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«1 UNIT Non - Mulbery Silkworms Structure 1.1 Introduction. 1.2 Distribution. 1.3 Salient features of non-mulberry silkworm Learning Objectives Afteer ...»

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1

UNIT

Non - Mulbery Silkworms

Structure

1.1 Introduction.

1.2 Distribution.

1.3 Salient features of non-mulberry silkworm

Learning Objectives

Afteer studying this unit, the student will be able to know

• Importance of non-mulberry silk worms.

• Study of distribution of non- mulberry silkworms.

• Study of life cycle of non- mulberry silkworms.

1.1 Introduction

Sericulture refers to mass scale rearing of silk producing organisms in order to obtain silk from them. There are five major types of silk of commercial importance, obtained from different species of silkworms which in turn feed on a number of food plants. India is unique in its biodiversity and bounty of nature in having varieties of silk. Except mulberry, other varieties of silks are generally termed as non-mulberry silks, namely: Tasar, Muga, Eri etc where these silkworms thrive on nature grown host plants. These groups have been now given a new identity and are collectively known as “Vanya silks”.

In recent times, Vanya silks have assumed more importance in view of the scope for the transformation of this age- old tradition into industry with 2 Sericulture immense potential. The country has rich natural resources and manpower and the challenge is to utilize these to bring about a balanced development without much disturbance to the forest ecology, traditional culture and the way of the way of life of the primary producers. Vanya silk culture is neither detrimental to the food plants available in forests, nor disturbing the forest ecology. When Vanya silkworms are reared on the food plants, they feed on the leaves and the litters spread in and around the plant, resulting in effective nutrient recycling.

1.2 Distribution Tasar : China is the largest producer of tasar followed by India. Tasar producing states in India are Jharkand, Chattisgarh contributing 70% of the production, Andhra Pradesh, Orissa, West Bengal in appreciable quantities, while U.P, Maharastra, Bihar and Madhya Pradesh are minor producing states.

In Andhra Pradesh tasar growing districts are Adilabad, Karim Nagar, Warangal, Khammam and East Godavari.

Muga : Muga production is confined mainly to the state of Assam to 95%, as it is an integral part of traditional culture of that state. To a small extent it is seen in Meghalaya.

Eri : The states of Assam, Nagaland, Meghalaya and Manipur contribute to 98% of countries production. Other states practicing in small scale are Arunachal Pradesh, Uttar Pradesh, Bihar, Orissa, West Bengal, Andhra Pradesh and Tamil Nadu. (Fig. 1.1) Fig. 1.1 Distribution Paper - I Rearing Requirements And Silkworm Pathology 3

1.3. Sailent features of Non-mulberry silkworms Tasar : The word tasar, apparently derives from the Sanskrit word Trasara {shuttle}. Tasar silk is mentioned in literature dating back to 1590 BC.

The Indian tasar silkworm, Antheraea myleTta is a natural fauna of tropical India. Wide distribution and Polyphagous of this insect species had resulted in extensive variation in the population. As high as19 ecoraces have been reported in this species which feed Terminalia species and Shorea robusta and also on number of secondary food plants.

The ecoraces are uni, bi or trivoltene depending upon the geo-ecological conditions and differ from each other in qualitative and quantitative traits. Tasar cocoons are reported to be largest among the silk producing in the world. Tasar silk fiber has its own distinctive color, is coarse to feel but has higher tensile strength, elongation than mulberry silk fiber. These properties have made tasar silk a competent and desirable as mulberry silk.

Muga : Muga silkworm is one of the economically important wild silk moths whose genome is among the least understood, is unique among saturnidae moths. Native of Assam and named after Assamese word “muga” which indicates the amber (brown) color of cocoon.It belongs to same family as Tasar. It is popular for its natural golden color, glossy fine textures and durability. A. assama is an endemic species prevalent in the Brahmaputra valley and adjoining hills. It is a polyphagous insect, which feeds on leaves of Som, Soalu and other plants.

By virtue of the narrow ecological distribution of host food plant, A. assama is confined to only Assam state of India. Empirical observations show that the population is declining due to depletion in genetic variability. The silk proteins of this species have not been studied so far despite their unique properties of providing golden luster to the silk thread. The popular items made from this silk are ‘dhothi’,’chaddar’,’chapkan,’ ‘pugree’and mekhala.

Eri : The name eri derives from the Assamese word ‘era’, which means castor-oil plant, the main food plant of this silkworm. Samia cynthia ricini a multivoltine silkworm commonly called as ‘eri silkworm’ is known for its white or brick-red eri silk.

The primary food plant of this polyphagous insect is castor (Ricinus communis L.), but it also feeds on a wide range of food plants such as Heteropanax fragrans, Manihot utilissima, Evodia flaxinifolia, Ailenthus gradulosa etc. The wild S. C. ricini silkworm completes one to three generations per year depending on geographical position and climatic conditions of the region, however, up to six generations occur in the domesticated cultures. Populations of S.C.ricini, that have been commercially exploited and are present in different regions of north-east India show wide morphological and quantitative variations 4 Sericulture in characters such as silk content, larval weight, cocoon weight, cocoon shell weight and silk ratio. Eri silkworms were successfully acclimatized in America and Europe, but could not take firm hold.





1.3.1 B. Life cycle of Tasar Scientific name : Antherea mylitta.

Food plants Primary : Terminalia tomentosa, Terminalia arjuna.

Secondary : Terminalia catappa, Zizyphus jujube.

Egg : The egg is oval, dorsoventrally symmetrical along the anterio posterior axis, about 3mm in length and 2.5 mm in diameter. It weighs approximately 10mg at Ovi-position and it is dark brown in color. Two brownish parallel lines along the equatorial plane of the egg divide the surface into three zones, disc, streak and edge. Eggs undergo incubation for 3-5 days.

Larva : The larva is typically cruciform and has a hypognathous head with biiting and chewing mouth parts. On hatching it is dull brownish yellow with black head. The body normally turns green and the head brown after 48 hours, but also yellow, blue and almond colored larvae are seen occasionally. The size and weight at maturity are about 13 × 2.1 cm and 50gms.The larva passes 5 instars within a period of 26-28 days for I crop, 42-45 days II crop, 55-60 days III crop.

The prothoracic hood of the first instar larvae dorsally bears an oval black spot, which early in the second instar becomes M-shaped and then, later on V-shaped with two dots. These marks are absent in the third instar, but reappear in the fourth and fifth instars as two semi lunar red markings.

The anal flap bears a triangular black mark early in the first instar, which becomes V-shaped and brownish from the second instar onwards. The triangular mark on each of the claspers is black in the first two instars and brown in the succeeding instars.

Early in the first instar the larvae have a black mid- dorsal line extending from the first to the seventh abdominal segment and a dumb-bell shaped black mark on the eighth segment. Laterally, the first seven abdominal segments have a pair of black vertical lines, and on the eighth abdominal segment there is a single oblique line. The vertical lines are replaced by V-shaped marks late in the first instar. Each of the abdominal feet bears a horizontal black mark. All these markings disappear in the later stages of development. First to fifth instar larvae have a redmid-ventral line extending the length of the abdomen.

Paper - I Rearing Requirements And Silkworm Pathology 5 Third-instar larvae have a yellow lateral line extending from the second to the tenth abdominal segment. This line is bordered by a brown upper line in fourth and fifth-instar larvae.

Tubercles : The body of silkworm larvae has prominent protrusions or out growths on thoracic and abdominal segments. These out growths are called tubercles. They work as sensors for changes in environment related to temperature and humidity. Based on the position five types of tubercles are recognized.Dorsal (DT), upper lateral (LT), lower lateral (LLT) and caudal (CT). They are black in the first instar, orange red in the second and violet in the third to the fifth instar.

Hairs and Setae : The larval body is covered by hairs and setae. The hairs are white, minute and irregularly distributed over the body. The number and arrangement of setae vary according to the type of tubercle, body segment and age of the larvae. Generally, the total number of tubercular setae remains constant until the third instar and diminishes thereafter.

Shinning Spots : Silvery white lateral shinning spots, either oval or triangular, appear during the third instar at the foot of upper lateral tubercles of the second to the seventh abdominal segment. Six regular and thirty-eight irregular patterns have recorded. Plain larvae are also common. Other shinning spots are present at the base of the dorsal tubercles. These are brick red in green and yellow larvae and white in blue and almond-colored larvae.

Sex markings : The sex markings appear late in the fifth instar as milky white spots on the ventral surface of the eight and ninth abdominal segments.

Pupa : The pupa is obtect, having a well-defined and segmental body.

It is dark brown in color and weighs 10 to 12gms.

Cocoon : The cocoon is single shelled, pendent, oval, closed and reelable, having a hard non-flossy shell with fine grains. At the anterior end, there is a well- formed dark brown peduncle with a ring at the distal end. The cocoons are generally yellow or grey. The females spin larger cocoons than the males.

Moth : The moths exhibit distinct sexual dimorphism. The females are bigger (4.5 cm), with a distended abdomen and narrow bi pectinate antennae (1.5 cm long). The males are smaller (4.0 cm), with a narrow abdomen and broad antennae. The females are polymorphic in color, being grey and yellow, whereas the males are brown.

There are two pairs of wings attached to the middle and hind thoracic segments which differ in size. Each wing carries a vividly colored eye like patterned referred as ocellus. The colored core of the ocellus is called the hyaline area.

6 Sericulture Life cycle of Tasar

–  –  –

Rearing Tasar silkworm being wild semi domesticated, polyphagous, their rearing is much different from the rearing of Bombyx mori.while mulberry silkworm is reared indoor, tasar silkworms are reared outdoors. Out door rearing causes 50-55% [during early instars]loss due to diseases and pests, predator. since the larva are exposed to natural conditions, the fate of the crop largely depends on choice of rearing site and food plants, brushing supervision and maintenance of larval population and other rearing operations. Tasar silk worms are mostly bi voltine or trivoltine. Bivoltine crops have the advantage of producing better quality cocoons per dfl due to the mild climatic conditions prevailing during that period.

The cocoons are collected from the branches of food plants and marketed. The produce is measured in” Kahans” [1Kahan=1280cocoons] and sold in numerical lots, which are as follows.

Ganda[4cocoons],Pan [80cocoons] Kahan(1280 in Bihar,1600 In orissa,1000 in M.P), Khandi(4000 in Maharastra).

–  –  –

Larva : The larva undergoes four moults and passes through five instars within a duration period of 26-28 days in summer, 50-65 days in winter.

The newly hatched larva is characterized by prominent black inter segmental markings over the yellowish body with brown head. After the first moult the body turns green, while head remains brown. At maturely The body weighs 15gms.

In the third instar, the prothoracic hood marking consists of two prominent rectangular black marks which in the subsequent instars; these are replaced by a pair of semi lunar deep brown markings.

The anal flap carries posteriorly, a rectangular black mark in the third instar. In the fourth instar, it becomes U-shaped, with the two arms joining the lateral line, and in the fifth instar, it changes to V-shape with a black inner and a deep brown outer border.

The dorsal, upper lateral and lateral tubercles (DT, ULT and LT) are bluish and lower lateral tubercles are brown in the third instar. However, the first three change to brick red subsequently. The larval duration period in summer is 28-35 days whereas in winter 50-65 days.

Pupa : The pupa is copper brown in color and weighs about 5.7gms.

Cocoon : The cocoon is single shelled, light brown, oblong, closed reelable and slightly flossy with a weak peduncle and weighs about 6.3 gms with a soft shell about 0.5 gms. The cocoon is golden brown or glossy white in color.

–  –  –

Moth : The moths exhibit distinct sexual dimorphism. The female moths are larger than males. The fore and hind wings are brown, rarely with a pinkish tinge. The females have larger wings than the males and the ocellus of the female moth has a strikingly reduced hyaline area, which in fore wing is almost dot like Paper - I Rearing Requirements And Silkworm Pathology 9 and the hind wing a horizontal slit. The area of the ocellus is greater in the hind wings.

–  –  –

Rearing : Muga silkworm is a multi voltine species with 5-6 generation in a year reared in out doors. Based on the assamese calendar the different generations are termed as follows jarua, chotua, jethua,Aherua.Bhodia and Katia.Of these six generations only autumn(kotia) and spring(jethua) are more favorable for commercial rearing. The other seasons are generally useful for seed stock maintenance and multiplication.



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