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Grasserie and flacherie — top silkworm diseases Photo: Ministry of Textiles · GODL-India · source ↗

Grasserie and flacherie — top silkworm diseases

Grasserie and flacherie are the two most economically damaging diseases of mulberry silkworm Bombyx mori in tropical Indian rearings. Together they account for the majority of late-instar crop failures recorded by Central Silk Board diagnostic centres. Grasserie is a viral disease caused by the Bombyx mori nucleopolyhedrovirus (BmNPV) and flacherie is a complex bacterial-and-viral gut disease. Both express most severely in 4th and 5th instar worms — exactly the stage where investment in leaf and labour is already committed — and both are far more prevalent in bivoltine rearings on CSR hybrids than in older crossbreed races.

Identification — grasserie (BmNPV)

Affected larvae swell and become shiny with a translucent intersegmental membrane. Movement slows; the integument tears easily under handling and releases a milky-white, turbid fluid containing virus polyhedra. Late-stage worms hang head-down from mountages and rearing trays. The condition is locally called milky disease or hanging disease. Polyhedral inclusion bodies are confirmed by simple wet mount under 400x microscopy at chawki rearing centres (Chawki Rearing Centre).

Identification — flacherie

Affected larvae stop feeding, become flaccid and shrunken with darkening at the head and anal regions. Vomiting of brownish gut content and watery, foul-smelling diarrhoea is common. Bodies progress rapidly to total collapse and decomposition. The complex involves cytoplasmic polyhedrosis virus (BmCPV), infectious flacherie virus (BmIFV), and secondary bacterial overgrowth (Bacillus, Streptococcus, Serratia) feeding on weakened gut tissue.

Hosts and lifecycle

BmNPV and the flacherie complex agents are obligate or facultative pathogens of Bombyx mori with no alternative hosts. They persist between batches in dried larval cadavers, contaminated rearing trays, mountages, floor cracks and unwashed worker clothing. Transmission is per-os — worms ingest leaf surfaces contaminated with virus or bacteria from previous batches. Predisposing factors are temperatures above 28°C, humidity above 85% (flacherie) or below 60% (grasserie), poor leaf quality (over-mature or wilted leaf), overcrowding and inadequate bed cleaning. Bivoltine hybrids (CSR Bivoltine Races CSR2 CSR4) are more susceptible than crossbreed races because of their lower stress tolerance.

Economic impact

Combined grasserie-and-flacherie crop loss in unmanaged commercial bivoltine rearings in Andhra Pradesh and Karnataka has been documented at 25-50% by CSRTI Mysore field surveys. Even partial expression reduces cocoon weight and shell ratio, downgrading the resulting silk to lower auction grades. In monsoon and post-monsoon batches the impact is greatest.

Management

Standard integrated package (CSRTI, Central Silk Board):

  • Pre-rearing disinfection: 2% formalin OR 0.3% bleaching powder solution sprayed on floors, walls and trays at 5 litres per 100 sq ft, 7 days before brushing. Sun-drying of trays, mountages and bed mats between batches.
  • Bed disinfection during rearing: Vijetha (5 g per sq ft), Sanjeevini, or Resham Keet Oushadh (formulated lime + bleaching powder mix) dusted on the worm bed after each moult, particularly before 4th and 5th instar.
  • Hygiene: Worker hand-wash and footwear-change between rearings; no movement from infected to healthy rearings.
  • Environment: Maintain temperature 23-26°C and humidity 70-80% by sprinkling, paddy-husk floor cover and ventilation; reject leaf that is over-mature, wet or yellowing.
  • Source-stage prevention: Use only certified pebrine-free DFLs (Disease Free Layings Dfl) from licensed grainages; complete first two instars at a controlled-environment chawki centre.
  • No chemotherapy: There is no effective antiviral or antibiotic in commercial use for grasserie; antibiotic dusting (oxytetracycline) for early flacherie has limited and inconsistent benefit and is not a substitute for hygiene.

See also: Bivoltine Silkworm, CSR Bivoltine Races CSR2 CSR4, Chawki Rearing Centre, Disease Free Layings Dfl, Silkworm Rearing Shed.

Sources

  1. Diseases of Silkworm. Central Sericultural Research and Training Institute, Mysore.
  2. Silkworm Disease Management. Central Silk Board.
  3. Pebrine, Grasserie, Flacherie and Muscardine in Bombyx mori. SILKS portal, Central Silk Board.