Managing Muscardine Disease in Silkworms: Fungal Infections and Their Control

Silkworm rearing is vulnerable to a range of diseases, but few are as visually striking – or as economically damaging – as muscardine. When a silkworm larva infected with this fungal disease dies, its body hardens into a mummified form and becomes coated in a dense white powdery layer. This is not just an aesthetic problem. Research from India estimates that white muscardine accounts for 10-40% of total silkworm disease losses, making it one of the most economically significant threats in sericulture. Understanding what causes it, how it spreads, and how to stop it is essential for anyone managing a silkworm rearing operation.

What is muscardine disease? 🔗

Muscardine is a fungal infection of insects. The name itself comes from the Italian word moscardino, meaning musk or grape confit – a reference to the chalky, calcified appearance of infected larvae. In sericulture, the term most commonly refers to white muscardine, the most prevalent and damaging form affecting the mulberry silkworm (Bombyx mori). Other types – green, brown, red, and black muscardine – exist and are caused by different fungal species, but white muscardine is by far the dominant concern for silk farmers.

The disease was first formally described in 1835 by Italian entomologist Agostino Bassi, who identified the causal agent as a fungus. This was historically significant: it was the first time a microorganism was demonstrated to cause disease in an animal, predating germ theory as widely understood. In Karnataka, India, the disease is locally known as Sunnakaddi or Sunnakattu Roga; in West Bengal, it is called Chuna-Kete.

The causative agent: Beauveria bassiana 🔗

Beauveria bassiana is the entomopathogenic fungus responsible for white muscardine. It grows naturally in soils throughout the world and parasitizes a wide range of arthropod species. The fungus belongs to the class Deuteromycetes and reproduces asexually by producing millions of microscopic spores called conidia. These spores are spherical, light in weight, and easily become airborne.

Infection begins when conidia from the environment or from a mummified larva land on the surface of a healthy silkworm. The spores germinate and penetrate the insect’s cuticle (outer skin) directly – no ingestion is required. Once inside, the fungal mycelium proliferates rapidly through the host’s body, eventually killing it. After death, the mycelium emerges through the body wall, covering the larva in the characteristic white growth, and produces fresh conidia that can infect surrounding worms.

Notably, B. bassiana has an extremely broad host range. It has been isolated from over 700 insect species across nine orders, which means alternate hosts such as mulberry leaf rollers and Bihar hairy caterpillars found near rearing areas can also harbor and spread the pathogen.

Symptoms and disease progression 🔗

Muscardine progresses rapidly once infection takes hold. The sequence of visible symptoms is fairly consistent:

In the initial stage, the infected larva becomes sluggish and stops feeding. Its color shifts from the normal creamy white to a pale or dull tone, and movement becomes minimal. At this point, the infection is internal and the fungus has already begun consuming the host’s tissues.

Within three to five days under low temperature and high humidity conditions, the larva dies. The body quickly stiffens as the fungal mycelium permeates the tissues. Within approximately 24 hours of death, the body becomes mummified and is covered with white fungal spores.

In some cases, infected larvae manage to spin a cocoon before dying – but the moth is unable to emerge, and the cocoon is rendered commercially unusable. Pupae and adult moths can also be infected, expanding the damage beyond just larval losses.

Why muscardine is worse in winter and the rainy season 🔗

Muscardine does not strike randomly. Specific environmental conditions create the conditions for an outbreak, and these conditions are most reliably met during two seasons.

Low temperature and high humidity 🔗

The Central Sericultural Research and Training Institute (CSRTIMYS), Mysore identifies low temperature combined with high relative humidity as the primary predisposing conditions for muscardine outbreaks. Favorable conditions for the fungus include temperatures below 25°C and relative humidity levels of 90-95%. Both conditions are common during India’s rainy season (June-October) and winter months (November-February).

Poor ventilation in enclosed rearing spaces 🔗

During winter, rearing houses are often closed to retain warmth. This traps moisture and allows fungal spores to accumulate in high concentrations within the enclosed space. A single diseased larva in the final instar can produce millions of conidia, each capable of infecting a new host. In a poorly ventilated room, these spores remain suspended in the air and spread rapidly across rearing trays.

Alternate hosts near rearing environments 🔗

Mulberry pests such as leaf rollers found on the mulberry garden can themselves carry B. bassiana infection. Their mummified bodies release conidia into the surrounding environment, which then drift into the rearing house. This is why controlling alternate hosts in and around the mulberry garden is considered part of muscardine management – not just the rearing room itself.

Breed susceptibility 🔗

Multivoltine silkworm breeds are generally less susceptible than bivoltine breeds to B. bassiana infection. Farmers rearing high-yielding bivoltine hybrids must therefore exercise greater caution during high-risk seasons.

Management of muscardine disease 🔗

There is no known cure once muscardine infection is established in a larva. Management is therefore entirely preventive and relies on three interconnected strategies: environmental control, bed hygiene, and chemical disinfection.

Environmental control: dry conditions and ventilation 🔗

Since high humidity is the primary enabler of muscardine spread, maintaining a dry rearing environment is the foundational management step. Rearing rooms should have good cross-ventilation to prevent moisture from accumulating. If the rearing house temperature falls below 22°C, it should be raised using a room heater or charcoal stove – not because warmth alone stops the fungus, but because it reduces the relative humidity inside the room.

Rearing beds should be kept thin and dry. Leftover mulberry leaves remaining in the bed after feeding can decompose and raise local humidity levels, creating micro-environments favorable to fungal growth. Removing excess leaves promptly after each feeding is therefore a routine but critical step.

Slaked lime dusting 🔗

Silkworm farmers traditionally dust their rearing trays with slaked lime to discourage fungal growth. Lime absorbs moisture from the bed surface, reducing local humidity and making conditions less hospitable for conidial germination. It is commonly applied when silkworms settle for moulting – a period when they are stationary and particularly vulnerable to infection through the integument.

Removing and destroying infected larvae 🔗

Infected larvae must be collected from the rearing bed before mummification occurs. Once a larva mummifies and the white conidia layer forms, it becomes an active spore reservoir. Collected infected larvae should be dusted with an anti-muscardine bed disinfectant and then burned. They should not be discarded on the street or fed to birds or animals, as this disperses the pathogen further into the environment.

Bed disinfectants: Vijetha and Ankush 🔗

Bed disinfectants are powdered antifungal formulations applied directly onto silkworm larvae during rearing. The most widely recommended for muscardine control in India are Vijetha and Ankush, both developed through sericulture research institutions.

Vijetha is a broad-spectrum silkworm bed disinfectant developed at the Central Sericultural Research and Training Institute (CSRTIMYS), Mysore. According to NRDC India, it is effective against muscardine, grasserie, pebrine, and flacherie, and is applied after each moult and after each bed cleaning before feeding. Field trials have shown that Vijetha application increased cocoon yield by approximately 30% in comparison to untreated rearing. A study evaluating bed disinfectants in Bihar found that Vijetha at 5 g/sq. ft. recorded the lowest disease incidence (20.50%) and the highest effective rate of rearing (79.25%) among treatments tested.

The recommended dusting schedule from CSRTIMYS is as follows:

Vijetha: Applied after each moult and on the 4th day of the final (5th) instar.
Vijetha Supplement: Applied on the 3rd day of the 4th instar, and on the 2nd and 6th day of the final instar.
Ankush: Applied after each moult, on the 3rd day of the 4th instar, and on the 3rd and 5th day of the final instar.

Fungicides and chemical measures 🔗

In addition to bed disinfectants, fungicidal compounds are used in some management protocols. The Central Silk Board recommends dusting a mixture of 1-2% Dithane M-45 (mancozeb) in slaked lime, or captaf in kaolin, directly onto the silkworm body as a specific antifungal measure. Azadirachtin and phytoalexin-based agents have also been evaluated against some muscardine pathogens. Researchers at a West Bengal extension centre demonstrated that dusting larvae with a mixture of Trichodermin-6 and lime powder could reduce muscardine infection, reflecting growing interest in biological and botanical alternatives to synthetic fungicides.

Disinfection of the rearing room and appliances 🔗

Regular disinfection of the rearing house, surroundings, and all rearing appliances (trays, nets, tools) is essential before each rearing cycle and after a disease outbreak. Recommended disinfectants include 2% bleaching powder in 0.3% slaked lime solution, or chlorine dioxide (500 ppm) in slaked lime solution – the latter has been found 100% effective against B. bassiana conidia in laboratory conditions, without adverse effects on silkworm growth or cocoon quality.

Why prevention matters more than treatment 🔗

Unlike bacterial or viral diseases where some curative interventions exist, there is currently no effective cure for muscardine once a larva is infected. The fungus kills the host from within before external symptoms are obvious enough to isolate the larva in time. By the time white mycelial growth is visible, the larva is already beyond saving – and actively releasing spores.

This makes the preventive approach non-negotiable. Maintaining dry and well-ventilated conditions, applying bed disinfectants on schedule, removing infected larvae promptly, and disinfecting the rearing environment consistently are the only reliable tools available. Research in sericulture has consistently found that the severity of muscardine outbreaks is closely tied to rearing hygiene and environmental management, not just to the presence of the pathogen – which, as a soil-dwelling organism, is virtually omnipresent in agricultural settings.

What do you think? Given that Beauveria bassiana naturally persists in soil and on mulberry plants, do you think it is realistically possible to eliminate the pathogen from the rearing environment – or is the focus better placed entirely on reducing conditions that allow it to thrive? And as bivoltine silkworm breeds are more susceptible to muscardine than multivoltine ones, how should farmers weigh higher silk yield potential against disease risk when choosing a breed?

References

1. https://www.researchgate.net/publication/288543787_Studies_on_white_muscardine_disease_of_mulberry_silkworm_bombyxmoril_in_India_-_A_review
2. https://en.wikipedia.org/wiki/Muscardine
3. https://en.wikipedia.org/wiki/Beauveria_bassiana
4. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/beauveria-bassiana
5. https://www.biologyjournal.net/archives/2025/vol7issue8/PartA/7-8-3-584.pdf
6. https://www.csrtimys.res.in/sites/default/files/menufiles/Forewarning-muscardine.pdf
7. https://nrdcindia.com/technologyDetals/238/VIJETHA%20(SILKWORM%20BED%20DISINFECTANT)
8. https://www.researchgate.net/publication/379629006_Know_how_on_essential_chemicals_used_in_Tasar_sericulture
9. https://mbimph.com/index.php/UPJOZ/article/view/5264
10. https://silks.csb.gov.in/churchandpur/diseases-and-pests-of-silkworms/
11. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/muscardine