Common Diseases in Muga Silkworms and Their Control Measures

Muga silk – the golden-hued fabric native to Assam – owes its production entirely to the health of one insect: Antheraea assamensis, the muga silkworm. But despite its cultural and commercial significance, this silkworm is highly vulnerable to a range of diseases that can devastate entire rearing batches. According to the Muga Eri Silkworm Seed Organization (MESSO), Central Silk Board, the four major diseases – Flacherie, Grasserie, Muscardine, and Pebrine – remain the most serious threats to muga sericulture, with losses especially severe during seed crop rearing seasons. Understanding each disease, how it spreads, and how to control it is the first line of defence for any muga rearer.

Flacherie: the bacterial threat that can destroy an entire crop 🔗

Among all muga silkworm diseases, Flacherie stands out for its sheer destructive potential. Known locally as Mukhlaga Bemar, it is primarily caused by the bacterium Pseudomonas aeruginosa and can result in crop losses of up to 100%. The MESSO disease management booklet notes that high temperature, high humidity, and fluctuating climatic conditions during rearing are the key predisposing factors, as these conditions actively promote bacterial proliferation.

Symptoms of flacherie 🔗

Infected larvae become lethargic and stop feeding. They begin vomiting gut juice and pass semi-solid, watery faeces. As the disease progresses, the larval body turns soft, translucent, and dark – eventually the worm loses its grip on the rearing branch and falls. The characteristic foul smell from dead larvae is a strong indicator of a bacterial flacherie outbreak. Death usually follows quickly once these symptoms appear.

Prevention and control of flacherie 🔗

Since no curative treatment exists once the disease takes hold, prevention is the only practical strategy. The Assam Directorate of Sericulture recommends using only disease-free seeds for rearing and avoiding brushing of worms at damp rearing sites. Feeding stage-appropriate, quality foliage reduces larval stress and susceptibility. Maintaining hygienic rearing conditions, prompt removal of dead or sick worms, and avoiding overcrowding at the rearing site are all critical preventive measures. Rearing appliances must be disinfected before each use.

Grasserie: the viral disease that makes larvae swell and die 🔗

Grasserie, locally called Phula Rog (meaning “bloated disease”), is caused by a nuclear polyhedrosis virus. The virus enters the larval body either through the skin or through contaminated food. Once inside, it invades body cells and multiplies rapidly, triggering a cascade of visible symptoms. According to data from the Central Silk Board, high temperature, low humidity, and poor quality foliage are the main predisposing environmental factors.

Symptoms of grasserie 🔗

One of the earliest signs is an unusual sheen on the larval skin just before moulting – infected larvae fail to moult properly. The inter-segmental membranes swell noticeably, giving the larva a bloated appearance. Infected worms move restlessly along the rearing bed or the rim of rearing trays. As the disease advances, the body becomes yellowish and the larval skin ruptures easily, releasing a turbid white haemolymph. This milky fluid is itself a major source of further contamination, spreading the virus to healthy larvae nearby.

Managing grasserie infections 🔗

Strict hygiene is the cornerstone of grasserie control. Since the virus persists in the environment for extended periods, all rearing surfaces and appliances must be thoroughly disinfected. The Assam Directorate of Sericulture recommends immediate isolation and removal of larvae showing restlessness or membrane swelling. Infected material – dead larvae, contaminated bedding, and soiled rearing branches – should be disposed of by deep burial or incineration to prevent environmental spread. A 0.3% slaked lime solution applied as an optional disinfection measure can help when disease incidence was high in the previous crop.

Muscardine: when fungi mummify living larvae 🔗

Muscardine is a fungal disease with a striking and unmistakable presentation – it literally turns infected larvae into hard, chalky, mummified structures. In muga silkworms, white muscardine and green muscardine are the most common forms, caused by Beauveria species and related entomopathogenic fungi. According to a study published in the Biology Journal, white muscardine alone accounts for 10-40% of total silkworm crop losses in sericulture areas of India, and it is particularly severe under low temperatures and high relative humidity conditions (90-95%).

How muscardine spreads 🔗

The mode of infection is percutaneous – fungal conidia (spores) land on the silkworm’s body surface and penetrate directly through the cuticle (skin). They then establish infection inside the body cavity. Mummified dead worms, contaminated rearing appliances, and alternate lepidopteran hosts in the rearing environment all serve as sources of spores. The MESSO booklet notes that infected larvae initially become inactive and lose appetite, then ooze an oily substance, develop diarrhoea and vomiting, and finally die. Within 24 hours of death, the body hardens and becomes covered in white or green fungal spore masses – the classic “mummified” appearance.

Fungal disease prevention strategies 🔗

Humidity management is the single most effective preventive measure. Keeping relative humidity below 80% significantly reduces the risk of spore germination and infection. Proper spacing between larvae on rearing branches ensures adequate air circulation. The Assam Directorate of Sericulture emphasizes proper disinfection of rearing sites and appliances, and strict, continuous monitoring to catch and remove hardened or discoloured larvae before they become sources of further spore dispersal. Fungicidal treatments such as copper sulfate or lime applications can help reduce environmental fungal loads during outbreaks.

Pebrine: the inherited disease with no direct cure 🔗

Pebrine is widely regarded as the most serious disease in muga sericulture – and historically, it is the disease that prompted Louis Pasteur to develop the first systematic disease management protocol for silkworms in the 19th century. In muga silkworms, it is known locally as Phootuka Bemar (spotted disease) and is caused by the microsporidian parasite Nosema antheraea (Nosema sp.) of the family Nosematidae. What makes pebrine uniquely dangerous is its transovarial transmission – the parasite passes directly from an infected mother moth to her eggs, meaning the disease can silently propagate through generations before visible symptoms appear.

Symptoms across life stages 🔗

Pebrine affects every stage of the muga silkworm’s development. According to the MESSO disease management guide, at the egg stage, infected batches show poor hatching, fewer fertilized eggs, and irregular emergence. In larvae, the parasite invades hypodermal cells, which die and undergo melanosis – appearing as dark, pepper-like black spots on the skin. Infected larvae show retarded, irregular growth, sluggish moulting, and reduced appetite. At the pupal stage, metamorphosis may fail entirely. Adult moths emerge with distorted antennae, clubbed or incompletely stretched wings, discoloured wing scales, and significantly reduced egg-laying capacity.

There are two main routes of infection. Primary (vertical) transmission occurs through infected eggs from the mother moth – when heavily infected, more than 50% of larvae may die before the third moult. Secondary transmission occurs through contamination during rearing, via infected leaves, rearing appliances, or litter in the soil. The Assam Directorate of Sericulture confirms that the life cycle of Nosema sp. completes in about a week in winter and as quickly as four days in summer, accelerating disease spread during warmer months.

Comprehensive pebrine management 🔗

No curative treatment exists for pebrine, making prevention and early detection entirely critical. The primary control measure, as established by the Assam Directorate of Sericulture, is individual mother moth examination during grainage using the Fujiwara microscopic method. Only disease-free egg lots should be used for rearing. Disease-free eggs must be surface-sterilised with 0.2% sodium hypochlorite solution for 5 minutes before use. All grainage equipment – kharika, chaloni, chakaripera – must be disinfected with 0.2% sodium hypochlorite or 5% bleaching powder solution before and after every grainage operation. If pebrine is detected during rearing, the entire infected crop must be destroyed immediately, rearing plants pruned and defoliated, and the site disinfected thoroughly. Dead larvae, excreta, and contaminated material must be burned – not simply discarded.

Integrated disease management: the bigger picture 🔗

While each disease has specific control measures, a unified, integrated approach provides the strongest protection. The MESSO management booklet identifies several common risk factors across all four diseases: poor quality seeds, low-nutrition foliage, overlapping rearing at the same site, contaminated rearing appliances, and unhygienic practices. Addressing these shared vulnerabilities creates a foundation that reduces susceptibility to all pathogens simultaneously.

Key integrated practices include:

• Use of certified, microscopically tested disease-free seeds – the most critical upstream intervention, particularly for pebrine prevention.
• Regular monitoring and immediate removal of diseased larvae – early detection limits pathogen spread across the rearing batch.
• Strict disinfection protocols – rearing appliances, grainage tools, and the rearing environment should be disinfected with appropriate agents (sodium hypochlorite, bleaching powder, formalin) before and after each rearing cycle.
• Environmental management – controlling temperature, humidity, and ventilation reduces conditions favorable to bacterial, viral, and fungal pathogens alike.
• Proper disposal of infected material – burning dead larvae, excreta, and contaminated material breaks the disease transmission cycle and prevents secondary spread.

The economic importance of disease control in muga sericulture 🔗

Muga silk is one of India’s most prized natural textiles, and the health of the muga silkworm is directly tied to the livelihoods of thousands of farming families in Assam and the northeastern states. The Biology Journal study on white muscardine estimates that silkworm diseases collectively cause annual cocoon crop losses of 30-40% across India’s sericulture zones. At the farm level, an uncontrolled outbreak of flacherie or pebrine during seed crop production can mean a total loss – not just of the current crop, but of the seed stock needed for the next rearing cycle. Investing in disease prevention is, in economic terms, far cheaper than recovering from a major outbreak.

As research institutions like the Central Silk Board continue to refine management protocols and develop disease-resistant seed stocks, the practical knowledge of rearers on the ground remains equally important. Timely observation, sound hygiene practices, and disciplined adherence to disinfection schedules are tools available to every muga farmer – and they make the most significant difference in protecting crop yields.

What do you think? Given that pebrine has no cure and can spread silently through generations via infected eggs, how feasible is it for small-scale muga rearers in rural Assam to consistently access microscopically tested, disease-free seeds? And considering that flacherie can wipe out 100% of a crop under poor environmental conditions, should climate-adaptive rearing practices be more central to training programmes for muga sericulture farmers?

References

1. https://messo.org.in/wp-content/uploads/2025/03/Muga-disease-booklet.pdf
2. https://sericulture.assam.gov.in/portlet-innerpage/flacherie-mukhlaga
3. https://silks.csb.gov.in/coochbehar/wp-content/themes/common_district/coochbehar/dpm-frame2.html
4. https://sericulture.assam.gov.in/portlet-innerpage/grasserie-phula-rog
5. https://www.biologyjournal.net/archives/2025/vol7issue8/PartA/7-8-3-584.pdf
6. https://sericulture.assam.gov.in/portlet-innerpage/muscardine
7. https://sericulture.assam.gov.in/portlet-innerpage/protozoan-disease-pebrine-of-muga-silkworm
8. https://sericulture.assam.gov.in/portlet-innerpage/prevention-and-control-of-the-disease