Unit 2(a) - Silkworm | Management of Beneficial Insects

Contents Types of silkworm, voltinism and biology of silkworm. Mulberry cultivation, mulberry varieties and methods of harvesting and preservation of leaves. Rearing, mounting and harvesting of cocoons.

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Types of Silkworm

Silkworms, also known as silkworm larvae or caterpillars, are the larval stage of silk-producing insects. They are primarily used in the production of silk, a valuable natural fiber. There are several types of silkworms, each with unique characteristics and habitats. Let's explore the most commonly known types:

1. Bombyx mori (Mulberry Silkworm):
• Mulberry silkworms are the most well-known and widely cultivated silkworms for silk production.
• They feed exclusively on the leaves of mulberry trees, hence the name.
• Bombyx mori has several strains developed for different purposes, such as sericulture, silk quality, and quantity of silk produced.
• These silkworms have been domesticated for centuries and are known for their high-quality silk fibers.

2. Antheraea mylitta (Tasar Silkworm):
• Tasar silkworms are native to India and are known for producing Tasar silk, which is coarser than mulberry silk.
• They feed on several species of trees, such as Terminalia and Shorea.
• Tasar silk has a distinctive golden-brown color and is often used for making fabrics like sarees and shawls.

3. Antheraea assamensis (Muga Silkworm):
• Muga silkworms are another variety native to India, specifically Assam.
• They feed on the leaves of Som and Soalu trees.
• Muga silk is prized for its natural golden color and luster and is used for making traditional Assamese garments.

4. Philosamia cynthia (Eri Silkworm):
• Eri silkworms are found in parts of India, China, and Japan.
• They feed on a variety of plants, including castor, tapioca, and kesseru.
• Eri silk is coarser than mulberry silk and is often referred to as "peace silk" as the cocoon is cut open to allow the adult to emerge, unlike other silks where the pupa is killed during the extraction process.

5. Bombyx mandarina (Oak Silkworm):
• Oak silkworms are wild silkworms found in China.
• They feed on oak trees and produce a type of silk known as "tussah" or "wild silk."
• Tussah silk is coarser and less lustrous than mulberry silk and is often used for making rustic textiles.

6. Attacus atlas (Atlas Moth):
• While not used for silk production on a large scale, the Atlas moth produces the largest cocoon of any moth species.
• The cocoon of the Atlas moth is sometimes used to make a type of silk fabric.

Each type of silkworm has its unique characteristics, feeding habits, and silk-producing qualities. They play a significant role in sericulture and the production of different types of silk, catering to diverse markets and preferences.

Voltinism

Voltinism refers to the number of generations or broods of insects that occur in a specific time period, usually a year. It is a term used in entomology to describe the seasonal life cycle patterns of insects. The concept of voltinism helps in understanding the population dynamics, behavior, and management of insects.

Voltinism of Silkworms: Voltinism in silkworms refers to the number of generations or broods of silkworms that occur in a specific time period, usually a year. Silkworms, which are the larvae of silk-producing moths, have a unique life cycle that is closely tied to their economic importance in silk production. Let's explore the voltinism of silkworms in detail:

1. Univoltine Silkworms:

• Univoltine silkworms have one generation per year.
• They are typically found in temperate regions with distinct seasons.
• The most well-known univoltine silkworm is the Bombyx mori, which is used for commercial silk production.
• Univoltine silkworms undergo a long diapause (resting stage) as pupae during the winter months.
• In the spring, they emerge as adult moths, mate, lay eggs, and complete their life cycle within a single year.

2. Bivoltine Silkworms:

• Bivoltine silkworms have two generations per year.
• They are common in regions with more moderate climates and fewer distinct seasons.
• Bivoltine silkworms often have a larger generation during warmer months and a smaller generation during cooler months.
• These silkworms also undergo a pupal diapause during unfavorable conditions.

3. Multivoltine Silkworms:

• Multivoltine silkworms have multiple generations per year.
• They are found in tropical regions with favorable weather conditions throughout the year.
• Multivoltine silkworms continuously reproduce and complete their life cycle within a short period.
• Unlike other types, they do not experience a prolonged diapause.

The voltinism of silkworms is crucial for silk production and the sericulture industry. Commercial silk production primarily focuses on the univoltine silkworm (Bombyx mori), which undergoes a specific and controlled life cycle to ensure high-quality silk production. The timing of mating, egg laying, and rearing of silkworms is carefully managed to optimize silk yield and quality.

Understanding the voltinism of silkworms helps sericulturists plan their activities, manage resources, and coordinate the rearing process. It also aids in predicting peak silk production periods and addressing potential challenges related to weather fluctuations, diseases, and pests.

Biology of Silkworm (Bombyx mori)

The silkworm (Bombyx mori) is a well-known insect that plays a vital role in the production of silk. Its life cycle is fascinating and involves various stages, each with unique characteristics and functions. Here's a detailed overview of the biology of the silkworm:

1. Egg Stage:
• The silkworm life cycle begins with the egg stage.
• The female moth lays eggs on specially prepared paper or leaves.
• The eggs are tiny, oval-shaped, and usually yellowish-white in color.
• They take about 10-12 days to hatch, but this time can vary depending on temperature and humidity.

2. Larval Stage (Caterpillar or Silkworm):
• Once the eggs hatch, the larval stage begins.
• The larva, often called a "caterpillar" or "silkworm," is the feeding stage of the silkworm's life cycle.
• The larva molts or sheds its skin multiple times as it grows. There are five instars (stages) in total.
• Silkworms primarily feed on mulberry leaves, which provide the nutrients necessary for growth.

3. Pupal Stage (Cocoon):
• After completing its fifth instar, the silkworm enters the pupal stage.
• The pupal stage is also known as the "cocoon" stage.
• The silkworm spins a protective cocoon using a fluid that hardens into silk fibers upon contact with air.
• Inside the cocoon, the silkworm undergoes metamorphosis, transforming into an adult moth.

4. Adult Stage (Moth):
• Once metamorphosis is complete, the adult moth emerges from the cocoon.
• The adult moth has a short lifespan of about 5-10 days, during which its main focus is reproduction.
• Adult moths have large wings, a furry body, and well-developed antennae.
• Female moths emit pheromones to attract male moths for mating.
• After mating, the female moth lays eggs to initiate a new generation.

Silk Production:

• The silkworm's silk production is a remarkable feature of its biology.
• During the pupal stage, the silkworm secretes a liquid protein solution through spinnerets located on its head.
• The liquid protein solidifies upon exposure to air, forming silk fibers.
• The silkworm uses these silk fibers to create its protective cocoon.

Importance of Biology: The detailed understanding of the biology of silkworms is crucial for sericulturists (silk producers) to successfully rear silkworms and harvest high-quality silk. Proper management of temperature, humidity, feeding, and hygiene is essential at every stage of the silkworm's life cycle. This knowledge helps ensure healthy growth, cocoon formation, and silk production.

In conclusion, the life cycle of the silkworm, from egg to adult moth, involves distinct stages: egg, larva, pupa, and adult. The silkworm's remarkable ability to produce silk during the pupal stage contributes to its significance in the sericulture industry. Understanding the biology of the silkworm is fundamental for effective sericulture practices and the production of valuable silk.

Mulberry Cultivation

Mulberry (Morus) is a deciduous tree that is primarily cultivated for its leaves, which serve as the primaryfood source for silkworms (Bombyx mori). The leaves are rich in nutrients and essential for the healthy growth of silkworms in sericulture. Here's a detailed overview of mulberry cultivation:

1. Climate and Soil:
• Mulberry trees thrive in tropical and subtropical climates.
• They require well-drained soil with good fertility and moisture retention capacity.
• The pH of the soil should be in the range of 6.0 to 7.5.

2. Varieties:
• Various mulberry varieties are cultivated based on climate and end-use purposes.
• Popular varieties include Morus alba, Morus indica, and Morus nigra.

3. Propagation:
• Mulberry trees can be propagated through seeds, cuttings, or tissue culture.
• Cuttings from mature trees can be used for vegetative propagation.

4. Planting:
• Planting is usually done during the rainy season.
• Spacing between plants varies based on the type of mulberry and the intended use.
• Proper spacing ensures good sunlight penetration and healthy growth.

5. Soil Preparation:
• The soil should be plowed and prepared to a fine tilth before planting.
• Adding organic matter and fertilizers can improve soil fertility.

6. Irrigation:
• Adequate and regular irrigation is essential, especially during the dry season.
• Drip irrigation is effective in conserving water and providing uniform moisture.

7. Fertilization:
• Mulberry trees respond well to balanced fertilization.
• Nitrogen, phosphorus, and potassium are essential nutrients for healthy growth.

8. Pruning and Training:
• Regular pruning helps maintain the shape of the tree and encourages new growth.
• Training involves removing unwanted branches and maintaining an open canopy.

9. Pest and Disease Management:
• Mulberry trees can be susceptible to pests like mealybugs, scale insects, and leaf-eating caterpillars.
• Diseases such as powdery mildew and root rot can also affect mulberry plants.
• Integrated pest and disease management practices should be adopted.

10. Harvesting:
• The leaves of mulberry trees are harvested when they are young and tender.
• Harvesting can be done by hand-picking or using tools, depending on the scale of cultivation.

11. Post-Harvest Handling:
• Harvested mulberry leaves should be handled carefully to prevent damage.
• Leaves should be transported to silkworm rearing units promptly.

12. Continuous Leaf Harvesting:
• Mulberry trees can tolerate continuous leaf harvesting if proper management practices are followed.
• Regular pruning and proper fertilization support continuous leaf production.

13. Value Addition:
• Apart from being a food source for silkworms, mulberry leaves have medicinal and nutritional value.
• Leaves can be processed into tea or used in traditional medicine.

In conclusion, mulberry cultivation is integral to sericulture, as the leaves are the primary feed for silkworms. Proper management practices ensure healthy growth, abundant leaf production, and optimal quality for silk production. A well-maintained mulberry plantation is essential for the success of sericulture activities and the production of high-quality silk.

Mulberry Varieties

Mulberry (Morus spp.) is a crucial plant in sericulture as it serves as the primary food source for silkworms. There are various mulberry varieties cultivated worldwide, each with specific attributes suited to different climatic conditions and sericultural practices. Here are some important mulberry varieties:

1. Morus alba (White Mulberry):
   • Korean White: Known for its high yield and leaf quality.
   • S1 (Chakwan 1): Developed in China, it is suitable for both leaf yield and cocoon production.
   • S36 (Chakwan 36): Adapted to tropical and subtropical regions, with resistance to diseases.
2. Morus indica (Indian Mulberry):
   • M5: Widely grown in India, this variety is known for its adaptability to various agro-climatic conditions.
   • S1635: Developed in India, it's suitable for irrigated areas and has good leaf quality.
   • V1 (Trayima 1): Developed in Vietnam, it's used for both leaf and cocoon production.
3. Morus multicaulis (Chinese Mulberry):
   • M4: Known for its quick growth and adaptability to diverse conditions.
   • S146: Well-suited for cocoon production, especially in subtropical regions.
   • K2: Developed in Korea, it has high leaf productivity and is resistant to diseases.
4. Other Varieties:
   • Morus macroura (Himalayan Mulberry): Suitable for cold regions and provides good quality leaves.
   • Morus nigra (Black Mulberry): Grown in some regions for silkworm rearing.

Important Terms:

• Leaf Yield: The amount of leaves produced by the mulberry plants, crucial for feeding silkworms.
• Cocoon Production: The yield of silkworm cocoons, which is directly related to the quality and quantity of mulberry leaves.
• Adaptability: The ability of a variety to thrive in specific environmental conditions.
• Resistance: The ability of a variety to withstand or tolerate diseases or pests.
• Agro-climatic Conditions: The combination of climate and soil characteristics of a specific region.

It's important to note that the choice of mulberry variety should consider local climate, soil, and the intended use (leaf or cocoon production) to maximize sericulture productivity. Proper selection and cultivation of suitable mulberry varieties play a vital role in the success of sericultural practices, contributing to the health and growth of silkworms.

Methods of Harvesting Mulberry Leaves

Harvesting mulberry leaves is a crucial aspect of sericulture, as the quality and timing of harvesting directly impact the health and growth of silkworms. Here are the methods and considerations for harvesting mulberry leaves:

1. Leaf Harvesting Time:
   • The timing of leaf harvesting is critical. Leaves are usually harvested when they are young, tender, and have optimal nutritional content for silkworms. This is generally done before the leaves become fully mature and lose their tenderness.
2. Methods of Harvesting:
   • Selective Picking: In this method, individual tender leaves are picked from the branches, leaving the older and mature leaves untouched. This approach ensures a continuous supply of fresh leaves for silkworms.
   • Branch Harvesting: In this method, entire branches are cut from the mulberry plant. This can be done using pruning shears or sickles. The leaves are then stripped from the branches and fed to the silkworms.
3. Considerations for Harvesting:
   • Frequency: Mulberry plants are usually harvested multiple times during the growing season. Frequent harvesting stimulates the growth of new shoots and leaves.
   • Harvesting Intensity: The intensity of harvesting should be moderate, avoiding excessive removal of leaves, which could stress the plants and hinder their growth.
   • Leaf Quality: Only healthy, disease-free, and pest-free leaves should be harvested to ensure the well-being of silkworms.
   • Avoid Flowering Stages: Mulberry plants tend to allocate resources to flowering and fruiting stages. It's advisable to avoid harvesting during these stages, as it might reduce leaf quality for silkworms.
   • Environmental Conditions: Harvesting is best done during cooler parts of the day, such as early morning or late afternoon, to prevent the leaves from wilting quickly in the sun.
4. Post-Harvest Care:
   • Harvested mulberry leaves should be transported to the rearing facility promptly to prevent wilting and loss of nutritional quality.
   • Leaves should be stored in clean, ventilated containers to maintain freshness.
   • If leaves are not immediately used, they can be stored in refrigerated conditions to extend their shelf life.

Harvesting mulberry leaves requires a delicate balance between ensuring a consistent supply for silkworms and maintaining the health and growth of the mulberry plants. Proper harvesting techniques contribute to the success of sericulture and the production of high-quality silk cocoons.

Preservation of Mulberry Leaves for Sericulture

Preserving mulberry leaves is essential to ensure a continuous and quality supply of food for silkworms, especially during periods when fresh leaves are not readily available. Here are the methods and considerations for preserving mulberry leaves:

1. Sun Drying:
   • Sun drying is one of the traditional methods of preserving mulberry leaves. Leaves are spread out in a thin layer under direct sunlight until they are completely dry. This method reduces the moisture content of the leaves and prevents mold growth.
   • Dried leaves can be stored in cool, dry, and well-ventilated storage areas. They should be protected from direct sunlight and pests.
2. Shade Drying:
   • In areas with high humidity or during rainy seasons, shade drying is preferred. Leaves are placed under shade or in well-ventilated drying sheds to prevent exposure to rain and excess moisture.
   • Shade-dried leaves retain their color and nutritional content better compared to sun-dried leaves.
3. Mechanical Drying:
   • Mechanical dryers, such as solar dryers or mechanical hot-air dryers, are used to quickly reduce the moisture content of mulberry leaves. These methods are efficient and can be controlled to maintain desired drying conditions.
4. Freezing:
   • Leaves can be frozen to preserve their nutritional content. This method involves blanching the leaves quickly in boiling water, followed by cooling and freezing. Frozen leaves are stored in sealed containers in freezers.
5. Vacuum Packing:
   • Vacuum packing involves removing air from the packaging to prevent spoilage. Mulberry leaves are packed in vacuum-sealed bags or containers to reduce exposure to oxygen, which can lead to deterioration.
6. Preservatives:
   • Some preservatives can be used to extend the shelf life of mulberry leaves. Common methods include adding a small amount of salt or sugar to the leaves before drying or packing.
7. Hybrid Methods:
   • Combining different preservation methods can enhance the effectiveness of preservation. For example, leaves can be partially dried using mechanical dryers and then sun-dried to reduce moisture to desired levels.

Considerations for Preservation:
• Leaves should be clean and free from dirt, pests, and diseases before preservation.
• Properly dried leaves should have low moisture content to prevent mold growth and spoilage.
• Preservation methods should be chosen based on the local climate and available resources.
• Proper packaging is crucial to prevent reabsorption of moisture and contamination.

Preserving mulberry leaves ensures a steady supply of nutritious food for silkworms throughout the year. It's essential to choose the right preservation method and maintain proper storage conditions to maintain leaf quality and support successful sericulture activities.

Rearing, mounting and harvesting of cocoons

Rearing

The rearing of cocoons is a crucial step in sericulture, where the silkworms complete their life cycle by spinning cocoons that encase their pupae. These cocoons are the primary source of raw silk. Here's a detailed overview of the rearing process:

1. Selection of Silkworm Eggs:
• The process begins with selecting healthy silkworm eggs. These eggs are usually obtained from reliable sources to ensure good quality larvae.
• Eggs are inspected for any signs of disease or damage before being distributed to farmers.

2. Hatching and Rearing Trays:
• Silkworm eggs are placed on special rearing papers or trays. These trays provide a controlled environment for the larvae to hatch and grow.
• The trays are kept in well-ventilated rooms with controlled temperature and humidity conditions.

3. Larval Feeding:
• Once the eggs hatch, the silkworm larvae start feeding on mulberry leaves. Feeding is the most critical phase, as it directly impacts the growth and health of the larvae.
• Fresh mulberry leaves are provided multiple times a day to ensure continuous and sufficient feeding.

4. Molting Stages:
• Silkworms go through several molting stages (instars) as they grow. During each molt, they shed their old skin and develop a new one.
• The duration of each instar and the number of molts can vary depending on factors like temperature, humidity, and silkworm species.

5. Rearing Conditions:
• Proper environmental conditions are maintained throughout the rearing process. The rearing room's temperature, humidity, and ventilation are adjusted to create an ideal environment for silkworm growth.
• Careful monitoring is essential to prevent stress or disease outbreaks among the larvae.

6. Pupal Formation and Cocooning:
• After completing the final instar, the mature silkworms stop eating and become sluggish. They then search for suitable spots to spin their cocoons.
• Silkworms secrete silk from specialized glands near their mouths and start spinning the cocoon in a spiral pattern.

Successful cocoon rearing requires careful attention to details, including proper feeding, maintaining the right rearing conditions, and ensuring the health and well-being of the silkworms. The quality of the cocoons directly affects the quality of the silk produced, making proper rearing practices essential for a successful sericulture industry.

Mounting

Mounting of cocoons is a crucial step in the sericulture process, where the cocoons produced by silkworms are prepared for silk extraction. Here is a detailed explanation of the mounting process:

1. Collection of Cocoons:
• Once the silkworms have completed their pupal stage and spun their cocoons, these cocoons are carefully collected from the rearing trays or frames.
• The collected cocoons are sorted based on their quality, size, and color. High-quality cocoons are preferred for silk extraction.

2. Stifling the Pupae:
• Before mounting, the pupae inside the cocoons need to be stifled to prevent them from breaking the delicate silk threads during the extraction process.
• Stifling is typically done by subjecting the cocoons to heat or steam, which kills the pupae inside.

3. Softening the Cocoons:
• Stifled cocoons are then soaked in warm water to soften the sericin, the natural protein that holds the silk threads together within the cocoon.
• Softening the cocoons makes it easier to unravel the silk threads without damaging them.

4. Mounting Process:
• The softened cocoons are gently spread out and mounted onto special mounting boards or frames.
• In traditional methods, the cocoons are secured onto the mounting boards using pins or nails, allowing easy access to the silk threads.

5. Unraveling the Silk Threads:
• As the cocoons are mounted, skilled workers gently unwind the silk threads from the cocoons.
• The process requires a delicate touch to ensure the silk threads remain intact and undamaged.

6. Reeling or Spinning:
• Once the silk threads are unwound from the cocoons, they are typically reeled or spun onto bobbins or spools.
• Reeling involves winding the silk filaments onto a single thread, creating a continuous length of silk.

7. Raw Silk Collection:
• The reeled silk threads are carefully collected and further processed to remove any impurities, such as pupal fragments or debris.
• The resulting raw silk is of high quality and is suitable for various silk-based products.

8. Handling and Storage:
• Throughout the process, proper handling is crucial to prevent damage to the delicate silk threads.
• The collected raw silk is stored under controlled conditions to maintain its quality until it's ready for further processing.

The entire process, from collecting the cocoons to unwinding the silk threads, is meticulously carried out to ensure the production of fine and valuable silk.

Harvesting

Harvesting of cocoons is a significant step in the sericulture process, as it marks the completion of the silkworm's life cycle and the beginning of silk extraction. Here's a detailed explanation of the cocoon harvesting process:

1. Identification of Mature Cocoons:
• Silkworms complete their pupal stage inside the cocoon, transforming into adult moths.
• As the pupae mature and are ready to emerge as moths, their presence can be detected by slight color changes and movements within the cocoon.

2. Selection and Segregation:
• Skilled sericulturists carefully inspect the trays or frames containing the cocoons to identify mature and healthy cocoons.
• Mature cocoons are selected for harvesting, while damaged or unhealthy cocoons are discarded.

3. Timing of Harvesting:
• Harvesting is done at the right moment when the pupae inside the cocoons are fully developed but have not yet started to emerge as moths.
• Timing is crucial to ensure the cocoon's quality and the preservation of the silk threads.

4. Manual Harvesting:
• The selected mature cocoons are manually harvested from the rearing frames or trays.
• The cocoons are gently detached from the substrate to avoid damaging the silk threads inside.

5. Sorting and Grading:
• The harvested cocoons are sorted based on their quality, size, and color.
• High-quality cocoons with uniform size and color are preferred for silk extraction.

6. Stifling the Pupae:
• To prevent the pupae inside the cocoons from emerging as moths and breaking the silk threads, the cocoons need to be stifled.
• Stifling can be achieved through heat treatment or steam exposure, which kills the pupae.

7. Storage of Harvested Cocoons:
• Once stifled, the harvested cocoons are stored under controlled conditions to prevent damage and preserve their quality.
• Proper storage ensures that the silk threads remain intact and suitable for the silk extraction process.

Harvesting of cocoons requires careful observation, timing, and skilled handling. It is a crucial step to ensure the production of high-quality silk. After the cocoons are harvested and stifled, they are ready for the next stages of silk extraction, such as softening, mounting, and unraveling the silk threads.

Note: Summarized key points for easy exam review.

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