Lifecycle of silkworm and sericulture

Introduction

• The silkworm is the larva or caterpillar or imago of the silk-moth that produces silk thread.
• The thread is used for making silk clothes and various other purposes.
• Raw silk is obtained from the cocoons of the silk-moth.
• The adult of silkworm is called silk-moth.
• Silk-moths are found in many indigenous varieties across the world.

1. History

• 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 silk-moth 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.

3. External morphology

• 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.

4. Life cycle

• The life-cycle of the silk-moth shows complete metamorphosis.
• It means that it has four stages i.e., egg, larvae, pupa and adults.
• Silk-moth 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.

a) 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 (18°C-25°C).

b) Larvae

• 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 pseudo-legs 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 up-to 5^th 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.
• 5^th 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 cocoon.
• A single caterpillar is said to produce nearly 1000-1500 m of silk thread in a period of about 3-4 days.
• Within a fortnight the caterpillar develops inside the cocoon and becomes a pupa, or chrysalis.

Image source: cgtn

c) Pupae

• 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 histo-genesis, inside the cocoon to become adult or imago.
• The imago secretes an alkaline fluid to moisten one end of the cocoon to escape outside.
• This stage last for about 12-14 days.

d) Adults

• On favourable condition, the imago comes out by tearing the cocoon.
• 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 18°C 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 18°C-25°C, 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 cocoons are treated with hot water or placed in a hot oven. It helps to kill the pupa inside the cocoon.
• 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 cocoons of good quality are kept as seeds for the next crop.

6. Economic importance of sericulture

1. It produces valuable silk fibres which is used to make clothes.
2. Raw silk and silk clothes can be exported to improve the economic status of the farmers.
3. Employment level will be increased as many women also get involved in this agro-industry.
4. It can be practiced with very low land holding.
5. All the things used in sericulture can be sold.
6. The pupae are fed to animals. Similarly, the spoilt cocoons can be used as food for fishes in fish farms.
7. It solves the problem of migration in search of jobs as well.

References: 

i) https://everythingsilkworms.com.au/silkworms/life-cycle-of-a-silkworm/

ii) https://www.biologydiscussion.com/animals-2/lifecycle-of-silkworm-with-diagram/2655

Lifecycle of silkworm and sericulture