Controlling Flacherie Disease in Silkworm Rearing: Causes and Prevention

Flacherie is one of the most damaging diseases in silkworm rearing, capable of wiping out 30 to 100% of a crop during peak seasons. The name comes from the French word flache, meaning flaccid – a direct reference to the most obvious sign of the disease: silkworms that become soft, limp, and lethargic instead of feeding actively and spinning cocoons. What makes flacherie particularly challenging is that it is not caused by a single pathogen. It can result from viral infection, bacterial infection, or both acting together, and poor rearing conditions dramatically increase its likelihood. Understanding how this disease develops and how to stop it is essential for anyone involved in sericulture.

What causes flacherie in silkworms? 🔗

Flacherie is broadly classified into two types – infectious (viral) flacherie and non-infectious flacherie – and the distinction matters for how you manage it.

Infectious flacherie: viral and bacterial pathogens 🔗

Viral flacherie is caused by a group of viruses that target the gut tissue of Bombyx mori, the domesticated silkworm. The primary culprits identified in research are Bombyx mori infectious flacherie virus (BmIFV), Bombyx mori densovirus (BmDNV), and Bombyx mori cypovirus 1 (BmCPV-1). BmIFV is a single-stranded RNA virus and is considered the most frequently occurring virus in commercial silkworm rearing worldwide. These viruses destroy the epithelial lining of the silkworm’s midgut, impairing digestion and nutrient absorption.

Bacterial flacherie follows closely, either independently or as a secondary infection after viral damage. The bacteria most commonly involved include species of Streptococcus, Staphylococcus, Serratia marcescens, Bacillus, and Escherichia coli. According to the Directorate of Sericulture, Government of Assam, the bacterial form is locally known as “Mukhlaga Rog” and is caused primarily by Streptococci and Bacillus bacteria. In Karnataka, another sericulturally significant state in India, the bacterial form is also referred to as “Sara hikke,” named after the beaded appearance of the fecal matter it produces.

In many real-world outbreaks, viruses weaken the gut lining first, and bacteria then invade and amplify the damage. This synergistic effect between viruses and bacteria results in much higher mortality than either pathogen would cause alone.

Non-infectious flacherie 🔗

Non-infectious flacherie, sometimes called touffée flacherie, does not involve a pathogen in the traditional sense. It is triggered by extreme heat waves and environmental stress that disrupt the silkworm’s digestive physiology. While it shares symptoms with the infectious form, the underlying mechanism is physiological collapse rather than infection. However, the resulting gut damage can still invite opportunistic bacterial colonization.

Who gets flacherie, and when? 🔗

Flacherie is not random. It follows predictable seasonal and environmental patterns. Research from the Central Silk Board of India confirms that the disease is most intense during summer and rainy seasons – particularly from June to August – when heat, humidity, and fluctuating weather create stress conditions for silkworms. The viral form peaks between April and September. These are the months when silkworm immune defenses are most taxed, and when pathogens proliferate fastest in rearing environments.

Young-age silkworms (early instars) are especially vulnerable because they are reared in high concentrations and any disease that goes undetected spreads rapidly across the entire batch. As the ScienceDirect overview on silkworm diseases notes, viral and bacterial diseases are more prevalent than other types and cause considerable crop losses – often going unnoticed until they have already spread significantly.

Predisposing conditions that trigger outbreaks 🔗

Pathogens alone do not always cause flacherie. The rearing environment plays a critical role in determining whether an infection takes hold and how severe it becomes. The following conditions are well-documented triggers:

Overcrowding is one of the most significant risk factors. When silkworms are packed too tightly in rearing trays, physical contact between healthy and infected individuals increases, and the accumulation of fecal matter and moisture creates an ideal breeding environment for bacteria.

Feeding wet or poor-quality mulberry leaves is another major contributor. Leaves with excessive moisture increase the water content in the silkworm’s gut, disrupting its pH balance and making it more susceptible to bacterial growth. Contaminated leaves – those touched by diseased larvae, their gut juice, or fecal matter – are a direct source of infection.

High temperature and humidity in the rearing room weaken the silkworm’s immune response and accelerate pathogen reproduction. Sudden temperature fluctuations are equally harmful, causing physiological stress that leaves silkworms vulnerable even without prior exposure to a pathogen.

Poor ventilation compounds humidity problems and allows infectious particles to accumulate in the rearing environment. Improper bed cleaning and the accumulation of feces and dead larvae in rearing beds also serve as ongoing contamination sources, reintroducing pathogens with every feeding cycle.

Starvation and underfeeding damage the gut lining directly. When silkworms do not receive enough food, the gut epithelium is compromised, making bacterial invasion easier. Even mild starvation accelerates bacteremia – a bacterial infection of the digestive tract.

Recognizing the symptoms of flacherie 🔗

Early identification is critical, because once flacherie spreads through a rearing batch, losses escalate quickly. The following symptoms, described by the Assam Directorate of Sericulture and corroborated by the Central Silk Board, help identify infected larvae:

Sluggish movement and lethargy are the earliest visible signs. Infected silkworms stop moving and feeding normally, sitting still while healthy larvae continue to feed actively. Loss of appetite follows quickly, leading to poor growth and underweight larvae.

Body flaccidity and softening of the skin is the hallmark symptom – the silkworm loses its normal turgid shape and becomes limp and inelastic. Shrinkage of the abdominal segments and a swollen thorax are also typical, particularly in bacterial flacherie.

As the disease progresses, infected larvae begin to vomit digestive fluid, and their fecal matter takes on an abnormal form – often appearing as a chain-like strand or showing rectal protrusion. The body color darkens, turning brown or black, and dead larvae putrefy quickly, emitting a foul odor that signals advanced bacterial decomposition.

In the viral form specifically, the head and thoracic region may appear translucent due to an empty foregut, and the body may develop a reddish tinge. This is sometimes referred to as “Red flacherie” based on this visual distinction.

Control and prevention measures 🔗

There is no curative treatment for flacherie once it takes hold. Control is entirely preventive, built around disinfection practices, rearing hygiene, and environmental management. The following measures are recommended by sericulture authorities and research institutions.

Disinfection of rearing facilities 🔗

Thorough disinfection before and after every rearing cycle is non-negotiable. The rearing house, appliances, trays, and surroundings should be treated with 0.5% bleaching powder solution or a bleaching powder-and-lime mixture at a 1:9 ratio. The Central Silk Board of India also recommends using Labex – a commercially available disinfectant for silkworm rearing – as part of the bed disinfection schedule from the chawki (young larval) stage onward. Dusting slaked lime on silkworms during molting helps reduce microbial load in the rearing bed.

Quality and handling of mulberry leaves 🔗

Only fresh, clean, and appropriately matured mulberry leaves should be fed to silkworms. Tender leaves are unsuitable for late-instar larvae and increase moisture-related gut stress. Harvested leaves should be covered with moist cloth to prevent wilting, but excessive surface water must be removed before feeding. Feeding contaminated leaves – including those that have been in contact with diseased larvae or feces – is a direct transmission route and must be strictly avoided.

Environmental management 🔗

Maintaining the recommended temperature and relative humidity (RH) is essential throughout the rearing period. Adequate ventilation should be ensured to prevent humidity buildup and pathogen concentration in the rearing space. Avoid sudden changes in temperature, particularly during sensitive molting periods. The Assam sericulture department specifically highlights maintaining optimum temperature and RH as a primary control measure against flacherie.

Rearing hygiene and bed management 🔗

Beds should be cleaned regularly and fecal matter removed before each feeding. Overcrowding must be avoided by maintaining proper spacing between silkworms. Any larvae showing signs of disease should be immediately removed, collected in disinfectant solution such as lime water or Shuchi, and then destroyed by burning or burial – never discarded carelessly where they can contaminate the rearing area. Dead larvae are a primary source of ongoing contamination and must be managed with strict hygiene protocols.

Using disease-resistant silkworm races 🔗

Where available, rearing of silkworm races that show greater resistance to viral and bacterial flacherie is a practical long-term strategy. Research into the gut microflora of Bombyx mori has also identified that beneficial bacterial strains – including certain Bacillus species – may act as probiotics that support the silkworm’s immune defense against pathogens. This is an emerging area of investigation that may support future disease management strategies in commercial sericulture.

Why early action matters in sericulture 🔗

The speed at which flacherie spreads in a rearing batch is directly tied to how early it is detected and how quickly infected individuals are removed. In large-scale rearing operations involving millions of larvae, even a brief delay in response can allow the disease to reach a point where it is economically unrecoverable. The principle – separating the healthy from the affected – was recognized as fundamental by Louis Pasteur himself during his early work on silkworm disease in the 19th century, and it remains just as relevant today.

Consistent record-keeping, regular observation of larval behavior, and adherence to disinfection schedules are the practical backbone of flacherie prevention. Sericulturists who treat disease prevention as a routine – rather than a reactive measure – consistently achieve better crop outcomes during high-risk seasons.

What do you think? Given that flacherie can cause up to 100% crop loss during peak seasons, which aspect of prevention – disinfection, rearing hygiene, or environmental control – do you consider the most critical line of defense? And how practical is it for small-scale sericulturists to maintain these standards consistently across every rearing cycle?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC4011363/
2. https://sericulture.assam.gov.in/portlet-innerpage/flacherie-mukhlaga
3. https://en.wikipedia.org/wiki/Flacherie
4. https://silks.csb.gov.in/pune/wp-content/themes/Common_District/dpm-frame2.html
5. https://www.sciencedirect.com/topics/medicine-and-dentistry/silkworm-disease