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

Package of Practices of Mulberry Sericulture for Eastern and North Eastern Region, Central Sericultural Research & Training Institute, Berhampore, West Bengal

Diseases and pests of muga silkworm

Pebrine disease

Pebrine is the most serious disease of muga silkworm caused by a protozoan of Nosema sp. It is unique in being transmitted to offspring by transovarial/ transovum means from other moths. If infection is primary, more than 50% larvae die before 3rd moult and rarely any larva goes for spinning. 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, whereas larvae infected during 5th larval stage form well formed cocoons.

Peak season: Throughout the year.

Symptoms

Early stage of infection

• The infected silkworm larvae appear normal. However, microscopic examination of silkworm larvae may indicate the presence of spores of the pathogen.

Later stage of infection

• The silkworm larvae loose appetite, vary in size, retarded growth, moult irregularly and the colour of the larvae become light yellowish green instead of deep green colour (normal healthy larvae).
• The infected late stage larvae show black dots or specks on the surface of the body and hence the disease is known as Phutuka (spotted disease in Assamese).

Sources of infection

Egg stage

• Transovarial.
• Surface contamination of eggs (transovum).
• Contaminated grainage appliances.

Larval stage

• Contaminated egg laying kharika.
• Transovarially infected larvae.
• Faecal materials of infected larvae.
• Contaminated foliage.
• Contaminated rearing site.
• Contaminated rearing appliances.

Moth stage

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

Spread of the disease: Pebrinized larvae extrude faecal material, gut juice and vomit which contaminate the rearing environment, appliances and host plant foliage. Mostly, consumption of contaminated foliage/ egg shell results in spread of the disease.

Management

• Scientific inspection of individual mother moths for detection of pebrine during egg production.
• Disinfection of grainage appliances with 2% formalin before and after every grainage operation.
• Use microscopically tested disease free disinfected eggs only.
• Surface sterilization of eggs with 2% formalin for 5 minutes.
• Maintain hygienic condition in egg production room and rearing sites.
• Follow cellular method of rearing for basic stock maintenance.
• Disinfection of rearing appliances before use.
• Check the faecal materials, unequal/ lathergic/ unsettled irregular moulters periodically.
• If pebrine spores are detected, reject the entire infected crop.

Source: Central Muga Eri Research & Training Institute, Lahdoigarh, Jorhat, Assam

Flacherie

Flacherie; commonly known as anal protrution is a syndrome of bacterial diseases in muga silkworm caused by Pseudomonas sp. Sometimes it is caused by an ultra virus, which acts as an exciting agent followed by secondary infection of bacteria. Sudden fluctuation in temperature and humidity, bad weather, poor quality leaves with high water content are predisposing factors. Peak season: Throughout, intensive during rainy summer months (June to August).

Symptoms

• The infected larvae become lethargic and motionless.
• The colour of the haemolymph turns black.
• Excreata looks like chain.
• Sealing of anal lips.
• Rectal protrusion.
• Infected larvae die within a short time.

Source of infection

• Larvae get infected upon feeding of contaminated/ poor quality leaves of host plants.
• Diseased larvae, its gut juice, faecal material, body fluid.
• Contaminated rearing site and appliances.

Spread of the disease

• Secondary infection of larvae due to feeding on contaminated leaves.
• Infected worms ooz-out body fluid containing pathogen throughout the incubation period and contaminate the rearing environment.
• Feeding of late stage worms with very tender succulent leaves.
• Sudden fluctuation of temperature and humidity during rearing period.

Management

• Use disinfected good quality seeds of disease free zones.
• Orient the brushing to protect the young larvae form direct sun shine.
• Disinfection of rearing site with 2% formalin solution before rearing.
• Dust 3% slaked lime in addition to formalin in case of high incidence of the disease in the preceding year.
• Inspect rearing field regularly and pick out stunted/ sluggish/ irregular moulters and destroy.
• Destruction of diseased/ doubtful worms by burying with 5% formalin solution.
• Wash the hands with formalin solution at the time of transfer of worms.
• Maintain hygienic condition during rearing.
• Feed with good quality disease free leaves.
• Do not feed late stage worms with tender succulent leaves.

Grasserie

It is a major viral disease of muga silkworm caused by a baculovirus. High temperature clubbed with high humidity, poor quality host plant leaves are predisposing factors.
Peak season: Throughout the year, predominant during rainy summer months of the year.

Symptoms

• The silkworm larvae fail to moult.
• The integument becomes fragile and intersegmental region becomes swollen and that is why the disease is known as Phularog (swelling disease in Assamese).
• The body tissues and haemolymph of the infected larvae get disintegrated into turbid white fluid and the larvae hang upside down with the anal claspers after dying.
• The turbid fluid contains large number of hexagonal polyhedral bodies.

Source of infection

• Feeding of contaminated foliage.
• Disintegrating diseased silkworms, their body fluids.
• Contaminated rearing sites and appliances.

Spread of the disease: The diseased silkworm larvae extrude the pathogen along with oozing of body fluid due to injury and breakage of dead and/ or diseased larvae. The body fluid and broken body parts of the larvae contaminate the foliage, rearing site and appliances. The disease spreads to healthy worms on feeding of the contaminated leaves and/ or use of contaminated appliances during rearing.

Management

• Disinfection of rearing site with 2% formalin solution before rearing.
• Dust 3% slaked lime in addition to usual disinfection in case of high incidence of disease in preceding rearing.
• Pick out growth retarded/ lethargic/ irregular moulters and destroy.
• Ensure destruction of diseased/ doubtful worms by burning or burying with 5% formalin solution.
• Ensure proper hygiene during rearing.
• Use certified disinfected disease free layings only.
• Ensure rearing on good quality leaves.

Mascardine

Mascardine is one of the major diseases of silkworm. However, it is less prevalent in muga silkworm and occurs under certain specific environmental influence only. The disease appears at an interval of 2-3 years. The causal organism is a fungus and it is yet to be identified for muga silkworm. Low temperature with high humidity is predisposing factors. Peak season: Winter months of the year when night temperature falls down and the day temperature remains comparatively high associated with high humidity i.e. foggy weather.

Symptoms

• Infected larvae loose appetite and become inactive.
• Colour of the larvae turns pale.
• Gradual cease of movement within 12-18 hours of infection.
• The larvae hang on tree twig/ trunk and harden.
• The larvae die within 16-18 hours of infection.
• A white encrustation appears on the larval body and it covers the whole larval body within 24 hours of death.
• The dead larval body becomes dry, brittle and mummified.

Source of infection

• Mummified / diseased larvae.
• Contaminated rearing environment.

Spread of the disease

• The conidia/ spores of the pathogenic fungus are dispersed by wind.
• The conidia on contact with larval integument germinate, penetrate into the larval body and cause infection.

Management

• Orientation of brushing towards sun shine during winter.
• Disinfection of rearing site with 2% formalin solution before rearing.
• Dust slaked lime in the field to control humidity at the time of rearing.
• Dust Tasar Kit Oushad developed by CTR&TI, Ranchi on the body of the larvae at the time of transfer.
• Spray 0.5% sodium hydroxide solution on the worms after 24 hours of each moult as prophylactic measure.
• Maintain hygienic condition during rearing.
• Collection and destruction of dead/ diseased larvae
• Pickout sick or dead worms with forceps/ chopstick and put in 2% formalin solution.
• Bury the carcasses in a pit and cover the soil.
• Wash hands with formalin or dettol solution after handling dead or infected larvae.
• Do not allow birds, ants or poultry to eat the carcasses.

Uzi fly (Exorista sorbillans)

Nature of damage: It is the major pest of muga silkworm. The fly lays eggs on the integument of the worms in the dorsal and dorso-lateral side. After hatching from the eggs, the maggots of the fly penetrate into the larval body and feed on the tissues of the worms. The mature maggots come out of the larvae/ pupae and undergo pupation in the rearing field or grainage hall. The uzi infested muga silkworm dies during larval or pupal stage. Season of incidence: Prevalent throughout the year, peak during December to march.

Management

• Rear the silkworm under nylon mosquito net during peak infestation period (December to March), which ensures 80-90% control.
• During transfer of late stage worms, remove the fly eggs from the integument of the silkworm larvae with the help of forceps.
• Keep the rearing field clean and dust with bleaching powder during rearing.
• Mount uzi infested worms in separate Jali.
• Harvest and stifle the uzi infested cocoons on 4th and 5th day of spinning.
• Collect and destroy the maggot/ pupae of the fly.
• Burn heavily infested worms.

Apanteles (Apanteles stantoni)

Nature of damage

Usually infect the early stage silkworms. Apanteles lays eggs inside the body of silkworm larvae by inserting the ovipositor through the tubercles after maturation. The mature maggots form pupae in aggregation outside the body of the silkworm larvae.
Season of incidence: Prevalent during summer and winter months of the year.

Management

• Rearing of silkworm under nylon mosquito net.
• Keep the rearing field clean and dust with bleaching powder during rearing.
• Collect and destroy the maggots/ pupae of the fly along with the silkworm larvae.

Spider: Attacks 1st instar worms

Wasp (Vespa orientalis)

It occurs during June-July to August-September months. It attacks early stage worms by lacerating and picking young age worms. It can be controlled by covering silkworm rearing by nylon nets and destroying hives.

Red ants

The red ants are also serious pest in many muga growing areas. It attacks 1st stage worms. The lost due to red ants are reported to be 5-10%. They can be controlled with the spray of 2% Rogor (insecticide) before 15 days of rearing or burning down their nest well before the rearing.

Grass hopper: They attack the 2nd to 3rd stage worms. Lost due to grass hoppers are minimal

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.