Identifying and Managing Diseases in Eri Silkworms

Eri silkworm (Samia ricini), the only fully domesticated non-mulberry silkworm, is a cornerstone of sericulture in Northeast India and produces a uniquely soft, spun silk known as eri or errandi silk. Yet despite their commercial importance, these larvae are highly vulnerable to disease. A single outbreak can wipe out an entire rearing batch within days – and annual cocoon crop losses in India due to silkworm diseases are estimated at 30-40%. Three diseases – Grasserie, Flacherie, and Muscardine – are the primary culprits. Understanding how each disease works, what symptoms to watch for, and how to respond is essential for anyone involved in eri culture.

Why eri silkworms are particularly disease-prone 🔗

Eri silkworms thrive in the humid, warm conditions of Northeast India – the same conditions that pathogens love. Production of eri silk in this tropical region is frequently threatened by microbial diseases, largely because the rearing environment – often characterized by warmth, humidity, and crowded trays – creates ideal conditions for viruses, bacteria, and fungi to spread. Compounding this is the fact that stress factors such as poor-quality leaves, temperature fluctuations, and overcrowding further weaken the larvae’s natural defenses. Once one larva is infected, the pathogen spreads rapidly through contaminated feces, shed skin, and shared food surfaces.

The three major diseases affecting eri silkworms – Grasserie, Flacherie, and Muscardine – each have a distinct causal agent, transmission route, and set of symptoms. Recognizing them early is the first step toward effective management.

Grasserie: the viral disease that bloats and shines 🔗

Grasserie is a viral disease caused by a Nuclear Polyhedrosis Virus (NPV). It is one of the most widespread diseases in silkworm rearing globally, and eri silkworms are no exception to its reach. The disease peaks during high-temperature, low-humidity periods, and the infection enters when silkworms feed on contaminated castor or other host plant leaves. The virus can also spread through milky-white fluid released by already-infected larvae, which contaminates the rearing bed, trays, and appliances.

Symptoms of grasserie 🔗

The name “Grasserie” comes from the French word for fat or greasy – a fitting description of what infected larvae look like. Key symptoms include:

• Shiny, translucent skin: The body surface becomes noticeably glossy before moulting, and the larva fails to moult properly.
• Swollen, bloated body: Inter-segmental areas swell up, giving the larva an abnormally inflated appearance.
• Yellowish discoloration: The body color shifts toward a pale yellow as the disease progresses.
• Restless movement: Infected larvae move erratically along the rim of rearing trays, unable to settle.
• Skin rupture: In the final stage, the weakened skin breaks open, releasing turbid, whitish haemolymph that contaminates everything around it.

This fluid is highly infectious and, if not removed immediately, will spread the virus to neighboring healthy larvae through the shared rearing surface and food.

Control of grasserie 🔗

Since there is no curative treatment once infection is established, control relies entirely on prevention and rapid response. Thorough disinfection of the rearing house and all appliances is the most effective method of destroying disease-causing pathogens. Practical steps include:

• Disinfecting rearing rooms and equipment before each rearing batch using recommended disinfectants such as 2% bleaching powder or slaked lime solution.
• Removing and destroying any larvae showing early symptoms – the infected larvae, their shed skins, and fecal matter must all be removed and burned or buried.
• Feeding only clean, dry, good-quality leaves and avoiding leaves that may have been contaminated by diseased insects.
• Maintaining optimal temperature and humidity to avoid stressing the larvae and lowering their resistance.
• Applying bed disinfectants like Vijetha or Amruth as per the recommended schedule after each moult.

Flacherie: when a virus opens the door for bacteria 🔗

Flacherie is a more complex disease than Grasserie because it involves two stages of infection. It typically begins as a viral infection – caused by Bombyx mori Infectious Flacherie Virus or Densonucleosis Virus – and is then followed by secondary bacterial infection by pathogens such as Streptococcus, Staphylococcus, Bacillus thuringiensis, or Serratia marcescens, either individually or in combination. Flacherie has been identified as the most predominant disease of silkworms, accounting for nearly 47.9% of disease-related mortality, with bacterial losses in the sericulture industry reaching up to 70% in some reports.

Grasserie and Flacherie incidence peaks during the summer and rainy seasons, making these months a critical period for monitoring. The disease spreads when silkworms eat leaves contaminated with the gut juice, fecal matter, or body fluid of already-infected larvae.

Symptoms of flacherie 🔗

Flacherie progresses quickly and has a distinctive set of signs:

• Loss of appetite and sluggishness: Infected larvae stop feeding and become increasingly lethargic.
• Soft, flaccid body: The larval body loses its firmness and becomes limp.
• Vomiting and loose feces: Larvae vomit gut juice and produce soft, watery droppings with high moisture content.
• Blackening of the body: As bacteria take hold, the haemolymph darkens and the body begins to turn black, typically progressing from the posterior end forward.
• Foul smell: Decomposing tissues produce a characteristic odor that makes the disease relatively identifiable – though by the time this odor is apparent, significant damage has already occurred.

Control of flacherie 🔗

The transmission route – contaminated leaves consumed orally – means that leaf hygiene and rearing conditions are the most important control points. Key measures include:

• Never feeding wet or poor-quality leaves; moisture on leaf surfaces dramatically increases bacterial activity.
• Ensuring adequate spacing between larvae to avoid overcrowding, which accelerates the spread of infection.
• Maintaining proper ventilation to reduce humidity levels in the rearing room.
• Promptly collecting and destroying all dead or diseased larvae along with fecal matter and rearing bed refuse.
• Applying bed disinfectant Vijetha (3 kg per 100 disease-free layings) after each moult and half an hour before feeding resumes.
• Feeding Amruth as per schedule, which is recommended specifically to control Flacherie in silkworm rearing.

Muscardine: the fungal disease that mummifies larvae 🔗

Muscardine stands apart from the other two diseases in both its cause and its distinctive visual outcome. It is a fungal disease, with white muscardine being the most prevalent form, caused by the entomopathogenic fungus Beauveria bassiana (also historically referred to as Botrytis bassiana). White muscardine alone accounts for 10-40% of crop losses, with peak incidence during rainy and winter seasons when temperatures fall below 25°C and relative humidity rises to 90-95%.

The disease has been known to sericulturists for over two centuries – it was first formally described by Italian entomologist Agostino Bassi in 1835 and is locally known in Karnataka as Sunnakaddi roga and in West Bengal as Chuna-Kete.

How muscardine infects and kills 🔗

The infection begins when fungal spores (conidia) land on the silkworm’s body surface. They germinate and penetrate the cuticle, entering through respiratory openings or tiny wounds. Once inside, the fungus grows through the body cavity, consuming internal tissues and killing the larva within four to five days. Within 24 hours of death, the larval body becomes mummified and covered with white fungal spores. The dead larva, now encased in a cotton-like white mycelium, becomes a spore-releasing factory that contaminates the entire rearing environment.

Symptoms of muscardine 🔗

• Loss of appetite and inactivity: Among the first visible signs, infected larvae stop feeding and become slow.
• Moist specks on skin: Small damp patches appear on the cuticle surface.
• Vomiting and flaccidity: The larva vomits and the body becomes soft.
• Mummification: After death, the body hardens and is progressively covered by white powdery fungal growth, eventually turning completely chalky white – the hallmark sign of muscardine.

Occasionally, infected larvae manage to spin cocoons but die inside before moth emergence, causing further economic loss.

Control of muscardine 🔗

Because fungal spores are highly resilient and persist on surfaces, rearing equipment, and in the air, muscardine is particularly difficult to eliminate once it appears. Disinfecting rearing houses with 2% bleaching powder and applying plant-based bed disinfectants like Ankush on schedule has shown measurable improvement in cocoon yields in trials conducted across sericulture farms in Andhra Pradesh. Additional measures include:

• Maintaining low humidity and good air circulation in rearing rooms – fungal spores thrive in stagnant, moist conditions.
• Removing and burning mummified larvae immediately; they should never be left in the rearing tray.
• Thoroughly disinfecting all rearing appliances between batches, as spores can survive on surfaces for extended periods.
• Avoiding injuries to larvae during handling, since wounds provide easy entry points for fungal spores.
• Applying bed disinfectants Vijetha and Ankush as per the recommended schedule throughout the rearing cycle.

General hygiene and disinfection: the foundation of disease management 🔗

Across all three diseases, the most consistent and effective defense is a strict hygiene and disinfection protocol. Silkworm diseases are better prevented than cured, and disinfection is one of the most important activities in silkworm rearing. Diseased larvae release pathogens that are highly stable and persist for long periods in the rearing environment, making thorough decontamination essential before each new batch.

Chemical disinfection using agents like 2% bleaching powder in 0.3% slaked lime solution is widely recommended by the Directorate of Sericulture, Government of Assam and endorsed by the Central Silk Board. Disinfection must cover the rearing house, rearing trays, feeding equipment, and the surrounding area. Importantly, disinfection must be carried out only when silkworms are not present, and sufficient drying time must be allowed before reintroducing larvae to treated spaces.

Beyond chemical disinfection, maintaining rearing conditions within the optimal range for eri silkworm health is equally important. Farmers must maintain adequate hygiene in their host plants to prevent the spread of diseases or pests from the field to the silkworm rearing area. Sun-drying of rearing appliances between batches, prompt disposal of rearing bed refuse, and controlled access to rearing areas all contribute to reducing pathogen load significantly.

Summary: key differences at a glance 🔗

The three diseases differ in causal agent, seasonality, and symptoms. Grasserie is viral, peaks in summer, and is identified by shiny, bloated larvae with rupturing skin. Flacherie involves viral and bacterial co-infection, peaks in summer and monsoon, and is marked by sluggish behavior, blackening body, and foul odor. Muscardine is fungal, peaks in winter and rainy seasons, and produces its unmistakable signature – mummified, chalk-white larvae. All three spread primarily through contaminated food, infected fecal matter, and direct contact between larvae. And all three are far easier to prevent than to treat once established.

What do you think? Given that eri silkworm rearing is largely practiced as a traditional cottage industry by rural and tribal communities in Northeast India, what challenges might small-scale farmers face in implementing rigorous disinfection protocols? And as climate patterns shift – with more erratic rainfall and temperature swings – how do you think disease pressure on eri silkworm farms is likely to change in the coming decades?

References

1. https://www.biologyjournal.net/archives/2025/vol7issue8/PartA/7-8-3-584.pdf
2. https://indianentomology.org/index.php/ije/article/view/3448
3. https://silks.csb.gov.in/coochbehar/wp-content/themes/common_district/coochbehar/dpm-frame2.html
4. https://egyankosh.ac.in/bitstream/123456789/9088/1/Unit-1.pdf
5. https://www.isroset.org/pub_paper/IJSRBS/13-IJSRBS-02606-19.pdf
6. https://koreascience.kr/article/JAKO200921752341398.page
7. https://sericulture.assam.gov.in/portlets/diseases-of-silkworm
8. https://www.researchsquare.com/article/rs-3252027/v1