Unit 3: Comprehensive Guide to Silkworm Rearing

Selection of Mulberry Variety and Establishment of Mulberry Garden

The successful rearing of silkworms, also known as sericulture, fundamentally depends on the quality and availability of mulberry leaves, which serve as the sole food source for silkworms. Therefore, careful selection of mulberry varieties and meticulous establishment of a mulberry garden are crucial first steps.

Selection of Mulberry Variety

Selecting the right mulberry variety is paramount for ensuring high-quality leaves that contribute to healthy silkworm growth and superior cocoon production.

• Nutritional Value: Varieties with high protein, carbohydrate, and moisture content are preferred.
• Leaf Yield: Choose varieties known for high leaf biomass production per unit area.
• Disease Resistance: Varieties that are resistant to common mulberry diseases and pests reduce cultivation challenges.
• Adaptability: Select varieties suitable for the local soil and climatic conditions.
• Palatability: Leaves should be tender and palatable for silkworms, especially for young larvae.

Important Observation: The quality of mulberry leaves directly influences the growth rate, health, and silk production efficiency of silkworms. Poor quality leaves can lead to stunted growth and increased susceptibility to diseases.

Establishment of Mulberry Garden

A well-planned and maintained mulberry garden ensures a continuous supply of nutritious leaves.

1. Site Selection: Choose a site with well-drained loamy soil, good sunlight exposure, and access to irrigation facilities. Avoid waterlogged or saline soils.
2. Land Preparation: Plough the land thoroughly to a depth of 30-45 cm. Incorporate farmyard manure or compost to enrich soil fertility. Form beds or ridges as per the planting method.
3. Planting Material: Use healthy saplings or cuttings from desired mulberry varieties. Cuttings are generally 15-20 cm long with 3-4 healthy buds.
4. Planting Methods: Common methods include row planting (for bush or tree types) or pit planting. Spacing depends on the variety and management practices (e.g., 90 cm x 90 cm for bush mulberry).
5. Irrigation: Provide regular irrigation, especially during dry spells and after pruning, to ensure vigorous leaf growth.
6. Fertilization: Apply balanced fertilizers (nitrogen, phosphorus, potassium) as per soil test recommendations to support healthy leaf development.
7. Weeding: Keep the garden free from weeds, which compete for nutrients and moisture.
8. Pruning: Regular pruning is essential to maintain the desired plant structure, promote new shoot growth, and facilitate leaf harvesting.

Rearing House and Rearing Appliances

An appropriate rearing environment and specialized appliances are critical for successful silkworm rearing, ensuring optimal growth and minimizing disease incidence.

Rearing House

The rearing house provides a controlled environment for silkworms. An ideal rearing house should have:

• Ventilation: Good cross-ventilation is essential to remove stale air and maintain fresh air circulation, preventing accumulation of harmful gases and moisture.
• Temperature and Humidity Control: Facilities to maintain optimal temperature (23-28°C) and humidity (70-90%) according to the silkworm stage.
• Light: Adequate indirect light is beneficial. Direct sunlight should be avoided.
• Hygiene: Easy to clean and disinfect, with smooth, washable surfaces.
• Pest and Predator Protection: Designed to prevent entry of ants, rodents, birds, and other pests.
• Space: Sufficient space for rearing trays, stands, and movement of personnel.

Exam Tip: A well-designed rearing house reduces stress on silkworms, leading to better health and higher silk yield.

Rearing Appliances

A range of specialized tools and equipment is used throughout the silkworm rearing process.

• Rearing Trays: Flat, shallow trays (often bamboo or plastic) where silkworms are reared and fed.
• Rearing Stands/Racks: Multi-tiered stands to hold rearing trays, optimizing space and ventilation.
• Chopping Knives and Boards: Used for finely chopping mulberry leaves for younger silkworms.
• Leaf Baskets/Bags: For collecting and transporting mulberry leaves.
• Thermometers and Hygrometers: To monitor and maintain optimal temperature and humidity inside the rearing house.
• Dusting Net/Sprayer: For applying disinfectants or disease-preventive powders.
• Feather/Brush: For gently handling young silkworms (brushing).
• Mountages: Devices for silkworms to spin cocoons (discussed in detail later).
• Cleaning Tools: Brooms, brushes, and scrapers for maintaining hygiene.

Disinfectants: Formalin, Bleaching Powder, RKO

Disinfection is a critical practice in sericulture to prevent and control the spread of silkworm diseases, which can cause significant economic losses. Various disinfectants are used for cleaning rearing houses and appliances.

Formalin

Definition: Formalin is a saturated solution of formaldehyde gas (typically 37% formaldehyde by weight), a powerful broad-spectrum disinfectant effective against bacteria, fungi, and viruses.

It is widely used in sericulture for sterilizing rearing rooms, trays, and other equipment before and after rearing.

• Application: Usually used as a 2-4% solution. It can be sprayed on surfaces or used for fumigation. For fumigation, potassium permanganate is often reacted with formalin to release formaldehyde gas.
• Precautions: Formalin is a pungent, irritating chemical. Adequate ventilation is necessary during and after application. Personnel should wear protective gear (mask, gloves). The rearing house should be kept sealed for a specific period after fumigation, then thoroughly aired out before introducing silkworms.

Bleaching Powder

Definition: Bleaching powder (calcium hypochlorite, CaOCl₂) is a potent oxidizing agent and disinfectant, often used as a source of chlorine for sanitation purposes.

It is effective against many microorganisms and is used for cleaning floors and general sanitation of rearing areas.

• Application: Typically used as a 2-5% solution for washing floors, walls, and trays. It can also be spread as a dry powder in specific areas or mixed with slaked lime for broader disinfection.
• Precautions: Bleaching powder is corrosive and can irritate skin and respiratory tracts. It should be stored in airtight containers away from moisture and light. Adequate ventilation is required during use.

RKO (Resham Keet Oushad)

Definition: RKO, or Resham Keet Oushad, is a specific sericultural disinfectant powder primarily developed for application directly on silkworms to prevent and control various silkworm diseases, particularly muscardine (fungal disease) and flacherie (bacterial/viral disease).

It is a prophylactic measure, meaning it is used to prevent disease outbreaks rather than just treating them.

• Application: RKO is dusted evenly over the silkworms and mulberry leaves immediately after feeding, typically once a day during the rearing period, especially during vulnerable stages.
• Precautions: While generally safe for silkworms, it should be applied in recommended quantities to avoid any adverse effects. Store in a dry place.

Silkworm Rearing Technology: Early Age and Late Age Rearing

Silkworm rearing is broadly divided into two distinct phases: early age rearing and late age rearing, each requiring specific environmental conditions and management practices for optimal growth and health.

Early Age Rearing (Chawki Rearing)

This phase covers the first two instars (ages) of the silkworm larval stage and is considered the most crucial period for healthy silkworm development.

• Duration: Approximately 7-10 days.
• Sensitivity: Young silkworms are very delicate and highly susceptible to environmental fluctuations and diseases.
• Ideal Conditions:
  • Temperature: Maintain consistently around 26-28°C.
  • Humidity: High humidity (85-90%) is crucial to keep the tender leaves fresh.
  • Ventilation: Moderate ventilation to avoid desiccation of leaves.
• Feeding: Feed young silkworms 4-5 times a day with very tender, finely chopped mulberry leaves. Leaf quality is paramount.
• Spacing: Initially, they require very little space, but spacing needs to be gradually increased as they grow within these instars.
• Hygiene: Strict hygiene is essential. Bed cleaning is performed frequently (often daily or alternate days) to remove leftover leaves and faecal matter.
• Brushing: Gently transfer newly hatched larvae (chawki worms) to fresh leaves using a soft brush or feather.

Important Note: Strong early age rearing forms the foundation for a healthy and robust late age rearing, directly impacting cocoon quality and yield. Any lapse during this stage can lead to significant losses.

Late Age Rearing (Main Crop Rearing)

This phase covers the third, fourth, and fifth instars, where silkworms grow rapidly and consume large quantities of leaves.

• Duration: Approximately 18-25 days.
• Robustness: Silkworms are more robust and less susceptible to minor environmental changes compared to early age larvae.
• Ideal Conditions:
  • Temperature: Maintained around 23-25°C.
  • Humidity: Gradually reduced to 70-75%.
  • Ventilation: Good ventilation becomes critical to remove metabolic heat, moisture, and CO₂ produced by the growing worms.
• Feeding: Feed 3-4 times a day with mature, slightly coarser mulberry leaves. The quantity of leaves consumed increases exponentially.
• Spacing: Requires significantly more space as larvae grow. Frequent bed expansion is necessary to prevent overcrowding, which can lead to diseases and uneven growth.
• Hygiene: Regular bed cleaning (once a day or every other day) is still important, though less frequent than in early age rearing.
• Disease Management: Continuous monitoring for disease symptoms and isolation of affected worms. Application of RKO may continue.

Common Mistake: Overcrowding in late age rearing is a major cause of disease outbreaks and reduced cocoon quality. Adequate spacing is crucial for healthy development.

Types of Mountages; Spinning, Harvesting and Storage of Cocoons

The final stages of silkworm rearing involve providing structures for cocoon spinning, carefully collecting the cocoons, and storing them properly.

Types of Mountages

Definition: Mountages are structures provided to mature silkworms (ready to spin cocoons) to facilitate the spinning of cocoons in an organized and hygienic manner. They offer a stable support and prevent larvae from falling or spinning defective cocoons.

Different types of mountages are used:

• Bamboo Chandrikes: Traditional, circular bamboo frames with spiral partitions. They are widely used, economical, and provide good ventilation.
• Plastic Collapsible Mountages: Modern, often rectangular, made of plastic with numerous small cells or chambers. They are easy to clean, sterilize, and can be reused.
• Netrika: A Japanese innovation, these are plastic sheets with small, conical depressions, designed for easy harvesting and uniform cocoon formation.
• Rotary Mountages: Often used in large-scale operations, these systems rotate to allow worms to move and settle into individual compartments.

Important Observation: A good mountage ensures proper aeration, prevents touching of cocoons (which can lead to double cocoons or stifled cocoons), and facilitates easy harvesting.

Spinning of Cocoons

Once silkworms mature (stop feeding, become translucent, and begin searching for corners), they are transferred to mountages. This process is called mounting.

• Process: The silkworm first discharges a sticky fluid to fix itself to the mountage, then starts secreting liquid silk through its spinnerets, forming a figure-eight pattern around itself. The liquid silk hardens upon contact with air, forming the continuous silk filament.
• Duration: Spinning typically takes 2-3 days, after which the silkworm transforms into a pupa inside the cocoon.
• Conditions: Optimal temperature (24-25°C) and moderate humidity (60-70%) with good ventilation are necessary during spinning to ensure proper hardening of silk and prevent fungal growth.

Harvesting of Cocoons

Harvesting is the process of collecting the cocoons from the mountages after the spinning is complete and the pupae have hardened.

1. Timing: Cocoons are usually harvested 5-7 days after mounting. Harvesting too early can damage the pupa or the silk filament, while too late can lead to moth emergence.
2. Identification: Mature cocoons will feel firm. Gently shake the mountage; if the pupa rattles inside, it indicates proper hardening.
3. Method: Carefully detach the cocoons from the mountages, removing any waste silk (floss) or dead larvae.
4. Sorting: Sort cocoons based on quality (good cocoons, defective cocoons like double cocoons, stained cocoons, pierced cocoons).

Exam Point: Proper timing and gentle handling during harvesting minimize damage to cocoons and maximize the reeling percentage.

Storage of Cocoons

After harvesting, cocoons need proper storage to prevent premature emergence of moths and deterioration of silk quality.

• Stifling: This is a crucial first step where the pupae inside the cocoons are killed (either by heat, sun exposure, or chemical methods) to prevent them from developing into moths and piercing the cocoon, which would break the continuous silk filament.
• Conditions: After stifling, cocoons should be stored in a cool (20-22°C), dry (relative humidity 60-65%) place with good ventilation.
• Pest Control: Protect stored cocoons from pests like dermestid beetles, which can damage the silk.
• Purpose: Proper storage ensures that the silk can be reeled efficiently without breakage, maintaining its value.