Management of silkworm rearing, mounting & production of cocoon

Sericulture, or silk farming, is the rearing of silkworms for the production of silk. Although there are several commercial species of silkworms, Bombyx mori is the most widely used and intensively studied silkworm. Silkworm is domesticated over thousands of years and the sole food for the growth of silkworm is mulberry leaf.

Silk Worm

Climate Requirements

Under ideal conditions silkworm completes cocoon formation in 24-28 days from the day of hatching. However, there is variation in number of days due to fluctuation in temperature and humidity. As the temperature increases, the development of larvae activates. The following required temperature/ humidity/spacing should be provided:

Sr Stage Temprature(°C) Humidity Spacing(for100dfl’s**) in Sq. ft
1 1st Instar* 26-28 85-90 4-14
2 2nd Instar 26-28 85-90 15-45
3 3rd Instar 25-26 80-85 46-90
4 4th Instar 24-25 75-80 91-100
5 5thInstar 23-24 70 181-360
NOTE: * Instar is the Phase between two periods of moulting in the development of an insect larva or other invertebrate animals ** Disease free laying (it refers to the eggs of caterpillars)

Leaf Quality

Leaf quality plays an important role in the production of quality cocoons. The young age worms are fed with tender, succulent leaves which contain sugar, less amount of fiber, starch but, high moisture and protein that are suitable for chawki worms. Hence, the top tender leaves of mulberry plant are used to feed the young age worms. The late age worms are fed with matured leafs available at the bottom of the mulberry trees.

Preservation of Leaf

Silk Worm

Harvested leaf should be transported in wet gunny bags or baskets made up of bamboo. Such leaf should be preserved in a separate room or in a corner of rearing room or in specially designed leaf preservation chamber made up of wood with sufficient number of ventilators.

The leaf chamber should contain a good number of ventilators covered with wet gunny cloth. This will help in maintaining leaf quality. Otherwise leaf should be scattered on the floor in a floor should be frequently single layer so that the water droplets on surface of leaf will be evaporated. Leaf should never be preserved on heaps. This will increase the humidity.

Leaf Requirement (About 400 Eggs per DFL’s)

The requirement of leaf will differ from race to race. The consumption of leaf also varies with season. During summer the quantity of leaf required will be a little higher since, drying of leaf will be faster hence additional feeding may be required.

Sr Silkworm Stage Quantity of Mulberry Leaf Required (approx.) (Kgs)
1 1st Instar 2-4
2 >2nd Instar 4-8>
3 3rd Instar 30-40
4 4th Instar 80-90
5 5th Instar 600-650

Transportation of Eggs

The required number of DFL’s should be collected from either Govt. Grainage or licenced Pvt. Grainage. The transportation of eggs plays an important role in the development of embryo and successful crop harvest. Therefore, the DFL’s are collected safely in a wet hand bag in the early morning or in the late evening. Care must be taken to not to cause any damage to the inside embryo. In case of loose eggs, the transportation box containing loose eggs should be covered with wet cloth; good aeration must be provided to the eggs.
Care should be taken while transporting the DFL’s and should be kept under incubation at 250C temperature and 80% humidity. Egg cards should be spread in the rearing trays, which should be kept in cooler places only.

Incubation

The eggs should be kept in cooler places at 25°C temperature and 80% humidity, lesser the humidity poorer the hatching percentage or more the temperature weak the larva and poorer the hatch percentage.

For uniform hatching all the egg cards or loose eggs should be kept in dark and cooler atmosphere. On the day of pinhead or blue egg stage all the eggs are covered with a black sheet or kept in black box and known as black boxing. On the day of hatching all the eggs are suddenly exposed to bright light in the early morning at around 8 A.M. so that 95% hatching can be achieved. It should be ensured that hatching should take place uniformly. With proper maintenance of temperature and humidity development of larvae will be uniform.

Brushing

Brushing is transferring of newly hatched larvae into rearing trays. The newly hatched larvae after one hour of hatching get ready to feed on mulberry leaf. They are fed with finely chopped tender mulberry leaf. Chawki worms are fed with tender leaves with high moisture content. The brushing of newly hatched larvae can be done in various methods.

Silk Worm

Newly hatched larvae can be covered with a net and chopped mulberry leaf can be sprinkled over the net. The larvae slowly crawls on to the net and start to feed on the mulberry leaf. Then transfer into rearing tray by gently tapping the net, this method is more convenient in case of the loose eggs. In case of egg cards, the egg cards are placed in the rearing trays and chopped mulberry leaf is sprinkled over the newly hatched larvae, the larvae crawls on to the mulberry leaf and later on the cards are removed.

Feeding of Leaf

The leaf is finely chopped as per the recommended size and sprinkled on the larvae and after half an hour the newly hatched larvae slowly crawl on to mulberry leaf. Chopped leaf should be preserved from drying and loss of moisture content. For this purpose it should be collected in a basin covered with paraffin paper to protect from loss of moisture content.

Spacing

Silk Worm

Larval growth will be very fast in the silk worms during the early stages. Too much crowd in the rearing tray results in increase in humidity, heat, fermentation of litter which results in un-hygienic conditions, wastage of leaf and under development of silkworm. The number of trays and space required for each instar will increase.

Silk Worm

Spacing should be given simultaneously at the time of bed cleaning or feeding and care should be taken so the worms are not disturbed very frequently.

Bed Cleaning

Silk Worm

Bed cleaning in silkworm rearing tray is done by various methods like using of paddy husk, straw, and bed cleaning net. During 1st instar bed cleaning should be done once during pre-moulting, during 2nd instar – twice, once after moult and before next moult. During 3rd instar thrice i.e. after moult, before next moult and once in the middle. During 4th and 5th stage once in a day in case of shelf rearing. However, in case of floor rearing or shoot rearing once in each instar.

Feeding

Not only the nutritive value but also the number of feeds in each instar plays a major role in the cocoon built. Three to four feeds are given to the silkworm. In a day the last feeding i.e., during the night should be a little more since the duration for the next feeding will be longer. During moulting period no feeding should be given. Before settling to moult and immediately after moult, first feeding should not be heavy. Feeding tender leaves to young age worms is essential. As the larval growth advances, the mature leaf can be fed.

Types of Rearing

Rearing of silkworm is done in various methods in different areas. Mainly the rearing of silkworm is of two types.

  • Chawki rearing:: Rearing of young age silk worms is called Chawki rearing. Here worms are reared upto third moult and distributed to the rearers for late age rearing. Chawki rearing will not only control the attack of diseases but also facilitate the rearing on most scientific lines and cost wise it is most economical. To maintain coolers, heaters, exhaust fans, etc., by individual rearers is a costly affair, however, all these facilities can be maintained in Chawki Rearing Centres (CRC).
  • Late age rearing:: Late age rearing after third moult does not require high temperature and humidity compared to chawki rearing. Late age rearing is a little easier process than chawki rearing. During late age the quantity of mulberry leaf required is more than 90% of total larval period. During 5th stage particularly the larvae eat voraciously worms feel maximum appetite larvae loses water from its body hence, less temperature, low humidity, good ventilation is required.

Mounting

Silk Worm

This is the last stage of rearing operation. At this stage silkworm becomes matured completely and ready to spin into cocoon. These silk worms after 5 days of 4th moult becomes yellowish and translucent, stops feeding on mulberry leaves, starts to search for a corner and try to crawl on the feeding trays further liquid like substance oozes out of the mouth from the spinneret. These movements clearly indicate to transfer the mature larvae into the mountages. Transferring of matured silkworm on to the mountage or cocoon frames is called "mounting". This is an important and skilled operation. Any deviation in identification of maturity of worms adversely effect on cocooning.
During mounting though, selection of ripen worms is not a difficult task, but, due care is needed. Most of rearers pick up ripen worms with hands to transfer them on to mountage. This is the most common method used in our country, however this is laborious and due care is needed while handling the mature worms. A few rearers use rearing nets, in some parts, rearers use mulberry branches to separate the matured worms. As the matured worms stop feeding they crawl on to the twigs searching for a support to spin the cocoon.
Another method called frame method is also used, fresh mulberry leaf is applied with a frame, matured worms crawl above the frame which are picked up and tranferred on to the mountages. While mounting care should be taken to avoid over crowding. Proper spacing should be given to avoid formation of double cocoons strained cocoons/urinated cocoons. For the purpose the density of worms in the mountage should be limited to 40-50 worms per sq.ft. to avoid formation of double cocoon, stained cocoon. However, the number of worms in each mountage depends upon the size of mountage and type of silkworm race.
The matured worms picked up in any of the methods mentioned above are transferred to the mountages. In an area of 1 mt 800-900 worms can be mounted to avoid too much density in the mountage. Mountage should be kept in inclined position so that, excreta of worms fall on ground.
In certain strains of silk worms around 250-300 worms can be conveniently mounted in a standard mountage of 90 x 60 cm size. However the size of mountage varies from place to place. In case of bamboo made chandrike that has a mat on the back ground of size 6’ x 4’, it easily carries above 1000 worms.

Cocooning

The matured silkworm spins the outer protective covering called cocoon and remains in dormant stage inside as pupa. After mounting the ripen worms in chandrike the larvae sticks on to the mountage by oozing out the silk fluid which will harden immediately after coming in contact with air and sticks itself to the mountage. It starts to ooz out the silk by continuous movement of its head in a very specific manner to form the silk filament in the shape of arithmetic figure.
Larvae moves its head continuously about 70-80 times per minute till the compact shell called cocoon is formed and detaches itself from the last layer of silk of then transforming into pupa. The Cocoon will have three layers.

  • Outer floss
  • Middle compact layers
  • Inner pelade

Silk Worm

The outer floss layer which will be 8-10% of weight of cocoon though formed by continuous filament, is not reelable. The floss percentage varies from race to race. The middle order compact layer only is reelable and is a continuous filament easily reelable and the inner pelade layer is not reeled. Silkworm completes the spinning in 2-4 days depending upon the silkworm strain and climatic conditions provided.

Harvesting of Cocoons:

Harvesting of cocoons is done on the fifth day of spinning. Whereas seed cocoons should be harvested on eighth day or ninth day of spinning depending upon atmospheric temperature. Harvesting should not be done immediately after pupation. Further, harvesting should be done before the moth emerges out. Too many days delay in harvesting will result in formation of pierced cocoons due to emergence of adult moth or uzi maggots. Cocoons are harvested generally with hand. In advanced temperate countries like Japan simple devices are used to harvest the cocoons from rotatory mountage.

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