Best Temperature And Humidity For Silkworm Growth

The Delicate Balance: Optimizing Temperature and Humidity for Silkworm (Bombyx mori) Rearing

The cultivation of the silkworm, Bombyx mori, represents one of the most ancient and intricate partnerships between humans and insects. For over five millennia, this domesticated lepidopteran has been the sole producer of genuine silk, a fiber synonymous with luxury, strength, and beauty. The success of sericulture, however, hinges not on grand technological interventions but on the meticulous maintenance of a microclimate. Silkworms are, in essence, biochemical machines exquisitely tuned by evolution and selective breeding to operate within a narrow band of environmental conditions. Among these, temperature and humidity stand as the twin pillars governing every aspect of their growth, health, metabolism, and ultimately, the quality and quantity of the silk they produce. Mastering this delicate balance is the difference between a profitable harvest and catastrophic loss.

The Physiological Imperative: Why Silkworms Are So Demanding

Unlike mammals, silkworms are poikilothermic (cold-blooded). Their internal body temperature and metabolic rate are directly governed by the ambient environment. This makes them incredibly sensitive to fluctuations in temperature and humidity. Their growth, digestion of mulberry leaves, enzyme activity, and hormonal cycles for molting and spinning are all thermohygrometrically driven processes.

Furthermore, the silkworm’s life cycle is a dramatic metamorphosis compressed into a brief, intensive period of larval growth. From a pinhead-sized newly hatched larva to a large, mature caterpillar ready to spin its cocoon, it increases its body mass by roughly 10,000 times in about 25-28 days. This explosive growth requires optimal conditions to facilitate efficient consumption, digestion, and conversion of mulberry leaves into body mass and, finally, silk proteins. Deviations from the ideal range force the larvae to expend energy on homeostasis—regulating their water balance or dealing with thermal stress—energy that is then diverted away from growth and silk synthesis.

The Goldilocks Zone: Detailed Requirements by Stage

The ideal temperature and humidity are not constants throughout the silkworm’s life but must be carefully staged according to their developmental phase.

1. Egg Incubation (Embryogenesis):

• Temperature: 24-25°C (75-77°F) is ideal for the gradual development of the embryo within the diapause (hibernating) eggs. For non-diapause eggs or at hatching time, a slightly higher range of 25-27°C (77-81°F) is used. Crucially, temperatures must never exceed 28°C (82°F), as this can kill the developing embryo. A stable temperature is vital.
• Humidity: 75-80% Relative Humidity (RH). This high humidity prevents the delicate eggs from desiccating. As hatching approaches (on the day before, known as “blue egg” stage), humidity is increased to 85-90% to soften the chorion (egg shell) and assist the larvae in emerging.
• Rationale: Stable, moderate warmth ensures synchronized embryonic development, while high humidity maintains egg viability. Successful hatching yields a uniform, vigorous “brush” of larvae, which is essential for synchronized rearing.

2. Larval Stage (The Feeding Period):
This stage is divided into five instars, separated by four molts (ecdysis). Requirements shift subtly between the young (1st-3rd instar) and late (4th-5th instar) stages.

• Young Age Silkworms (1st, 2nd, 3rd Instar):
  • Temperature: 26-28°C (79-82°F). Younger larvae have a higher surface-area-to-volume ratio, losing heat and moisture more rapidly. They require slightly warmer conditions to maintain metabolic activity for rapid early growth.
  • Humidity: 80-85% RH. High humidity is critical here for two reasons: it prevents the tender leaves from wilting too quickly, ensuring the small larvae can consume them, and it prevents the larvae themselves from dehydrating. Their cuticle is thinner, and they are more susceptible to water loss.
  • Management: This is often done in a “rearing tray” system with direct coverage to maintain a humid microclimate. Ventilation is still necessary but controlled to prevent humidity from dropping.
• Late Age Silkworms (4th and 5th Instar):
  • Temperature: A gradual reduction is beneficial. The 4th instar thrives at 24-25°C (75-77°F), and the 5th instar, the most critical for silk gland development, performs best at 23-24°C (73-75°F). Avoid temperatures above 28°C (82°F) at all costs during the 5th instar, as heat stress severely impacts silk yield and quality.
  • Humidity: Lowered to 70-75% RH. The larvae are now larger, with a tougher integument less prone to dehydration. More importantly, lower humidity helps keep the rearing bed (the mass of larvae, feces, and leftover leaves) drier. A damp bed is a breeding ground for pathogenic bacteria (e.g., Bacillus thuringiensis) and fungi (e.g., Beauveria bassiana, the causal agent of muscardine disease).
  • Rationale: Cooler temperatures in the late stages, particularly the 5th instar, prolong the feeding period slightly, allowing for greater consumption and more efficient conversion of nutrients into silk proteins. Reduced humidity is a primary disease prevention strategy.

3. Mounting & Cocoon Spinning:
When the mature 5th instar larvae stop feeding, clear their gut, and seek a place to spin (a behavior called “ripening”), they are transferred to spinning frames or mountages.

• Temperature: 24-25°C (75-77°F). Stability is paramount. Fluctuations can cause the larva to stop and start spinning, leading to weak spots or defects in the cocoon filament.
• Humidity: 65-70% RH. This is the lowest humidity in the entire cycle. Dry conditions are essential for the silk filament, a composite of fibroin protein bound by sericin gum, to dry and solidify properly as it is spun. High humidity results in a soggy, poorly formed cocoon, prone to collapse and bacterial degradation, which makes it impossible to reel (the process of unwinding the continuous filament).
• Rationale: The correct thermo-hygrometric conditions ensure the physical chemistry of silk solidification happens correctly, yielding a cocoon with optimal reelability, luster, and tensile strength.

4. Pupation & Moth Emergence:
If the cocoons are kept for seed production (rather than silk reeling), they are maintained under specific conditions to allow the pupa to develop and the moth to emerge.

• Temperature: 24-25°C (75-77°F).
• Humidity: 75-80% RH. Increased humidity softens the sericin gum at one end of the cocoon, allowing the adult moth to secrete an alkali solution and break its way out without drowning in its own fluids.

The Consequences of Imbalance: A Delicate System Disrupted

High Temperature & Low Humidity (Hot & Dry):
This is one of the most dangerous combinations. Elevated temperature accelerates metabolism, shortening the larval period, but without adequate humidity, mulberry leaves wilt rapidly. Larvae cannot consume desiccated leaves, leading to malnutrition. They become dehydrated, lethargic, and undersized. The result is reduced cocoon weight, thin cocoon shells, and dramatically lower silk yield. The silk filament itself may be brittle.

Low Temperature & High Humidity (Cold & Damp):
This scenario slows everything down. Metabolism drops, larvae eat slowly and irregularly, and the growth period is prolonged. A damp, cold environment is a haven for pathogens. Fungal spores germinate readily on both the larvae and the wet bed. Bacterial diseases flourish. Larvae may fail to molt properly or become susceptible to vomit disease (flacherie). Even if they survive, cocoon quality is poor—the cocoons may be malformed, and the silk gum does not set correctly, making reeling difficult.

High Temperature & High Humidity (Hot & Humid):
While initially seeming favorable for young larvae, this becomes catastrophic for late-age worms. It creates a “tropical stew” effect in the rearing bed, promoting the rapid proliferation of germs. The larvae are thermally stressed, eating ravenously but inefficiently. They are more prone to viral diseases like grasserie (Nuclear Polyhedrosis Virus). Mortality rates spike, often leading to a total crop loss. Spinning in these conditions produces sticky, gummy cocoons of very low commercial value.

Low Temperature & Low Humidity (Cold & Dry):
Growth is severely retarded. Larvae may enter a semi-dormant state. Development becomes highly unsynchronized, making management a nightmare. Feeding is minimal, and the resulting cocoons are small and lightweight. The extended rearing period also increases the risk of opportunistic infections.

Modern Management Practices & Technological Aids

Traditional sericulturists relied on experience, the feel of the air, and the timing of seasons. Modern sericulture, especially in controlled environments, uses technology to maintain precision.

• Environmental Control Rooms: Industrial sericulture farms use insulated rearing houses with automated systems.
• Heaters & Thermostats: For maintaining optimal temperature, especially during cooler nights or seasons.
• Humidifiers & Dehumidifiers: Ultrasonic humidifiers can increase RH, while dehumidifiers or exhaust fans coupled with heaters are used to lower it. The choice depends on the external climate and the silkworms’ stage.
• Hygrothermographs: Digital devices that continuously monitor and log temperature and humidity, providing data for timely adjustments.
• Ventilation Systems: Crucial for removing stale air, excess CO2 from larval respiration, and ammonia from feces, while also helping to regulate humidity. The rate is carefully calibrated—too much draft can desiccate the bed; too little leads to stagnation.
• Bed Management: This low-tech practice is as important as any gadget. Regularly removing leftover leaves and feces (litter cleaning), sprinkling bleaching powder or lime to dry the bed, and spreading fresh, dry mulberry stems are all practices that directly affect the micro-humidity around the larvae.

The Mulberry Factor: An Intrinsic Link

It is impossible to discuss silkworm environment without considering the mulberry leaf. The leaf is both food and a source of moisture. The quality of the leaf—its moisture content (ideally 75-80%), tenderness, and nutritional value—is itself dependent on agronomic practices and weather. Feeding wilted or overheated leaves to silkworms is a direct pathway to failure, regardless of how perfect the rearing room air might be. Therefore, the “best” temperature and humidity for silkworms also encompass the conditions that preserve the quality of their sole food source from harvest to consumption.

Here are 15 frequently asked questions (FAQs) on silkworm growth:

1. How long does it take for a silkworm to complete its life cycle?
The full life cycle from egg to adult moth typically takes about 6 to 8 weeks, depending on temperature, humidity, and food quality.

2. What do silkworms eat? Can they eat anything besides mulberry leaves?
Silkworms primarily and almost exclusively eat fresh mulberry leaves (Morus spp.). In emergencies, they can sometimes accept artificial diets or lettuce, but this often results in weaker silk or health issues.

3. At what temperature and humidity should I keep silkworms?
Ideal conditions are a stable temperature between 23-28°C (73-82°F) and humidity around 60-70%. Avoid drafts, direct sunlight, and extreme dryness or dampness.

4. How often should I feed silkworms?
Young worms need finely chopped, fresh leaves available at all times. Larger worms should be fed whole leaves 2-4 times a day, as they have a voracious appetite.

5. How can I tell when a silkworm is ready to spin a cocoon?
They stop eating, become translucent, start wandering to find a suitable spot, and their body shortens. They will also expel excess fluid.

6. What do I need to provide for cocoon spinning?
Provide twigs, cardboard egg cartons, or specially made frames with lots of nooks and crannies. They need a secure, upright structure to anchor their silk.

7. How long does it take for a silkworm to spin its cocoon?
The spinning process takes approximately 2 to 3 days.

8. What happens inside the cocoon?
Inside the cocoon, the silkworm (now a pupa) undergoes metamorphosis to transform into an adult moth. This stage lasts about 10-14 days.

9. Should I let the moths emerge from the cocoons?
If your goal is silk production, you must harvest the cocoons before the moths emerge (typically by boiling or baking them) to preserve the long, continuous silk thread. If your goal is breeding, allow some to emerge.

10. How do adult silkworm moths behave?
The domesticated silkworm moth (Bombyx mori) cannot fly. They emerge, mate, the female lays eggs (200-500), and then both males and females die within about a week.

11. How do I collect and store silkworm eggs?
After the moths lay them, the eggs (tiny, yellow, and pinhead-sized) will darken if fertile. Store them in a sealed container in the refrigerator to simulate winter (diapause) until you are ready to hatch them.

12. How do I hatch silkworm eggs?
Remove eggs from the refrigerator and place them at room temperature. They will hatch in 7-14 days. Keep them in a ventilated container and provide fresh, tender mulberry leaves immediately.

13. Why are my silkworms dying?
Common causes include: starvation (no fresh leaves), contaminated food (pesticides), excessive moisture (mold), extreme dryness, overcrowding, or bacterial/viral infections (often signaled by black spots or liquefaction).

14. How do I clean their container without harming them?
Gently move the worms and leaves to a temporary container. Remove old leaves and droppings (frass). Do not use chemicals; simply wipe the container clean. Avoid touching young worms directly; use a soft brush.

15. Can I use any type of mulberry tree?
Yes, but white mulberry (Morus alba) is the best and most nutritious. Ensure leaves are from a tree not treated with pesticides or herbicides, and always wash them thoroughly. Avoid leaves from roadside trees due to pollution.