Silkworm Rearing

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Silkworm Rearing

Silkworm rearing is an extensive month-long exercise starting from egg stage and terminating in adults laying eggs and dying their natural death. During this course, they pass through five larval instars intervened by four moults, cocoon and pupal stage. Silkworm rearing effectively means the culturing of five larval instars as other stages like egg, pupa and adults are non-feeding stages. Whole life cycle spans through 45-55 days with 10-12 days of egg stage, 25-30 days of larval stage, 2-3 cocoon spinning days, 5-7 days as pupal duration and 4-5 days in adult stage. Prevailing environmental conditions especially, temperature and relative humidity conditions are vital in determining silkworm physiology as it is a cold-blooded organism. Hence, maintenance of recommended temperature, relative humidity (RH), light and ventilation conditions for every stage of rearing are of utmost importance for successful silkworm rearing. Dark condition, room temperature and 65% RH is required for incubation of silkworm eggs. 27-28 °C and 80-85% RH is required for first and second instar larvae (Chawki silkworm rearing), while 24-25 °C and 60-65% RH is required for third, fourth and fifth instar larvae (late-age silkworm rearing). During the intervening moulting stage of 24 hour each between two instars, temperature of 25-26 °C and RH 60% is recommended for the smooth integument change over. Room temperature and 65% RH is required during spinning, cocoon preservation, moth emergence, coupling, decoupling processes. Dark conditions with room temperature and 75-80% RH is required for oviposition process. A day in silkworm culture consists of various activities like harvesting of mulberry leaves, food preparation, feeding, bed cleaning etc. Silkworms are fed four times in a day – morning (9-10 A.M.), afternoon (1-2P.M.), evening (4-5P.M.) and night (9-10P.M.). Leaves after harvesting from plantations are first washed with plain running water and then treated with mild KMnO4 for general disinfection. After adequate drying they are fed to silkworms. During first and second instars, silkworms are fed with chopped tender and succulent mulberry leaves with high moisture content from apical portions of the plant. During third instar, 3-4 pieces of medium sized leaves are given to the silkworms. Later on, the entire leaf and complete shoot is given during fourth and fifth instar after required treatment. Bed cleaning is an important process to ensure the hygiene in the immediate vicinity of silkworms in order to protect them from infection. Four different mesh-sized bed cleaning nets are used for cleaning the rearing beds. Bed cleaning is done once in first instar, twice in second instar and preferably daily in third, fourth and fifth instar. Bed cleaning nets are spread just before the morning feed. Before the afternoon feed, nets with the silkworms are shifted to new beds and feeding is then resumed. The litter, leftover food and dead silkworm, if any, are removed carefully and disposed off away from the rearing house. Moulting is the process when silkworms seized feeding, becomes immobile and prepare themselves for shedding their old skin to accommodate the fast growth. Four moults takes place during the entire larval period. During this period, special care is required for the moulting worms. Lime powder is dusted in the rearing bed to reduce the humidity to 60-65% RH to facilitate the moulting process. Moulting period lasts for about 24 hrs and care should be taken not to disturb silkworms during this period. During late fifth instar, after completing the feeding silkworms reaches the ripened stage (ready –to-spin silk). Ripened silkworms are identified by their characteristics movement to the corners of the rearing treys, reduction in size by one-third and transparent yellow appearance. These ripened silkworms are transferred to the mountages (equipment to provide support for cocoon formation) for spinning cocoons. Plastic collapsible mountage, bamboo-brush mountage and mountage made out of locally available materials (dried leaves and branches of different plants arranged in a zig-zag manner in card-board boxes). After two-three days of spinning, cocoons are harvested from the mountages. Cocoons can be used for either propagating the generation or extraction of silk fibre. For propagating generation, cocoons are left at the room temperature and 65-70 % RH for moths to emerge from cocoons after passing through intermediate pupal stage after 6-7 days. After emergence, males and females are coupled for four-five hours, decoupled, and females are kept for oviposition process. Males can be used for second coupling after short-term refrigeration at 5 °C for 1-2 days. After this, the adult approaches their natural deaths in 1-2 days. The silk worm larvae hatched out of oviposited eggs in 10-12 days after completing embryonic growth. For extracting silk, the cocoons are subjected to the stifling process in which, the pupa inside the cocoons are killed by subjecting them to high temperature treatment via. sun drying, steam or hot air in order to maintain the continuity of silk filament making up the cocoon. Then, cocoon is boiled or cooked for 3-4 mins at 95-96 °C to make the serecin soft to dissolve upto 25-26%. Then, silk filament can be extracted out easily on suitable reeling apparatus by finding the true end in brushing process in which the coarser floss layer is removed.

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