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Jhansi, Uttar Pradesh
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Diesases & Pests of Tasar Silkworm (Antheraea Mylitta)

I. Diseases of Tasar Silkworm

Pebrine

Tasar silkworm is susceptible to Nosema mylittansis which belongs to Phyllum – Protozoa, Class – Sporozoa, Order – Microsporidia, Family – Nosemetidae. The disease caused is known as Pebrine.

Symptoms: Pebrine disease generally does not show visible symptoms to the naked eye until unless the infestation is heavy and advanced. The following symptoms are visible in the different stages of tasar silkworm when the pebrine infection is higher.

Egg stage
  • Infected eggs have less muconium leading to poor adherence to substratum
  • Poor egg number.
  • Reduction in size and weight.
  • Egg laying is not uniform.
  • Number of dead and unfertilized eggs increases.
  • Irregular hatching.     
Larval stage
  • Diseased worms lose their appetite.
  • Worms show disparity in growth leading to unequal in size.
  • The worms become sluggish and slow.
  • Irregular moulting with increased moult duration in many.
  • In severe case black pepper like spots appear over the whole body of silkworm from III instar onwards
  • If eggs are infected, heavy mortality is observed after 2nd moult.
  • When the infection occurs in later instar, the larvae may spin good or flimsy cocoons and adults may lay eggs also.
Pupal stage
  • Pupa looks flaccid.
  • Pupa infected with pebrine is light in weight with shrunken and deformed abdomen.
  • Heavy mortality at pupal stage.
Moth stage
  • Infected moths are generally deformed with crumpled wings and scale less abdomen.
  • Scale of wings and abdominal area come off easily.
  • Poor mating and egg laying.
  • Virosis (Cytoplasmic polyhedrosis)

Among the silkworm pathogens, virus causes comparatively higher level of mortality in most of the tasar culture areas. Bad weather, ineffective disinfections and poor management leads to outbreak of the disease and severe crop loss. Virosis accounts for 25-30% of the total crop loss from diseases and pose a serious threat to tasar culture. It is difficult to control once the worms become infected.

Occurrence : Virosis prevails throughout the year but is intensive during 1st and 2nd crop rearings.The severity is far less during 3rd crop.

Causative agent: An occluded virus infecting tasar silkworm is a Reovirus, the cytoplasmic polyhedrosis virus (CPV) causing virosis disease. Cytoplasmic polyhedrosis virus of tasar silkworm Antheraea mylitta is abbreviated as AmCPV.

Symptoms:  
  • The larvae lose appetite, become sluggish, disproportionate large head or long bristles, loses its natural shape, distends lengthwise and turns brownish
  • The inner content of the body disintegrates and the larva dies.
  • Looses clasping power of prolegs.
  • The larva hangs head downwards, attached to the host twigs with its caudal legs and dark brown fluid oozes out as drops from the mouth with an obnoxious smell.
Management:
  • The symptoms of this disease are atypical at the initial stages of the infection. However as the disease develops, clear symptoms are manifested.
  • AmCPV in tasar silkworm could be managed by practicing the preventive measures.
  • All the tasar eco-races are susceptible to the infection by the AmCPV. However, the susceptibility level is differs in different eco-races.
  • Several chemicals such as 0.01% Sodium hypochlorite, Slaked lime, Chlorinated lime, Formalin, TKO, Jeevan Dhara, Jeevan Suraksha and LSM effective to control of virosis in tasar silkworm.
  • Bacteriosis

Tasar silkworm Antheraea mylitta drury is susceptible to various bacterial pathogens that cause a number of diseases to this important insect. Death of worms due to bacteriosis occurs in every stage of its life cycle. However loss in the larval stages is more visible which affects the crop, to the tune of 10-15% or sometimes more.

Occurrence : The occurrence of bacterial disease in tasar silkworm is more pronounced during June – July than that of the other seasons.

Causative agents :
  • The major types of pathogenic bacteria causing sealing of anal lips and rectal protrusion in tasar silkworm were reported to be gram positive bacillus and gram negative coccus (Micrococcus)
  • Chain type of excreta was micro bacterium.

Symptoms:The initial symptoms in the larvae are immobility and sluggishness. Diseased larvae lose appetite and become irritable. With the advancement of disease worms become flaccid (soft), long and thin followed by loss of gripping ability. Three types of distinct features develop in tasar silkworm larvae.

  • Sealing of anal lips: Soil coloured sticky semisolid fluid oozing from colon seals the anal lips.
  • Chain type excreta: The faecal beads excrete out of the anal aperture embedded in a jelly like substance in the form of a chain.
  • Rectal protrusion: The rectum protrudes out as transparent bag filled with haemolymph.
Management:
  • A healthy silkworm is generally more resistant to infection than stressed one.
  • Stress brought about by the malnutrition, metabolic imbalance, physical and other factors results in weakened larva and increased susceptibility to bacterial infection.
  • The nutritional stress lower the resistance of silkworm by rendering them inefficient in production of antibacterial and antiviral factors in the gut and hemolymph.
  • The ability of larvae to produce such factors in the gut is dependent on quality of the leaves.
  • For preventing outbreak of bacteriosis, Tasar Keet Oushad (TKO), Jeevan Suraksha, Jeevan Dhara and Leaf Surface Microbes (LSM) have been developed and found effective.
  • Muscardine (Fungal disease)

Fungal diseases in insects are called muscardine or mycosis. These are found throughout the world and are most contagious. The incidence of muscardine is noticed particularly in silkworm rearing during September to November.

Causative agents: Muscardine or mycosis in tasar silkworm is caused by the infection of Penicilium citrinum and Paecilomyces varioti.  The species belongs to Division: Eumycota, Class: Plectomycetes, Order: Eurotiales and Family: Eurotiaceae.

Symptoms:
  • The infected larva becomes inactive and loses its appetite. The colour turns pale and the body gets hardened.
  • In about 12-14 hours the larva hangs with its anterior or posterior half obliquely downward giving a characteristic dorsal bending.
  • The worm at this stage looks very hard and pale and dies in another 6-8 hours. Eight hours after death the worms become spongy and very fragile.
  • In the next 16-18 hours a white encrustation appears round each segmental ring and the larva gets more compressed laterally. After another 24 hours the encrustations cover whole the body. The dead worms become completely compressed laterally.
  • The white encrustation turns slightly greenish powdery material after 24 hours indicating the formation of conidiospores. The dead larva becomes dry, brittle and mummified.
Management:
  • Alternate hosts for the pathogen are an important factor in occurrence of muscardine in addition to primary sources in the silkworm rearing environment.
  • High humidity and low temperature predispose the disease in silkworm.
  • The application of antifungal disinfectant such as Jeevan Suraksha, TKO as per recommended schedule and quantity is most essential.
  • As a specific measure, 1-2% of Dithane M45 in slaked lime or captaf in Kaolin is dusted on silkworm body.

II. PESTS / PREDATORS OF TASAR SILKWORM

PREDATORS:
  • Reduviid bug (Sycanus collaris):  Incidence of Reduviid bug was observed from July to November.  Its peak period of incidence recorded in the month of August
  • Stink bug (Canthecona furcellata Wolf): The stink bug was observed throughout the rearing season (June – January). The reduced incidence of stink bug was observed in September, October and January months and higher incidence was in the month of November.
  • Wasp (Vespa orientalis): The prevalence of wasp also followed the same trend of stink bug. The peak period of wasp infestation was in the month of November
PARASITOIDS:
  • Ichneumon fly (Xanthopimla pedator): 
  • Uzi fly (Blepharipa zebina):

The prevalence of both the parasitoids Ichneumon fly and Uzi fly was observed from September to January.  The peak period of infestation of Ichneumon fly was recorded in the month of December and January. Similarly, the peak incidence of uzi fly also recorded in the month of December and January.

CONTROL MEASURES

IPM  package for control of UZi fly Blepharipa zebina

Mechanical control:
  • Uzi fly infested / dead silkworm larvae should be collected and destroyed.
  • Uzi fly maggots / Pupae should be collected from rearing Fields / Grainage houses and destroyed.
  • Flimsy and Uzi fly infested cocoons should be harvested early and stifled or sun dried.
  • Sticky trap (Lassa-adhesive) should be used for catching / collecting and killing the adult Uzi flies.

Chemical control: Bleaching powder solution (2%) as an ovicide should be sprayed on silkworm body to kill the glued eggs of Uzi fly. This treatment has to be repeated four to five times.

Schedule of spray:

IV instar:  3rd day once

V instar:   3rd, 5th and 7th day once in each day (if larval duration is prolonged, one

     additional spray should be done on 9th day).

Biological control:  Nesolynx thymus (Hymenoptera: Eulophidae) a bio-control agent of Uzi fly to be released at the rate of 1,00,000 adults for 100 dfls of silkworm rearing.

Schedule of parasitoid release:

Once at the time of cocoon harvest  : 30000 adults
Once after seven days of cocoon harvest in Rearing field  : 40000 adults
Once after seven days of cocoon harvest in Grainage house: 30000 adults
Management of Yellow fly / Ichneumon fly Xanthopimpla predator

Mechanical control:

During rearing period:

  • Rearing of tasar silkworm must be conducted under nylon net protection during the final stage to cocoon formation in the field up to the harvest of cocoons.
  • Trapping of adult Yellow Fly males and females manually with the help of gummy sticks during the rearing period.

After cocoon harvest:

  • Collection and destroy of infested host pupae with developing embryo inside.
  • Collection and killing of adult Yellow flies inside the Grainage houses after their emergence by piercing the anterior end of the preserved cocoons.
  • Thorough sorting of infested cocoons immediately after cocoon harvest followed by stifling

IPM Package for the Control of Silkworm Predators

Mechanical Control:
  • Use of nylon net (mesh size 2 mm) especially during chawki rearing to prevent silkworm from the attack of predators.
  • Mechanical capturing of predators in the rearing field throughout rearing period and specially at the time of incidence of the predator by using sticky country made adhesive (lassa) glued to bamboo pole or other sticks.
  • Collection and destruction of egg mass / oothecae / nests / nymphs and adults of predators.
  • The rearing trees should be cleaned of ants and their nests before brushing or transferring of silkworm on it.
Chemical control:
  • The base of the trees should be dusted with insecticide (Methyl-parathion 2% dust) to prevent any ant attack on the trees.
Source:

Central Tasar Research & Training Institute, Central Silk Board, Ranchi