Effective Acid Treatment for Bivoltine Silkworm Eggs

In sericulture, the quality of silkworm eggs – often called “seeds” – directly determines the success of the entire silk crop. Silkworm egg production is considered the most critical and delicate stage of sericulture, and nowhere is this more evident than with bivoltine breeds. Unlike tropical multivoltine silkworms whose eggs hatch naturally within 10-11 days, bivoltine silkworm eggs enter a dormant state called diapause and will not hatch on their own. To manage this and ensure timely, synchronized hatching, sericulturists rely on a well-established technique – acid treatment. This post explains the complete process of acid treatment for bivoltine eggs, with a focus on the hot acid method used in Indian sericulture.

What is diapause and why does it matter? 🔗

Diapause is a physiological arrest in development – essentially a state of suspended animation in the egg. It is a natural survival mechanism, allowing the embryo to withstand adverse seasonal conditions. During diapause, glucose is converted to trehalose and glycerol, which help the egg resist environmental stress. When diapause is terminated, these compounds are broken down and embryonic development resumes.

Bivoltine silkworm eggs are defined as those that produce diapausing-type eggs in the tropics. Under natural conditions, they do not hatch unless subjected to acid treatment or a low-temperature hibernation schedule. For commercial sericulture – where farmers need to plan rearing cycles precisely – this natural delay is a major constraint. Acid treatment solves this problem by artificially terminating diapause, converting hibernating eggs into non-hibernating ones that will hatch predictably.

Why hydrochloric acid? 🔗

Inorganic acids like hydrochloric acid (HCl) are preferred for acid treatment in sericulture. HCl acts by penetrating the eggshell and disrupting the biochemical signals that maintain diapause. HCl treatment induces a rapid elevation of esterase A activity within the egg – an enzyme closely linked to diapause termination – stimulating the egg to resume normal embryonic development. At the molecular level, HCl treatment can induce expression of DNA methyltransferase genes and terminate diapause in the early embryonic stage.

The hydrochloric acid hatching method originated in Italy and France in the 19th century and was established as a practical artificial hatching technology in Japan in 1914. It has remained the most widely used diapause-termination method in sericulture practice across the world.

Correct age of eggs for treatment 🔗

Timing the acid treatment correctly is just as important as the treatment itself. The optimal age for acid treatment is 20 to 24 hours after oviposition. Eggs are held at 25°C with a relative humidity of 75 ± 5% during this period. Freshly laid eggs are pale or dark yellow, and the onset of diapause is marked by the eggs turning brown due to Ommochrome pigment forming in the serosal cells. Treating eggs outside this optimal window – either too early or too late – reduces hatchability and can compromise larval health.

Step 1: Surface sterilization with formalin 🔗

Before any acid treatment begins, the eggs must be surface sterilized. Egg sheets are dipped in a 2% formalin solution for 10-15 minutes. This removes pathogens adhering to the eggshell and prevents secondary contamination. Washing the eggs in formalin solution also helps them adhere firmly to the egg card.

This step is not optional. The formalin soak is followed by thorough washing to remove all formalin traces from the egg surface before proceeding to acid treatment. Residual formalin on the eggs can interfere with the acid’s action and harm the developing embryo. Formalin also plays a role in the fixation of eggs to their substrate, preventing them from falling off during the subsequent immersion in acid.

Step 2: Hot acid treatment – the core process 🔗

For bivoltine eggs, the standard procedure used in Indian sericulture is the hot acid treatment method. The conventional treatment involves immersing silkworm eggs in a hydrochloric acid solution with a specific gravity of approximately 1.075 g/mL at 46°C for 5 minutes. In Indian practice, as outlined in sericulture training manuals from the Central Silk Board, the specific gravity of HCl is 1.075 recorded at 15°C (approximately equivalent to 1.072 at operational temperatures), the temperature is maintained at 46.1°C, and the immersion time is 5 to 5½ minutes depending on the race being treated.

Why are these parameters so precise? 🔗

Each parameter – specific gravity, temperature, and immersion time – plays a distinct and non-negotiable role. The specific gravity of the acid determines its concentration, which affects how deeply and rapidly it penetrates the eggshell. The temperature at 46°C ensures the acid is active enough to terminate diapause without cooking the embryo. The immersion time of 5 to 5½ minutes is the window within which the acid achieves its effect; anything shorter may be ineffective, and anything longer risks damaging the embryo. Acid treatment parameters – specific gravity ranging from 1.076 to 1.1, temperature from 25 to 48.6°C, and time from 5 to 90 minutes – vary by method and breed. The hot acid method uses the narrower range for speed and precision.

Advantages of the hot acid method 🔗

The hot acid method allows large quantities of eggs to be treated in a short time and requires less quantity of concentrated acid compared to cold acid treatment. It does not affect non-diapausing eggs, making it suitable for treating mixed batches. These advantages make it the preferred method in commercial seed production centers where time efficiency and throughput matter.

Step 3: Washing and drying after acid treatment 🔗

Once the immersion time is complete, the acid must be removed immediately and completely. After dipping, the eggs are washed in water at a temperature of 15-30°C for 15-20 minutes. Washing is followed by drying in the shade. The washing step halts the acid’s action precisely, preventing over-treatment. Drying must always be done in shade – not direct sunlight – to avoid heat stress to the treated embryos.

After treatment, the eggs behave as artificial non-diapause eggs. They can either be directly incubated for hatching or cold-stored at 5°C for up to three weeks to adjust the hatching date as per the farmer’s rearing schedule.

Cold acid treatment: an alternative approach 🔗

While hot acid treatment is standard for immediate hatching needs, cold acid treatment (also called room-temperature acid treatment) is another option. Cold acid treatment uses HCl at a specific gravity of 1.10 at 15°C with a concentration of 20%, at temperatures of 23-30°C, with an immersion time of 40 to 90 minutes depending on temperature. In Karnataka, India, a standard cold acid treatment is used year-round to ensure synchronized hatching of bivoltine silkworms. Cold acid treatment is safer and more reliable, and it is better suited for younger eggs, though it requires larger volumes of acid and longer exposure times.

What happens after acid treatment: incubation and hatching 🔗

Non-hibernating and acid-treated eggs are incubated by the constant temperature incubation method, maintaining eggs at 23-25°C until hatching. Optimal incubation conditions are 25 ± 1°C with a relative humidity of 75 ± 5%, with 16 hours of light and 8 hours of darkness. Before two days of expected hatching, the eggs are placed in total darkness – a process called black boxing – to ensure uniform embryo development and synchronized larval emergence on a single day.

Research on the long-term preservation of acid-treated bivoltine eggs has shown that hatchability remains high when acid-treated eggs are preserved continuously at 5°C for up to 30 days. This gives sericulturists flexibility to align hatching with mulberry leaf availability and seasonal rearing windows – a crucial advantage in commercial silk production.

Key precautions during acid treatment 🔗

Precision is everything in acid treatment. Critical precautions include monitoring acid quality, avoiding re-use of acid, maintaining the correct temperature throughout the dip, adhering strictly to the dipping duration, ensuring coordination of the process, and conducting test hatching after treatment. Workers must use protective gear – gloves, goggles, and aprons – since hydrochloric acid is a corrosive chemical. The color of eggs during and after treatment also serves as a quality indicator; any irregular discoloration signals a problem with the process.

In tropical countries like India, bivoltine silkworm eggs require very detailed and precise scientific methods – either cold storage or hot or cold acid treatment – for quality seed production. These methods demand a high degree of organisation and accuracy. A failure in any step – incorrect acid concentration, wrong temperature, insufficient washing – can result in reduced hatching rates, weak larvae, and ultimately a failed rearing crop.

The bigger picture: why acid treatment matters for silk production quality 🔗

Acid treatment is not merely a technical step – it is the foundation of planned, efficient silk production. Bivoltine silkworms are commercially preferred because they produce cocoons with better reelability and lower renditta (the quantity of cocoons needed per kilogram of reeled silk) compared to multivoltine races. However, this commercial advantage only materializes when the eggs hatch reliably, at the right time, and in good health. Acid treatment is what makes that possible.

Recent research has even explored whether elevated acid treatment temperatures – such as treating at 47.5°C for 7 minutes – can simultaneously break diapause and inhibit the spread of the pathogen Nosema bombycis, the cause of the damaging pebrine disease. This shows that acid treatment is an evolving practice, with ongoing research refining its parameters for both efficiency and biosecurity.

What do you think? Given how tightly controlled acid treatment parameters must be, how should seed production centers balance the precision required for small-scale quality control with the throughput demands of commercial sericulture? And as newer alternatives like corona treatment emerge, do you think traditional HCl-based methods will remain the industry standard for bivoltine egg diapause termination?

References

1. https://www.notesonzoology.com/sericulture/management-of-egg-larvae-and-cocoons-of-silkworm/194
2. https://en.wikipedia.org/wiki/Bombyx_mori
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC5187757/
4. https://www.quickcompany.in/patents/a-method-for-preventing-diapause-in-bivoltine-silkworm-eggs-before-laying-of-eggs
5. https://www.slideshare.net/slideshow/zl-104-devarsh-kumar/249982562
6. https://www.sciencedirect.com/science/article/abs/pii/0022191076901529
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC9406231/
8. https://www.mdpi.com/2075-4450/16/8/862
9. https://tnsericulture.gov.in/sericultureNov12/silkwormeggproduction.htm
10. https://eurekamag.com/research/002/052/002052261.php
11. https://koreascience.kr/article/JAKO200811237154536.page
12. https://www.scribd.com/document/912985139/Manual-on-Silkworm-Egg-Production