Diseases and pests of mulberry silkworm

I. DISEASES

1. GRASSERIE:

Causative agent: Bombyx mori Nuclear Polyhedrosis Virus

Occurrence: The disease prevails all through the year but its severity is more during Summer and Rainy seasons.

Source of infection: Silkworm gets infected when it feed on contaminated mulberry leaves. The milky white fluid released by the grasserie larvae, contaminated silkworm rearing house and appliances are the sources of infection.

Predisposing factors: High temperature, low humidity and poor quality mulberry leaves.

Symptoms:

• The skin of infected larvae becomes shining before moult and fails to moult.
•  Inter segmental swelling appears and the colour of the body becomes yellowish.
• The infected larvae move restlessly in the rearing bed/ along the rim of the trays.
• Infected larval body ruptures easily and turbid white haemolymph oozes out.

Management:

• Practice thorough disinfection of rearing house, its surroundings and appliances with any recommended disinfectant. 
• Conduct an optional disinfection with 0.3% slaked lime solution when high incidence of disease noticed in the previous crop.
• Practice personal and rearing hygiene.
• Collect the diseased larvae and ensure its proper disposal.
• Maintain optimum temperature and humidity in the rearing house.
• Feed quality mulberry leaf and avoid overcrowding.
• Apply recommended bed disinfectant as per schedule and quantity.
• Feed Amruth as per schedule to control grasserie disease.

2. FLACHERIE:

Causative agent: Bombyx mori Infectious flacherie virus/Bombyx mori Densonucleosis virus or different pathogenic bacteria viz., Streptococcus sp./Staphylococcus sp./Bacillus thuringiensis/Serratia marscesence individually or in combination of bacteria and viruses.

Occurrence: The disease is common during Summer and Rainy seasons.

Source Infection: Silkworm gets infected by eating contaminated mulberry leaf. Dead diseased silkworm, its faecal matter, gut juice, body fluid are the sources of pathogen contamination. The infection can also takes place through injuries/cuts/wounds.

Predisposing factors:Fluctuation in temperature, high humidity and poor quality of leaves.

Symptoms:

• The larvae become soft and flaccid.
• The growth of infected larvae retarded, becomes inactive and vomit gut juice. The faeces become soft with high moisture content. Sometimes chain type excreta and  rectal protrusion also observed.
• Larval head and thorax become translucent.
• When infected with Bacillus thuringiensis symptoms of toxicity such as paralysis and sudden death are observed. After death, larvae turn black in color and gives foul smell. 
• Some times, the dead larvae turn red when infected with Serratia sp.

Management:

• Disinfect the rearing house, its surroundings and equipments with recommended disinfectant mentioned above.
• Pick up diseased larvae and dispose them by burning.
• Provide good quality leaf grown under good Sunlight and recommended inputs. Do not provide over matured/over stored /dirty leaf to the silkworms
• Avoid starvation, overcrowding and accumulation of faeces in the rearing bed. 
• Rear silkworms under optimum temperature and humidity.
• Avoid injury to the larvae.
• Apply recommended bed disinfectant as per  schedule and quantity.
• Feed Amruth as per schedule to control flacherie disease.

3. MUSCARDINE:

Causative agent : Among fungal diseases, White Muscardine is common. The disease is caused by Beauveria bassiana.

Occurrence: The disease is common during Rainy and winter seasons.

Source of Infection:  The infection starts when conidia come in contact with silkworm body. Mummified silkworms / alternate hosts (most are lepidopteron pests), contaminated rearing house and  appliances are sources of infection.

Predisposing factors : Low temperature with high humidity.

Symptoms:

• The larvae loose appetite and become inactive.
• Presence of moist specks on the skin.
• The larva vomits and turns flaccid.
• After death, larva gradually becomes hard followed by mummification due to growth of aerial mycelia and conidia over the body and body turns chalky white.

Management:

• Disinfect the rearing house, its surroundings and equipments with recommended disinfectant as mentioned above.
• Control mulberry pests in the mulberry garden.
• Pick up diseased larvae before mummification and dispose them by burning
• Avoid Low temperature and high humidity in the rearing house. If required use heater/stove to raise the temperature.
• Regulate bed humidity during rainy season by dusting slaked lime powder during moult.
• Apply bed disinfectant, Vijetha and Vijetha supplement/Ankush/any recommended bed disinfectant as per schedule and quantity.

4. PEBRINE:

Causative agent: Nosema bombycis / different strains of microsporidia. 

Occurrence: Non-seasonal

Sources of Infection: Silkworm gets infected through eggs (Transovarian/Transovum transmission) or by eating contaminated mulberry leaf.  Infected silkworms, faecal matter, contaminated rearing house and appliances and alternate hosts (mulberry pest) are the sources of infection.

Symptoms:

• Irregular hatching of silkworm eggs.
• Irregular size of the larval body and moulting.
• The infected larva looses its appetite and becomes inactive with wrinkled skin.
• Black pepper-like spots appear on the body of the infected worms.
• White postules appear on the silkgland when examined under microscope with presence of shining oval spores.

Management:

• Disinfect the rearing house, surroundings and with recommended disinfectant as mentioned above.
• Conduct strict mother moth examination and surface disinfection of silkworm eggs to produce and rear disease free layings.
• Follow strict hygiene maintenance during rearing.
• Control mulberry pests in and around the mulberry garden.
• Apply recommended bed disinfectant, Vijetha/Ankush as per schedule and quantity.
• Monitor seed crops constantly to eliminate the microspodian infection.

Disinfection of rearing house, its surroundings and appliances:
Select any recommended disinfectant for disinfection purpose. CSR&TI, Mysore has recommended the following disinfectants:

• 0.05% Asthra solution (Add 50g Asthra powder in 100 liters of water and stir thoroughly and keep for 2 hours for dissolution of the powder).
• 2.5 % Sanitech/Serichlor in 0.5% Slaked lime solution (To prepare 100 liters of solution, take 250g of activator in to a basin/bucket and add 2.5 liters of Sanitech/Serichlor solution.  Keep it for 10 minutes. Add activated solution to the rest of water.  To this solution, add 500 g slaked lime powder and mix thoroughly). 
• 2% Bleaching powder in 0.3% slaked lime solution (To prepare 100 liters of solution, add little water to 2 kg bleaching powder and 300g slaked lime powder and make a paste. Add this paste to the rest of water and stir thoroughly. Keep for 10 minutes and use the supernatant).
• 0.3 % Slaked lime solution (optional disinfection if viral diseases noticed in previous crop – Add 300g of slaked lime to 100 liters of water and stir thoroughly. Keep for 10 minutes and use supernatant). 
• The total requirement of disinfectant solution for disinfection is estimated based on the rearing house floor area (Length × Breadth of floor).
• The quantity of disinfectant solution required is 1.5 lt./sq. m or 140 ml/sq. ft. floor area of rearing house (height 3 m /10 ft.) + 10% of total quantity of disinfectant solution. 
• Disinfect the rearing house, appliances and surroundings by spraying the solution with power sprayer. Two times disinfection recommended for each crop (once 3days before initiation of rearing and after completion of rearing).

II. PESTS

1. Uzi fly

Uzi fly

Egg of uzi fly

Black scar

Damage to cocoons

Occurrence & Symptom:

The uzi fly, Exorista bombycis is a serious endo-larval parasitoid of the silkworm, Bombyx mori, inflicting 10-15% damage to the silkworm cocoon crop in the premier silk producing states of Karnataka, Andhra Pradesh and Tamil Nadu.

Uzi fly occurs throughout the year, but severe during rainy season. Presences of eggs or black scar on the silkworm body and maggot emergence hole at the tip of the cocoon are the typical symptoms of uzi fly attack.

As soon as the uzi fly enters into rearing house, it lays one or two eggs on each silkworm larva. After 2-3 days, egg hatches, enters inside the larva and feed on internal contents for 5-7 days, after which it comes out by rupturing the larva. The maggot pupates in a dark corner or cracks & crevices in about 18-24 hours. The pupal stage lasts for 10-12 days. If the uzi fly infests at last instar, the uzi maggots come out after cocoon formation by making a circular hole.

Control measures

Exclusion Method

• Provide wire mesh/nylon net on all windows/doors.
• Provide doors with automatic closing mechanism.
• Provide anteroom at the entrance of the rearing house. 
• Keep the leaf in the verandah of the rearing house and observe for the uzi fly before shifting leaf into the rearing house.

Physical (using uzi trap)

Dissolve one table in 1 litre of water and keep the solution in white trays both inside and out side the rearing house at window base from 3rd instar onwards up to spinning.
Place uzi traps inside the rearing house/mounting hall after spinning up to 20 days under close-door condition to trap uzi flies emerging inside.

   

Biological

• Release Nesolynx thymus (a pupal parasitoid of the uzi fly) inside rearing house on 2nd day of V instar.
• After mounting of all spinning worms transfer the same pouches near the chandrikes.
• After harvesting of cocoons keep the same pouches near the manure pit.
• Two pouches are required for 100 dfls.

   

Proper disposal of silkworm litter after cocoon harvest

• Separate the silkworm litter from mulberry twigs.
• Do not throw silkworm litter in open space/litter pit, as it contains hundreds of
• Uzi fly pupae. Instead, pack it in plastic bags and keep for 15 to 20 days to prevent the emergence of uzi fly from   litter. Alternatively, it can be buried in soil or burnt immediately.

Availability of Nesolynx thymus:

Available at Pest Management Lab., CSRTI, Mysore. Place indents on the day of brushing, indicating the number of pouches required and brushing date of silkworms. Supply is made by courier on receipt of advance payment @ Rs 25 per pouch.

2. Dermestid beetles

  

Occurrence & Symptom:

Dermestid beetles, Dermestis ater are known to attack pierced cocoons in cocoon storage rooms. The female beetles lay about 150-250 eggs in the floss of cocoons.  The beetles migrate from cocoon storage room to grainage and attack green cocoons as well as moths also. Generally they attack the abdominal region of the moth. The damage is estimated to be 16.62% on cocoons and 3.57% on moths.

Management of Dermestid beetles:

Preventive measures

• Storage of rejected cocoons and perished eggs for long period should be avoided.
• Rearing house & cocoon storage rooms should be cleaned periodically.
• Grainage premises should be cleaned before & after moth emergence.
• Provide wire mesh to door & windows in pierced cocoon (PC) storage rooms.
• Wooden articles of storage room & grainage should be dipped in 0.2% malathion solution for 2-3 minutes. Trays etc., should be thoroughly washed & sun dried for 2-3 days before reusing.

Mechanical control: Collect the grubs and adults by sweeping or by using a vacuum cleaner, destroy by burning or dipping in soap water.

Chemical control:

• Store pierced cocoons in Deltamethrin treated bags ie., soak the bags in 0.028% Deltamethrin solution (1 ltr : 100 ltr water) and dry in shade.
• Spray 0.028% Deltamethrin solution on walls and floor of PC room once in 3 months.
• Sprinkle bleaching powder (200 gm/sq.mt) all around inner wall of PC room to prevent crawling of grubs from PC room.

Source:

Central Sericulture Research & Training Institute, Mysore, Karnataka

Diseases and pests of eri silkworm

Pebrine

Causal organism: Protozoan- Nosema sp. Peak season: Summer season of the year.

Symptoms

At early stage of infection: The infected early stage worms do not show any morphological abnormality. Only microscopic examination of the silkworm larvae may indicate the presence of spores of the pathogen.

Later stage of infection: The silkworm larvae loose appetite. Varies in size, retard in growth, moult irregularly and the colour of the larvae becomes pale.

Infection: It is unique in being transmitted to offspring by trasovarial / transovum means from mother moth and this is called primary. If infection is primary, more than 50% larvae die before third moult and rarely any larva goes to spinning stage. When healthy larvae get infected through contamination during rearing, it is called secondary infection. Secondary infection during early 4th larval stage leads to formation of flimsy cocoons, where as larvae infected during 5th larval stage spun well formed cocoons.

Source of infection

Egg stage

• Transovarial.
• Surface contamination.
• contaminated grainage appliances.

Larval stage

• Contaminated egg laying kharika.
• Transovarially infected larvae.
• Faecal matters of infected larvae.
• Excreta of infected larvae.
• Contaminated foliage.
• Contaminated rearing room.
• Contaminated rearing appliances.

Moth stage

• Purchase of infected seed cocoons.
• Infected moth.
• Infected grainage appliances.
• Meconium and moth scales.
• Grainage dust.

Spread of disease: Perbrinized larvae excreat faecal matter, gut juice and vomit containing pathogens, which contaminate the rearing environment, appliances and foliage. Mostly, consumption of contaminated foliage or egg shell results in infection and spread of the disease.

Management

• Follow the scientific inspection method of individual mother moth testing for detection of pebrine during egg production.
• Practice disinfection of grainage hall and appliances before and after every grainage operation with 2% formalin.
• Ensure use of microspically tested disease free disinfected eggs only.
• Practice surface sterilization of eggs with 2% formalin for 5 minutes.
• Maintain hygienic conditions in egg production room and rearing room.
• Practice disinfection of rearing appliances and rearing room before use.
• During rearing, test the faecal matters, unequal/ lethargic/ unsettled/ irregular moulters periodically. If pebrine spores are detected, reject the entire infected crop.
• Ensure the measures for destruction of diseased silkworm larvae/ cocoons/ moths/ eggs.

Flacherie

• Flacherie is a syndrome of bacterial diseases in eri silkworm. Flacherie disease is caused by an ultra virus, which is an exciting agent, followed by secondary infection of bacteria.
• Peak season: All seasons of the year, intensive during rainy summer months (June to August).
• Symptoms: Infected silkworm larvae become lethargic and motionless. The colour of the haemolymph turns black. Chain type excreta, sealing of anal lips, rectal protrusion are some of the easily detectable symptoms of the disease. Infected larvae die within short time.

• Infection: Feeding of contaminated/ poor quality foliage.
• Source of infection: Diseased larvae, its gut juice, faecal matters, body fluid and contaminated rearing site and appliances.
• Pre-disposing factor: Sudden fluctuation in temperature and humidity, bad weather, poor quality leaves with high water content.
• Spread of disease: The disease is transmitted by secondary infection of the larvae feeding on the contaminated/ poor quality leaves. Infected worms ooze out body fluid containing pathogen throughout incubation period of infection and contaminate the leaves of the rearing bed and rearing environment. The disease spreads to healthy worms on feeding of the contaminated leaves. Feeding of late stage worms with very tender succulent leaves and sudden fluctuation of temperature & humidity during rearing period also lead to outbreak of the disease.

Management

• Use disinfected quality seeds of disease free zone.
• Disinfection of rearing room before rearing with 2% formalin solution.
• Dusting of 0.3% slaked lime in addition to usual disinfection for rearing room and appliances in case of high incidence of the disease in preceding rearing.
• Inspect rearing room regularly and pick out stunted/ doubtful worms by burying with 5% formalin solution.
• Practice washing of hands with formalin solution at the time of handling of worms.
• Maintain hygienic condition during rearing.
• Feeding of good quality leaves, because food is major source of infection.
• Do not allow late stage worms to feed on succulent tender leaves.

Insect pests

Unlike mulberry and muga silkworm, attack of insect pests is less in eri silkworm. However, use of nylon net in the windows & doors of the rearing room prevents uzi fly infestation in eri rearing. Usually fly pests come to the rearing room through the food plant foliage. Hence, preservation of plucked leaves in separate site also helps in checking the entry of fly pests.

Source:

• B.K. Singh, N. Tiken Singh, 2010, Muga Silkworm Seed Organization (MSSO), P-4 Unit, Mendipathar, East Garo  Hills, Meghalaya.
• Package of practices of Muga, Eri and Mulberry Sericulture for North Eastern region of India, 2005, Central Muga  Eri Research & Training Institute, Lahdoigarh, Jorhat, Assam.
• Directory of Sericulture Technology 2008, Karnataka State Sericulture Research and Development Institute,  Bangalore- 560 062.