Silk, stemming from the saliva of an insect (mainly Bombyx Mori also called mulberry silk) is a natural fiber. It is obtained from cocoons spun by silkworms. Silk is preferred over other fibers for its remarkable properties like water absorbency, heat resistance, dyeing efficiency, tensile strength, and luster.
The mulberry silkworm is being domesticated for approximately 5000 years. For that reason, they are very delicate, highly sensitive to environmental fluctuation. Therefore, any natural fluctuations in temperature, humidity, air quality, or light could affect cocoon weight, shell weight, and cocoon shell ratio.
Traditionally Mulberry silkworm-based sericulture in India is practiced in tropical environmental regions such as Karnataka, Tamil Nadu, Andhra Pradesh, and West Bengal and to a limited extent in the temperate region of Jammu and Kashmir.
The rapidly changing environmental conditions during the last decade demand the need for managing environmental factors like temperature, and relative humidity for sustainable cocoon production.
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Silk cocoon production is determined by various factors including the environment and genotype (genetic setup) of the silkworm. The figure describes the contribution of genetic and environmental factors on growth and development of silkworms and quality of cocoons.
Some important cocoon quality parameters to know before going on to the impact of temperature, humidity, air quality, and light.
Cocoon weight (g)
Cocoon weight was recorded by weighing cocoons individually using a sensitive electronic balance.
Shell weight (g) & ratio (%)
Shell weight was recorded by removing the floss layer and cutting open the cocoon to remove pupa and the last larval skin i.e., exuvium.
Pupation (%)
Pupation is the process of larvae growing into a pupa and pupation % is the percentage of DFLs reaching the pupa stage.
Renditta
The Renditta test is a grading system that measures the quantity of cocoons(in kg) it takes to produce 1kg of raw silk.
Role of Temperature on Cocoons
As silkworms are cold-blooded animals, the temperature will have a direct effect.
The early instar larvae are resistant to high temperatures which also helps in improving survival rate and cocoon quality parameters.
The temperature has a direct correlation with the growth of silkworms. Wide fluctuation of temperature is harmful to the development of silkworms. The increased temperature during silkworm rearing particularly in late instars accelerates larval growth and shortens the larval period. On the other hand, at low temperatures, the growth is slow and the larval period is prolonged. The optimum temperature for normal growth of silkworms is between 20°C and 28°C and the desired temperature for maximum productivity ranges from 23°C to 28°C. Temperature above 30°C directly affects the health of the worm. If the temperature is below 20°C all the physiological activities are retarded, especially in early instars. As a result, worms become too weak and susceptible to various diseases. The temperature requirements during the early instars (I, II, III) are high and the worms feed actively, grow very vigorously, and lead to a high growth rate.
The health of silkworms is a very determinant factor for the quality of cocoons. The temperature has a direct effect on the growth, activity, and strength of silkworms. The increased temperature during silkworm rearing particularly in late instars fastracks larval growth and shortens the larval period. Low-temperature results in slower growth resulting in a prolonged larval period. Thus the optimum temperature for normal growth of silkworms is between 23°C and 28°C. Temperature above 30°C directly affects the health of silkworm while anything below 23°C results in activities at a slow pace, mainly in early instars. Silkworms are more sensitive to temperature and humidity during IVth and Vth Instars.
The table below shows the results of research conducted by professors from Kuvempu University(Y L Ramachandra and Geetha Bali) and Manipal Academy of Higher Education (Padmalatha Rai)
The experiment they ran was simple, 3 rearing sheds, 3 batches of mulberry chawki, reared at 22°C, 25°C, and 38°C. The study showed significant differences in the quality of output they got. All the other parameters were maintained at optimum levels.
Role of Humidity on Growth of Silkworm
The influence of humidity on the cocoon quality and quantity is indirect but significant. It determines the withering of leaves in silkworm rearing beds. In dry conditions, the leaves wither fast. Decreasing the consumption of mulberry leaves by larvae.
As mulberry leaves are the sole source of energy. So, if the consumption of leaves decreases the silkworms grow weak and fail to spin a cocoon of good quality. They could also die as they’re more susceptible to diseases.
At a humidity of 90 percent or higher, if the temperature is maintained at 26°C–28°C, they can grow without being greatly affected. Like temperature, humidity also fluctuates widely not only from season to season but also within the day itself. Therefore, it is necessary for the silkworm rearers to regulate it for their successful crop. However, it is important to lower humidity to 70 percent or below during the moulting time in each instar to facilitate uniform and good moulting.
The following table shows the results of an experiment run at Kuvempu University where 3 different values of relative humidity were maintained with the help of Lime powder. All other environmental parameters were maintained at optimum levels.
The table below shows the optimum temperature and relative humidity for each instar, during spinning and cocoon preservation.
Role of Air and Light on Growth of Silkworm
Like most living organisms, silkworms also face health problems in bad quality air. By respiration, silkworms release carbon dioxide gas so the freshness of air can be determined by its CO2 contents. Although atmospheric CO2 Content is generally 0.03-0.04% in the rearing rooms, carbon monoxide, ammonia, sulfur dioxide, and so forth are also released in the rearing room when farmers burn charcoal to raise the temperature.
These gases are injurious to silkworms. Therefore, care should be taken to allow fresh air through proper ventilation to keep the toxic gases at a low level. If CO2 exceeds above 2 percent concentration, the growth of silkworm is retarded. Insecticides and disinfectants are also avoided in the rearing room. Young silkworm larvae are more susceptible to poisonous gases and hence the artificial circulation of air is extremely useful in bringing down the contaminated air. The air current of 1.0 m/sec during 5th-age rearing reduces the larval mortality and improves ingestion, digestibility, larval weight, cocoon weight, and pupation rate compared to those recorded under zero ventilation conditions.
Silkworms are photosensitive and have a tendency to crawl towards dim light. They do not like either strong light or complete darkness. Rearing of silkworms in continuous light delays the growth. Further, it causes the reduction of both larval and cocoon weights. Silkworms are fond of the dim light of 15 to 20 lux and avoid strong light and darkness. Late-age worms survive better in 16-hour light and 8-hour dark periods. However, young-age worm prefers 16 hr darkness and 8 hr light period. The silkworm larvae are fed in complete darkness during the life cycle, their larval duration is longer, and cocoon quality becomes poor.
Environmental Care during Silkworm Spinning
If the temperature rises beyond 22°–25°C during spinning, the shell becomes very loose and folded with wrinkles and knots making it a bad quality cocoon. It also changes the properties of sericin. Low temperature slows down the secretion of silk thread and resulting in large-sized cocoons. Relative humidity (60–70%) induces good health, good reelability, and a good-quality cocoon. Low humidity causes double-layered cocoons and loose cocoons. Air current is also important as fast air could crowd the silkworms resulting in double cocoons. The mounting room requires moderate, evenly illuminated dim light. Strong light causes the crowding of silkworms at one side as silkworms prefer dim light and finally results in double cocoons or uneven-thickness cocoons. Complete darkness will slow down the spinning process resulting in low-quality cocoons.
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Sources: https://www.hindawi.com/journals/psyche/2012/121234/
