sericulture boost to the sericulture industry in the developing countries, e.g., india and south...

2Sericulture

The word ‘sericulture’ derive from the Greek word ‘Sericos’ meaning ‘silk’ and ‘culture’ meaning ‘rearing’. Thus, sericulture is the art and technology of raising silk worms for production of raw silk yarn. From the view point of its economic and religious signifi cance it is referred as “queen of textiles”. Sericulture was introduced to China by the Queen Hoshomin. For a long time, it was kept as a national secret by the Chinese Government. Later, it was introduced into Europe and Japan and into India through Korea, Japan and Tibet about 400 years ago and till 1857 silk industries fl ourished as an agro-industry. It faced a havoc loss due to outbreak of Pebrine disease during 1857 to 1895 which gradually regained its position but it faced fi erce competition from advanced sericulture countries, such as Japan, China and European countries. After the Independence, the industry is fl ourishing as an agro-industry. Since the beginning of this industry, China has been occupying the fi rst position in the world. Up to 1986, Japan, was the premier silk-producing country next to China but owing to its recent industrialization, high cost of labour, shortage of land available for mulberry cultivation, heavy internal consumption showed limitations in increasing its production. Gradually, it became an importer of silk. This situation has given a boost to the sericulture industry in the developing countries, e.g., India and South Korea and at present India is occupying the second position from the viewpoint of yarn production in the world. At present it is the largest agro-based cottage industry giving employment to over 3.5 million people. India’s silk production stood at 19,600 tonnes in 2010 against the demand of 28,000 tonnes. Karnataka accounts for 38% of the nation’s produce. India stands second in the world raw silk production contributing to around 15% of the global raw silk production trailing far behind China that produces 98,620 metric tonnes annually accounting to around 81% of global silk production.

2.1 CHARACTERISTIC FEATURES OF SERICULTURE INDUSTRY

(i) Sericulture is a farm based, labour-intensive and commercially attractive economic activity falling under the cottage and small scale sector. It is the oldest agro-based small scale industry.

Sericulture 73

(ii) It provides employment that has positive impact on the employment generation in mitigating unemployment problems.

(iii) It is mainly labour-based job hence, educationally/fi nancially backward sections can have opportunities for earnings.

(iv) It yields maximum fi nancial return with minimum investment so in comparison to other crops it is highly profi table.

(v) It doest not require much land, money and recurring investment. (vi) By culturing multivoltine varieties 4–5 crops/yr can be harvested which has direct

impact on the unemployment problem. (vii) For this industry no well developed infrastructure is required, even the mulberry

plants can be grown on barren non irrigated land. (viii) It indirectly helps to boost up the economy of the country by earning foreign

currency.

2.2 ROLE OF SERICULTURE IN THE UPLIFTMENTOF RURAL ECONOMY

In developing countries, e.g., India, agriculture and agro-based industries play a vital role in the improvement of rural economy. Limited availability of land, and cash and agriculture being confi ned to one or two seasons in the year, have made villagers to look for other supporting rural industries. Thus, agriculture and sericulture are adopted simultaneously by the agriculturists in regions where the ecological conditions are favourable. (i) In drought-prone areas where other crop production is impossible, in those areas

also sericulture can be a lucrative crop for employment generation and fi nancial support.

(ii) In tribal belts where in spite of no labor problem, other crops are not produced, sericulture can be practised successfully.

(iii) Aged people, women untrained and even physically handicapped persons can also be employed in this industry.

(iv) In comparison to other industries it is highly profi table. From one bigha land 130 kg cocoon can be harvested and if it is repeated six times/yr then a small family can earn about ` 90,000 per year.

(v) The by-products of this industry can be used as fuel, food of the cattle, manure and even the pupae are used for oil extraction as well as food of the cattle.

(vi) From the outer portion of the stem of mulberry cotton like fi bre is made used as artifi cial leather for the production of net and strings.

(vii) From the unused shoot of mulberry polyphenol like substance is produced which is highly benefi cial for the growth of hair.

74 A Textbook of Economic Zoology

2.3 TYPES OF SILKMOTHS AND THEIR HOST PLANTS

Of the total demand of silk, a major share is produced by mulberry silkworms – Bombyx mori. It is reared extensively in the states of Karnataka, WB and J&K. About 85% of the country’s production is contributed by Karnataka by rearing multivoltine hybrids of silkworm and this activity enables the sericulturists to harvest 5–6 crops a year. J&K, owing to its salubrious climate during autumn and spring, is producing silk by rearing univoltine silkworms. Other states, namely AP, Assam, TN, UP, HP and Punjab, contribute roughly 1.8 % to the total production of mulberry silk in India.

Table 2.1 Types of silkworms and their host plants

Silkworms Host plantsMulberry silkmoth: Bombyx mori, (B. textor, B. nistri, B. fortunatus, B. meridionalis, B. sinensis)

Mulberry leaves: Morus alba, M. indica, M. laevigata

Tasar silkmoth: Antheraea mylitta, A. proyeli and A. pernyi

Arjuna (Terminalia arjuna, T. tomentosa), Sal (Shorea robusta), Ber ( Zizyphus jujube).

Muga silkmoth: Antheraea assama Som (Machilus bombycina) and Soalu (Litsea polyandra)

Eri silkmoth: Phylosamia ricini Castor (Ricinus communis,) Papaya (Carica papaya), Heteropanax, Manihot, Plumeria, etc.

Fig. 2.1 Different types of Silkmoths.

2.4 VOLTINISM OF BOMBYX

It is a term used to indicate the number of broods or generations of an organism in a year. The term is particularly in use in sericulture, where silkworm varieties vary in this aspect. The number of breeding cycles in a year is under genetic control in many species and they have been evolved in response to the environment. Following varieties of silkworms are found on the basis of voltinism.

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UnivoltineReferring to organisms having one brood or generation per year. Their larvae are of robust size and consume much more food. These produce larger sized cocoons having 200–300 mg shell weight. Such cocoon yields 800 – 1200 m silk. They show diapause.

BivoltineReferring to organisms having two broods or generations per year. Their larvae are comparatively of moderate size. Shell weight of the cocoon is 150 – 200 mg. They yield 600 – 800 m silk.

MultivoltineReferring to organisms having more than two broods or generations per year. Their larvae are comparatively of small size. Shell weight of the cocoon is 100 – 150 mg. They yield 300 – 400 m silk.

2.5 MOLTINISM IN BOMBYX

The larval phases are called instars. They moult several times to attain pupa. Variation in the time of moulting is called moultinism. Following races of Bombyx are found on the basis of moltinism: (i) Trimoulters: Their larvae moult three times in their larval period. The weight of

cocoon, shell ratio, the length of silk obtained from their cocoon is much less. (ii) Tetramoulters: Their larvae moult four times in their larval period. The weight of

cocoon, shell ratio, the length of silk obtained from their cocoon is comparatively better.

(iii) Pentamoulters: Their larvae moult fi ve times in their larval period. The weight of cocoon, shell ratio, the length of silk obtained from their cocoon is of higher quality.

2.6 RACES OF BOMBYX ON THE BASIS OF THEIR ORIGIN

Four races of Bombyx are found on the bases of origin: (i) Japanese races: Uni- or bivoltine, produces green, yellow or white coloured cocoon,

larval phase is prolonged, silk is thick, of short length and are better adapted in unfavourable conditions. They usually produce double cocoons.

(ii) Chinese races: Uni-, bi- or multivoltine; larval growth rate high, feeding rate is high, cocoon is oval, white or golden, yields much longer fi ne silk with less diameter.

(iii) European races: Univoltine, eggs are larger, cocoon is long, or oval, white or yellow coloured, yield much longer silk. Larvae are with higher feeding rate, larval phase is prolonged, can’t endure higher temperature and humidity.


(iv) Indian races: Multivoltine, takes less time to complete life cycle; cocoon is small, elliptical, yellow or green coloured and yields silk of considerable length.

2.7 LIFE CYCLE OF BOMBYX

Among different silkworms Bombyx is only domesticated and are reared in culture rooms. Karnataka, where the temperature ranges from 16–31°C, enjoys favourable climatic conditions for rearing Bombyx throughout the year, while in W.B. the multivoltine silk-worm rearing is practised even under adverse conditions of temperature. In J&K, the rearing of silkworms is practised only during May-June. Bombyx is a holometabolous insect. It shows four stages in its life cycle – adult, egg, larva and pupa.

Adults(i) Body is divided into head, thorax and abdomen. Whole body is covered with fi ne scales. (ii) They are 25 mm long and 50 mm in width. (iii) Head bears paired compound eyes, antennae and reduced mouth parts. (iv) Thorax shows pro-, meso- and metathorax. Each segment bears paired legs. Meso- and metathorax bear paired wings. (v) Abdominal segments bear lateral stigmata. (vi) Abdomen shows eight narrow segments in case of male and seven swollen segments in case of females. The last segment is modifi ed to form reproductive organs. In case of males, on each side of the penis there is a hook called herpes. In case of females at the ventral side of 7th and 8th segments there is genital aperture to which ovipositor is attached. Genital papilla occurs in the last abdominal segment.

Fig. 2.2 Adult male and female of Bombyx.

EggsOn the basis of hatching two types of eggs are found – normal egg and diapausing eggs. Normal eggs are light yellow or white coloured and complete embryonic development within 10 – 12 days while hibernating eggs are blackish brownish or copper coloured, they undergo diapause up to eight months. Usually, eggs stick on the egg card but some varieties produce non-sticky eggs. These are called loose eggs.

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Fig. 2.3 Structure of egg in Bombyx.

Structure of Egg(i) Univoltine/bivoltine moths produce 500 – 675 eggs having 0.06 mg weight and 1.075 specifi c gravity while multivoltine moths produce 350-400 eggs having 0.5 mg weight and 1.064 specifi c gravity. (ii) Each egg measures 1.3 mm long, 1 mm wide and 0.5 mm thick. (iii) Ventral surface of the eggs is convex and dorsal side is slightly fl at. (iv) It is externally covered by a thick shell or chorion that is beset with 5,000 – 10, 000 pores. (v) Beneath the shell there is a fi ne vitelline membrane. (vi) At the anterior side of the egg there is a fi ne aperture called micropile. (vii) Nucleus occurs at the anterior side beneath which occurs the yolk encircled by a fi ne fi lm of cytoplasm called periplasm.

Larval StagesWithin 10–12 days larva hatches from the eggs. In tetramoulters, larvae moult four times to complete larval stages. From fourth instar onwards, sexual dimorphism becomes prominent. In male larvae, in the ventral aspect, in between 8th and 9th segments there is white gland called Herold’s gland while in female larvae in both 8th and 9th segments there are two pairs of Ishiwatta’s gland.

Table 2.2 Characteristic features of silkworm larvae

Larval instars Characteristic features Leaf consumption/400 DFLs Moulting durationIst instar larvae 0.67 cm long, lasts for 3-4 days 2–4 kg 20 hrs2nd instar larvae 1.15 cm long, lasts for 3 days 4–8 kg 20 hrs3rd instar larvae 2.75 cm long, lasts for 4 days 30–40 kg 24 hrs4th instar larvae 4.78 cm long, lasts 4-5 days 80–90 kg 24 hrs5th instar larvae 10 cm long, lasts for 6-10 days 600–650 kg 48–72 hrs

Structure of Fift h Instar Larva (i) Light grey coloured larva whose body is divided into head, thorax and abdomen. (ii) Head consists of six segments showing 3 pairs of simple eyes, two antennae and

mouth parts (one labrum, two mandibles, two maxillae and a labium).


(iii) Thorax is three segmented – pro-, meso- and metathorax. (iv) Dorsal part of meso- and metathorax are hump like. Mesothorax bears a dorsal

eye spot. (v) Each thoracic segment bears a pair of legs showing coxa, trochanter, femur, tibia

and tarsus which ends in claw. (vi) Abdomen is 11 segmented of which only 9 are visible because 9th, 10th and 11th

segments collectively form 9th segment. (vii) 3rd, 4th, 5th and 6th abdominal segments bear paired lateral muscular projections

called pseudolegs/prolegs which end with a circular disc containing curved hooks. (viii) Dorsal part of 8th segment bears a mid dorsal anal horn and 9th segment bears

two ventral projections called caudal legs. (ix) Three thoracic and eight abdominal segments bear paired lateral spiracles. (x) Last abdominal segment bears anus.

Fig. 2.4 Different life cycle stages of Bombyx.

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Prothorax

MesothoraxMetathorax Abdomen Caudal Horn

HeadLegs Spiracle Pseudolegs Caudal leg Anus

Male

Ischiwata’sgland

VIII

IX

Herold’sgland

(B)Female(A)

Fig. 2.5 Structure of fi fth instar larva of Bombyx (A) and sexual dimorphic featuresof the mature larvae (B).

2.8 STRUCTURE OF SILK GLAND

These are transformed labial glands, situated on the ventrolateral sides of the mid gut. The structure of the silk gland can be clearly understood in the third instar larvae after which it becomes so toturous that the morphology of the gland becomes completely obscured. The whole gland is divided into anterior, middle and posterior portions. The anterior portion shows fi ne twisted ducts that unite anteriorly and open at the base of the labium through a fi ne needle-like spinneret. On each side of the junction of the anterior ducts there is a gland called Filippi’s or Lyonnet’s gland. Its secretion perhaps increases the lustre of the silk fi bre. The middle portion is the largest and thickest portion showing three fl exions. The anterior and posterior parts of the middle part are

Fig. 2.6 Silk gland of third instar larva of Bombyx.


comparatively narrower than the middle part. The middle part leads into thin coiled posterior portions that extend up to the end of the intestine. The wall of the silk gland shows three layers outer tunica propria, middle glandular layer and innermost tunica intima. From the posterior portion of the gland, fi broin, the outer layer of the silk fi bre is secreted. It remains stored in the middle part for maturation. Sericin forming the inner core of the silk fi bre is secreted from the middle portion.

2.9 STRUCTURE OF PUPA

(i) Dark brown coloured body divided into head, thorax and abdomen. (ii) Head is hidden at the ventral side of the thorax and consists of two compound eyes, antennae and ill developed mouth parts. (iii) Thorax is large with two pair of wing pads and three pairs of legs. (iv) Abdomen is 11 segmented though only 9 segments are externally visible. First abdominal segments bear lateral paired spiracles. Pupae exhibit sexual dimorphism. Male pupa is smaller in size and shows a point-like structure in the ventral portion of 9th segment. Female pupa is larger in size and shows a ‘X’ shaped structure in the ventral portion of the 8th segment.

Fig. 2.7 Male (A) and female pupa (B) of Bombyx.

2.10 MULBERRY SILKWORM REARING

Bombyx is the only domesticated silkworm. For their rearing a rearing house is constructed and it needs many appliances.

Rearing House(i) Rearing house should be built depending on the brushing capacity and the method of rearing. The rearing area of 2 ft2/dfl for fl oor rearing and 3 ft2/dfl for shoot rearing is the general criteria. 480 ft2 area is required for rearing 100 dfl s. (ii) The rearing house should have suffi cient number of windows to permit cross ventilation. (iii) It shouldbe well high (9’ – 10’) from the ground. (iv) It should be surrounded by open verandah (7’ wide). (v) Doors and windows should be made up of glass. Its roof should be made up of straws and wood. (vi) The number of ventilators should be of considerable number. (vii) The house should be divided into fi ve rooms; the fi rst room - chawki room

Sericulture 81

(10’ × 14’) should contain 1st, 2nd and 3rd instar larvae, the next room (12’ × 16’) should contain 4th and 5th instar larvae, the 3rd room can be used as laboratory and the 4th room should be used for leaf preservation. Before entering the fi rst room there must be an anteroom (8’ × 8’). (viii) Temperature and humidity of the house must remain under control. (ix) Rearing house has to be built in such a way to provide optimum temperature of 26–28 ºC and RH of 60–70% for the growth of silkworm at minimum operational cost. In tropical climate the house should face east-west while in temporal climate it should face north-south direction. (x) Doors and windows must be protected by fi ne net to check pest infestation. (xi) Provision should be made to make it airtight for proper disinfection. (xii) Rearing house must avoid damp condition, stagnation of air, direct and strong drift of air, exposure to bright sunlight and radiation. (xiii) Growing trees around rearing house help to maintain favourable environment. (xiv) Rearing house should have facilities for disinfection, washable fl oor, etc.

Fig. 2.8 Life cycle of Bombyx.


Fig. 2.9 Rearing house of Bombyx silkworms.

Rearing AppliancesFollowing appliances are required for rearing of Bombyx larvae: These may be non- recurring or recurring items.

Non Recurring ItemsRearing trays: Bamboo made round or square trays are used for rearing of the larvae. For rearing of 1st, 2nd and 3rd instar larvae the measurement of the trays for 100 larvae should be 0.9/0.7 m and 7.5 – 15 cm deep. These trays are kept in stands made up of bamboo or wood. Each stand can accommodate 16 trays. In between two trays there is 20 cm gap. Each room can accommodate 6 stands.

Foam pads: For maintaining humidity, the chawki rearing trays are encircled by 2.5 cm thick rubber ribbons.

Feathers: For transferring the newly hatched larvae white feathers are used.

Chopsticks: For transferring the 1st and 2nd instar larvae 17.5–20 cm long fi ne bamboo sticks are used.

Chopping board: For chopping the leaves 0.9 × 0.9 m wooden boards are used.

Knives: For cutting the leaves 0.3 – 0.5 m long knives are used.

Mats: For keeping the chopped leaves 1.2 × 1.8 m mats are used.

Leaf chamber: 1.5/0.9/0.8 m wooden chambers are used for keeping leaves for different instars. It is covered with jute bags for keeping proper humidity in summer.

Nets: Larvae are reared on trays over cotton or nylon nets which is used for bed cleaning.

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Mountage: It is a bamboo plate made structure showing spring-like arrangement (having 5–6 cm gap) to which mature larvae are transferred for cocoon formation. It is called chandika. 0.12 m space is provided for 50 larvae.

Feeding stands: For keeping the trays during bed cleaning, these stands are used.

Ant wells: For protection against ants, the stands are kept on grooved (2.5 cm deep) ant wells containing water.

Egg transportation box: For carrying the purchased eggs from the egg selling centres specifi c boxes are used to keep the eggs undisturbed. After eggs are transported the center, these are kept in incubation chamber and just prior to hatching these are transferred to black box for undisturbed hatching.

Fig 2.10 Different rearing appliances.


Other accessories: For keeping humidity, hygrometers; temperature, room thermometer; for bringing leaf, basket; for disinfection, disinfection mask and sprayer; for egg production, egg card and cellulae; are used, earthen pot, plastic basin, buckets, mug, etc. are required.

Recurring ItemsParaffi n paper: For chawki rearing, each tray is covered dorsally and ventrally by paraffi n papers.

Disinfectant: Formalin (40%), bleaching powder, lime powder, bed disinfectants.

Gunny cloth: For carrying leaves bags made up of gunny cloths are used.

Rearing of SilkwormsHybrid varieties: CSR & TI have evolved some high yielding hybrid varieties that are recommended for different states suitable for different seasons.

Table 2.3 State-wise and season-wise suitability of hybrid silkworms

Hybrids Suitable Season Suitable Region1. P2D1 × NB18 Winter, Spring Andhra Pradesh, West Bengal, Odisha,

Madhya Pradesh2. MY1 × NB 18 Spring/Autumn West Bengal, Assam, Bihar3. N × (NB 18 × P5) Autumn, Summer/Autumn West Bengal, Odisha, Madhya Pradesh4. SH6 × KA Spring/Autumn/Winter West Bengal, Odisha, Madhya Pradesh5. CA2 × NB4D2 Spring/Autumn/Early Winter West Bengal, Odisha, Madhya

Pradesh J & K/Uttar Pradesh6. PM × NB 18 Summer Odisha, Madhya Pradesh7. NB18 × P5 Winter Odisha, Madhya Pradesh8. YS3 × SF19 Spring J & K/Uttar Pradesh9. SH6 × NB4D2 Spring J & K/Uttar Pradesh

10. PAM101 × NB4D2 Autumn/Early Winter J & K/Uttar Pradesh11. CC1 × NB4D2 Autumn/Early Winter J & K/Uttar Pradesh12. PAM111 × SF19 Autumn/Early Winter J & K/Uttar Pradesh13. RD1 × NB18 Summer/Early Winter Uttar Pradesh

Types of RearingRearing of silkworm is mainly of two types: (a) Chawki rearing: Rearing of young age silk worms is called chawki rearing.

Here worms are reared up to third moult and distributed to the rearers for late age rearing. Chawki rearing is quite troublesome because young age larvae are vulnerable to pathogens. To avoid this risk, at each government rearing centres, 1st – 3rd instars larvae are reared with great care and after that these larvae are handed over to the rearers.

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(b) Late age rearing: Late age rearing after the 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.

Various Steps of Rearing are as Follows(a) Collection and transportation of eggs: Eggs can be obtained from two sources — the rearers can produce eggs in their own house by covering the post-mated females with cellulae on a cardboard or they can purchase eggs from either government Grainage or licenced Pvt. Grainage. The second option is safer because eggs can carry the spores of Nosema, the causative parasite of the most dreadful disease, pebrine. DFL’s are transported safely in a wet hand bag in early morning or late evening so that no damage to the embryo is done. In case of loose eggs, these are carried by transportation box covered with wet cloth. During transportation DFLs should be kept at 25°C temperature and 80% humidity with proper aeration. Egg cards are spread in the rearing trays kept in cooler places only.(b) Incubation: For uniform hatching, all the egg cards or loose eggs should be kept in dark and cooler atmosphere. On the day of blue egg stage eggs are covered with a black sheet or kept in black box. On the day of hatching newly hatched larvae are exposed to bright light in the early morning at around 8 a.m. so that 95% hatching can be achieved. (c) Brushing: It involves transferring of newly hatched larvae into rearing trays. For loose eggs these are covered with a net and chopped mulberry leaf are sprinkled over the net. The larvae slowly crawl on to the net and start to feed on the leaves. Then these are transferred into the rearing trays by gently tapping the net. In case of egg cards, the cards are placed in the rearing trays and chopped leaves are sprinkled over the larvae. The larvae crawl on to the mulberry leaf and later on the cards are removed. The larvae should not be touched with hands instead chop sticks are used to spread the worms in the rearing tray.(d) Feeding of leaves: In addition to the nutritive value, the number of feeds in each instar plays a major role in the cocoon built. Three to four feeds/day are given to the silkworm of which the last feeding, i.e., during the night should be a little more since the duration for the next feeding will be longer. Two hours before and after moult no food is given. Overfeeding is also avoided. Feeding of tender leaves to young age worms is essential. Tender leaves are fi nely chopped and sprinkled on the larvae. First feeding should be given at 9 a.m. During the 1st instar 2-2.5 kg leaf per 100 DFLs are provided. As the larvae advances the mature leaves are supplied. More number of feedings is given during summer since moisture content in the leaf will not be suffi cient during summer and leaves will dry very easily. Maximum leaf consumption and growth occur during the 5th instar. About 50% of the total weight is increased in the 5th instar alone. During this stage maximum growth of silk gland occurs.


Table 2.4 Feeding schedule of the fi fth instar larvae of Bombyx mori

Sl. No. 1st feeding 2nd feeding 3rd feeding 4th feeding 5th feeding 1. 6 a.m. – 6.30 a.m. 11-11.30 a.m. 3-3.30 p.m. 7-7.30 p.m. 2. 7 a.m. – 7.30 a.m. 11-11.30 a.m. 2-2.30 p.m. 5-5.30 p.m. 3. 6 a.m. – 6.30 a.m. 10-10.30 a.m. 2-2.30 p.m. 6-6.30 p.m. 10-10.30 p.m.

(e) Spacing: During the early stages larval growth is very fast. For proper growth suffi cient spacing is required since overcrowding in the rearing tray results in increased humidity, heat, fermentation of litter which results in unhygienic conditions, wastage of leaf and improper growth of silkworm.

Table 2.5 Different developmental parameters of Bombyx mori (for 100 DFLs)

Instars No. of trays Leaf (kg) Temp oC RH -% Tenure Space needed (ft2)1st 2 2–2.5 26–28 85–90 3–4 days 1–15 ft2.2nd 2–6 4–8 26–28 85–90 3 days 15–45 ft2.3rd 5–12 30–40 25–26 80–85 4–5 days 46–90 ft2.4th 10–20 80–90 24–25 70–75 4–5 days 91–100 ft2.5th 20–40 600–650 23–24 70 6–10 days 181–360 ft2.

(f) Bed cleaning: All the leaves supplied to the silkworm may not be consumed. Dried and waste leaves in the bed, excreta of the silkworms, dead worms, diseased and putrifi ed larvae, exuviae of the worms all increase the humidity, fermentation and temperature in the bed. Thus, cleaning of the bed is done for maintaining hygiene and proper growth. Bed cleaning is done by various methods like using of paddy husk, straw and bed cleaning net. During the instar bed clearing should be done once during pre-moulting, during the 2nd instar – twice, once in pre moult and once in post moult. During the 3rd instar thrice, i.e., in post moult, in pre moult and once in the inter moult. During the 4th and the 5th instars once in a day in case of shelf rearing. However, in case of fl oor rearing or shoot rearing once in each instar.

(g) Mounting: Transferring of mature silkworm on to the mountage or cocoon frames is called “mounting”. At the end of the 5th instar these become yellowish and translucent, stops feeding, and move by raising their head. A liquid-like substance oozes out of the mouth from the spinneret. These movements clearly indicate that they are ready to spin. They are now transferred to the mountages manually. Proper spacing should be given to avoid formation of double cocoons, strained cocoons/urinated cocoons. Usually the number of worms should be 40–50 worms/ft2 or 800–900 worms/m2. The size of mountage varies from place to place. In case of bamboo made chandrika built over a mat of size 6’ × 4’, can accommodate above 1000 worms. Mountage should be

Sericulture 87

kept in inclined position so that excreta of worms falls on the ground. Different types of chandrika are used for mounting purpose. Plastic mountage, bamboo made chandrika, straw mountage, bottle brush, revolving mountage, etc.

(h) Formation of cocoon: Mature 5th instar larvae stop food intake and they become restless and move by raising their head. Then these are transferred to a cocoon forming material called chandrika which is made up of bamboo plates arranged like watch spring. Usually 0.1 m2 space is provided for 50 larvae. After transfer to chandrika they eject that last excretory substance as paste or watery fl uid. After this, the larva emits a drop of silk fl uid by which they stick their head with the chandrika. It starts to ooze out the silk by continuous movement of its head in a very specifi c manner to form the silk fi lament in the shape of ‘8’. Larvae move its head about 70–80 times per minute till the cocoon is formed and detaches itself from the last layer of silk and then transformed into pupa. The cocoon will have three layers – outer fl oss, middle compact layers and inner pelade silkworm completes the spinning in 2–4 days.

(i) Harvesting of cocoons: Harvesting of commercial cocoons is done manually on the fi fth day of spinning. Whereas seed cocoons should be harvested on the 8th or 9th day of spinning.

Cocoon quality: A series of natural circumstances will produce variations in cocoon quality. Some of the most noteworthy include: (i) Differences in cocoons produced in the same location by different farmers. (ii) Seasonal infl uences, e.g., in Japan, cocoons produced in the spring and late autumn

are higher in quality than those in early autumn and summer. (iii) Environmental conditions such as temperature and humidity affect cocoon

reelability. (iv) Bivoltine cocoons are of superior quality than cocoons from multivoltine silkworms. (v) Recent breeding operations develops cross-breeds of multivoltine with bivoltine

silkworms to improve overall cocoon quality.

Factors Infl uencing Cocoon Quality

Temperature and humidity during mounting: 25º +/– 1oC temperature and 65% relative humidity is optimum for the production of good quality cocoons with a high reelability.

Mounting device: Although different mount practices are employed among producer countries, rotary mounting frames provide good ventilation. The result is improved reelability of cocoons.

Harvesting and handling of fresh cocoons: Cocoons should be harvested only following complete pupation. In practice, the appropriate harvesting day would be the fi fth day in tropical countries, and the seventh or eighth day in temperate countries, from the mounting date. If premature harvesting takes place, the silkworm will still be in its larval stage, weigh more, have fragile skin, and could likely be crushed, which would cause stains to the cocoon during handling and transportation.


Classifi cation of CocoonsWhen cocoons are sold at the market, price is assessed on the basis of cocoon quality. This is judged by grading shell per cent, fi lament length, reelability and the percentage of defective cocoons.

Defective CocoonsDouble cocoons: A double cocoon is spun by two worms, producing a fi lament, which does not unwind smoothly and tangles easily. As these cannot be reeled along with normal cocoons, double cocoons are used for manufacture of a coarse, non-uniform, stubby yarn called “doupion”. Double cocoons may be caused by crowded mounting conditions, high temperatures, high humidity and mutation of silk species.

Inside stained cocoons (dead cocoons): In this case, the pupa inside the cocoon is dead and sticks to the inside shell of the cocoon causing a stain. Melted cocoons are called mutes because they do not make a sound when shaken. These cocoons are diffi cult to process and will result in silk, which is dull in colour.

Outside stained cocoons: These are recognized by a rusty colour spot on the cocoon shell caused by absorption of intestinal fl uid/urine of the mature worm formed during mounting. Reelability is very poor in this case.

Printed cocoons: This defect happens due to improper mounting frames. These are also called scaffold pressed cocoons.

Malformed cocoons: These are abnormal shaped cocoons, which may arise due to feeding with mulberry leaves stained with agrochemicals.

Flimsy cocoons: Here, the shell is loosely spun in layers and has a low silk content. These cocoons are easily overcooked and produce waste.

Th in-end cocoons: One or both ends of the cocoon are very thin and can be easily damaged when processed. Improper temperature and humidity during rearing and mounting may be the basic cause of this defect.

Pierced cocoons: This happens when a moth has emerged due to infestation of the beetles or any parasite. Pierced cocoons are unfi t for reeling and can be used only for hand spinning or as raw material of machine spun silk yarn.

Seed CocoonsThe journey of the development of silk worm eggs ends in the formation of cocoon around the pupa of silk moth. These cocoons are the main stages for harvesting silk or to renew the process of development through new generation silkworms. For practical purposes, thus seed cocoons and reeled cocoons are separated. The reeled cocoons are stifl ed, boiled and silk is extracted but seed cocoons are kept for the emergence of adult silk moths which are used for the beginning of new generation of silkworm development. For transportation, the seed cocoons are to be loosely packed either in perforated plastic crates or bamboo baskets during cooler hours of day.

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Before selection of seed cocoons, it is very important to know the disease freeness of a lot and good cocoons are also to be separated. The gut portion of any pupa of a lot is taken out and subjected to microscopic examination. In case there is incidence of pebrine, the lot has to be rejected. Before the arrival of new lots, it is very important and essential to disinfect the contaminated rooms. Immediately after the receipt of seed cocoons, they are to be spread on trays in a single layer to facilitate good aeration. The healthy cocoons should be preserved in trays for further processing. There should be cross-ventilation in the preservation room, 25 +/– 1°C temperature, 75 +/– 5% RH, 12 hour light and 12 hour dark conditions to be maintained in the cocoon preservation rooms. Complete darkness to be maintained on the previous day of emergence, to avoid irregular emergence of moths.

Early Eclosion/Artifi cial Eclosion/Forced Eclosion of MothsThis helps in determining the disease freeness of a batch and helps in minimizing the loss to grainage.

Early Moth Eclosion BoxA simple box made up of wood and plywood sheet with a glass door having dimension 90 × 75 × 60 cm is used. The bottom is fi tted with asbestos sheet. A heating element is connected to the electric main through a thermostat (0 – 60°C). On the top of the box at the centre, a 15 cm diameter ventilator covered with wire mesh is provided. It is fi tted with a sliding top to regulate ventilation. At the bottom of the box and on the lower portion of sidewalls, small holes are drilled to facilitate aeration. For reading the temperature, a thermometer is fi xed from inside of glass door. Within the box, a portion has been made to place 4-5 plastic trays in two tiers for keeping seed cocoon inside. For early emergence of moths, 50–60 seed cocoons are taken from individual lots and placed into an artifi cial eclosion box. The temperature in the box is adjusted to 32-33oC with the help of thermostat. This accelerates the development of pupae and moth emerges early. The early-emerged female moths of respective lots are taken and subjected for microscopic examination to know the disease freeness of lots.

Synchronization of Emergence of Moths, Pairing, Depairing and OvipositionBefore the expected day of emergence of moths, the cocoon preservation rooms should be kept dark. In case of variation in development of male or female pupae, the development of male pupae can be arrested by preserving them at 5–7°C and 75 +/– 5% relative humidity for 3-4 days. Only healthy and active moths are taken for pairing. After 1-2 hours of emergence, the male and female moths of respective combinations are allowed for 3.5 to 4 hours of pairing. At the time of depairing, the male and female moths are to be moved sideways so that the moths are separated easily without causing injury to reproductive organs. The matted female moths are taken in a separate container and induced for urination. Moths are placed on egg sheet and covered with cellules and kept in dark for oviposition. Under proper preservation (5–7°C) male moths can be


used for second pairing by giving 1-2 hours rest. Throughout the process of pairing, depairing and oviposition, optimum temperature of 25 +/–1°C and relative humidity of 75 +/– 5% should be maintained.

2.11 STIFFLING

Before marketing or reeling of the cocoons, these are stored in the grainage house. Before storage the cocoons should be subjected to hot sun or steam to kill the inside pupae because if the pupae metamorphose into adults then they emerge from the cocoon by piercing the shell. This greatly reduces the market value of the cocoons. This artifi cial killing of the pupae is called stifl ing.

2.12 REELING

The silk fi bre consists of two proteins, the inner core of fi broin (80%) and an outer covering of gum sericin (20%). Reeling involves loosening of the silk fi bres on the cocoon shell enabling easy unwinding of the silk without break. During reeling, the cocoons are processed in hot water at 95 – 97oC for 10–15 minutes. This process is called cooking. It enables the sericin portion to get softened and make unwinding easy. Cocoon cooking: It unwinds the cocoon fi lament spun by the silkworm. The sericin covering around the cocoon fi lament is agglutinated after silkworm spinning, then hardened through the cocoon drying process. In preparation for reeling, it should be softened. Processing softens sericin by heat, water and steam. Ideally, there will be uniform softening of the outer and inner cocoon shell.

Cocoon Cooking MethodsPan cooking (a) Cocoons are put into a pan of boiling water. The groping ends of the cooked

cocoon can be completed with the stirring rod in the pan. (b) The cocoons contained in the wire cage are placed into the boiling water and

then boiled for a few minutes. After boiling, the cocoons with the wire cage are moved into a low temperature bath in another pan. If there is water permeation inside cocoon shell and swelling of the cocoon shell, this work can be repeated.

Machine cookingNowadays, machine cooking is widely used in most silk reeling factories. The process is divided into six sub-phases:

(a) Soaking part: It involves dipping the outer layer of the cocoons into a water bath at 55ºC resulting in the swelling of the cocoon layer at the soaking part.

(b) High temperature and low temperature permeation part: The wet cocoons from the soaking part are exposed to steam at about 90–95ºC at an appropriate steam

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pressure. Next, the air inside the cocoon cavity is heated. When cocoons treated at high temperature permeation are moved into low temperature permeation around 65ºC, partial condensation occurs in the cocoon cavity. Then the cocoon sucks water in, evenly wetting all the layers of the shell. In this way uniform cooking of the outer and inner layers of the cocoon can be easily attained by replacing the air of the cocoon cavity with water since water transfers heat faster than air. The amount of water permeated into the cocoon cavity is controlled by the difference in temperature between high temperature and low temperature permeation parts, as well as the air permeability of the cocoon shell.

(c) Steam cooking part: Now the cocoons treated in steps (a) and (b) are moved to the steam processing. This part causes the sericin to swell and soften the silk layers and the steam to fi ll up the cocoon cavity by diffusing the permeated water out of the cocoon. For cocoons anticipated having poor reelability, the steam cooking part has to be prolonged. Sudden variations in steam pressure can adversely affect the cooking process by producing over-processed or insuffi ciently processed cocoons. These poorly cooked cocoons seriously deteriorate reeling effi ciency by decreasing raw silk yield and quality and boosting cleanliness defects during reeling.

(d) Cooking adjustment part: At this point the steam content of the cocoon cavity is replaced with water through gradual condensation of steam in the cocoon. This is effected by gradually cooling of the water from 98º to 65ºC. Sericin swollen by steam cooking becomes stable. As this step consumes large volumes of water for cocoon permeation, it needs to be longer in duration and requires more fresh water than other parts.

(e) Low temperature-fi nishing partTypes of reeling: The cocoons after cooking are reeled in hot water in different types of machines.

Country Charka SystemIn India, 61% of the silk is reeled on country-charka type reeling system. It shows the following basic parts:

(i) Mud platform: A mud platform, measuring 6-75 cm high, 120 cm long and 90 cm wide is used to operate the whole system of reeling. In front of this platform, there is a furnace, above which a metal pot is placed. In the side of these appliances there is kept a cool water pot, a straw brush and a metal ladle. The reeler sits behind this place.

In the hot water pots, several cocoons are made warm and the loose silk ends of 5 cocoons are assembled to pass through the tharpatti.

(ii) Th arpatti: Above the hot water pot, there is a metal plate like part (15 cm/5 cm) called tharpatti. It contains several apertures through which some fi ne threads


are passed to form a common thick thread. It regulates the denier of the reeled silk and removes the dirts stuck to the silk thread.

(iii) Croissure: It is a metal made part through which two unifi ed threads from the tharpatti are passed to squeeze the threads to remove excess water and tightly woven to form a unifi ed thread. This type of croissure is called Chambon type croissure.

Reel

Distributor

Thread guide

Croissure

Tharpatti

Mud platform

Fig. 2.11 Country charka reeling apparatus.

(iv) Th read guide: These are two pulley-like structures that are tightly bound with a metal rod-like distributor. It is used to pass the threads from the croissure to remove excess water and other impurities.

(v) Distributor: From the croissuer threads are passed through a hot metal rod-like thread guide to make the silk thread dry and free from sticky substances.

(vi) Reel: It is a pulley-like structure that is used to reel the threads manually.

Advantages of Country Charka Type (i) It is a traditional process which requires no special training. (ii) It is very economical because the appliances are cheap and made from the

indigenous materials. (iii) In this process even from the defective cocoons, silk can be reeled. (iv) The silk reeled in this process can be used for weaving immediately.

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Disadvantages of Country Charka Type (i) In this process high quality silk cannot be produced. (ii) The temperature of water is too high to be suitable for proper reeling.

Cottage Basin TypeThe improved cottage-basin types have several advantages. (i) Cooking and reeling pans are different and from reeling pans water is changed

time to time. (ii) An improved jettebout is used in place of tharpatti. (iii) From the jettebout, silk thread is passed through a complex croissure to obtain

silk of better quality. In this type the provision for button-hole type tharpatti and proper croissure system maintain the thickness of the fi bre and controls the defects of neatness producing of better-quality silk. The silk produced from the multivoltine races is poor in quality and have greater defects, such as lousiness, and defects in neatness and cleanliness and is of very poor quality. It belongs to D grade. The silk produced by the bivoltine races of silkworms possesses superior neatness and cleanliness, is without lousiness and has high tensile strength and stands to the international A grade.

Factors Aff ecting the Quality of SilkWater: The water used in silk reeling plays an important role. The water used for cooking cocoons should be colour less, odour less and transparent. The pH of water should be between 6.8 to 8.4. It is estimated that to produce one kg of silk from charka, 100 litres of water is required. If the quality of water is poor, the impurities suspended in the water will adhere to the silk thereby the colour, and lustre of silk would be infl uenced. Cocoon drying: The cocoons contain live pupae which will emerge out within 10–12 days of cocooning hence, cocoons are stifl ed. Once the moth emerges out, the shell becomes of no use to reel since the adult emerges out by piercing the cocoon shell. The cocoons contain live pupae, which will emerge out within 10-12 days of cocooning hence, cocoons are stifl ed. Once the month emerges out, the shell becomes of no use to reel since the adult emerges out by piercing the cocoon shell. The primary object of drying cocoons is to kill the pupae and to reduce the water content of fresh cocoons.

2.13 PHYSICAL PROPERTIES OF SILK

Silk fi bres from the Bombyx mori silkworm have a triangular cross section with rounded corners, 5–10 μm wide. The fi broin-heavy chain is composed mostly of beta-sheets, due to a 59-mer amino acid repeat sequence with some variations. The fl at surfaces of the fi brils refl ect light at many angles, giving silk a natural shine. The cross section from other silkworms can vary in shape and diameter: crescent-like for Anaphe and elongated


wedge for tussah. Silkworm fi bres are naturally extruded from two silkworm glands as a pair of primary fi laments (brin), which are stuck together, with sericin proteins that act like glue, to form a bave. Bave diameters for tussar silk can reach 65 μm. See cited reference for cross-sectional SEM photographs. Silk has a smooth, soft texture that is not slippery, unlike many synthetic fi bres. Silk is one of the strongest natural fi bres but loses up to 20% of its strength when wet. It has a good moisture regain of 11%. Its elasticity is moderate to poor: if elongated even a small amount, it remains stretched. It can be weakened if exposed to too much sunlight. It may also be attacked by insects, especially if left dirty. One example of the durable nature of silk over other fabrics is demonstrated by the recovery in 1840 of silk garments from a wreck of 1782: ‘The most durable article found has been silk; for besides pieces of cloaks and lace, a pair of black satin breeches, and a large satin waistcoat with fl aps, were got up, of which the silk was perfect, but the lining entirely gone ... from the thread giving way ... No articles of dress of woollen cloth have yet been found. Silk is a poor conductor of electricity and thus susceptible to static cling. Unwashed silk chiffon may shrink up to 8% due to a relaxation of the fi bre macrostructure, so silk should either be washed prior to garment construction, or dry cleaned. Dry cleaning may still shrink the chiffon up to 4%. Occasionally, this shrinkage can be reversed by a gentle steaming with a press cloth. There is almost no gradual shrinkage nor shrinkage due to molecular-level deformation. Natural and synthetic silk is known to manifest piezoelectric properties in proteins, probably due to its molecular structure. Silkworm silk was used as the standard for the denier, a measurement of linear density in fi bres. Silkworm silk therefore has a linear density of approximately 1 den, or 1.1 dtex.

Table. 2.6 Comparison of silkworm silk and spider silk

Comparison of silk fi bres Linear Density Diameter (μm) Coeff. VariationMoth: Bombyx mori 1.17 12.9 24.8%Spider: Argiope aurentia 0.14 3.57 14.8%

Chemical PropertiesSilk emitted by the silkworm consists of two main proteins, sericin and fi broin, fi broin being the structural centre of the silk, and serecin being the sticky material surrounding it. Fibroin is made up of the amino acids Gly-Ser-Gly-Ala-Gly-Ala and forms beta pleated sheets. The high proportion (50%) of glycine, which is a small amino acid, allows tight packing and the fi bres are strong and resistant to breaking. The tensile strength is due to the many interceded hydrogen bonds and when stretched the force is applied to these numerous bonds and they do not break. Silk is resistant to most acids, except for sulfuric acid, which dissolves it.

Sericulture 95

2.14 SILKWORM DISEASES

In spite of domestication and rearing in artifi cially controlled atmosphere silkworms are infected by many pathogens causing great loss to the industry.

Protozoan DiseasesBombyx is infected by a number of parasitic protozoans like Nosema, Pleistophora, Leptomonas, etc. But several strains of Nosema (NIK-2r, NIK-3 h and NIK-4 m) cause the most dreadful disease called pebrine. Nosema bombycis is a microsporidium. Its spores infect a larva through dead larvae, faecal matters of larvae, exuvae of larva or pupa and discarded leaf. It affects the ovary of the mature moth and is carried to the next generation through eggs.

Fig. 2.12 Spore of Nosema

Symptoms of pebrine (i) Shapes of the eggs appear different. (ii) Production of unfertilized eggs increases. (iii) Hatching of the eggs becomes irregular. (iv) The 1st instar larvae become black and die. (v) Moulting is delayed and even they may not moult at all. (vi) The 2nd and 3rd instar larvae appear rust coloured with black spots at the sides

of caudal horn or pseudolegs. (vii) Mature larvae become restless and do not form pupa. If pupate, their body

becomes soft with black spots appear on the body wall. (viii) Infected adults are scaleless and show curly wings. (ix) Inside the body, while swellings are noted and the silk glands and the glands

remain immature.


(x) The body muscles become very weak. (xi) Haemocytes burst out and the haemolymph appear thick.

Control measure (i) Freshly laid eggs are examined for the spores of Nosema. If spores are found, all

the eggs should be destroyed. (ii) If spores are found in the faecal matter of the larvae, in that case also all the

larvae should be destroyed. (iii) If the eggs are immersed in hot water or hot dilute HCl, cocoons are kept in high

temperature, and Fumagillin, Benomy or Bavistin are fed with food then Nosema infection is checked.

Bacterial DiseasesThese are the most common type of diseases. Warm and moist weather, stored faecal matter and rotten leaves, favour this type of infection. Three specifi c bacterial diseases have been identifi ed.

(a) Bacterial septicaemia: It is caused by Bacillus or Serratia sp. From the dust particles, stored leaves, or from infected appliances this bacterium enters into the body of the worms through any sores on the body wall. They rapidly multiply in the haemolymph to cause metabolic disorders.

Fig. 2.13 Symptoms of different diseases of silkworms.

Sericulture 97

Affected larvae become very lazy, show loss of appetite, liberate loose faecal matters, swollen thoracic portions and constricted abdominal segments. Dead larvae become stiff, discoloured, body wall of the dead larvae ruptures to liberate stinky liquid. Whole body becomes black and easily putrefi ed.

(b) Bacterial gastrointestinal disease: It is caused by Streptococcus sp. In unfavourable conditions, shortly after moulting the larvae are affected though contaminated food.

These larvae become lazy, short sized with reduced growth rate. Their head becomes transparent, defecates loose faecal matter. Dead larvae become black and quickly become putrefi ed.

(c) Bacterial toxicosis or soto: It is caused by Bacillus thuringensis. From the dust particles, stored moist leaves, or from infected appliances this bacterium enters into the body of the worms. Their number greatly increases in moist weather. Infection usually is noted in 5th instar larvae.

Affected larvae stop feeding and they keep their head in raising posture, show muscular tremor and dies quickly. Dead larvae become black, putrefi ed and blackish-brown fl uid is liberated from their body.

Control measures against bacterial diseases (i) Disinfection (specially the leaf reservoir) with 2% formalin after each batch of

rearing is essential. (ii) Proper aeration in the rearing room is a must.

Viral DiseasesAmong all the diseases 70–80% is viral disease. Infected leaves, high density, fl uctuating temperature and humidity, improper aeration favours viral infections. They show 4 types of viral diseases.

(a) Nuclear Polyhedrosis or Grasserie: It is caused by Nuclear Polyhedrosis Virus (NPV). The virus enters through the food and inside the gut it forms many hexagonal polyhedrons that are liberated out through the faecal matters. If it is observed in chawki stage, then these larvae must have been infected while hatching or during rearing. The disease can also spread if the rearing house is not disinfected effectively.

Affected larvae appear shinning, their intersegmental portions become swollen, body wall cracks through which haemolymph oozes out. Body wall shows sores, spiracles appear blackish, larvae become restless. The dead larvae become blackish and quickly putrefi ed.

(b) Cytoplasmic Polyhedrosis or White Flacheriae: It is caused by Cytoplasmic Polyhedrosis Virus (CPV). The virus enters through the infected leaf and from the gut it enters into the cells of the gut wall and the activity of the mid gut is disrupted. The polyhedra are liberated out through the faecal matters.


Affected larvae show reduced growth rate, loss of appetite. Body wall becomes whitish, thorax appears transparent and head is enlarged. They vomit and defecate whitish faecal matter.

(c) Infectious Flacheriae: Among viral diseases it is the most infectious and dangerous disease. It is caused by Bombyx mori Infectious Flacheriae virus. The virus enters through the infected leaf and from the gut it enters into the goblet cells of the gut wall. They multiply in these cells that results in death of the host. The virus particles are liberated out through the faecal matter.

Affected larvae show reduced growth rate, loss of appetite, irregular moulting and constricted body segments. They vomit and defecate whitish faecal matter.

(d) Densonucleosis: It is caused by Densonucleosis virus. The virus enters through the infected leaf and from the gut it enters into the goblet cells of the gut wall. They multiply in these cells that result death of the host. The virus particles are liberated out through the faecal matter.

Affected larvae appear transparent. Mid gut appears yellowish green.

Control Measures of Viral DiseasesProper disinfection, aeration, scientifi c storage of leaves are prophylactic measures. After moulting anti viral powder (1.2 gm paraformaldehyde, 1 gm benzoic acid and 97 gm lime) should be sprinkled on the body wall. Some varieties have been evolved that are resistant against viral diseases. A protein - BmSP-2, an insect digestive enzyme, has been proved to be a potential antiviral factor against BmNPV at the initial site of viral infection.

Fungal DiseasesIt is the most usual disease among other silkworm diseases. Fungal diseases are of two types – Muscardine and Aspergilosis.

Muscardine: It is caused by the fungus Beauveria bassiana. Their air-borne spores from dead larvae, faecal matters, exuvae, come in contact with the body wall and cause scars on the body wall through which they enter the body of the larvae. Infection rate is higher in chawki larvae. 10–28oC temperature and 90–100% RH and improper aeration favour their infection and it never occurs above 33oC temperature and 70% RH. Affected larvae show oily streaks on the body wall, they vomit and excrete liquid faecal matter, head becomes soft, and body of the dead larvae becomes stiff. 1-2 days after death, the body of the larvae becomes covered by soft cotton like hyphae. If affected larvae pupate, they show hyphae in the intersegmental portions.

Aspergillosis: It is caused by Aspergillus fl avus or A. oryzae. Mode of infection is as in Muscardine. Affected larvae becomes lazy, show many scars on the body wall. From the scars, many yellowish conidia emerge that covers the body causing rottening of the body

Sericulture 99

wall. Before death head and thorax enlarge and the affected larvae vomit. Affected pupae become stiff.

Botrytis bassiana: It is a fungus that destroys the entire silkworm body. This fungus usually appears when silkworms are raised under cold and high humidity. Actually, the infected silkworms cannot survive to become moths and lay eggs.

Control Measures of Fungal DiseasesDisinfection of the rearing house and appliances, maintenance of optimum temperature, humidity, proper aeration are the main prophylactic measures. Affected larvae should be immersed in lime water, or anti-muscardine powder should be sprinkled on the body wall, dead larvae should by burnt.

2.15 PEST OF BOMBYX

In addition to be affected by many pathogens Bombyx larvae are also target of some insects that act as pest. Uzi fl y (Exorista (=Tricholyga) sorbillans, E. bombycis) is considered as the major and Alphitobius, Lypropos, Nicrobie, Tribolium, Desmestes, Labia, Crossocosmina, Ctenophorocera, Pediculoids, etc., act as minor pests.

Uzi Fly [Exorista (=Tricholyga) sorbilans)]It is cosmopolitan in distribution and in every silk producing country it acts as major pest of silkworms. Its bionomics and control measures are as follows:

Adults (i) Body is divided into head, thorax and abdomen. (ii) Head shows two short antennae and compound eyes. (iii) Thorax shows two forewings and six legs. Haltere present. (iv) Abdomen is 11 segmented without any appendage. They live for 10–21 days.

Life cycle: They are holometabolous insects and life cycle shows four stages – adults, egg, larva and pupa.

Egg: The adults, within 24 hours of the emergence, mate 1-2 times and oviposit with 44-45 hours on the body wall of the worms. One fl y produces about 300 – 1,000 eggs. The eggs are white or cream coloured and measure 0.45/0.25 mm. Within 2–5 days, maggots emerge.

Maggot: Newly hatched maggots pierce the body wall of the worms and they moult twice to form the 3rd instar larva or maggot. These are 1.3–1.6 cm long, 11 segmented and grow with the expense of different organs of the worms. Within 5–8 days they come out of the body of the worm and pupate within 10–12 hours in the cracks and crevices of the rearing room.


Pupa: They measure 0.9-1.2/0.6 cm. They are light reddish brown coloured. Body shows head, thorax and abdomen. Head shows immature eyes, antennae and mouth parts; thorax shows two wing pads, six legs and a 11 segmented abdomen. Adult emerges within 10–12 days.

Nature of damage: If the 4th and 5th instar larvae are affected they die before spinning. If infection occurs just prior to pupation, maggots emerge from the cocoon so that the quality of cocoon is degraded.

Control measures (i) Protection of doors and windows by fi ne net and repairing of all cracks in the

rearing house are good prophylactic measures. (ii) Dusting the larvae with lime powder (3-4 gm/100) larvae kills the eggs. (iii) Exoristobia or Spilomicrus can affect the maggots while Nosolynx, Dirhinus and

Brachymena can affect pupae of Uzi fl y as parasites. These act as hyperparasites.

2.16 BOTTLENECKS IN THE EXPANSION OF MULBERRYSERICULTURE IN DROUGHT-PRONE LATERITE BELTS

Though sericulture is practised in defi nite room under relatively regulated conditions, yet natural environment plays a vital role in its development. It involves a combination of agricultural, biological, and marketing processes. Natural conditions have a direct infl uence on the completion of its life cycle and disease susceptibility. Mulberry

Fig. 2.14 Life cycle of uzi fl y.

Sericulture 101

cultivation is usually concentrated in the districts of Malda, Murshidabad, Bankura, Birbhum, Purulia, W. Dinajpur and Darjeeling of West Bengal. Except Darjeeling it is done on alluvial soil. Adverse temperature and humidity may affect the cultivation of mulberry. Mulberry prefers mild temperature not exceeding 25oC. High humidity is also found to be injurious for its cultivation. Therefore, mulberry was considered a plant of temperate regions. But now it thrives well in tropical and subtropical climates. Bankura and Purulia districts are considered to be drought-prone areas of West Bengal having annual rainfall 1320 mm and 1310 mm; temperature 20 – 45oC and 21 – 48oC and humidity ranges from 67% and 58% respectively. Thus, in comparison to other districts, Bankura and Purulia districts are considered to be drought prone areas. Due to climatic adverseness, these two districts have mainly shown preference to weaving than cultivation and rearing. However, these regions have developed both mulberry and non-mulberry sericulture with mulberry being limited to only 340 ha land area and non-mulberry to 1,25,000 ha land area. Thus, climatic harshness seems to be one of the main constrains for sericulture in these areas.

Role of Temperature in Mulberry SericultureThe mulberry silkworm (Bombyx mori L.) is one of the most thermal-sensitive organisms, thus temperature seems to a play a crucial role in sericulture.

Harmful eff ects (i) Temperature is the vital climatic factor. It exerts adverse role in declining soil

moisture that affects leaf production as well as moisture content of the leaf for smooth maintenance of body moisture of the larvae.

(ii) Heat shock is a thermal injury caused by increased temperature in biological molecules like DNA, RNA, lipids, etc., of the cell which are vulnerable to heat stress. This leads to a number of abnormalities at cellular level. Normal pattern of protein synthesis halts; tRNA and rRNA degrade; DNA loses ability to function properly; there is aggregation of intermediate fi lamentous proteins at the nucleus instead of forming the cytoskeleton; pH of body fl uid also drops; proteins become unfolded which may lead to inappropriate interactions with one another or with other cellular components. Once unfolded, it can interact with folded proteins and induce them to unfold. Such interactions result in aggregates of unfolded protein that is cytotoxic since it diminishes the pool of functional proteins. Increased temperature leads to increase in kinetic energy of macromolecules, decrease of ionic bonds, hydrogen bonds, van der Waals forces, etc., and increase its hydrophobic interactions. Denatured proteins also get adhered to DNA and restrict enzymatic access to DNA causing large-scale DNA damage. These ultimately lead to death of cells.

Beneficial effects: The only beneficial effect of higher temperature is the less susceptibility to fungal diseases since it adversely affects the spread of fungal spores.


Management strategy for overcoming the problems ofadverse temperature in mulberry sericulture (i) Mulberry prefers plenty supply of water during the growing season. Such perennial

supply of water throughout the year yields good leaf harvest. Water supply in summer and winter is very scanty in these areas. In these seasons average leaf yield rate is 11,400 kg/ha while it appears 15,200 kg/ha if proper irrigation is maintained. Thus, artifi cial supply of water is of utmost importance to increase leaf yield.

(ii) CSR & TI has evolved some drought-tolerant hybrid varieties like PM × NB18 and RD1 × NB18.

(iii) Bivoltine races are more vulnerable to thermotolerant than multivoltine ones. Thus, many quantitative characters decline sharply at higher temperatures. Therefore, one of the key considerations in developing bivoltine hybrids for tropics could be needed for thermotolerant bivoltine races. This could be achieved through hybridization of multivoltine with bivoltine races. In India earlier efforts in this direction using Pure Mysore, a comparatively robust but poor silk yielding Indian race did not yield expected outcome but recent advances in silkworm breeding and those in stress induced protein synthesis have opened up new avenues to evolve robust productive silkworm hybrids.

(iv) Problem of lower temperature is solved by putting artifi cial source of temperature in the culture room.

2.17 RESEARCH POTENTIALITIES TO IMPROVERACES AND VARIETIES

Mulberry VarietiesThe success of mulberry leaf production depends on three factors namely, variety, cultivation practices (agronomic characters like good rooting, fast growth, high yield, wide adaptability to environmental and soil conditions and resistance to diseases and pests are important with respect to leaf production which is the primary activity in sericulture) and plant protection measures. Besides that, climatic conditions are essential for the growth of mulberry. Different varieties require different climatic conditions. 1. Mysore local or local cultivar or Natikaddi: It is a low yielding variety known for

its adaptability to low agronomic inputs and poor management practices both under rainfed and irrigated conditions. It is cultivated mainly in Karnataka.

2. Improved cultivar or Kanva-2 (K2) or Mysore 5 (M5): It is an open pollinated hybrid (OPH) selection from the seeding population of Mysore Local variety. It grows vigorously and responds well to agronomic inputs. It can be grown under varied agro-climatic conditions with suitable system of planting. It is cultivated in almost all the Indian states and recently introduced to some south east Asian countries like Sri Lanka, Bangladesh, Philippines, Vietnam and Thailand.

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3. MR2 (Mildew Resistant Variety –2): This variety was developed by the Tamil Nadu Sericulture Department at their experimental station, Coonoor during 1970s. MR2 variety is resistant to powdery mildew disease caused by Phyllactinia corylea and is very popular in the plains of Tamil Nadu and better suited for high altitude areas also where high temperature prevails. This variety is good in rooting, leaf yield and growth. It is rated high for its nutritional level of leaves and hence susceptible for thrips attack. It yields 25,000 to 30,000 kg leaf per hectare under irrigated conditions of Tamil Nadu.

4. High yielding varieties: To boost productivity of mulberry per unit area, experiments were carried out for evolving high yielding varieties at different sericultural research stations. A few such evolved varieties suited for irrigated conditions are already under cultivation and or fi eld multiplication programmes namely, S13, S36, S54, Viswa (DD) and V1 for irrigated conditions and S13 and S34 for rainfed conditions. They yields from 40,000 kg to 60,000 kg per hectare per year. Leaf yield of S13 is 18 tonnes/ha/year. Leaf yield of S34 is about 17 tonnes/ha/year.

Silkworm RacesThe trend of sericulture development in India clearly depicts a quantum jump in mulberry silk production during the last three decades. Renditta (Quantity of Silk Cocoons required to produce 1 kg of raw silk) in 1960 was as high as 16-17, which came down to 6-7 in 2010. This was possible mainly because of advancement and improvement in mulberry genotypes and cultivation practices. Till recently, India had no system of authorisation of silkworm hybrids, which caused undue delay in release of breeds/hybrids evolved by the breeders with great caution. This also deprived industry from the benefi ts of having wider choice of silkworm hybrids for commercial exploitation. Central Silk Board (CSB) has brought out a system of auhorisation of silkworm races. The system involved 14 test centres spread in different regions of the country and test rearings are conducted twice in a year. Popular silkworm hybrids authorised by CSB for commercial exploitation and already in use are given in the following table.

Sl. No Hybrids Combinations Season States / Region1. P2D1 × NB18 Mv × Bv Winter

Spring Summer/Early autumn

A.P.W.B., Assam, Bihar, Odisha, Jharkhand, M.P., Chattisgarh /U.P., Uttaranchal

2. MY1 × NB18 Mv × Bv Spring/Autumn W.B., Assam, Bihar, Jharkhand, Odisha, M.P.

3. N × (NB18 × P5) Mv × Bv AutumnSummer/Autumn

W.B., Assam, BiharOdisha, M.P., Chattrisgarh

4. PM × NB18 Mv × Bv Summer Assam, Bihar, Odisha, M.P.


5. RD1 × NB18 Mv × Bv Summer/Early Winter U.P., Uttarakhand6. BL23 × NB4D2 Mv × Bv Spring/Autumn Rainfed Areas of Tropics7. BL24 × NB4D2 Mv × Bv Spring/Autumn Irrigated areas of Tropics8. SH6 × KA Bv × Bv Spring/Autumn

Spring/WinterW.B., Assam, Bihar, Odisha, M.P

9. SH6 × NB4D2 Bv × Bv Spring U.P., Uttarakhand, J&K10. CA2 × NB4D2 Bv × Bv Spring

Spring/Autumn/Early Winter

W.B., Assam, BiharJharkhand, Odisha, M.P.,Chattisgarh

11. NB18 × P5 Bv × Bv Winter Assam, Bihar, Odisha, M.P.12. YS3 × SF19 Bv × Bv Spring J&K, U.P., Uttarakhand13. PAM101 × NB4D2 Bv × Bv Autumn/Early Winter J&K, U.P., Uttarakhand14. CC1 × NB4D2 Bv × Bv Autumn/Early Winter J&K, U.P., Uttarakhand15. PAM111 × SF19 Bv × Bv Autumn/Early Winter J&K, U.P.16. CSR12 × CSR6 Bv × Bv Spring/Autumn Temperate & tropical zones17. CSR18 × CSR19 Bv × Bv Autumn Temperate & tropical zones18. CSR16 × CSR17 Bv × Bv Spring/Autumn Temperate & tropical zones19. CSR3 × CSR6 Bv × Bv Spring/Autumn Temperate & tropical zones20. CSR2 × CSR4 Bv × Bv Spring/Autumn Temperate & sub-tropical

zones21. CSR2 × CSR5 Bv × Bv Sring/Autumn Temperate & tropical zones22. KS01 × SP2 Bv × Bv Spring Temperate zone23. S K U A S T - 1 ×

SKUAST-6Bv × Bv Spring Temperate zone

24. APS5 × APS4 Bv × Bv Spring/Autumn Temperate & tropical zones25. APM1 × APS8 Mv × Bv Spring/Autumn Tropical zone26. BL43 × NB4D2 Mv × Bv Spring/Autumn Tropical zone

Multivoltine × Bivoltine Silkworm HybridsIn India, over 95% of the commercial silk being produced is from Multivoltine female × Bivoltine male parent (cross-breed) and the existing cross-breed Pure Mysore × NB18/NB4D2 has some limitation specially lower shell weight high renditta, low cocoon shell ratio and poor fi bre quality. In order to overcome the limitations of PM × NB4D2, new hybrids involving multivoltine breeds have been evolved. PM × CSR2 hybrid is meant for irrigated areas in all the seasons and is superior over the existing hybrid in yield, cocoon weight, Shell weight, SR%, fi lament length, raw silk%, neatness and renditta.

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Questions

Short Answer Questions 1. ‘Japan loses, China gains’. Explain in the light of sericulture. 2. What are the main characteristic features of sericulture? 3. Give a brief account on the role sericulture in the upliftment of rural economy. 4. Give an account of silkworms and their host plants. 5. What do you mean by voltinism? 6. What do you mean by moultinism? 7. What are DFLs and loose eggs? 8. Describe a mature 5th instar larva of Bombyx. 9. What are the differences between legs and prolegs? 10. Describe the silk gland of mulberry silkworm. 11. What do you mean by Chawki rearing? 12. Describe the life cycle of Uzi fl y.

Broad Answer Questions 1. Give an account of the life cycle of Bombyx mori. 2. Describe a model rearing house for silkworm rearing. 3. Describe different rearing appliances for rearing of Bombyx silkworms. 4. Describe the steps of scientifi c rearing of the silkworms. 5. Discuss different processes of silk reeling. 6. Discuss different diseases, their causal organisms, symptoms and control measures. 7. Discuss different research potentialities to improve the mulberry and silkworm

races. 8. Discuss different bottlenecks in the expansion of mulberry sericulture in drought-

prone belts.

Multiple-Choice Questions 1. Arjuna is the host plant of (a) Mulberry (b) Philosamia (c) Antheraea (d) All 2. Som and Soalu are the host plants of (a) Mulberry (b) Tasar (c) Muga (d) Eri silk worms 3. Castor is the host plant of (a) Mulberry (b) Tasar (c) Muga (d) Eri silk worms 4. In India, the leading state in silk production is (a) J&K (b) W.B. (c) A.P. (d) Karnataka


5. The mouth parts of Bombyx larva is of (a) biting and chewing. (b) piercing and sucking. (c) siphoning type. (d) sponging type. 6. The fi rst proleg in silkworm occurs in (a) second abdominal segment (b) third abdominal segment (c) fi rst abdominal segment (d) fourth abdominal segment. 7. The abdomen of silkworms show only (a) 10 externally visible segments (b) 9 externally visible segments (c) 11 externally visible segments (d) 12 externally visible segments 8. Spinneret of the silk gland opens in (a) Labrum (b) Labium (c) Mandible (d) Maxillae 9. Sericin of the silk is synthesized from (a) anterior portions of the silks gland. (b) middle portions of the silks gland. (c) posterior portions of the silks gland. (d) neither of these portions. 10. In male larvae in the ventral aspect, in between the 8th and 9th segments there is (a) Herold’s gland. (b) Ishiwata’s gland. (c) Exuvial gland. (d) Neither of these glands. 11. In female larvae in the ventral aspect, in between the 8th and 9th segments

there is (a) Herold’s gland. (b) Ishiwata’s gland. (c) Exuvial gland. (d) Neither of these glands. 12. Chawki rearing means rearing of (a) young age silkworms. (b) late age of silkworms. (c) hybrid silkworms. (d) neither of these.

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13. In chandrika, the average density of worms is (a) 40 – 50/sq. ft. (b) 50 – 60 sq. ft. (c) 20 – 30/sq. ft. (d) 30 – 40 sq. ft. 14. Stiffl ing involves (a) proper hatching. (b) proper cocooning. (c) killing of pupa inside the cocoon. (d) putting cocoons in warm water. 15. Pebrine is caused by (a) Beauveria (b) Streptococcus (c) Nosema (d) Aspergillus 16. Soto is caused by (a) Beauveria (b) Streptococcus (c) Bacillus (d) Aspergillus 17. Tricholyga sorbilans (Uzi fl y) is a (a) predator of silkworms. (b) pest of silkworms. (c) parasite in the silkworms. (d) neither of these. 18. Eggs of silkworm are stored at (a) 5oC (b) 4oC (c) 10oC (d) 2–5oC 19. All the four types of silkworms are cultured in (a) China (b) Japan (c) India (d) Korea 20. One DFLs contain (a) 200–300 (b) 300–400 (c) 400–500 (d) 400–600 21. Lifespan of adult Bombyx is (a) 2 days (b) 3 days (c) 3-4 days (d) 6 days 22. Silk fi bres are held together in cocoon by (a) Fibrin (b) Fibroin (c) Sericin (d) Casein 23. Silk is the secretion of (a) cephalic gland. (b) salivary gland. (c) gastric gland. (d) buccal gland. 24. Silk is produced by (a) Caterpillar (b) Pupa (c) Adult (d) Cocoon 25. Central Silk Board was established in Bangalore in (a) 1959 (b) 1949 (c) 1953 (d) 1956


26. Central Sericulture Research & Training Institute was established in Mysore in

(a) 1959 (b) 1949 (c) 1953 (d) 1962 27. The Silkworm and Mulberry Germplasm Station was established in 1991 at (a) Hesur (b) Ranhi (c) Mysore (d) Bangalore 28. Goldenostepkil variety of mulberry is resistant against the disease (a) leaf spot. (b) powdery mildew. (c) bacterial blight. (d) doagre blight.

Answers to MCQs

1. (c) 2. (c) 3. (d) 4. (d) 5. (a) 6. (b) 7. (b) 8. (b) 9. (b) 10. (a) 11. (b) 12. (a) 13. (a) 14. (c) 15. (c) 16. (c) 17. (b) 18. (d) 19. (c) 20. (d) 21. (c) 22. (c) 23. (b) 24. (a) 25. (b) 26. (d) 27. (a) 28. (b)

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