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Diseases and Pest of Silkworms

Diseases and pests of mulberry silkworm

I. DISEASES

  • 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.
  • 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.
  • 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.
  • 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

  • Uzi fly

    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.

  • 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

DIESASES & PESTS OF TASAR SILKWORM (ANTHERAEA MYLITTA)

  • 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.
  • 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.

    Mechanical control:

    IPM package for control of UZi fly Blepharipa zebina

    PARASITOIDS:

    • 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