Posts
Sericulture
Introduction
Silk is another valuable product
from the insect world. The silk is obtained as fine threads from the cocoons of
various species of silkworms. In the recent years, in the field of sericulture,
a great progress has been made by developing many techniques and introducing
new technologies in order to increase production and improve silk quality. An
important item of this progress is the increase of mulberry leaf yield through
breeding, cultivation, pruning and other management practices. Improvements
have been made in the silkworm hybrids and their use, in the rearing
techniques, on prevention of diseases, on multiple rearings per year, in the
mounting method and in the silk manufacturing progress. The silkworm is
completely domesticated. Its development can be controlled by regulating mainly
nutrition and the environmental conditions.
Sericulture is the scientific
management of production and marketing of natural silk from silkworms.
Sericulture is practiced in India
and India is the 5th largest producer of silk in the World. It has been
identified as employment oriented industry. All the sections of sericulture
industry, viz. mulberry cultivation, silkworm seed production, silkworm
rearing, reeling and weaving of silk and collection of byproducts and its processing
provide a large scale employment, thereby a source of livelihood for the rural
and tribal people. Sericulture industry is rated as the second largest employer
in India.
Owing to this peculiar nature,
the Indian planners have identified sericulture as one of the best-suited
occupations for ideal growth and development of rural India. Mulberry
sericulture has been traditional occupation in Karnataka, Tamil Nadu, A.P. and
Kashmir; Tasar one, in M.P., Chota Nagpur Division and Orissa; Muga one, in Assam,
Nagaland,
Tripura, Assam and West Bengal.
North-eastern part of India is the only region in the world where all four
varieties of silk are produced.
The sericulture plays a
significant role in the rural economy of our country. The Chinese have the credit of discovering silk. It was
kept as a secret for several centuries. India now occupies a unique position in
the world for its rich sericigenous fauna to produce pure mulberry silk as well
as the three other varieties of non-mulberry silks.
The silk is used for various
purposes since ancient times. Pure silk is one of the finest natural fibre and
is said to be the “queen of fibers”. Due to its great value and usefulness,
various methods were adopted to rear silk worms on a large scale with great
care. Rearing of silkworm and adoption of different rearing techniques for the
production of silk threads of fine quality is known as Sericulture.
Types of Silk.
The silk is produced by certain
moths. The moths belonging to the families Saturnidae and Bombycidae of order
Lepidoptera and class Insecta. Mainly four types of silks have been recognized
which are secreted by different species of silkworm.
(1) Mulberry Silk : This silk is
superior in quality due to its shining and creamy white colour. It is secreted
by the caterpillar of Bombyx mori, which feeds on mulberry leaves.
(2) Tasar Silk : It is secreted
by the caterpillars of Antheraea mylitta, A.paphia, A.royeli etc. This silk is
of coppery colour. The caterpillars feed on leaves of Arjun, Asan, Sal, Oak .
(3) Eri Silk : It is produced by
the caterpillars of Attacus ricini which feed on castor leaves. Its colour is
creamy white like mulberry silk and is less shining than the latter.
(4) Munga Silk : It is obtained
from caterpillars of Antheraea assama, which feeds on leaves of Som, Champa and
Moyankuri. It is shining like tasar silk.
Type of silk insect food plants
1. Mulberry silkworm -Bombyx mori
: This is a completely domesticated insect. Since the natural food of this worm
is mulberry leaves, it is called mulberry silk worm. The silk produced by this
moth is creamy white in colour. It is called the mulberry silk.
2. Tasar Silk worm : Antheraea
paphia This caterpillar feeds on ber,
oak, sal, plum arjun and fig plants. The cocoon produced by this worm is
smooth and hard. It is of hen’s egg size. The cocoon yields reelable, brown
coloured Tasar silk.
3. Eri Silk worm : Attacus
ricinii The caterpillar of this worm
feeds on castor leaves. The cocoons of this worm have very loose texture and
the silk produced is called as Arandi silk locally. The threads are not glossy
but much durable.
4. Muga Silk worm: Antheraea
assamensis: The native place of this species is Assam where it has now become
a good source of cottage industry. The silk produced by this moth is known as
Muga silk.
Biology of Silk moth
The Silk Moth, Bombyx mori,
belong to Class insect and is a member of a small family of about 300 moth
species in the Order Lepidoptera. . It is a flightless insect. Adults of this
family have heavy, rounded, furry bodies and cannot feed because of undeveloped
mouth parts. Wings and body are usually white, but may also have varied shades
of light brown. Wingspan is 4-6 cm. Wings are membranous and weak. The adult moths have a very short life, and
live only for 2 or 3 days after emergence. The larva of Bombyx mori is an
elongated caterpillar commonly called a silkworm. Larvae are monophagous and
feed only on mulberry plants. The larva will moult five times before spinning a
silk cocoon of one continuous fiber for pupation. Silk cocoons from this
species are the commercial source of silk.
Formation of silk:
The main purpose of silk worm is
to produce silk which is the result of secretion of silk glands which is
nothing but modified salivary gland. There are two tubular, coiled glands lying
one on each side of the alimentary canal of the caterpillar. These glands are
connected to a narrow tube like structure known as spinneret. The spinneret is
a part of the hypopharynx (tongue). The
secretion of salivary gland contains fibroin, a sort of liquid fibrous
protein.Fibroin is insoluble in water and is made up of glycine, alanine and
tyrosin. As the liquid secretions of the two glands pass through the Spinnert,
it transforms them into a single thread. Another secretion called Sericin,
produced by a pair of accessory glands, helpful for to unite two fibers of fibroin. Two streams of fibroin along with sericin are
expelled through the spinnert due to contraction and expansion of the body of
the caterpillar. This sticky secretion when comes into contact with the air is
converted into a fine, long and solid thread of silk.
Life cycle of silk worm :
The life cycle of Bombyx mori
completes into four distinct stages . They are as follows.
1. Egg,
2. Larva,
3. Pupa and
4. Imago.
EGG: Incubation 7-10 days
Fig Life-cycle of mulberry
silk-moth
The egg of Bombyx mori is a very
small and hard structure about the size of a pin head and resembling a poppy
seed. The egg shell provides a protective covering for embryonic development.
When the eggs are first laid, they are light yellow in colour and later the
eggs become darken to a blue-grey within a few days.
Larva (Caterpillar): 45 days (5 instars)
This is the state where growth of larva is takes place. The larva of Bombyx mori,
commonly called a silkworm, ( caterpillars) . During this stage of life the
larva moults four times. The period between successive moults is called an
instar. The body of caterpillars have segmented appearance with strong
mandibular jaws. With the help of these mandibles larva consumes mulberry
leaves throughout the five instars. During this
salivary glands (silk glands)are fully grown and silk concentrate gets
secreted out of this gland.
Pupa : 15 days
The secreted silk spins in to a
silk cocoon. The silk cocoon serves as protection for the pupa. Cocoons are
shades of white, cream or yellow in colour. After a final moult inside the
cocoon, the larva develops into the brown, chitin covered structure called the
pupa. Metamorphic changes of the pupa result in an emerging moth.
Imago: 5 - 7 days
An adult insect produced after
metamorphosis is called imago .This adult stage completes the life cycle of
Bombyx mori. It is the reproductive stage where adults mate and females lay
eggs. Moths are flightless and lack functional mouth parts, so they are unable
to consume nutrition.
Sericulture in Mulberry Type:
Majority of the silk is obtained
from Mulberry silk worms are of domesticated type because they can be reared
indoors. Large and healthy cocoons are selected and they are kept in well
ventilated cages. Young ones emerge after few days and male and females are
easily distinguished. Females are larger than males. Once they attain maturity,
they are kept as pairs in the coupling jars for about 24 hours. After
copulation females are transferred to egg-laying boxes. Each female lays about
300 - 400 eggs of small size. These eggs are commercially called seeds. A great
care should be taken in the selection of seeds because the quality of the seeds
depends on the silk production. The eggs are kept in incubators at 70oF. Small
larvae hatch between 7-10 days, and they are known as caterpillars.
The sericulture practice involves
following different stages. They are
(1) Cultivation of food plants.
(2) Rearing practice
a. Rearing of larvae.
b. Brushing
c. Preparation of bed and feeding
d. Bed cleaning
e. Spacing
f. Mounting
g. Harvesting of cocoon
h. Post cocoon process
(3) Reeling of thread from cocoon.
a. Stiffing
b. Reeling
5) Cultivation of mulberry and pruning
Cultivation of food plant
The first stage of sericulture is
cultivation of mulberry tress The larvae of Bombyx mori feed on leaves of
mulberry. Regular supply of healthy mulberry leaves is essential. Mulberry
leaves have high nutritional food value. It is a perennial woody plant. The
propagation of mulberry trees can done by sapling, cutting and grafting
methods.
Rearing Practices
As Silkworms are susceptible for
various diseases must be reared with
utmost care. Therefore, hygienic rearing techniques must be applied to prevent
diseases and malformation during rearing . Room temperature should be maintained
around 250C. The appliances and the rearing room should be thoroughly cleaned
and disinfected with 2-4% formaldehyde solution.
The
most important step in silkworm rearing is the procurement of quality seeds
free from diseases. Cocoons should be
purchased from the certified seed cocoon producers. The cocoons are
placed in well-ventilated rooms with proper temperature about 23 to 25o C and
humidity about 70-80 %. Grainage rooms
may be kept dark, and light may be supplied suddenly on the expected day of
emergence to bring uniform emergence. Emerging moths are sexed and used for
breeding purposes to produce seed eggs. Three hours of mating secures maximum
fertilized eggs. The females are then allowed to lay eggs on paper sheets or
cardboard coated with a sticky substance. The egg sheets are disinfected with
2% formalin, and then washed with water to remove traces of formalin and then
dried up in shades. The eggs are transported in the form of egg sheet. To
loosen the eggs, the sheets are soaked in water. The loose eggs are washed in
salt solution of 1.06-1.10 specific gravity to separate out unfertilized eggs
and dead eggs floating on surface. Prior to the final washing, the eggs are
disinfected with 2% formalin solution. Eggs are dried, weighed to the required
standard and packed in small flat boxes with muslin covers and now they are
ready to dispatched to silk producer.
Rearing of larvae tiny
caterpillars -
Initially eggs are allowed to
hatch in well maintained conditions(Temperature, humidity light, etc.)The
caterpillars measure 5-7 mm in length are transferred to feeding trays
containing chopped tender leaves of mulberry. They start feeding on the tender
leaves and grow faster. The caterpillar thus formed stop feeding and become
inactive within four to five days. The larvae then undergo morphological
changes after every moult. They undergo 5 moults. The maturity is achieved in
about 45 days. The salivary glands, otherwise called silk glands start
secreting silk. At this stage, the matured caterpillars are transferred to
spinning trays. They excrete their last excreta and begin to secrete sticky
secretion from the silk glands through a very narrow pore situated on the
hypopharynx. With in 5 days, the caterpillars are transformed into pupa or cocoon.
After 15 days acute metamorphic changes take place during pupation and the
young imago tries to come out. The young one secretes an alkaline fluid to
soften one end of the cocoon. Later it escapes by forcing its way out of the
softened area. Just after the formation of cocoons, healthy cocoons are
selected and kept in cages for the next crop.
Brushing
The process of transferring the
silkworm to rearing trays is called brushing. Best and suitable time for brushing is about 10.00 am. Eggs are
kept in black boxes on the days prior to hatching. The next day they are
exposed to diffused light so that the larvae hatch uniformly in response to
photic stimuli. About 80 to 90% hatching
can be obtained in one day by this method.
Newly hatched caterpillars measure 5-7 mm in length are transferred to
feeding trays containing chopped tender leaves of mulberry. They start feeding
on the tender leaves and grow faster. Worms start crawling over the leaves on
the net; the net with worms is transferred to rearing tray.
Preparation of feed bed and
feeding
After brushing, the worms and the mulberry leaves collected together by using a feather. The
first feeding is given after two hours of brushing. Feed bed is a layer of chopped leaves spread
on a tray. The first and second instar larvae are commonly known as chawki
worms. For chawki worms, paraffin paper sheet is spread on the rearing tray.
Chopped mulberry leaves are sprinkled on the sheet and hatched larvae are
brushed on to the leaves. A second paraffin paper sheet is spread over the
first bed. In between two sheets water soaked foam rubber strips are placed to
maintain humidity.
The 4th and 5th instars are reared in wooden
or bamboo trays. The rearing trays are arranged one above the other in tiers on
a rearing stand which can accommodate 10 -11 trays. This method provides enough
space for rearing, but it is uneconomical as it requires large number of
laborers to handle the trays. Chopped leaves are given as feed in this method.
In floor rearing, fixed rearing sheets of 5-7x1-1.5m size are constructed out
of wooden or bamboo strips in two tiers one meter apart. These sheets are used
for rearing. Chopped leaves are given as feed. This method is economical than
the first one because it does not involve much labour in handling of trays.
Shoot-rearing is most economical of the three methods. The rearing sheet used
is one meter wide and any length long in single tier and the larvae are offered
fresh shoot or twigs bearing leaves. This method can be practiced both outdoors
and indoors depending upon the weather.
Each age of the silk worms could
be conveniently divided into seven stages. First feeding stage, sparse eating
stage, moderate eating stage, active eating stage, premoulting stage, last
feeding stage, moulting stage. The larvae have good appetite at first feeding
stage and comparatively little appetite at sparse and moderate eating stages.
They eat voraciously during active stage to last feeding stage after which they
stop feeding.
Bed Cleaning
Periodical removal of left over leaves and
worms’ excreta may be undertaken and is referred to as bed cleaning. It is
necessary for proper growth and proper hygiene. Four methods are adopted:
conventional method, husk method, net method, and combined husk and net method.
Spacing
Provision of adequate space is of
great importance for vigorous growth of silkworms. As the worms grow in size,
the density in the rearing bed increases and conditions of over crowding are
faced. Normally it is necessary to double or triple the space by the time of
moult from one to other instar stage, with the result that from the first to
third instar the rearing space increases eight fold. In 4th instar, it is
necessary to increase the space by two to three times and in 5th instar again
twice. Thus, the rearing space increases up to hundred folds from the time of
brushing till the time of maturation of worms.
Mounting
Transferring mature fifth instar
larvae to mountages is called mounting. When larvae are fully mature, they become
translucent, their body shrinks, and they stop feeding and start searching for
suitable place to attach themselves for cocoon spinning and pupation. They are
picked up and put on mountages. The worms attach themselves to the spirals of
the mountages and start spinning the cocoon. By continuous movement of head,
silk fluid is released in minute quantity which hardens to form a long
continuous filament. The silkworm at first lays the foundation for the cocoon
structure by weaving a preliminary web providing the necessary foot hold for
the larva to spin the compact shell of cocoon. Owing to characteristic
movements of the head, the silk filament is deposited in a series of short
waves forming the figure of eight. This way layers are built and added to form
the compact cocoon shell. After the compact shell of the cocoon is formed, the
shrinking larva wraps itself and detaches from the shell and becomes pupa or
chrysalis. The spinning completes within 2-3 days in multi-voltine varieties
and 3-4 days in uni- and bi-voltine.
Harvesting of Cocoons
Harvesting of cocoon is crucial
The larva undergoes metamorphosis inside the cocoon and becomes pupa. In early
days, pupal skin is tender and ruptures easily. Thus, early harvest may result
in injury of pupa, which may damage the
silk thread. Late harvest has a risk of threads being broken by the emerging
moth. It is, therefore, crucial to harvest cocoons at proper time. The
harvesting of cocoonsis done by hand. After harvesting the cocoons are sorted
out. The good cocoons are cleaned by removing silk wool and faecal matter
before market. The prices of cocoons are
on the basis Rendita and reeling parameters. Rendita may be defined as number
of kg of cocoon producing 1 kg of raw silk.
Post Cocoon Processing
The post cocoon processing involves all
processes to obtain silk thread from cocoon which are summarized as below.
1. Stifling
2. Reeling
3. Cultivation of mulberry and pruning
Stifling
The killing pupa inside cocoon is termed as
stifling. Good-sized cocoon are selected for further processing (8-10 days
old). Stifling is done by subjecting cocoon to hot water, steam, dry heat, sun
exposure or fumigation etc.
Reeling
The process of removing the
threads from killed cocoon is called reeling.
This process is carried out by cooking of cocoon in hot water at 95-97oC
for 10-15 minutes to soften the adhesion of silk threads among themselves,
loosening of the threads to separate freely, and to facilitate the unbinding of
silk threads. This process is called cooking. Due to the sericin protein to get softened and make
unwinding easy without breaks.
The reeling is done with the help
of a suitable machine in hot water. Free ends of threads of four or five cocoon
are passed through eyelets and guides to twist into one thread and wound round
a large wheel. The twisting is done with the help of croissure. The silk is
transferred finally to spools, and silk obtained on the spool is called the Raw
Silk or Reeled Silk. The Raw silk is further boiled, stretched and purified by
acid or by fermentation and is carefully washed again and again to bring the
luster. Raw Silk or Reeled Silk is finished in the form of skein and book for
trading.
The waste outer layer or damaged
cocoons and threads are separated, teased and then the filaments are spun. This
is called Spun Silk. Cultivation of
food plants is generally avoided, as tasar silkworms are wild in nature and
need to be reared outdoors. However, modern sericulturists prefer to cultivate
the food plants for better supervision. Cultivation is done with seeds or
saplings being raised in nurseries. Saplings are transplanted to fields 20-25
feet apart. Agronomic practices are carried on regularly. Pruning is done
regularly to maintain better foliage growth.
Cultivation of mulberry and
pruning
The cultivation of food plant is
the prime need of sericulture. Cultivation is done with seeds or saplings being
raised in nurseries. Saplings are transplanted to fields at about 20-25 feet
apart. Timely, watering and application of fertilizer/ manure for better growth of these food plant is
necessary. Pruning is done regularly to maintain better foliage growth.
Requirements of rearing silk
worms.
House : Silkworm requires a well
aerated building or mud walled thatched houses which are the best as they are
cool in summer and warm in winter. The range of temperature for maintaining
larvae is 70o -75oF.
Feeding Trays: Freshly hatched
worms are kept in flat trays made up of bamboos along with chopped small pieces
of mulberry leaves.
Machan : Machan are made by
fixing two pairs of bamboos in the ground and tying across bars of bamboos.
They are needed to accommodate large number of trays in a limited space.
Nets : cleanness is most
important for larvae. Large amount of
excreta and remains of leaves may fall on the trays from the holes of the upper
trays. If worms are not protected from these wastes they may get diseased. To
prevent form any infextion, trays are covered with the nets.
Spinning Trays : Before cocoon
formation mature worms are transferred to specific type of trays known as
spinning trays or Chandraki made up of bamboo strips.
Initially the start of rearing
the machan, feeding trays, spinning trays, nets and everything used in rearing
except the leaves are washed with Copper sulphate solution or other antiseptic
chemicals. The remaining germs are killed by fumigation.
Precautions to be taken during
rearing of mulberry silkworms.
1) The worms should never be
kept, overcrowded in a tray.
2) Dried or dusty leaves should
never be fed to the worms.
3) Free ventilation is a must.
4) There should be equal
distribution of leaves among the worms.
5) Worms which are under process
of moulting should not be disturbed.
6) There should be no dust at the
floor of the house. It should be well plastered with cow dung or mud at regular
intervals.
7) Smoking should be strictly
prohibited in the rearing room.
8) Worms should not be handled
with dirty hands to avoid infection. Hand should be washed before handling.
9) Water may be sprinkled
periodically over the feeding trays, during the hot season.
Stage 3 -Reeling of raw silk from
cocoon
For the production of standard
variety of raw silk latest technology is employed for the speedy and economical
reeling of raw silk. Reeling process is an important aspect of sericulture
because cocoon production is directly related to reeling industries. The process of removing the threads from
killed cocoon is called reeling. This
process is carried out by cooking of cocoon in hot water at 95-97oC for 10-15
minutes to soften the adhesion of silk threads among themselves, loosening of
the threads to separate freely, and to facilitate the unbinding of silk
threads. This process is called cooking. Due to
the sericin protein to get softened and make unwinding easy without
breaks. Reeling is the process of unwinding the silk filament from a required
number of cocoons so as to form a single composite thread, after stifling the
cocoons. Before reeling the thread, the cocoons are dipped in a container of
hot water for more than 10 minutes. During this period, continuous stirring
with a rod is necessary. Due to this, their outer portion is loosened and
removed in the form of long tapes. The end of the continuous filament is now
found. The filaments of several cocoons are picked up and passed through the
‘glass eye’ on to the reel. The thread thus reeled forms the “raw silk” of
commerce. About 1 kg of raw silk is obtained from nearly 55,000 Cocoons.
TASAR CULTURE
Silkworms show three types of
voltinisms ,univoltine and bivoltine occur in Central India and bivoltine and
multivoltine in north eastern India. Its Holometabola, having four stages in
life cycle (Egg,larva, pupa (enclosed in cocoon) and adult (moth). Emergence of
moth take place in the evening, the males fly long distance in search of
females ( which are tied to a straw bundle called KHARIKA ) and placed on host
trees so that wild males may couple with females ensuring proper mating
.Copulation period is for eight hours. The females are decoupled and kept in
bamboo baskets (called MONIA) for 2 days to lay eggs. 150¬200 eggs are laid.
The eggs are oval and dorso-ventrally flattened, having two brown lines running
along the periphery and coated with gummy substance.
The larvae hatch in about 10 days
after eggs are laid. The eggs are kept in leaf cups of host and distributed
uniformly on host trees. Larval period is 30¬35 days undergoing four moults and
has five instars which feed voraciously .When all host leaves are defoliated ,
silkworms move downwards of the host to find new hosts trees. The normal tasar silkworm at final instar is
generally green in colour with violet tubercles distributed over the body and
weighs 50¬60gm and 12.5¬15 cm in length.
The matured larvae first spin a
kind of hammock by webbing together a few leaves of the host plant. Then it
constructs a ring made up of silk to attach the future constructed cocoon to
tree twig, in¬between it forms a peduncle which attaches the cocoon to the
ring. Completely constructed cocoon is large, compact and the bivoltine variety
is used for commercial purpose .The bivoltine cocoons are harvested in Nov
–December, go into diapause at the pupal stage and the moth emerge in May ¬June
of the following year. Hence ,upon cocoon harvest a thick thread is passed
through the ring to form cocoon garlands, which are sold. Few to be retained
for next rearing are hung indoors during diapause, on moth emergence; females
are tied to kharika and hung in jungles for mating with wild males. Rearing is
completed in June¬July .This is summer crop and the cocoon does not undergo
diapause.
MUGA CULTURE
Muga silk is the monopoly of
India, culture is not seen in any other part of the world. Muga is an Assamese
word which indicates the brown(amber) colour of cocoon or golden yellow .Muga
silk is secreted by semi domesticated multivoltine species. Muga larvae feeding
on Litsea citrate produce a kind of silk which is favoured for its durability,
lustre and creamy white shade. The
golden silk produced by the muga silkworm, Anthraea assamensis found only in
Assam India .This is semi domesticated. Muga silkworm is multivoltine in nature
and therefore rearing could be conducted all through the year. It is
polyphagous , feeding on mostly som (Machlius bombycina ) and soalu (Listea
polyantha). Only larval stage is wild and spent in open, ripening worms are
brought in indoors for cocooning.
Mature worms are collected from
base of trees on structures called CHANDALI (Triangular bamboo sieves with long
handle). Cocoons have rudimentary peduncle which does not allow attachment to
host. Moth emergence is in the evening, pairing is in the tray itself .Next
morning ,mated females are separated and placed in egg basket called MONIA ,for
egg laying. The muga silk worm is multivoltine and passes through four moults
and five instars. Moth lays 150¬200 eggs.
The tribal takes the Monia with
eggs and hang them on host plant in the forest/ jungles. When the leaves are
fully consumed, the silkworms climbing down the tree trunks are collected on
Chandali , and hung on hosts with leaves .The larval period is 30¬35 days. The
ripe worms come down the trunk of the tree during the evening at night. They
are then collected and put in baskets containing mango twigs which act as a
cocoon age for spinning. Spinning take place two to three days and is golden or
light brown 4¬6cm long 2¬3cm broad with rudimentary peduncle. Muga culture is
often hazardous and risky on account of attack by several kinds of pests and
predators. Constant vigilance is required.
ERI CULTURE
Eri culture is among the
commercially exploited non mulberry silkworms. The eri silkworm, Attacus
ricini, is the only species domesticated completely and adopted to indoor
rearing all through the year. Eri silk is considered as poor man’s silk. Eri
silkworm is polyphagous and rearing on leaves of castor, Ricinus communis L,
alone can ensure good quality cocoons. About 25 percent of total leaf yield can
be harvested and used for eri cocoon production without affecting castor seed
yield.
The female moth lays between
300¬500 eggs. Eri silkworms are hardy and not easily susceptible to diseases.
The fertilized female moth is tied with a piece of thread to kharika for egg
laying. The Eri cocoons are open mouthed and do not contain continuous filament
and hence are not reliable. Takli spinning is practiced for eri cocoons. The
larval period is 22¬28 days, depending upon the host plant.
Eri silkworms are reared
following tray method, or branch feeding method wherein branches of Castor
leaves are brought indoors and placed in containers with water, to keep leaves
fresh. Rearing of Eri silkworm for
cocoon production is not yet followed in Karnataka. Mostly practiced by tribals
in North eastern India. Tribals are known to consume the larvae/pupae. Castor
can be cultivated for Eri culture as well as seed production, as capsules are
harvested and sold. The eri silkworm are
of two species namely Philosamia ricini Hutt and Philosamia cynthia Drury . The
latter is a wild species now named Philosamia cynthia ricini and commonly
called castor silk worm, is domesticated producing white or brick coloured
cocoons .Since filament of cocoons spun by these worms is neither continuous
nor uniform in thickness it cannot be properly reeled and therefore the moths
are allowed to emerge naturally and then silk is used. It is called Ahimsa
silk.
Uses of Silk :
1) Silk fibers are utilized in
preparing silk clothes. Silk fibers are now combined with other natural or
synthetic fibers to manufacture clothes like Tere-Silk, Cot-Silk etc.
2) Silk is used in industries and
for military purposes.
3) It is used in the manufacture
of fishing fibers; parachutes, cartridge bags, insulation coils for telephone
and wireless receivers, tyres of racing cars, filter clothes, in medical
dressings and as suture materials.
Diseases and enemies of silk worm
The profitable silk industry is
threatened not only by various diseases caused by the virus, fungal, bacterial
and protozoan infections but also by insect predators, birds and other higher
animals. Ants, crows, kites, rats, feed upon silk worms thereby causing a great
loss to silk industry. Various types of silk worm diseases, the symptoms and
methods of control have been given in the following chart.
Diseases
Pebrine: Pebrine is also known as pepper
disease or corpuscle disease. The disease is caused by a sporozoan, Nosema
bombycis (family Nosematidae). The main source of infection is food
contaminated with spores. Infection can be carried from one larva to another by
the spores contained in faeces or liberated in other ways by the moths carrying
infection. Pebrinized eggs easily get detached from the egg cards. They may be
laid in lumps. The eggs may die before hatching. The larva shows black spots. They
may become sluggish and dull, and the cuticle gets wrinkled. Pupa may show dark
spots. Moths emerging from pebrinized cocoons have deformed wings and distorted
antennae. The egg laying capacityof the moth becomes poor.
Flacherie: Flacherie is a common term to
denote bacterial and viral diseases. It has been classified into following
types:-
i) Bacterial diseases of
digestive organs: Due to the poor supply of quality mulberry leaves, the
digestive physiology of the silkworm is disturbed, and multiplication of
bacteria occurs in the gastric cavity. Bacteria like Streptococci, Coli, etc.
have been found associated with this disease. Symptoms, like diarrhoea,
vomiting, shrinkage of larval body may be seen.
ii) Septicemia: Penetration and
multiplication of certain kinds of bacteria in haemolymph cause septicemia. The
principal pathogenic bacteria are large and small Bacilli, Streptococci, and
Staphylococci etc. Symptoms like diarrhoea, vomiting, shrinkage of larval body
may be seen. Appearance of foul odor is also a common symptom.
iii) Sotto disease: It is caused
by toxin of Bacillus thuringensis. The larvae become unconscious, soft, and
darkish and rot off.
iv) Infectious Flacherie: It is caused by a virus
called Morator Virus which does not form polyhedra in the body of silkworm
larvae. The infection occurs mainly through oral cavity. The virus multiplies
in the midgut and is released into the gastric juice and is excreted in faeces.
v) Cytoplasmic polyhedrosis: It
is caused by a virus called Smithia which form Polyhedra are formed in the
cytoplasm of the cylindrical cells of the midgut. The larva loses appetite. The
head may become disproportionately large. Infection occurs through the oral
cavity.
Vi)Grasserie: The disease is also
known as Jaundice or Nuclear Polyhedrosis It is caused by a virus called Borrelina, which form polyhedra in the nuclei
of the cells of fatty tissues , dermal
tissues, muscles, tracheal membrane ,basement membrane , epithelial cells of
midgut and blood corpuscles. The infected larvae lose appetite, become
inactive, membranes become swollen, skin becomes tender and pus leaks out from
skin. The larvae finally die.
Muscardine or Calcino: It is of 3 types-
i) White Muscardine: It is caused
by the fungus, Beuveria bassiana. The larva loses appetite, body loses
elasticity and they cease to move and finally die.
ii) Green Muscardine: It is
caused by Metarrhizium anisopliae. The larva loses appetite, appears yellowish, becomes
feeble and dies.
iii) Yellow Muscardine: It is
caused by Isaria farinosa. Many small black specks appear on the skin. Larvae
lose appetite and dies.
Pests
a) Tricholyga bombycis, a dipteran fly of the family tachinidae,
commonly known as Uzi fly.It is a serious pest of silkworm larvae and pupae. It
parasitizes Mulberry and Tasar silkworm.
b) Dermestid beetles: These insects belong to the order
Coleoptera, family dermestidae. This family contains many genera and a large
number of destructive species. Some of them are: Dermestes cadverinus, D.valpinus,
D.vorax, D.frishchi, and Trogoderma versicolor. The larvae bore inside the
cocoon and eat the pupa. These pests cause great damage and economical loss, as
the damaged cocoons cannot be reeled.
c) Mites: Pediculoides ventricosus (order Acarina, class
Arachnida) damage the larvae. The toxic substance produced by the mite kills
the silkworms.
In addition, ants, lizards,
birds, rats and squirrel also cause considerable damage to silkworm larvae as
well as the cocoons.
The Silk
The raw silk is used in the
manufacture of woven materials, knitted fabrics and garments. It is also used
in parachutes, parachute cords, fishing lines, as sieves in flour mills,
insulation coil for telephones and wireless receivers, and tyres of racing
cars.
Properties of the silk:
Silk contains 70-75% fibroin and
25-30% sericin protein. The biochemical composition of fibroin can be
represented by the formula C15H23N5O6. It has the characteristic appearance of
pure silk with pearly lustre. It is insoluble in water, ether or alcohol, but
dissolves in concentrated alkaline solutions, mineral acids, and glacial acetic
acid and in ammoniacal solution of oxides of copper. Sericin, a gummy covering
of the fiber is a gelatinous body which dissolves readily in warm soapy
solutions and in hot water, which on cooling forms a jelly with even as little
as 1% of the substance. It is precipitated as a white powder from hot solutions
by alcohol. Its chemical formula is C15H25N5O8. It can be dyed before or after
it has been woven into a cloth. The weight in gram of 900m long silk filaments
is called a denier which represents size of silk filament.
Silk has following peculiar
properties: -
A. Natural colour of Mulberry silk is white , yellow or yellowish
green
B. Tasar brown;
C. Muga, light brown or golden
D. Eri, brick red or creamy white or light brown.
2. Silk has all desirable
qualities of textile fibers, viz. strength, elasticity, softness, coolness, and
affinity to dyes. The silk fiber is exceptionally strong having a breaking
strength of 65,000-lbs/sq. inch.
3. Silk fibre can elongate 20% of
original length before breaking.
4. Density is 1.3-1.37g/cm3. S
5. Natural silk is hygroscopic
and gains moisture up to 11%.
6. Silk is poor conductor of heat
and electricity. However, under friction, it produces static electricity. Silk
is sensitive to light and UV- rays.
7. Silk fibre can be heated to
higher temperature without damage. It becomes pale yellow at 110o C in 15 minutes
and disintegrates at 165o C.
8. On burning it produces a
deadly hydrocyanic gas.
Use of silk: Silk is used in the manufacture
of following articles:
a) Garments in various weaves like plain, crepe, georgette and
velvet.
b) Knitted goods such as vests, gloves, socks, stockings.
c) Silk is dyed and printed to prepare ornamented fabrics for
saries, ghagras, lehengas and dupattas, Jackets, shawls and wrappers.
d) Caps, handkerchiefs, scarves, dhotis, turbans.
e) Quilts, bedcovers, cushions, table-cloths and curtains
generally from Eri-silk or spun silk.
f) Parachutes and parachute cords.
g) Fishing lines.
h) Sieve for flour mills.
i) Insulation coil for electric and telephone wire.
j) Tyres of racing cars.
k) Artillery gunpowder.
l) Surgical sutures.
ESTABLISHMENT OF A MULBERRY
PLANTATION
Mulberry leaves are the only food
for silkworms and without mulberry leaves, it is impossible to complete a
silkworm rearing. There are also other plants on which the silkworms can feed.
With some of these plants, the silkworms can reach the silkworm’s spinning
stage. However, this alternative food does not ensure a productive sericulture.
Sericulture could not be developed without suitable cultivation of mulberry
trees and the production of leaves of good quality and large quantity.
The mulberry tree grows well in
tropical, subtropical and temperate climates and can stand temperatures of 25
ºC below the zero. Mulberry tree has the ability to grow on unfertile and less
moist soils, however when it is cultivated on fertile soils with regular
irrigation it produces large quantities of leaves of the high quality. In order
to produce large quantity of leaves, a mulberry tree should have many branches
of leaves and should grow fast.
Satisfactory and sufficient leaf
production is the most important step of sericulture , a modern mulberry
plantation should have at least 1600 trees per hectare (planting distance of
3X2 metres). In temperate zones the most suitable number is 2,500 trees per
hectare. Following are the steps for
cultivation of mulberry trees.
Planting pattern
In temperate climate planting
pattern is dictated by mulberry varieties, types of soil, cultivation practices
etc. There are three basic planting patterns:
1. Single rows in high plant density, 1.5-1.8X0.5-0.8m (about
9,000 trees/hectare).
2. Rows with paired plants, wide ridge 1.8 m, narrow ridge 0.6 m X
0.5 m (about 16,000 trees/hectare).
3. Single rows in lower density, 2.5X1.5 m (about 2,500
trees/hectare)
Requirements of mulberry cultivation.
1. Soil conditions
Mulberry trees grow on fertile, well-drained soils with pH 6.5 – 7.5. Addition of organic matter
(compost and others), enriches the soil with nutrients, improves its structure and
increases the action of micro-organisms.
2. Irrigation
Despite the fact that mulberry
trees resist to drought, their irrigation increase leaf production. For summer
and fall rearing, irrigation is very important since we ensure optimum leaf
moisture, of course with the combination of the suitable pruning. Both extreme
wet and dry soils are harmful to the mulberry growth. Basically an average daily rainfall of 5 mm
is required to maintain normal growing. There are two water conservation
methods: direct and indirect.
Direct method includes drip irrigation,
sprinkler and furrow methods. Since sprinkler is ideal but costly method and
furrow method is of low cost but some water is wasted, the most effective
method is drip irrigation.
Indirect method includes mulching
of the soil surface in order to reduce evaporation and the organic matter will
increase the water holding capacity of the soil by enhancing aggregation of
soil particles.
3. Fertilization
Fertilization increases the
quantity and improves the quality of leaves. The type and the quantity of
fertilizer depended on the soil fertility and on the irrigation. When we have
many rearings per year, fertilization should be frequent. Usually, for a medium
fertile soil, the average quantity of fertilizers is 230 kg of N, 150 kg of P
and 170 kg of K per hectare, applied in three doses.
4. Tillage- Weeding
Mulberry tree has a deep root
system and therefore good soils conditions are prerequisite for its continuing
productivity. Tillage is considered to be one of the common practices for
improving soil conditions. Tillage improve
mulberry productivity in the following manner:
1. Air and water permeability and supply are improved.
2. Soil temperature is raised whereby the function of roots is
activated
3. The insoluble nutrients are made soluble and the soil fertility
is improved.
4. The soil is soften, which encourage the growth of the roots and
regeneration of a large number of new roots.
5. Weeds on the soil are removed and damage from pests is
minimized. For weed control, chemical, mechanical and cultural methods are
applied. The question as to the method to use can only be answered with local
land conditions. When applying pesticides, particular measures must be taken
not to drift to mulberry leaves. Pesticides on mulberry leaves will negative effects
on the reared silkworms.
5. Pruning
Pruning is depending up on the
height of trees and the leaf collection methods, various types of pruning are
applied. From the planting of the tree up to the 4th-year tree, only shaping
pruning is performed. After the 4th year and according to the number of rearing
units harvesting pruning is applied.
Depending on the density of planting, three types of cutting are
distinguished: low cut (plant cut at a level less than 0.5 m from the ground),
medium cut (plant cut at a level between 0.5 and 1.5 m) and high cut (plant cut
at a level more than 1.5 m) For example
for a mulberry plantation of 2.000 trees per hectare (2.5 X 2 m), the most
suitable type of cutting is the medium cutting.
Rearing Equipments
i) Rearing house:
The rearing house should meet certain specification, as the silk worms are
very sensitive to weather conditions
like humidity and temperature. The rearing room should have proper ventilation
optimum temperature and proper humidity. It
should be ensured that dampness, stagnation of air, exposure to bright
sunlight and strong wind should be avoided.
ii) Rearing stand:
Rearing stands are made up of wood or bamboo and are portable. These
are the frames at which rearing trays are kept. A rearing stand should be 2.5 m
high, 1.5 m long and 1.0 m wide and should have 10 shelves with a space of 20
cm between the shelves. The trays are arranged on the shelves, and each stand
can accommodate 10 rearing trays.
iii) Paraffin paper:
This is a thick craft paper coated with paraffin
wax with a melting point of 55o C. It is used for rearing early stages of silk
worms and prevents withering of the chopped leaves and also help to maintain
proper humidity in the rearing bed.
iv) Rearing tray:
These are made of bamboo or wood so that they are
light and easy to handle. These are either round or rectangular.
v) Ant well:
Ant wells are provided to stop ants from crawling on
to trays, as ants are serious menace to silk worms. They are made of concrete
or stone blocks 20 cm square and 7.5 cm high with a deep groove of 2.5 cm
running all round the top. The legs of the rearing stands rest on the centre of
well filled with water.
vi) Chopsticks:
These are tapering bamboo rods (1cm in diameter) and
meant for picking younger stages of larvae to ensure the hygienic handling.
vii) Feathers:
Bird feathers
preferably white and large are important items of silkworm rearing room. These
are used for brushing newly hatched worms to prevent injuries.
viii) Foam rubber strips:
Long foam rubber strips 2.5 cm wide and 2.5 cm
thick dipped in water are kept around the silkworm rearing bed during first two
instar stages to maintain optimum humidity. Newspaper strips may also be used
as a substitute.
ix) Chopping board and Knife:
The chopping board is made up of soft
wood it is used as a base for cutting leaves with knife to the suitable size
required for feeding the worms in different instar stages.
x) Leaf chambers:
These are used for storing harvested leaves. The
sidewalls and bottom are made of wooden strips. The chamber is covered on all
sides with a wet gunny cloth.
xi) Mountages:
These are used to support silkworm for spinning
cocoons. These are made up of bamboo, usually 1.8 m long and 1.2 m wide. Over a
mat base, tapes (woven out of bamboo and 5-6 cm wide) are fixed in the form of
spirals leaving a gap of 5-6 cm. They are also called chandrikes. Other types of mountage such
as centipede rope mountage, straw cocooning frames etc. are also used.
xii) Cleaning net:
These are cotton or nylon nets of different mesh
size to suit the size variations of different instars of the silk worm. These
are used for cleaning the rearing beds, and at least two nets are required for each rearing tray.
xiii) Hygrometers and Thermometers:
These are used to record humidity and temperature of the rearing room.
xiv) Feeding stands: These are
small wooden stands (0.9 m height) used for holding the trays during feeding
and bed cleaning.
Sericulture in India:
India stands fifth in the
production of silk. The other main silk producing countries in the order of
production are Japan, China, South Korea, USSR, Brazil, Bulgaria and Italy.
India accounts for 5% of the total global output of mulberry raw silk and 10%
of Tasar. The production of natural silk has increased to about 40% in the last
15 years and the global output of 1974 was 45 thousand tones. At present the production
rate is higher and the value of silk product in India is about Rs. 80 crores
per annum.
In India the major silk producing
states are Mysore, West Bengal, Jammu and Kashmir, Assam, Bihar, Orissa,
Madhyapradesh, Uttra Pradesh, Andhra Pradesh, Tamil nadu, Punjab, Manipr,
Tripura and Maharashtra. 75% of total production of raw silk is in Mysore.
Bihar has the privilege of producing Tasar in larger quantity. The Eri silk is
limited in the state at Gangetic plains (Assam & Bihar). Munga silk is produced
only in Assam. Rearing of silk worms on a large scale is
carried on in villages and remote forests by villagers and tribals. They are
assisted by State Government and Central Government agencies. A qualitative and
quantitative breakthrough has been made by “Central Silk Board” running under
the Union Ministry of Trade and Commerce. The increased silk production in
India is due to research conducted on various aspects of the 268 industry by
the respective research centres running under this board.
Distribution and statistics of
silk production in the world
Sr.No. Country Production
in MT %Share
1 China 84000
81.06
2 India 16322
15.75
3 Brazil 811
0.78
4 Thailand 665
0.64
5 Vietnam 550
0.53
World Mulberry raw silk
production 1,03627 MT
No comments:
Post a Comment