Overview of Common Silkworm Diseases and Their Impact on Cocoon Production

Sericulture – the practice of rearing silkworms (Bombyx mori) to produce raw silk – supports the livelihoods of millions of farmers across Asia and beyond. But silkworms are highly susceptible to disease, and outbreaks can devastate an entire cocoon crop within days. Pathogens including viruses, bacteria, fungi, and microsporidia cause an estimated 20% annual loss of potential cocoon production in commercial sericulture. The four diseases responsible for the bulk of these losses are Grasserie, Flacherie, Muscardine, and Pebrine. Understanding each disease – its cause, how it spreads, what it looks like, and how to manage it – is fundamental to protecting silk yield and farm income.

Grasserie: the viral threat 🔗

Grasserie is considered one of the most destructive silkworm diseases globally. It is caused by Bombyx mori nucleopolyhedrovirus (BmNPV), a double-stranded DNA baculovirus that replicates inside the nuclei of host cells. The disease occurs throughout the year in tropical countries and is linked to more than 15% loss in cocoon yield. In some Indian states, NPV infection accounts for nearly half of all disease-related mortality.

The name comes from the French word grasse, meaning thick or sticky – a reference to the fluid that oozes from dying larvae. The virus spreads primarily through oral ingestion of contaminated mulberry leaves, though it can also enter through skin wounds. The incubation period is typically six to ten days, and most infections become visible during the fourth to fifth larval instar.

Symptoms of grasserie 🔗

Infected larvae show a distinct set of symptoms that experienced farmers learn to recognize quickly. The body becomes swollen, particularly in the intersegmental regions, and the skin takes on a shiny, turgid appearance. Larvae become restless and stop settling for moulting. In the terminal stage, the fragile integument ruptures easily, releasing a milky white fluid packed with viral polyhedra. Dead larvae characteristically hang upside down from the rearing surface, suspended by their hind legs.

Management of grasserie 🔗

There is no cure once larvae are infected, making prevention the only effective strategy. Rearing appliances should be sun-dried for one to two days, and the rearing room disinfected with 5% bleaching powder. Infected larvae, their faecal matter, and bed refuse must be collected and burned immediately. Bed disinfectants such as Vijetha or Resham Keet Oushadh are applied after each moult. Maintaining proper bed spacing, adequate ventilation, and a supply of high-quality mulberry leaves significantly reduces disease pressure.

Flacherie: the bacterial disease 🔗

Flacherie is a broad syndrome caused by bacterial pathogens – primarily Streptococcus and Staphylococcus species – though viruses such as the infectious flacherie virus (BmIFV) can act as a predisposing agent that opens the door to secondary bacterial invasion. Viral diseases of silkworms, which include infectious flacherie, are responsible for nearly 80% of total cocoon loss when combined across disease categories. Flacherie thrives under poor rearing conditions – fluctuating temperatures, high humidity, overcrowded beds, and feeding with wet or low-quality leaves all create ideal conditions for bacterial proliferation.

Symptoms of flacherie 🔗

Diseased larvae are stunted, sluggish, and soft. They vomit gut juice, and their faeces become soft, moist, and chain-shaped. The cephalothoracic (head and chest) region may appear translucent. As the disease progresses, larvae become completely flaccid, emit a distinct foul odour, and their body fluids turn dark. The smell is often the first sign noticed in a rearing room with a developing outbreak. Unlike grasserie, the skin does not rupture – the body simply rots internally.

Management of flacherie 🔗

Hygiene and environmental control are the primary defences. Rearing rooms and all equipment should be thoroughly disinfected before and after each crop cycle. Overcrowding must be avoided, and injured or weak larvae removed promptly since wound entry is a common infection route. Antibiotics including Streptomycin, Tetracycline, or Ampicillin can be applied in severe cases, though these are supportive measures rather than cures. Feeding larvae only with fresh, dry, nutritious mulberry leaves is equally critical – wet leaves are a primary carrier of bacteria into the rearing bed.

Muscardine: the fungal disease 🔗

Muscardine is a group of fungal diseases caused by entomopathogenic fungi. The most economically important is white muscardine, caused by Beauveria bassiana, while green muscardine is caused by Spicaria prasina (also referred to as Metarhizium anisopliae). In India, white muscardine alone accounts for 10-40% of cocoon crop losses, with the highest incidence during rainy and winter seasons when low temperatures and high relative humidity favour fungal spore germination.

Unlike viral and bacterial diseases, infection in muscardine is percutaneous – fungal conidia (spores) land on the larval body surface and penetrate directly through the cuticle (skin). Spores are spread through air, physical contact, mummified cadavers, and contaminated rearing equipment. Favourable conditions for germination are temperatures below 25°C combined with relative humidity above 90%.

Symptoms of muscardine 🔗

Early signs include loss of appetite, sluggishness, and the appearance of small oily or moist specks on the larval skin. Larvae vomit and eventually stop moving. After death, the body does not decompose – instead, the cadaver hardens, mummifies, and white mycelia emerge from intersegmental membranes before gradually covering the entire body with white or green powdery conidia. This distinctive mummification is the hallmark sign of muscardine and makes identification straightforward in the field.

Management of muscardine 🔗

Controlling humidity is the single most effective preventive measure. Rearing beds should be kept thin and dry, with good air circulation throughout the rearing room. All appliances should be disinfected with 5% bleaching powder, and mummified larvae removed and destroyed immediately to prevent spore dispersal. Applying Dithane M45 (3 kg per 100 disease-free layings) or Vijetha supplement as a bed disinfectant is recommended in affected rearing batches. For Aspergillus-related muscardine infections in young silkworms, rearing rooms and trays should be disinfected with 4% pentachlorophenol.

Pebrine: the microsporidian disease 🔗

Pebrine holds a unique place in sericulture history. In the 19th century it nearly wiped out the European silk industry, and it was Louis Pasteur’s investigation of this disease that established foundational principles of germ theory. The disease is caused by the microsporidian parasite Nosema bombycis, which infects eggs and is transmitted to the next generation – a property known as transovarial transmission. This makes pebrine categorically different from other silkworm diseases: it is inherited, not just acquired.

The name comes from the French word for pepper – poivre – because infected larvae develop small, dark spots on their skin resembling pepper grains. If silkworms acquire the parasite in their larval stage, there may be no visible signs, but infected mother moths pass the microsporidium to their eggs, and all larvae hatching from those eggs die before completing their development.

Symptoms of pebrine 🔗

Primary infection – when larvae hatch from infected eggs – typically results in more than 50% mortality before the third moult, with very few larvae surviving to spin cocoons. Secondary infection, contracted through contamination during rearing, is more variable in its effects: larvae infected during the early fourth instar produce thin, flimsy cocoons, while those infected during the fifth instar may produce normal-looking cocoons with reduced internal quality. Diseased larvae show slow growth, undersized bodies, poor appetite, and pale, flaccid skin with tiny black spots on the integument. Crucially, dead larvae from pebrine remain rubbery and do not undergo rapid putrefaction – a distinguishing feature from flacherie.

Management of pebrine 🔗

Because the disease is seed-transmitted, management centres on ensuring disease-free layings (DFL) – eggs produced only from mothers confirmed to be free of Nosema spores. The standard method, developed by Pasteur, involves examining the body fluid of each mother moth under a microscope after egg-laying; any batch linked to a positive moth is destroyed. Egg cards should be surface-disinfected in 2% formalin for 10 minutes before incubation. Strict hygiene throughout the rearing cycle, prompt destruction of diseased material, and disinfection of all rearing rooms and utensils are essential supporting measures. If spores are detected during rearing, the entire affected lot must be rejected.

The economic impact of silkworm diseases on cocoon production 🔗

The cumulative toll of these four diseases on sericulture is substantial. Annual cocoon crop losses in India due to silkworm diseases are estimated at 30-40%, with flacherie and grasserie together accounting for the majority of disease-related mortality. China, which produces around 80% of the world’s cocoons, faces the same pressure – and the economic stakes are enormous given that silk commands premium prices in global textile markets.

Beyond direct crop losses, diseases affect the quality of cocoons that do survive. Thin shells, irregular filament lengths, and discoloured cocoons all reduce reeling efficiency and the market value of raw silk. In regions where sericulture is a primary household income source, a single diseased crop can be financially catastrophic.

Prevention and integrated disease management 🔗

Because effective curative treatments are largely unavailable once disease is established, prevention is the cornerstone of disease management in sericulture. A few core principles apply across all four disease categories:

Disinfection: The rearing house, all appliances, and surroundings must be disinfected before each batch using appropriate disinfectants. Sun-drying appliances for one to two days, followed by disinfection with 2% bleaching powder, is widely recommended. Different pathogens require different approaches – 5% formalin is effective against viral particles, bleaching powder targets bacteria and fungi, and formalin dipping addresses pebrine spores on egg cards.

Hygiene: Contaminated mulberry leaves, dead larvae, excreta, and bed refuse are the primary vectors of pathogen spread. Immediate removal and destruction of infected material – through burning or deep burial – is non-negotiable. Rearing beds should be kept dry and thinned regularly.

Environmental management: Silkworms should be reared at temperatures between 24-26°C with good ventilation and adequate bed spacing. High humidity is a shared predisposing factor for both fungal and bacterial diseases. Overcrowding accelerates disease spread.

Early detection: Daily monitoring for changes in feeding behaviour, larval size, skin appearance, and mortality patterns allows farmers to catch outbreaks before they escalate. Infected batches should be isolated or rejected at the earliest sign of infection rather than waiting for symptoms to become widespread.

Disease-free seed: For pebrine specifically, sourcing certified disease-free layings from reputable production centres is the single most important preventive step. No amount of rearing hygiene can compensate for planting infected seed material.

What do you think? Given that most silkworm diseases have no cure once infection sets in, how feasible is it for smallholder sericulture farmers – who may lack access to laboratory testing – to implement consistent early detection and disinfection protocols? And with pebrine being seed-transmitted, what systemic changes in egg supply chains could most effectively reduce its prevalence at the farm level?

References

1. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/sericulture
2. https://www.researchgate.net/publication/286369095_Grasserie_-_Nuclear_polyhedrosis_virus_NPV_disease_of_silkworm_Bombyx_mori_L
3. https://agritech.tnau.ac.in/sericulture/disese%20mgt_silkworm.html
4. https://www.sciencedirect.com/topics/medicine-and-dentistry/silkworm-disease
5. https://www.biologyjournal.net/archives/2025/vol7issue8/PartA/7-8-3-584.pdf
6. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/muscardine
7. https://en.wikipedia.org/wiki/P%C3%A9brine
8. https://egyankosh.ac.in/bitstream/123456789/9088/1/Unit-1.pdf