Notes on Silkworm Diseases and Pests

Diseases are the behavioral and physiological changes induced by agents calledpathogens in an organism. All diseases have specific symptoms and features. Similarly, silkworms or larva are also affected by various types of diseases caused by pathogens like protozoa, fungi, bacteria and viruses. Silkworm diseases  cause substantial financial loss to the industry, their prevention and control assumes utmost importance.

Pebrine:

Pebrine is caused by a protozoan organism called Nosema bombycis. In the initial stages the larvae appears to be healthy, but when observed under a microscope one  can see oval, shinning spores of Nosema. Pebrine disease is infected to the silkworms in two methods; per oral and transovarial infection. In advanced stages of infection, silkworms stops feeding resulting in unequal size damaged larvae, they become sluggish, and die. The dead larvae turn black in colour due to secondary infection by bacteria . If infection occurs in late Vth 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 decreaed

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

Pebrine disease can be controlled by disinfecting the rearing room of larva , equipments and rearing surroundings.At the time of  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 of sericulture.

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 virus (DNV), Cytoplasmic polyhedrosis virus  (CPV) and Bacterial diseases and several types of bacteria. It  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.