Silkworm Diseases

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Silkworm Diseases
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 disease:
Pebrine is caused by Nosema bombycis, which is a protozoan. The disease is transmitted
through contaminated leaves or from mother moths through eggs. It is the most serious disease
in silkworms as it is highly infectious and infects the young silkworm larvae causing
considerable loss.
Symptoms of egg stage:



Poor egg number.
Reduction in size and weight.
Lack of adherence of substratum, disuniform with more of dead and unfertilized
eggs.
 Irregular hatching.
Symptoms at larval stage:


Loss of appetite, retarded growth and disuniformity in size
Irregular moulting.
 Heavy mortality after 2nd moult if infected at egg stage.
 Larvae shrink in size and vomit gut juice.
 Dark brown or black spots may be seen sometimes on the body.
Symptoms at the pupal stage:



Pupa looks floppy and swollen.
Irregular black spots on the body.
Heavy mortality at pupal stage.
Symptoms at the moth stage:




Improper development of moth.
Deformed wings and distorted antennae.
Poor mating and egg laying.
Scale of wings and abdominal area come off easily.
Control measures of pebrine disease:


Follow strict mother moth microscopic examination method to produce disease
free layings. Individual moth examination or group moth examination for
industrial seed must be resorted to, in order to eradicate pebrinized layings.
Periodical microscopic examination of silkworm larvae during rearing. If pebrine
spores are detected, the whole lot should be discarded /rejected.
1

Carry out surface sterilization of disease free layings by dipping egg cards in 2%
formalin solution for 10 minutes followed by washing in running water.
 Follow strict disinfection of rearing room, appliances and surroundings of rearing
area.
 Maintain strict sanitation and hygienic conditions during rearing.
 Apply bed disinfectant as per recommended schedule and quantity. Infected
silkworm faeces and bed refuse are important source of infection and should be
disposed off to prevent cross infection and spread of disease.
Grasserie disease:
Grasserie is a viral disease in silkworm caused by Nuclear Polyhedrosis (NPV),
Cytoplasmic polyhedrosis (CPV) and infectious flacherie. Nuclear polyhedrosis (NPV) is a
major viral disease in silkworm. It is caused due to the presence of high temperature, high
humidity and feeding of poor quality mulberry leaves. It is highly infectious.
Symptoms:
In the early larval stage of infection, it is difficult to detect the disease. Microscopic
examination of larvae may indicate the presence of polyhedral bodies. As the disease advances,
the larvae loose appetite and skin becomes shiny before moulting. The inter-segment membrane
becomes swollen.
The haemolymph or body fluid becomes turbid white.
Microscopic
examination shows presence of large number of polyhedral bodies.
Control measures for grasserie:








Rear the larvae under clean and hygienic condition.
Thorough disinfection of rearing room, appliances and surroundings.
Ensure proper disinfection of egg surface.
Incubate eggs under hygienic conditions. Avoid touching with hands.
Provide suitable and timely feed during rearing.
Maintain proper spacing and adequate ventilation.
Pick out diseased, weak and injured larvae and destroy them properly.
Apply bed disinfectants as per recommended schedule and quantity.
Flacherie disease:
Flacherie disease of silkworm is caused by bacteria or virus individually or in association.
Depending on symptoms and cause they are called as bacterial flacherie disease, septicemia,
sotto disease, etc. The disease may occur due to fluctuations in temperature, humidity and
feeding poor quality mulberry leaves.
Symptoms:



Larvae become sluggish and loose appetite.
Body shrinks, becomes soft and flaccid.
Growth is retarded, becomes dull and vomits gut juice.
2


Loose clasping power of prolegs.
Body ferments, turn to different colour and oozes out foul smell.
Control measures for flacherie:


Raise only healthy and strong silkworm races.
Maintain proper temperature (22-25°C) and humidity (80-85%) during incubation
of eggs.
 Attend thorough disinfection of rearing room, appliances and surroundings.
 Take strict hygienic measures during rearing.
 Isolate infected larvae from the healthy one immediately and destroy by burning
or dumping deep in the soil.
 Provide quality leaves for feeding and maintain proper spacing and ventilation.
 Maintain proper temperature and humidity during rearing.
Muscardine (Fungal disease):
Muscardine is a fungal disease in silkworm. Various types of muscardine diseases have
been reported i.e. white, green, yellow etc. but white muscardine is commonly found in Kashmir
valley. The white muscardine in silkworm is caused by a fungus, Beuveria bassiana. The disease
is caused due to body contamination by the fungus. The fungus grows well under low
temperature.
Symptoms:



The larvae loose appetite, become sluggish, ceases to move and finally die.
The dead larvae become harder and mummified.
The body after death is covered with white mycelia. Conidia develops in the
body.
 Finally whole body of dead larvae looks chalky white.
 The pupa and moth are also infected by white muscardine disease. In heavy
infection, pupal body gets cowered with conidia.
Control measures for muscardine disease:











Infected larvae should be picked up and burnt.
Maintain good ventilation and proper humidity in rearing room.
Regulate humidity by dusting lime at the time of each moult and in between.
Apply bed disinfectants as per recommended schedule and time.
Provide quality leaves and maintain proper spacing during rearing. Avoid dusty
leaves.
Follow strict disinfection of rearing house, rearing appliances and surrounding
area.
Practice proper disinfection of silkworm egg surface.
Maintain hygienic conditions during rearing.
As a specific measure, dust Diathene M-45 in Kaolin or Captan in slaked lime on
silkworm body immediately after every moult as per following schedule.
I-II instar -1% dust
III-V instar-2% dust
3

Apply formalin chaff to silkworm body. The formalin chaff is prepared by
partially burning paddy husk and mix formalin of 0.6% to 0.8% concentration
(0.6% for chawki and 0.8% for late stage) in the ratio of 10:1. The chap is
sprinkled uniformly on the silkworms 30 minutes before feeding.
Aspergillosis
Aspergillosis is a major silkworm disease in several sericultural countries. This is caused
by a number of Aspergillus species of fungi. Aspergillus is a facultative fungus and is able to live
saprophytically in the silkworm rearing environment like soil surface and rearing appliances,
silkworm faeces etc. These form the source of the fungus and thus disease spreads rapidly. The
early instars i.e., first and second instar silkworm larvae are more susceptible and later stage
silkworms are fairly resistant to this diseases. High temperature and high relative humidity
conditions maintained during young stage are reportedly contributing factors to greater disease
incidence during young age. Aspergillus flavus and Aspergillus tamarii are commonly found
strains in India.
Aspergillus spp. infects the silkworm through the integument. The conidia are the
infectious units and these on coming in contact with host-integument under congenial conditions
of temperature and humidity germinate to put forward the germ tube that penetrates through
integument. The germ tube after entering through the epithelium branches at the spot of entry. At
the point of entry black marking may be noticed. The fungus does not form short hyphae as in
case of B. bassiana and grows only at the site of infection and finally the larva die due to the
secretion of aflatoxin.
Symptoms



Infected larvae stop feeding, become letharginc, show body tension and lustrousness and
the victim die soon due to Aflotoxin produced by the fungus in the host.
Aerial hyphae appear a day after death and later conidia cover the body giving particular
colour according to the Aspergillus species.
Hardening of the body is limited to the site of infection and the rest of the body decay.
Disease management



Preventive measures such as disinfection and hygiene maintenance in the rearing
environment is the best way to keep the disease at bay.
Sun drying of rearing equipments is an effective way of destroying Aspergillus pathogens
to some extend.
Sick worms discovered before conidification should be incinerated or placed in lime jars
and never thrown around indiscriminately.
4

The faeces and bed refuses should be disposed off properly and disinfection with anti
muscardine powder should be carried out immediately.
Integrated silkworm disease management:
Comprehensive integrated steps can result in prevention of silkworm diseases. Prevention is
better than cure is the correct approach for controlling silkworm diseases. The following
preventive measures will result in controlling silkworm diseases:

Follow strict disinfection of rearing houses, rearing equipments and surrounding areas.

Rear only disease free layings. Infected layings should be isolated through strict moth
examination and discarded.

Control mulberry pests (alternate hosts of muscardine causing pathogens) in the field and
provide disease free and quality leaves according to age of silkworms.

Maintain strict sanitation and hygienic conditions during rearing.

Enhance vigour of silkworms by providing suitable and timely feeding. This will enhance
resistance against disease.

Isolate sick and diseased worms and dispose off properly.

Maintain appropriate spacing during rearing.

Isolate infected material/equipment and follow strict disinfection.

Faeces (silkworm litter) and bed refuse should be disposed off properly by burying it.

Adjust the humidity of rearing room and trays. Dust lime at the time of each moult.

Apply bed disinfectant (Vijetha/Labex/RKO) as per recommended schedule and time.

Intensive rearing management and close monitoring is essential.
Pests of Slkworm
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
5
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 to 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.
Insect Pests Of Grainages:
6
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 Dermester 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:

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


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

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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 to 3 minutes.
After
10
days,
wash
them
thoroughly,
sundry
and
use.
Spray stands and room with 0.076% DDVP solution
7

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