GOK > Sericulture > English > Silkworm Diseases and Pest Control
Last modified at 24/08/2016 13:40 by System Account
Silkworm Diseases and Pest Control Diseases are the behavioral and physiological changes induced by pathogens in an organism. All diseases have specific symptoms and characteristics. Similarly, silkworms are also affected by various types of diseases caused by protozoa, fungi, bacteria and viruses. Since they cause substantial financial loss to the industry, their prevention and control assumes utmost importance. Pebrine: Pebrine is caused by a protozoan called Nosema bombycis. In the initial stages the larvae appears to be healthy, but when observed under a microscope we can see oval, shinning spores of Nosema. Pebine disease is infected to the silkworms in two methods; peroral and transovarial infection. In advanced stages of infection, silkworms stops feeding resulting in unequal size larvae, they become sluggish, and die. The dead larvae turn black in colour due to secondary bacterial infection. If infection occurs in late V instar, the larvae spin the cocoons, and the moth may also emerge. Infected female moths lay pebrine contaminated eggs in lumps one above the other. The number of eggs per laying is also drastically reduced.

Pebrine is commonly observed during rainy and winter season. The spores spread through the faecal matter and digestive secretions of the infected larvae, contaminate the mulberry leaves, rearing equipments and rearing environment.

Pebrine disease can be controlled by disinfecting the rearing room, equipments and rearing surroundings. During rearing, unequal size worms and faecal matter should be microscopically examined for the presence of pebrine spores and if observed, larvae, cocoons and layings should be collected and burnt or buried. In the grainages, scientific methods of mother moth examination should be employed. If pebrine disease is detected, effective disinfection should be undertaken before starting the next rearing or grainage operations.

Microscopic smears are to be prepared by crushing the abdominal region of the mother moth in 2ml, 0.6% Potassium carbonate solution. Similarly, smears can be prepared from dead and unequal larvae, layings, facial matter and also from the rearing room dust. The smears should be subjected to microscopic examination and if pebrine spores are detected, the crop should be destroyed followed by disinfection. Flacherie : Flacherie is a syndrome associated with infectious flacherie, Densonucleosis (DNV), Cytoplasmic polyhedrosis (CPV) and Bacterial diseases and several types of bacteria. The disase is caused by Infectious flacherie virus, densonucleosis virus and kenchu virus. Flacherie may be caused by virus individually as well as in association other virus or bacteria.

There are different types of flacherie, white flacherie, red flacherie, chained faecal matter. The body of the infected larvae become flaccid, pale in colour and become soft. Sometimes, the facial matter is execrated in a chain form. The infected larvae stop feeding, become weak and retarded growth, fail to settle for moult and starts vomiting releasing contaminated body fluid. Dead larvae putrefying on the rearing bed and starts emitting bad smell. If the infection is during the late larval stage, larvae spin the cocoon, but they die inside the cocoons, some larvae fail to spin the cocoons and die and hang on the mountage. Flacherie is observed throughout the year, but incidence is very high during rainy season.

To control flacherie, the rearing room, equipments and surroundings should be disinfected. During rearing, the rearing bed should be disinfected with recommended bed disinfectants. If the incidence is high, the rearing bed should be dusted with active lime powder before feeding. Diseased, weak larvae are to be separated from the rearing bed and disposed into a bowl containing lime water. Feeding the larvae with good quality leaves, good ventilation in the rearing room and adequate bed spacing helps to prevent the incidence of flacherie. Maintenance of recommended temperature and humidity also helps to contain flacherie. Muscardine and Aspergillosis: This is a fungal disease caused by Beauveria bassiana and Aspergillus flavus, A. oryze and A. tameri. Fungal disease in silkworms is caused mainly through cross infection. Initially the infected larvae appears normal and do not show any external symptoms. As the disease spreads, the feeding rate reduces and larvae go below the bed. The body become flaccid and soft, after death, the fungal conidia grow on the body surface and larvae become hard like white chalk. If the infection is during the fifth instar, the larvae spin the cocoons, but die inside the shell. Aspergillosis infected larvae also show the same symptoms of white muscardine but on death of the larvae, they turn green in colour.

Generally fungal disease is seen during rainy season but incidence is very high during winter. High temperature and humidity required during chawki stage is also highly congenial for fungal spores to multiply. Therefore,chawki stage is also highly congenial for fungal spores to multiply. Therefore, care should be taken in observing the worms frequently.

To control the spread and incidence of muscardine, disinfection of rearing room and appliances should be done scientifically. Muscardine infected larvae should be immediately separated from the bed and disposed into bowl containing lime water. Maintenance of hygiene during rearing, dusting of active lime powder on the rearing bed during moulting periods, good ventilation in the rearing room and providing adequate spacing in the bed helps to prevent the occurrence of muscardine. Dusting of 1-2% Dithane M45 in Kaolin or Captan in claked lime on silkworm body immediately after every moult and on the 4th day of fifth instar @ 3-5 g/sq.ft. Old news paper or paraffin paper is covered for 30 min. Feeding should be given afterwards.

Practice disinfection of silkworm rearing house, surroundings, appliances and silkworm egg surface. Aspergillus sp. are more tolerant to formalin hence, 3% formalin solution is suggested. Uzi Fly, Exorista bobmycis: The Uzi Fly, Exorista bobmycis is a serious endo-larval parasitoid of the silkworm. The loss estimated from this fly is 10-20%. The pest occurs throughout the year. Its incidence is high from August to November. Presence of black scar on the body of parasitized silkworms and maggot emerge hole in cocoon indicate the uzi infestation. The silkworms parasitized in early instars are killed before attaining spinning stage, while those parasitized in the late fifth instar spin cocoons of weak built and from such cocoons uzi maggots emerge by piercing, thus rendering cocoons unfit for commercial reeling. Management: Physical Method: � Collect and destroy the uzi infested silkworms and uzi maggots and pupae.
� Keep the rearing house floor free from cracks and crevices.
� Uzi infested larvae spin cocoons a day or two earlier than other silkworms and such cocoons are flimsy and bad quality.
� Fixing wire mesh or nylon mesh to the window of the rearing house with an arrangement to construct anteroom would physically prevent uzi fly entering the rearing house. Chemical methods: Uzi Powder: It is an ovicidal dust formulation. It is dusted on the body of silkworms on the 2nd day during III instar, 2nd and 4th days during IV instar and 2nd, 4th and 6th days in V instar. Uzi powder should be dusted after bed cleaning and silkworms should be fed half an hour after dusting. 4-5 kg uzi powder is required for 100 dfls. Uzitrap: It is a chemo trap used for attracting and killing the adults. Dissolve one tablet in one liter of water and the solution should be poured in light coloured flat trays or plates and placed near the windows of the rearing house. Uzi flies are attracted and get trapped in the solution. 12 tablets are required for 100dfls rearing. Uzicide: It is an ovicidal formulation. It is sprayed on the body of silkworms starting from 2nd at in III instar through 4th or 6th day of V instar on alternate days except during moulting. Uzicide should be sprayed after bed cleaning or 2�3 hours after feeding. Silkworms should be fed half an hour after spraying uzicide. 4 to 5 liters is required for rearing 100 dfls. Biocontrol Agent : Nesolynx thymus is a ecto-pupal parasitoid to kill the pupae. One lakh adult females should be released in 3 doses corresponding to IV and V instars and within one or two days after cocoon harvest at 8000, 16000 and 76000 adults, respectively. The parasitoids are released immediately after sunset in the rearing house, places of mountage storage, near mountages with spinning worms and also near the manure pits. Insect Pests Of Grainages : A number of insects are known to attack B. mori in grainages. Dermestid beetles and earwigs cause heavy loss in grainages. The most important pest is Dermestes ater beetles which are omnivorous. They attack and feed on eggs and silk moths, besides boring into cocoons to feed on pupae within the cocoons. They also feed on dead organic matter.

The predatory earwigs, Labia arachidis attack gravid females placed on the egg sheet for oviposition. They cut open the abdominal region of the moths and the injured moths die before egg laying.
Management: � The grainage premises and place of storage of cocoon should be kept clean.
� The pest insects should be collected using a vacuum cleaner and destroyed by burning.
� The pierced cocoons should be thoroughly sundried before storing them in airtight bags.
� The pierced cocoons (PC) should be shifted from the grainages as and when they are available and stored in place away from the grainages.
� Fix wire mesh to the doors and window of the grainage.
� Use of plastic cocoon storing trays reduced the incidences of pests. Chemical Control : � Dusting of bleaching powder (36% chlorine) on the floor near the walls @ 200g/sq.ft.
� The cocoon storage bags should be dipped in 0.028% deltamethrin solution (1 part of deltamethrin in 100 parts of water), followed by shade drying.
� Spray 0.028% deltamethrin solution on floor and walls of PC storage rooms.
� Dust 5% malathion on PC, before storing them (5kg dust to cover 50kg PC)
� Dip wooden trays harbouring the pests in 0.076% DDVP solution for 2 � 3 minutes. After 10 days, wash them thoroughly, sundry and use.
� Spray stands and room with 0.076% DDVP solution.
Late age Rearing

Scientific cultivation of mulberry and adoption of new technologies in silkworm rearing plays an important role in the production of quality raw silk. The larval duration in the life cycle of the silkworm, ranges from 24-28 days. The larval stage comprises of 5 instars, and 4 moults. Rearing of silkworms from 3rd or 4th instar up to spinning stage is called the late age silkworm rearing. During this period the silkworms consumes more than 94% of total mulberry leaves required, 133 times increase in body size, 125 times increase in body weight and 1000 times increase in the silk gland weight from the time of hatching. Since the late age silkworms are sensitive to high temperature and humidity, scientific rearing methods and rearing skill method are necessary for achieving maximum growth and survival of the larvae, thereby increasing the cocoon yield and silk production.

Rearing House:

A separate rearing house with good ventilation, light and space has to be constructed for late age rearing. The height of the rearing building has to be appropriate, with provisions for maintaining the required temperature and room humidity. The rearing room has to be clean and hygienic and suitable for conducting effective disinfection. It should have an anti chamber for the prevention of uzifly entry and have a separate leaf storage room.

In tropical country like India, rearing houses should be constructed East-West longitudinally with corridors all around and window on the South and North sides of the building. For easy exchange of air, the windows should be constructed opposite to one another. All the doors and windows should be fixed with uzifly mesh. For effective exchange of air, ventilators are to be fixed above and below the windows. The rearing space required for one dfls is 8 sq.ft.

The rearing rooms and rearing appliances to be disinfected before and after the completion of silkworm rearing.

Silkworm Rearing in Bamboo Trays:

Rearing silkworms in bamboo rearing trays has been the traditional method of rearing in our country. Late age silkworm larvae require a temperature of 22-25� C, 70-80% RH, 16:08 hrs of light and darkness. The rearing room has to be adequately ventilated. Depending on the season and environment, the silkworms are to be fed 3 to 4 feedings per day. The mulberry leaves to be harvested during the cooler hours of the day and transported in bamboo leaf baskets coved with wet gunny cloth to the leaf storage room. The leaves are stored in leaf storage bins which are coved with wet gunny cloth; this will preserve the leaf moisture for a longer period and maintain the quality of leaves. Perforated PVC pipes can be placed inside the heap, this helps to reduce temperature buildup inside the leaf chamber.

During moulting periods, the rearing bed has to be dried by dusting lime powder and aeration inside the room increased by opening the window and ventilators to the required extent. After more than 95% of the larvae come out of moult, the rearing bed disinfection has to be done by dusting the bed disinfectants as per recommendation and feeding resumed after 30 minutes. If any diseased larvae or unequal size larvae are observed they have to be immediately separated and disposed to a basin containing formalin or bleaching powder solution. Regular rotation of the position of rearing trays from the top to the bottom will provide uniform temperature and humidity to the larvae. Depending on the season and weather conditions the ventilators and windows can be opened or closed to facilitate maintaining of rearing temperature and humidity inside the room.

Shoot Feeding Method Of Rearing:

To bring down the labour cost and increase the productivity, shoot feeding methods of rearing has been developed, standardized for tropical conditions and recommended for the sericulturists. This new technology reduces the labour cost to an extent of 50%. Irrigated mulberry gardens are suitable for adoption of shoot feeding method. 5 to 6 crops can be harvested in a unit area of mulberry garden with an interval of 40-45 days between crops. In this method of rearing, 15 to 20% leaf saving can be achieved which can be used by brushing additional dfls, increasing the cocoon productivity per unit area of land. Since the leaves are attached to the shoots, the quality of leaves is maintained for a longer period, facilitates effective feeding and reduces the leaf cocoon ratio significantly.

The mulberry shoots to be cut using sharp secateurs or sharp sickle leaving 2 -3 buds from the previous cut and harvested during the cooler hours of the day, covered with wet gunny cloth and immediately transported to the leaf storage room. The shoots are place upright and covered with wet gunny cloth.

In this method of rearing, bamboo trays or rearing stands are not required, instead shoot rearing racks are constructed/fabricated. The rack should be 5 ft in width and length as long as the rearing hall. The rearing rack can have 3 to 4 tiers with a gap of 2 to 2.5 ft between the racks for effective aeration. 600 to 750 sq.ft. rearing space is required for hybrids while the CSR hybrids require 800 to 850 sq.ft. bed area.

After resuming from the moult, the 2nd or 3rd instar larvae are to be evenly spread on lime powder dusted old new paper spread on the shoot rearing rack. Mulberry shoots should be gently placed on the rearing bed opposite to one another. Depending on the season 3 to 4 feedings can be given per day and as the larvae grow, bed spacing should be increase.

When more than 90% of the larvae settle for 4th moult, feeding should be stopped, aeration inside the rearing room increased and rearing beds should be dried by dusting active lime powder. When more than 90% of the larvae come out of the moult, the rearing bed should be disinfected by dusting the recommended quality of disinfectants and after 30 minutes feeding should be resumed. In the shoot feeding method, rearing bed is cleaned once after 4th moult by using rearing nets or rope, larvae spread evenly as per the recommendation and feeding resumed. If the rearing room humidity is very low, the rearing beds can be covered with old news paper.

Handling of spinning larvae, harvesting of cocoons and transportation:

Once the larvae exhibits spinning behavior, feeding quantity has to be reduced and stopped. Spinning larvae has to be separated from the bed and shifted to the mountages. During spinning stage, 24 - 25� C temperature and 60-70% relative humidity has to be maintained in the mounting hall. This helps in getting quality cocoons. Adequate aerating should be provided during spinning period and the spinning larvae should not be exposed to direct sun light or heat. Cocoons can be harvested after 5 to 7 days, dead, flimsy, irregular and stained cocoons should be separated and good cocoons transported to the cocoon market in a cotton bags.

Types of Mountages:
  1. Bamboo Mountage:

    Bamboo mountages are the traditional type use in Karnataka. Bamboo strips are fixed in a spiral/circular fashion on the bamboo mats. The idea size of the mountage is 1.8 x 1.2 meters and gap between the spirals should be 5 to 6 cms. 40 to 60 larvae per sq. ft. can be mounted. During spinning stage, two mountages placed one behind the other at an inclination of 45�, this reduces the number of bad or rejected cocoons.

  2. Plastic corrugated mountage:

    Plastic corrugated nets are specially designed for mounting spinning larvae. The height of the corrugation should be 6 cms and each mounatge should have 11 corrugations in number. The ideal size of the mountage should be 60 x 90 cms, and this can be easily placed in wooden rearing tray. 300 to 400 can be mounted on these mountages. Before mounting the larvae, old news paper should be spread below the mountage, this will help to absorb the urination, reducing the humidity build up. These types of mountage, require less mounting space and can be easily disinfected. Cocoons can be easily harvested from this type of mountage. Adequate aeration should be provided in the mounting hall or rack particularly for CB larvae, as urination is more in these larvae.

  3. Rotary Mountages:

    This is an improved, Japanese type of mountage made with thick paper boards. Each mounting frame has 13 rows and 12 columns containing 156 mounting slots. Each slot is 4.5 x 3 x 3 cms in size and the dimension is 55 cms in length, 40 cms in width and 3 cms in depth. 10 such mountages are arranged in metal frame with a distance of 8 cm from one another. The rotary mounting frames are hung from the ceiling during spinning stage and can be packed and stored after cocoon harvesting.

    Before mounting the larvae, the rotary frames are placed on old news paper; about 1200 larvae are measured and distributed between the rows on the news paper. Once all the larvae climb on the mountage, the frames are lifted and hanged from the ceiling. Since the spinning larvae exhibit negative-geotrophic behavior, they climb and move up, because of the weight displacement, the mountage rotates, helping the larvae to search an empty slot and settle for spinning. Therefore, this type of mounatge is called a rotary mountage.

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