made.
Silkworm Farming – Life Cycle of Mulberry Silkworm:
The silk moth is dioecious, i.e., the sexes are separate. Fertilisation is internal, preceded by copulation. The development includes a complicated metamorphosis.
After fertilisation, each female moth lays about 300 to 400 eggs. These eggs are placed in clusters on the leaves of mulberry tree The female covers the eggs by a gelatinous secretion which glues them to the surface of the leaves The eggs are small, oval and also usually slightly yellowish in colour. The egg contains a good amount of yolk and is covered by a smooth hard chitinous shell.
After laying the eggs the female moth does not take any food and dies within 4-5 days. In the univoltine (a single brood per year) they may take months because overwintering takes place in this stage but the multivoltine broods come out after 10-12 days. From the egg hatches out a larva called the caterpillar.
The larva of silkworm moth is called caterpillar larva. The newly hatched larva is about 4.00 to 6.00 mm in length. It has a rough, wrinkled, hairless and yellowish white or greyish worm-like body. The full grown larva is about 6.00 to 8.00 cm in length. The body of larva is distinguishable into a prominent head, distinctly segmented thorax and an elongated abdomen. The head bears mandibulate mouth and three pairs of ocelli.
A distinct hook-like structure, the spinneret, is present for the extrusion of silk from the inner silk-gland. The thorax forms a hump and consists of three segments. Each of the three thoracic segments bears pair of jointed true legs. The tip of each leg has a recurved hook for locomotion and ingestion of leaves.
The abdomen consists of ten segments of which first nine are clearly marked, while the tenth one is indistinct. The third, fourth, fifth, sixth and ninth abdominal segments bear ventrally a pair of un-jointed stumpy appendages each.
These are called pro-legs or pseudo-legs. Each leg is retractile and more or less cylindrical. The eighth segment carries a short dorsal anal horn. A series of respiratory spiracles or ostia are present on either lateral side of the abdomen.
The larva is a voracious eater and strongly gregarious. In the beginning chopped young mulberry leaves are given as food but with the advancement of age entire and matured leaves are provided as food. The caterpillar moves in a characteristic looping manner. The larval life lasts for 2-3 weeks. During this period the larva moults four times.
After each moult, the larva grows rapidly. A full-grown larva is about 8.00 cm long and becomes transparent and golden brown in appearance. A pair of long sac-like silk-glands now develops into the lateral side of the body. These are modified salivary glands.
The full-grown larva now stops feeding and hides itself in a corner under the leaves. It now begins to secrete the clear and sticky fluid of its salivary glands through a narrow pore called the spinneret situated on the hypo pharynx The sticky substance turns into a fine, long and also solid thread or filament of silk into the air.
The thread becomes wrapped around the body of the caterpillar larva forming a complete covering or pupal case called the cocoon. The cocoon-formation takes about 3-4 days. The cocoon serves a comfortable house for the protection of the caterpillar larva for further development.
The cocoon is a white or yellow, thick, oval capsule which is slightly narrow in the middle.
It is formed of a single long continuous thread. The outer threads, which are initial filaments of the cocoon, are irregular but the inner ones forming later the actual bed of the pupa, is one long continuous thread about 300 metres in length, wound round in concentric rings by constant motion of the head from one side to the other about 65 times per minute.
The irregular surface threads are secreted first and the inner continuous thread later. The silk thread is secreted at the rate of 150 mm per minute. Within a fortnight the caterpillar larva transforms into a conical brownish creature called the pupa or the chrysalis.
The pupa lies dormant, but undergoes very important active changes which are referred to as metamorphosis. The larval organs such as abdominal pro-legs, anal horn and mouth parts are lost. The adult organs such as antennae, wings and copulatory apparatus develop. The pupa finally metamorphoses into the imago or adult in about 2-3 weeks time.
Imago or Adult:
The adult moth emerges out through an opening at the end of the cocoon in about 2 to 3 weeks time, if allowed to live. Immediately before emergence, the pupa secretes an alkaline fluid, that softens one end of the cocoon and after breaking its silk strands, a feeble crumpled adult squeezes its way out. Soon after emergence, the adult silk moths mate, lay eggs and die.
Silkworm Farming – Sericulture
Cultivation of the silkworm is known as sericulture. Although many insects produce silk, only the filament produced by Bombyx mori, the mulberry silk moth and a few others in the same genus, is used by the commercial silk industry.
Silkworm Farming – Types
Mulberry silkworms can be categorized into three different but connected groups or types. The major groups of silkworms fall under the univoltine (‘uni-‘=one, ‘voltine’=brood frequency) and bivoltine categories. The 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. The second type is called bivoltine and is normally found in China, Japan, and Korea.
Silkworm Farming – Hybrids recommended
- Multivoltine: PM × CSR2, L14 × CSR2
- Bivoltine: CSR2 × CSR4, FC1 × FC2 (Double hybrid)
The breeding process of this type takes place twice annually, a feat made possible through the slightly warmer climates and the resulting two lifecycles. The polyvoltine type of mulberry silkworm can only be located 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 lifecycles throughout the year..
Silkworm Farming – Rearing house (for 200-250dfls.)
Mulberry silkworm rearing, being completely domesticated, demands specified environmental conditions like optimum temperature (24-28oC) and relative humidity (70-85%). It is therefore necessary to evolve measures for economic cooling through selection of proper material for wall and roof fabrication, orientation of building, using the right construction method/design, etc. Further, enough space must be available to carry out leaf preservation, chawki rearing, late age rearing and moulting and also its effective cleaning and disinfection.
The size of the rearing house depends upon the quantum and type of rearing The different types of rearing are shelf or stand rearing, platform rearing and floor rearing Shelf rearing also requires minimum space and is most common in India. A floor area of 150 sft. can provide rearing space for 100 Cross bred(CB) DFLs or 75 Bivoltine(BV) DFLs.
In platform rearing, the larvae are also shifted to shoot rearing platforms after 3rd moult. Ideal size of each platform will be 5 x 25 ft. which can accommodate 50 DFLs upto the stage of spinning Platforms can also be arranged in two/ three tiers with a gap of 2-3 ft. A building floor area of 250-300 sft is required to rear 100 DFLs of CB layings or 75 DFLs of BV layings.
- Construct rearing house of 50 feet x 20 feet x10 feet size on an elevated and shady place to accommodate 200-250 dfls.
- Provide 3 feet verandah surrounding the rearing house.
- Provide sufficient windows and ventilators for free circulation of air inside the rearing house.
- Cover the windows and ventilators with nylon net to restrict the entry of uzi flies.
Silkworm Farming – Diseases and pests of mulberry silkworm
Causative agent: Bombyx mori Nuclear Polyhedrosis Virus
Occurrence: The disease prevails all through the year but its severity is more during Summer and Rainy seasons.
Source of infection: Silkworm ge
ts infected when it feed on contaminated mulberry leaves. The milky white fluid released by the grasserie larvae, contaminated silkworm rearing house and appliances are the sources of infection.
Predisposing factors: High temperature, low humidity and poor quality mulberry leaves.
Silkworm Farming – Symptoms:
- The skin of infected larvae becomes shining before moult and fails to moult.
- Inter segmental swelling appears and the colour of the body becomes yellowish.
- The infected larvae move restlessly in the rearing bed/ along the rim of the trays.
- Infected larval body ruptures easily and turbid white haemolymph oozes out.
Silkworm Farming – Management:
- Practice thorough disinfection of rearing house, its surroundings and appliances with any recommended disinfectant.
- Conduct an optional disinfection with 0.3% slaked lime solution when high incidence of disease noticed in the previous crop.
- Practice personal and rearing hygiene.
- Collect the diseased larvae and ensure its proper disposal.
- Maintain optimum temperature and humidity in the rearing house.
- Feed quality mulberry leaf and avoid overcrowding.
- Apply recommended bed disinfectant as per schedule and quantity.
- Feed Amruth as per schedule to control grasserie disease.
Silkworm Farming – FLACHERIE
Causative agent: Bombyx mori Infectious flacherie virus/Bombyx mori Densonucleosis virus or different pathogenic bacteria viz., Streptococcus sp./Staphylococcus sp./Bacillus thuringiensis/Serratia marscesence individually or in combination of bacteria and viruses.
Occurrence: The disease is common during summer and Rainy seasons.
Source Infection: Silkworm gets infected by eating contaminated mulberry leaf. Dead diseased silkworm, its faecal matter, gut juice, body fluid are also the sources of pathogen contamination. The infection can also takes place through injuries/cuts/wounds.
Predisposing factors:Fluctuation in temperature, high humidity and poor quality of leaves.
Silkworm Farming – Symptoms:
- The larvae become soft and flaccid.
- The growth of infected larvae retarded, becomes inactive and vomit gut juice. The faeces become soft with high moisture content. Sometimes chain type excreta and rectal protrusion also observed.
- Larval head and thorax become translucent.
- When infected with Bacillus thuringiensis symptoms of toxicity such as paralysis and sudden death are observed. After death, larvae turn black in color and gives foul smell.
- Sometimes, the dead larvae turn red when infected with Serratia sp.
Silkworm Farming – Management:
- Disinfect the rearing house, its surroundings and equipment with recommended disinfectant mentioned above.
- Pick up diseased larvae and dispose them by burning.
- Provide good quality leaf grown under good Sunlight and recommended inputs. Do not provide over matured/over stored /dirty leaf to the silkworms
- Avoid starvation, overcrowding and accumulation of faeces in the rearing bed.
- Rear silkworms under optimum temperature and humidity.
- Avoid injury to the larvae.
- Apply recommended bed disinfectant as per schedule and quantity.
- Feed Amruth as per schedule to control flacherie disease.
Silkworm Farming – PESTS
- Uzi fly
Occurrence & Symptom:
The uzi fly, Exorista bombycis is a serious endo-larval parasitoid of the silkworm, Bombyx mori, inflicting 10-15% damage to the silkworm cocoon crop in the premier silk producing states of Karnataka, Andhra Pradesh and Tamil Nadu.
Uzi fly occurs throughout the year, but severe during rainy season. Presences of eggs or black scar on the silkworm body and maggot emergence hole at the tip of the cocoon are the typical symptoms of uzi fly attack.
As soon as the uzi fly enters into rearing house, it lays one or two eggs on each silkworm larva. After 2-3 days, egg hatches, enters inside the larva and feed on internal contents for 5-7 days, after which it comes out by rupturing the larva and also the maggot pupates in a dark corner or cracks & crevices in about 18-24 hours. The pupal stage lasts for 10-12 days. If the uzi fly infests at last instar, the uzi maggots come out after cocoon formation by making a circular hole.
Silkworm Farming – Control measures
- Provide wire mesh/nylon net on all windows/doors.
- Make Provision doors with automatic closing mechanism.
- Provide anteroom at the entrance of the rearing house.
- Keep the leaf in the verandah of the rearing house and observe for the uzi fly before shifting leaf into the rearing house.
Physical (using uzi trap)
Dissolve one table in 1 litre of water and keep the solution in white trays both inside and out side the rearing house at window base from 3rd instar onwards up to spinning.
Place uzi traps inside the rearing house/mounting hall after spinning up to 20 days under close-door condition to trap uzi flies emerging inside.
Silkworm Farming – Hatching the Eggs
The first stage of silk production is the laying of silkworm eggs, in a controlled environment such as an aluminum box, which are then examined to ensure they are free from disease. The female deposits 300 to 400 eggs at a time.
In an area the size of your monitor screen, 100 moths would deposit some 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.
Silkworm Farming – The Feeding Period
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 very finest silk. The larva will eat 50,000 times its initial weight in plant material.
For about six weeks the silkworm eats almost continually. 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.
Silkworm Farming – 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 8 day period This period is also termed pupating.
Silkworms possess a pair of specially modified salivary glands called sericteries, which are used for the production of 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 also The secretions harden on exposure to the air and form twin filaments composed of fibroin, a protein material. A 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 also producing about a kilometer of silk filament.
Silkworm Farming – Reeling the Filament
At this stage, the cocoon is treated with hot air, steam, or boiling water. The silk is then unbound from the cocoon by softening the sericin and then delicately and carefully unwinding, or ‘reeling’ the filaments from 4 – 8 cocoons at once, sometimes with a slight twist, to create a single strand.
As the sericin protects the silk fiber during processing, this is often left in until the yarn or even woven fabric stage. Raw silk is silk that still contains sericin. Once this is washed out (in soap and boiling water), the fabric is left soft, lustrous, and up to 30% lighter. The amount of usable silk in each cocoon is small, and about 2500 silkworms are required to produce a pound of raw silk.