Understanding and Managing Grasserie Disease in Silkworms

Grasserie disease is one of the most damaging threats facing silkworm farmers today. Viral diseases alone account for nearly 70% of all silkworm crop losses due to disease, and grasserie – caused by Bombyx mori Nuclear Polyhedrosis Virus (BmNPV) – sits at the center of that problem. For anyone involved in sericulture, understanding how this disease works, how to identify it early, and how to manage it effectively can mean the difference between a healthy cocoon crop and a total loss.

What is grasserie disease?
How does the virus spread?
Recognizing the symptoms
Progressive signs to watch for
Why the summer and rainy seasons are high-risk periods
Management strategies
Rearing house and appliance disinfection
Bed disinfectant application – Vijetha and Ankush
Feeding Amruth to suppress viral activity
Isolation and removal of diseased larvae
Temperature, humidity, and ventilation control
Mulberry leaf quality and harvesting practices
Emerging research in grasserie management
Summary of key management practices

What is grasserie disease?

Grasserie, also commonly called Nuclear Polyhedrosis or Milky Disease, is a viral infection of the mulberry silkworm Bombyx mori. It is caused by BmNPV, a baculovirus classified as a polyorganotrophic pathogen – meaning it attacks multiple organ systems within the larva simultaneously. The virus replicates inside the host’s cells and forms characteristic polyhedral inclusion bodies, which can be seen under a microscope as hexagonal or pentagonal structures.

The disease is present year-round but becomes particularly severe during summer and rainy seasons. High temperature and low humidity in the rearing environment, combined with poor quality mulberry leaves, are the key predisposing factors that trigger outbreaks. Under these stressful conditions, silkworms become physiologically weakened and far more susceptible to BmNPV infection.

How does the virus spread?

Understanding the infection route is essential for effective prevention. The primary route of infection is per os – through the mouth. Silkworms become infected when they feed on mulberry leaves that have been contaminated with BmNPV polyhedra. The main source of contamination is the milky fluid that oozes from infected larvae when their skin ruptures. This fluid contains vast numbers of polyhedra and quickly contaminates leaves, rearing trays, and the surrounding environment.

Other sources of infection include the body fluids and waste of diseased larvae, as well as contaminated rearing appliances and the rearing house itself. Alternate host insects in the environment can also harbor BmNPV, making thorough sanitation of the broader rearing area critical. Once shed into the environment, the virus remains stable and infectious for extended periods, which is why the disease can recur across successive rearing batches if the rearing space is not properly disinfected.

Recognizing the symptoms

Early identification of grasserie is critical because infected larvae continue feeding and contaminating the rearing bed until their skin breaks. In the early stages, the skin develops an oily, shining appearance; as infection progresses, the skin becomes thin and fragile, and inter-segmental swelling gives the larva a bloated look. The body color shifts to a yellowish hue.

Progressive signs to watch for

As the disease advances, the following signs become visible:

Shiny integument: The larval skin takes on an unusually shiny, oily appearance early in infection.
Failure to moult: Infected larvae do not settle properly for moulting and may move restlessly along the edges of the rearing tray.
Inter-segmental swelling: The body segments swell unevenly, giving the larva a distorted, swollen appearance.
Fragile skin and milky ooze: The integument ruptures easily, releasing a turbid milky fluid (haemolymph) packed with BmNPV polyhedra that immediately contaminates the rearing environment.
Organ degeneration: At the time of death, most internal organs show signs of fatty degeneration and liquefaction. The dead larva essentially becomes a bag of polyhedral viral bodies.
Inverted hanging: Some diseased larvae are found hanging upside down on the mountages.

Why the summer and rainy seasons are high-risk periods

Grasserie can strike at any time of year, but the summer season poses a particularly major threat to silk cocoon production in tropical regions. High ambient temperatures weaken the larvae’s physiological defenses and reduce the nutritional quality of mulberry leaves available for feeding. Leaves harvested under heat stress have lower moisture content and reduced protein and carbohydrate levels – exactly the nutritional shortfall that predisposes silkworms to BmNPV infection.

During the rainy season, fluctuating humidity and warmth create similarly favorable conditions for viral spread. The combination of stressed larvae, contaminated rearing environments, and high viral load in the bed makes these seasons the most critical windows for proactive disease management.

Management strategies

There is no cure for grasserie once a larva is infected. All management is therefore focused on prevention, environmental control, and limiting the spread of BmNPV within the rearing batch. A multi-pronged approach is most effective.

Rearing house and appliance disinfection

Thorough disinfection before and during every rearing cycle is the single most important preventive step. The Central Silk Board recommends disinfecting the rearing house, surroundings, and all appliances using one of the following approved disinfectants: 0.05% Asthra solution, 2.5% Sanitech or Serichlor in 0.5% slaked lime solution, or 2% bleaching powder in 0.3% slaked lime solution. An additional round of disinfection using 0.3% slaked lime solution is recommended as a supplementary measure.

All surfaces, floors, walls, and rearing equipment must be thoroughly treated. Given that BmNPV is highly stable in the environment, incomplete disinfection can leave residual viral load that triggers disease in the very next batch of worms.

Bed disinfectant application – Vijetha and Ankush

Bed disinfectants are applied directly to the rearing bed (the surface where larvae are kept and fed) to suppress pathogens in real time. Vijetha is one of the most widely recommended and researched bed disinfectants for grasserie management. Studies have shown that Vijetha applied at 5 g per square foot recorded the shortest larval duration, highest larval weights, lowest disease incidence at 20.50%, and the highest effective rate of rearing among tested treatments.

Vijetha is formulated to be effective against all major silkworm diseases including grasserie, flacherie, pebrine, and muscardine, and is recommended for use throughout the year. It should be applied after every moult and before feeding resumes, allowing at least 30 minutes between application and the next feed. Ankush is another widely used bed disinfectant that can be used alongside or alternated with Vijetha depending on the season and disease pressure.

Feeding Amruth to suppress viral activity

Beyond disinfection, Amruth – a feed supplement developed for silkworm disease control – is recommended to be fed as per schedule specifically to suppress and control grasserie disease. It is applied according to a structured schedule during the rearing cycle and works to bolster the worms’ resistance to BmNPV, complementing the protective effect of bed disinfectants.

Isolation and removal of diseased larvae

Prompt removal of visibly infected larvae before their skin ruptures is essential to preventing further contamination of the rearing bed. Infected worms should be collected carefully using gloves or forceps, without breaking the fragile integument, and disposed of properly – ideally by burying or burning – well away from the rearing area. Allowing diseased larvae to remain in the bed or rupture on the leaves dramatically increases the viral load in the rearing environment.

Temperature, humidity, and ventilation control

Maintaining the right microclimate in the rearing house directly reduces disease pressure. During summer, humidity inside the rearing house should be raised by using wet gunny cloth screens on doors and windows, and the walls may be sprayed with water to bring the temperature down. Silkworms should ideally be reared between 24-26°C. Good ventilation – especially during late-age (fourth and fifth instar) rearing – helps prevent the warm, stagnant conditions that favor viral spread. Rearing houses with asbestos or tin roofs should be avoided during summer as they trap heat.

Mulberry leaf quality and harvesting practices

The nutritional quality of mulberry leaves directly impacts larval immunity. During summer, mulberry should only be harvested during cooler hours of the day, and stored wrapped in wet gunny cloth in a dark, humid room with frequent sprinkling of water to maintain leaf moisture. Feeding larvae with wilted, low-moisture, or nutritionally poor leaves weakens them and increases susceptibility to BmNPV. Where possible, silkworm hybrids that are specifically bred or selected for summer tolerance should be used for warm-season rearing.

Emerging research in grasserie management

Beyond conventional chemical and cultural practices, researchers are exploring newer approaches to control BmNPV. Studies have investigated the use of botanical extracts as eco-friendly antiviral agents. Research evaluating plant-based treatments found that chloroform leaf extract of Psoralea corylifolia at 1000 ppm was the most effective botanical tested against BmNPV, with treated larvae showing significantly reduced mortality.

Other studies have examined the potential of bioactive compounds like azadirachtin (derived from neem), curcumin, and allicin, with azadirachtin showing the greatest effectiveness in limiting damage from BmNPV in experimental settings. Nanoparticle-based interventions have also shown early promise in laboratory research, though these technologies are not yet in routine field use. For now, the foundation of grasserie management remains rigorous hygiene, timely bed disinfection, optimal rearing conditions, and early removal of infected larvae.

Summary of key management practices

Effective grasserie control comes down to consistent, disciplined rearing hygiene combined with targeted chemical interventions. The following practices form the recommended management framework:

Pre-rearing disinfection of the rearing house and all appliances using recommended disinfectants (Asthra, Sanitech/Serichlor, or bleaching powder in slaked lime solution).
Bed disinfectant application with Vijetha or Ankush after every moult and during the fifth instar, as per the recommended schedule and dosage.
Feeding Amruth as per schedule throughout the rearing cycle to help suppress BmNPV activity.
Prompt isolation and disposal of diseased larvae before skin rupture to prevent contamination.
Temperature and humidity regulation – maintain 24-26°C and adequate humidity, with good ventilation especially during late-age rearing.
Provision of quality mulberry leaves with adequate moisture, harvested during cooler hours.
Use of summer-tolerant silkworm hybrids during high-temperature rearing seasons.

What do you think? Given that grasserie has no cure once larvae are infected, how important do you think early visual monitoring of rearing beds is compared to routine preventive disinfection – and can one compensate for the other? With botanicals like azadirachtin showing antiviral promise in laboratory studies, do you think eco-friendly alternatives could realistically replace chemical bed disinfectants in commercial sericulture in the near future?

References

PDF 📄

Table of Contents

What is grasserie disease? 🔗

How does the virus spread? 🔗

Recognizing the symptoms 🔗

Progressive signs to watch for 🔗

Why the summer and rainy seasons are high-risk periods 🔗

Management strategies 🔗

Rearing house and appliance disinfection 🔗

Bed disinfectant application – Vijetha and Ankush 🔗

Feeding Amruth to suppress viral activity 🔗

Isolation and removal of diseased larvae 🔗

Temperature, humidity, and ventilation control 🔗

Mulberry leaf quality and harvesting practices 🔗

Emerging research in grasserie management 🔗

Summary of key management practices 🔗