Managing Common Diseases in Tasar and Oak Tasar Silkworms

Tasar silk is one of India’s most valued wild silks, produced by the tropical tasar silkworm Antheraea mylitta and the temperate oak tasar silkworm Antheraea pernyi. Unlike mulberry silkworms reared in controlled indoor environments, these species are raised outdoors – A. mylitta on food plants like Terminalia tomentosa, T. arjuna, and Shorea robusta, while A. pernyi feeds on oak (Quercus species) in the sub-Himalayan belt. That outdoor exposure makes them highly vulnerable to a range of pathogens. Research published in ScienceDirect estimates that tasar crop losses due to silkworm diseases collectively reach nearly 40% in India – a staggering figure with direct consequences for the livelihoods of tribal and rural farming communities. Understanding the three major diseases – Microsporidiosis (Pebrine), Virosis (Polyhedrosis), and Bacteriosis – and how to manage them is critical for every tasar sericulturist.

Microsporidiosis (Pebrine): the most persistent threat 🔗

Pebrine is caused by Nosema mylittensis, a microsporidian protozoan classified under Phylum Protozoa, Class Sporozoa, Order Microsporidia, Family Nosematidae. The name “pebrine” – a French word – was given to the disease in 1860 by De Quatrefages because the black spots appearing on infected silkworms resemble pepper grains. It is historically one of the most destructive diseases in sericulture worldwide, and the Central Silk Board’s regional extension centers identify it as the cause of 20-25% of total tasar crop losses.

What makes Pebrine particularly dangerous is its dual mode of transmission. The pathogen spreads transovarially – meaning an infected female moth passes the spores directly to her eggs – as well as perorally, through larvae ingesting contaminated food. Once the spore enters the host, it infects cells across virtually all tissues and developmental stages, from egg to moth.

Symptoms of Pebrine across life stages 🔗

Early-stage infections are often invisible to the naked eye, which makes Pebrine especially hard to detect and contain. As infection advances, a clear pattern of symptoms emerges. Infected eggs show reduced mucin, resulting in poor adhesion to the substrate, irregular hatching, and higher numbers of unfertilized or dead eggs. In larval stages, worms become sluggish and lose appetite; growth becomes uneven, moulting turns irregular, and – in heavy infestations from the third instar onwards – the characteristic black pepper-like spots appear across the body. Infected pupae are underweight with shrunken, deformed abdomens, while adult moths show crumpled wings, loss of scales, and poor mating and egg-laying ability. Studies on larval pathology confirm that mortality due to Pebrine accelerates from the third instar and peaks in the fifth, with significant loss of larval body weight and a corresponding drop in silk gland weight.

How Pebrine spreads and why stress matters 🔗

The transovarial transmission route means that if a female moth is infected, 100% of her F1 progeny can carry the disease – even if the parents appear outwardly healthy. Research from the Central Tasar Research and Training Institute (CTR&TI), Ranchi shows that environmental stressors – high larval density, leaf maturity, and temperature-humidity fluctuations – significantly increase the rate of spread. In experiments, Pebrine incidence ranged from 36% to nearly 80% depending on stress conditions and disease inoculum levels.

Control measures for Pebrine 🔗

Since there is no cure once larvae are infected, prevention and early detection are the only effective strategies. The most important step is using disease-free layings (DFLs) – eggs certified free from Pebrine through mother moth examination. This method, pioneered by Louis Pasteur in the 1860s and still in use today, involves microscopically examining the crushed body of the female moth before her eggs are used for rearing. Surface disinfection of eggs using agents like Depuratex (developed by CTR&TI) helps eliminate transovum contamination on the egg chorion. Additional measures include burying diseased larvae and rearing waste away from the rearing site, and thorough disinfection of all rearing equipment. Dusting with 5% bleaching powder mixed with slaked lime has been shown to inactivate N. mylittensis spores effectively. Rearing under lower temperature and disinfected conditions also helps minimize Pebrine incidence.

Virosis (Polyhedrosis): the leading cause of crop loss 🔗

Among all tasar silkworm diseases, Virosis is responsible for the highest crop losses – estimated at 25-30% of total disease-related losses. The causative agent is Antheraea mylitta Cytoplasmic Polyhedrosis Virus (AmCPV), a Reovirus that produces polyhedral inclusion bodies (polyhedra) in the cytoplasm of infected midgut epithelial cells. In oak tasar silkworms (A. proylei), a distinct but related pathogen – the nucleopolyhedrovirus (AnprNPV) – causes what is known as tiger band disease, characterized by dark stripes across the larval body. A study published in the Journal of Insect Science confirmed that this virus can spread through both horizontal (oral ingestion of occlusion bodies) and vertical (trans-ovum) transmission pathways, and infects all developmental stages of the silkworm.

Virosis is most severe during the first and second crop rearings and is intensified by poor sanitation, bad weather, and inadequate disinfection. All tasar eco-races are susceptible to AmCPV, though susceptibility levels differ between them. Once larvae are infected, the disease is extremely difficult to control – making prevention the priority.

Recognizing the symptoms 🔗

Early symptoms of virosis are atypical and easy to miss. As the disease progresses, larvae lose appetite and become sluggish. The body elongates and turns brownish, the head appears disproportionately large relative to the body, and bristles become unusually long. Larvae lose the clasping power of their prolegs and eventually hang head-downwards from host plant twigs, attached only by their caudal legs. In terminal stages, the internal body contents disintegrate, and dark brown foul-smelling fluid oozes from the mouth. The larva dies shortly after. In oak tasar silkworms infected with AnprNPV, distinctive dark tiger-like bands appear across the body before death.

Control measures for Virosis 🔗

According to the Central Silk Board, several chemical disinfectants are effective against AmCPV, including 0.01% Sodium hypochlorite, slaked lime, chlorinated lime, formalin, and proprietary formulations like TKO (Tasar Keet Oushad), Jeevan Dhara, and Jeevan Suraksha. Leaf Surface Microbe (LSM) treatment has also shown effectiveness. These should be applied preventively to the rearing environment and host plant foliage before and during rearing. Removing and destroying infected larvae immediately, avoiding overcrowding, and using leaves of optimal maturity – neither too young nor overly mature – all reduce the risk of outbreak. Because stress factors like high larval density and adverse weather directly amplify virosis spread, maintaining optimal rearing conditions is as important as chemical management.

Bacteriosis: a recurring seasonal threat 🔗

Bacteriosis accounts for 10-15% of tasar crop losses and can sometimes exceed that figure during peak seasons. It is most prevalent during June and July, when warm, humid weather favors bacterial proliferation. CSB extension resources document that the main bacterial pathogens responsible for the characteristic anal and rectal symptoms in tasar silkworms include gram-positive Bacillus species and gram-negative cocci such as Micrococcus, with chain-type excreta linked to microbacterial infection. More recent field isolations have also identified Serratia nematodiphila and Serratia marcescens as agents causing anal lip sealing and related septicemic symptoms.

A key factor in bacteriosis is host resistance. Healthy, well-nourished larvae produce antibacterial and antiviral factors in the gut and hemolymph that keep pathogens in check. When larvae are nutritionally stressed – from low-quality leaves, overcrowding, or metabolic imbalance – this resistance drops sharply, making them far more susceptible to bacterial infection. A study from Research Square (2024) isolated a highly virulent new bacterial strain from diseased second instar larvae that caused up to 96% mortality, underscoring the continuing risk of emerging bacterial pathogens in tasar culture.

Symptoms of Bacteriosis 🔗

Initial signs are behavioral: larvae become immobile and sluggish, lose their appetite, and grow irritable. As the disease advances, the body becomes soft (flaccid), elongated, and thin, and the larva loses its grip on the host plant. Three distinct clinical presentations emerge in affected larvae. Sealing of anal lips occurs when soil-colored sticky semisolid fluid oozes from the colon and clogs the anal opening. In chain-type excreta, fecal beads are expelled embedded in a jelly-like substance that strings them together. In rectal protrusion, the rectum bulges outward as a transparent sac filled with hemolymph – one of the most visually distinct signs of advanced bacteriosis. Death follows as the internal systems fail.

Control measures for Bacteriosis 🔗

Because bacteriosis is closely tied to larval stress and host plant quality, management begins with ensuring larvae receive fresh, appropriately mature leaves and are not overcrowded on rearing trees. Prompt removal and disposal of diseased larvae and contaminated frass is essential to prevent spread within a rearing batch. Chemical management involves spraying or dusting with formulations like Tasar Keet Oushad (TKO), Jeevan Suraksha, Jeevan Dhara, and Leaf Surface Microbe (LSM) treatment, all developed and validated for use in tasar rearing environments. Maintaining general field hygiene – clearing away dead plant debris, disinfecting rearing tools, and avoiding rearing during peak bacterial disease seasons without preventive cover – significantly reduces outbreak risk.

Integrated disease management: the practical approach 🔗

In reality, Pebrine, Virosis, and Bacteriosis rarely occur in complete isolation. Stress conditions that favor one disease often predispose silkworms to others. This is why researchers at the Central Tasar Research and Training Institute advocate an integrated disease management module that combines seed health monitoring, preventive chemical application, field hygiene, and stress reduction into a single coordinated framework. Technologies like Pebrine Visualization Solution (PVS) for easier spore detection, Depuratex for egg surface disinfection, and validated chemical protocols for virosis and bacteriosis control are now being actively adopted by farmers across tasar-producing states.

For oak tasar sericulture specifically – practiced across the sub-Himalayan states including Arunachal Pradesh, Manipur, Meghalaya, and Uttarakhand – disease management must also account for the tiger band disease caused by AnprNPV, which shares cross-infectivity with related wild silkworm species and can spread rapidly in the autumn rearing season when temperature and humidity are high. Early detection, rearing in sanitary conditions, and strict quarantine of diseased batches remain the most reliable defenses.

Managing diseases in outdoor silkworm rearing is inherently more challenging than in indoor systems. There is no single silver bullet – consistent hygiene, quality seed selection, timely application of approved disinfectants, and attention to larval nutrition together determine whether a tasar rearing season is a success or a loss.

What do you think? Given that tasar silkworms are reared outdoors and exposed to natural stress factors year-round, do you think chemical disinfection alone is sufficient for long-term disease management – or does sustainable tasar sericulture ultimately depend more on developing disease-resistant silkworm strains? And how can the adoption of new diagnostic tools like Pebrine Visualization Solution be accelerated among smallholder farmers who may lack access to laboratory facilities?

References

1. https://www.sciencedirect.com/science/article/abs/pii/S0580951721000106
2. https://silks.csb.gov.in/jhansi/diseases-and-pests-of-silkworms/
3. http://kpubs.org/article/articleMain.kpubs?articleANo=E1IEAM_2013_v26n1_1
4. https://www.researchgate.net/publication/285143634_THE_EFFECT_OF_DIFFERENT_STRESS_FACTORS_ON_THE_SPREAD_OF_VIROSIS_AND_PEBRINE_DISEASES_IN_TASAR_SILKWORM_REARING
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC9543400/
6. https://silks.csb.gov.in/mahoba/diseases-and-pests-of-silkworms/
7. https://www.researchsquare.com/article/rs-3631387/v1
8. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/antheraea-mylitta