Silkworm is reared for the production of cocoons, which is the raw material for silk production. It provides gainful employment and helps in the improvement of the socio-economics life of the peoples in the rural areas. During rearing, the silkworm are affected by various types of pathogenic agents viz. protozoa, virus, fungi and bacteria.
It has been reported that about 30-400 % of total crop loss is due to the occurrence of four major diseases namely Pebrine (protozoan disease), Flecherie (bacterial disease), Grasserie (viral disease) and Muscardine (fungal disease). Other than diseases Uzi fly is considered as serious endo-larval parasitoid of the silkworm and yield of cocoons is reduced tremendously due to the infestation of Uzi fly.
Diseases of Silkworm and their Management
Pebrine is the most important disease of silkworm that is caused by by the protozoa called Nosema bombycis. Pebrine is commonly observed during rainy and winter season and transmitted to offspring by transovarial.
Initial stage of infection: The infected silkworm larvae appear to be healthy. Only microscopic examination of the silkworm larvae may indicate the presence of shinning spore of Nosema.
Advanced stage of infection:
The silkworm larvae loose appetite, unequal in size, retard in growth, moult irregularly. The dead larvae turn black in colour on the surface of the body due to secondary bacterial infection. If infection occurs in late 5th instar, the larvae spin flimsy cocoons. Infected female moths lay pebrine contaminated eggs in lumps one above the other. The number of eggs per laying is also drastically redused.
Source of infection:
The diseases are transmitted from the infected mother to the offspring by transovarial this is called primary infection. Secondary infections occur during early 4th larval stages to the formation of flimsy cocoon. Whereas larvae infected during 5th larval stages form well formed cocoons. The spores spread through the faecal matter and digestive secretions of the infected larvae.
Control of Pebrine:
- Pebrine disease can be controlled by disinfecting the rearing room, equipments and rearing surroundings with 2% formalin.
- During rearing, unequal size worms and faecal matter should be microscopically examined for the presence of pebrine spores and if observed, larvae, cocoons and laying should be collected and burnt or buried.
- In the grainages, scientific methods of mother moth examination should be employed.
- Surface sterilization of the eggs with 2% formalin for 5 minutes.
It is a syndrome associated with bacterial disease and the affected worms are bombysepticus. The disease is caused by Infectious flecherie virus, densonucleosis virus and kenchu virus.
Flacherie may be caused by virus individually as well as in association other virus or bacteria. Flacherie occurs in all seasons of the year but occurs mostly during the rainy season.
The infected silkworm become motionless and lethargic. Body of the infected silkworm become soft, facial matter or excreta is like chain type, sealing of anal lips, infected larvae stop feeding, become weak and retarded growth, fail to settle for moult and starts vomiting releasing contaminated body fluid, larvae on death putrefy on the rearing bed and starts emitting foul smell.
Source of infection:
The disease is transmitted by secondary infection of larvae feeding on the poor quality leaves, faecal matters, body fluid, contaminated rearing site and appliances. Feeding of late instar larvae with tender leaves is also fector of infection.
Sometimes, sudden fluctuation of temperature and relative humidity during the rearing period also may lead to outbreak of the disease.
Control of Flacherie:
- Disinfection of rearing room and appliances with 2% formalin solution.
- Use of disease free quality seeds.
- Destroy sluggish, irregular moulters and diseased worms.
- Do not allow late stage worms to feed on tender succulent leaves.
- Avoid injury to the worms, overcrowding of trays.
- Apply antibiotics like Streptomycin/Tetracyclin.
It is the most common dreadful disease caused by Beauveria bassiana. Generaly fungal disease is common mostly during rainy season but incidence is very high during winter.
High humidity above 80% and low temperature of 19-22oC during rearing is also congenial for fungal spores to multiply.
Diseased larvae loss their appetite and become inactive. The colour of larvae turn pale gradually ceases movement within 12-18 hours of infection. During later stages the body becomes harder, brittle and mummifies covered with white encrustation or white coating looking like a wite chalk pencil.
Source of infection:
Mummified larvae in the rearing tray. Dead silkworm containing millions of conidia or spores which are dispersed by wind or foliage of the host plants. These conidia on contact with host integument germinate into the host body and cause infection.
Control of Muscardine :
- Disinfection of rearing site before rearing with 2% formalin solution and sun drying of appliances.
- Dusting of slaked lime in the rearing bed to maintain humidity during moulting periods.
- Collection and destruction of alternate hosts.
- Dusting of Tasar Kit or Oushad developed by CTR&TI, Ranchi on the body of the larvae at the time of transfer.
- Collection and destruction of mummified larvae from the rearing bed.
Nuclear Polyhedrosis, commonly known as grasseria caused by baculovirus. This disease prevails throughout the year but is predominant during rainy summer months of the year.
The larvae will be sluggish with swollen intersegmental region integuments becomes fragile and rapture easily, the body tissues and haemolymph of the infected larvae get disintegrated into turbid white fluid.
Source of infection:
Body fluid and contaminated rearing site and appliances.
Control of Grasserie:
- Sun drying of rearing appliances for one/two days.
- Ensure proper ventilation and air circulation.
- Collect and burn infected larvae.
- Early diagnosis and rejection of infected lots.
- Dusting the bed disinfectant with Vijetha or Resham Keet Oushadh on the larvae and rearing
Important Predators of Silkworm
1. Uzi fly:
Exorista bombycis is an endoparasitoid of silkworm. The fly lay eggs on the integument of the worms and after hatching from the eggs maggots of the fly penetrate into the larval body and feed on the tissue of the worms.
- Using of nylon net to the doors and windows of rearing room ensure 80-90% of control.
- Collect and destroy the maggots of the fly.
- Dusting of Uzicide and Uzipowder uniformly over the silkworm body is very much effective against Uzifly.
- Some of the hymenopteran parasitoids viz. Exorista phillippinensis, Trichopria sp., Nesolynx thymus etc. is effective for control of Uzi fly.s
Ramesh kumar sanp1, Harjindra singh2 and V.S. Acharya3
1 & 2 Ph.D. Entomology, College of agriculture , Bikaner
3Assistant professor, Entomology, College of agriculture , Bikaner