- The silkworm is the larva or caterpillar or imago of the silkmoth that produces silk thread.
- The thread is used for making silk clothes and various other purposes.
- Raw silk is obtained from the coccons of the silkmoth.
- The adult of silkworm is called silkmoth.
- Silkmoths are found in many indigenous varieties across the world.
- Sericulture, or silk farming is the rearing of silkworms for the commercial purposes i.e. for silk fiber.
- It is an agro-industry which implies a technique of silk production.
- It plays a vital role in the rural economy of many countries.
- It was introduced for the first time in China, around 2700 BC.
- It was considered to be a national secret by Chinese government, as it was not known in other countries.
- Later it was introduced in Europe and other Asian countries.
- It has become one of the important cottage industries.
- Many countries after China like Japan, Korea, India, Brazil, Russia, Nepal, etc are involved in it.
- Today, China and Japan are the two main producers of silk fibres.
- They are manufacturing 50% of the world production each year.
2.Sericulture in Nepal
- In Nepal, sericulture was introduced for the first time in Khopasi, Kavre.
- It is being flourished to various parts of the country as well.
- There are eight branches of sericulture which are located in Syangja, Dhading, Itahari, Dhankuta, Chitapur, Pokhara, Bandipur, Bhandara.
- Seri silkworm (Bombyx mori) and Eri silkworm (Attacus ricini) are commonly being reared in Nepal.
- Bombyx mori is the most common species of silkmoth found in Nepal.
- It is a blind, flightless moth with a life span of (9-10 days).
- Eri silkworm is reared indoors, which is not easily susceptible to diseases.
- Seri silkworm feeds on mulberry leaves and Eri silkworm on castor leaves.
- It is a medium sized insect with a robust body.
- It is creamy-white in colour.
- Its body can be divided into head, thorax and abdomen.
- The head bears a pair of compound eyes, a pair of feathery antennae and sucking mouth parts.
- Thorax bears 2 pairs of wings covered with scales, 3 pairs of jointed legs and 2 pairs of spiracles for breathing purposes.
- The abdomen is made of few segments, which contains seven pairs of spiracles.
- Female abdomen is larger as compared to the male.
- The life-cycle of the silkmoth shows complete metamorphosis.
- It means that it has four stages i.e. egg, larvae, pupa and adults.
- Silkmoth is dioecious i.e. unisexual, sexes are separate.
- Copulation takes place in air and the fertilization is internal.
- The male dies after mating and female dies after laying eggs.
- Eggs are small, rounded and yellowish white in colour.
- 300-400 eggs are laid by females after copulation.
- The eggs are laid in cluster on a mulberry leaves.
- A sticky secretion from the moth helps the eggs to adhere on the leaves.
- Eggs hatch into larvae after 10-12 days on favourable condition (18C-25C).
- Larva is creamy-white and about 6mm in length.
- It moves with the looping movement and is voracious in nature.
- It feeds on mulberry leaves for about 25-32 days.
- It contains a head with biting and chewing mouth parts.
- The mouthparts can easily cut the mulberry leaves.
- The thorax is 3 segmented with 3 pairs of true legs.
- The abdomen is of 10 segments, which has 5 pairs of pseudolegs and a dorsal anal horn on the 8th segment.
- Moulting occurs four times and it passes through 5 instar stages.
- The stages of larva between two successive moultings is called an instar.
- They grow maximum in size upto 5th instar as they just feed and sleep continuously for several days.
- All the instar stage feeds for about 3-4 days and sleeps for about 20-30 hours except the fifth instar.
- 5th instar feeds for about 7 days and sleeps for 36-42 hours.
- The caterpillar stops feeding after they are fully grown.
- Body colour changes and a pair of salivary glands develop which help in secretion of liquid silk.
- The liquid silk flows in two ducts to a common exit tube in the worm’s mouth.
- On emerging the liquid silk hardens and form silk fibres.
- The silk fibres wrap around the body of caterpillar to form the pupal case, or coccon.
- A single caterpillar is said to produce nearly 1000-1500 m of silk thread in aperiod of about 3-4 days.
- Within a fortnight the caterpillar develops inside the coccon and becomes a pupa, or chrysalis.
- It is the third stage of the life cycle which is completely inactive.
- It is non-feeding stage and does not move as well.
- But metamorphosis occurs by histolysis and histiogenesis inside the coccon to become adult or imago.
- The imago secretes an alkaline fluid to moisten one end of the coccon to escape outside.
- This stage last for about 12-14 days.
- On favourable condition,the imago comes out by tearing the coccon.
- It contains head, thorax and abdomen, with 2 pairs of wings and 3 pairs of legs.
- It flies away after drying its wings.
- Soon, the male and female moths mate.
- The female lays eggs and both dies after 3-4 days.
In this way, on favourable condition, the lifecycle of silk moth completes within 45 days.
5.Extraction of silk fibres
- The eggs are stored at temperature below 18C for long term storage when mulberry leaves are not available.
- Mulberry is a deciduous tree so leaves are not available throughout the year.
- But the larvae of silk moths are voracious feeder, which needs mulberry leaves immediately after hatching. If not then they will die.
- When the leaves are available, the eggs are kept in an incubator for hatching.
- When the eggs are kept at 18C-25C, eggs hatch into tiny caterpillars within a week.
- Clean and dry mulberry leaves are fed to the larvae as they will die if wet leaves are given.
- The coccons are treated with hot water or placed in a hot oven. It helps to kill the pupa inside the coccon.
- If they are allowed to hatch, they will cut the silk thread on emerging into fragments.
- It also helps to enable the hard sericin portion to get softened and make the unwinding easy without breaks.
- A few coccons of good quality are kept as seeds for the next crop.
6.Economic importance of sericulture
- It produces valuable silk fibres which is used to make clothes.
- Raw silk and silk clothes can be exported to improve the economic status of the farmers.
- Employment level will be increased as many women also get involved in this agro-industry.
- It can be practiced with very low land holding.
- All the things used in sericulture can be sold.
- The pupae are fed to animals. Similarly, the spoilt coccons can be used as food for fishes in fish farms.
- It solves the problem of migration in search of jobs as well.