Diseases of Silkworm
A number of diseases are faced by sericulture industry in tropical regions of South East Asia. The common diseases of silkworm include maggot disease, pebrine, polyhedrosis and flacherie that causes extensive damage to sericulture industry.
Maggot disease is caused by Tricholyga sorbillans, a fly belonging to the order Diptera and family Tachinidae. It is found in India, Japan, Korea, China, Vietnam and Thailand. The symptom of this disease is the presence of milky white cylindrical eggs on the skin of silkworm larvae. This fly lays eggs on silkworm larvae that vary from several to more than fifty but usually they are two or three. After 30-40 hours of egg laying, maggot is developed inside the egg shell. This maggot makes a hole on the ventral side of egg shell and on the skin of the silkworm. In this way, the maggot penetrates into the body of the silkworm larvae and starts eating the tissues of larvae. When the maggot penetrates into the larval body, the big black mole is formed on that portion of the skin. The segments where maggot exists, bends and swell up, as a result the attacked larva becomes inactive and loses appetite. Maggots usually attack fourth and fifth stadia. The larvae which are attacked upto fourth stadia usually die before making cocoon, while those which are attacked in fifth stadium make cocoons but usually do not reach the pupal stage.
There are number of natural enemies of T. sorbillans like parasitic insects, fungus etc. they control the population growth of T. sorbillans. The only means of preventing this disease is by avoiding the entrance of this fly into the rearing room of silkworm. For this purpose nets are fixed around the windows.
This disease is one of the worst diseases of silkworm. Once sericulture industry was damaged by this disease in all the countries but some of the countries have overcome this disease and succeeded in getting pebrine-free silkworm eggs for reeling cocoons. This disease is caused by Nosema bombycis belonging to Microsporidia.
This infection reaches through the mouth of the larvae at the time of feeding or through the ovary of the mother. When new spores are formed in the tissues of alimentary canal of silkworm, they are discharged with the faeces and make a source of infection. When the spore enters into the digestive tract of silkworm, two nuclei present in the sporoplasm are divided into four nuclei and at the same time the polar filament projects and penetrates into the cells of the alimentary canal. Further, they enter into the blood and swim in it. They are spread and distributed throughout the host body, attacks various tissues especially in the fat bodies and organs, excluding chitinous tissues and nucleus of cells. When the hypoderm is attacked and its cell die, the affected part becomes black due to the formation of melanin. Inside the body, the milky white spots or marks are developed on the silk gland or on the surface of the alimentary canal. In case of severe infection of the eggs the whole of yolk gets almost filled with the microorganisms resulting in their death (death of eggs). In case of slight infection, the eggs hatch but the larvae carrying infection die at the third moult without making cocoon.
The diseased larva loses appetite showing inclination towards food and exhibit irregular and differential growth resulting in the formation of small larvae of different sizes. They becomes tardy, shrunken or moult quite late and finally die.
Counter measures against Pebrine
When infection occurs at the embryo stage, the larva die in the third moulting stage. So it is important to use pebrine free eggs during the process of rearing of reeling cocoons. The infection by pebrine should be prevented at any stage of the life cycle of the silkworms for rearing seed cocoons in order to get the healthy eggs. Therefore it is important to keep the rearing houses free from pebrine germs. Following measures should be applied:
- Examination of mother moths: All the mother moths producing eggs should be carefully examined one by one and eggs laid by healthy moths only be taken for further use. The unqualified eggs should be burnt away.
- Forecasting and correcting examination: In order to make the pebrine examination more reliable, the forecasting and correcting examinations are carried out for the eggs of the seed cocoons.
The materials employed for forecasting examinations are the excrement of mature larvae, late moulting larvae, dead larvae, cocoons or pupae and moths accelerated to emerge out of cocoons. For the correcting examination a few eggs of each reproductive egg-batch are taken and incubated and thus, emerged larvae are used as the material for examination of pebrine germs.
- Removal and disinfection of pebrine germs: The spores of pebrine may survive for a number of years in an ordinary type of rearing room if the environmental condition is humid. Therefore, rearing rooms, tools and other utensils should previously be cleaned and washed to remove the pebrine diseased eggs, carcases of infected larvae, pupae, moths and dead cocoons, faeces of diseased larvae and so on. The pebrine spores can be destroyed by treatment with 2% formalin for 30 minutes, 0.5% sublimate for 5 minutes, 5% chlorinated lime for 30 minutes, current steam for 30 minutes and sun shine in the summer for 7 hours.
- Care on rearing silkworms: The tendency of the infestation by the pebrine spores has been observed to occur more when silkworms are reared in dry and cool conditions than in the hot and wet conditions. If the larval period becomes longer, the infection of pebrine is severe, so care should be taken on these points during the course of selection of the seed cocoons by the rearers.
Polyhedrosis in Silkworms
Three types of polyhedrosis are found in silkworms:
- Nuclear polyhedrosis.
- Intestinal cytoplasmic polyhedrosis.
- Intestinal nuclear polyhedrosis.
Nuclear polyhedrosis (Grasserie, Jaundice):
It is caused by a kind of virus which forms polyhedra in the nuclei of the cells of fatty tissues, dermal tissues, muscles, tracheal membranes, basement membrane, epithelial cells of midgut and blood corpuscles. The polyhedra are commonly hexagonal and rarely tetragonal in shape, containing large number of virus in them. The polyhedra form the white pus after they are released into the body fluid. The virus present inside the polyhedra maintains its pathogenic power for a number of years in the rearing room but the isolated virus loses its pathogenic power in a period of short time.
The larvae infected by this virus become inactive and lose appetite and the membrane between segments swell up. Further, the whole body swells up showing the loose skin. In the last phase of the disease the body becomes purulent and the skin becomes tender from which pus leaks out. The larvae in this phase crawl around up and down and finally die. The infection of grasserie occurs through the mouth of the larvae, the wounds of skin and induction under extremely adverse conditions (cold treatment, heat treatment, chemical treatment).
- The rearing rooms, tools and utensils should be washed, cleaned and disinfected.
- The highly nutritive mulberry leaves should be given sufficiently to the infant larvae. The fresh air should be circulated in the rearing room especially in fifth stadium.
- The high temperature, low temperature and high moisture content should be avoided especially during the infant stages to maintain good health.
- The diseased larvae or dead bodies should be removed and the room disinfected completely by 2% formalin.
- The silkworms should be reared carefully, so that they may not suffer from the wound of skin.
Intestinal cytoplasmic polyhedrosis:
This type of polyhedra are formed in the cytoplasm midgut cells but in a few cases they are formed in the goblet, too. The polyhedra of this disease contains plenty of virus. The infected cells of midgut rupture and polyhedra are released into the gut. Thus, the faeces, excreted becomes whitish, containing plenty of polyhedra.
The symptoms of this disease are the occurrence of translucent cephalothorax and shrinkage of body size but in the advanced phase the larvae excrete white coloured loose faeces. The body fluid of the diseased larvae remains in the normal condition. The mode of infection of this disease and countermeasures are similar to those of the nuclear polyhedrosis.
Intestinal nuclear polyhedrosis:
In this disease the virus makes polyhedra inside the cytoplasm and nucleus of the midgut cylindrical cells. The polyhedra formed in this disease are large sized The translucent cephalothorax, shrinkage of body and diarrhoea are the symptoms of this disease also. The mode of infection and countermeasures are similar to those of nuclear polyhedrosis.
Flacherie is the genetic name of some kinds of silkworm diseases, carcases of which rot due to the attack of bacteria. Flacherie may be divided as follows:
- Infectious flacherie caused by a kind of virus: The various symptoms of flacherie are loss of appetite, translucent cephalothorax, vomiting and diarrhoea but the real diagnosis can be made after the microscopic observation of the virus. The pathogen of this disease is a spherical virus which does not form polyhedra in the body of silkworm larvae. The virus multiplies in the tissues of midgut and is released into the gastric juice and is excreted along with faeces which is the source of infection. The virus infects the larvae of silkworm orally.
- Resistant silkworm races should be selected.
- The rearing room, tools and utensils should be well disinfected.
- In order to maintain the good health of silkworm, high quality mulberry leaves should be provided.
- Favourable conditions like temperature and humidity should be maintained in rearing rooms.
- The faeces, diseased larvae and dead bodies should be piled in the compost.
- Some of the chemicals like hydrochloric acid, formalin, chlorinated lime may be used as disinfectants for this virus.
- Gastric injury caused by physiological disturbance of silkworms followed by the multiplication of bacteria: Due to the supply of bad quality of mulberry leaves the digestive physiology of the silkworm is disturbed and multiplication of bacteria in the gastric cavity takes place. Thus, the combined action of physiological disturbances and bacterial activity in the gut are major causes of this disease. In unfavourable climatic conditions, the bacteria like Streptococci sp. Coli aerogenous bacilli or proteus group bacilli attack the weakened silkworms. The control ineasures against this disease is to keep healthy conditions of rearing silkworms.
- Bacterial intoxication: This disease is caused by a toxin of some bacilli, Bacillus thuringiensis Var. The larvae attacked by this toxin become unconscious, later soften, become darkish and finally rot off. The infection occurs orally and can retain the toxicity as long as for seven years in some cases. The countermeasures are the disinfection of the rearing room and instruments.
- Septicaemia: This disease is caused by infection of some bacteria as Bacillus megatherium, B. proteus, B. prodigiosus, B. pyocyones in the blood of silkworms. This disease is rare in the larval stage but it causes severe damage to the pupae and the moths during the period of egg production. The infection is caused through the wounds on the skin.
- The rearing tools and rearing houses should be kept sterilized.
- The diseased cases should be kept away and put into the fermenting compost.
- During the period of egg laying the temperature and humidity of the room should be maintained in proper order specially at lower range of humidity.
It is a fungal disease of silkworms. There are a number of muscardine in silkworm but only green muscardine (Spicaria prasina) has been noticed to affect the larvae of silkworm in Vietnam. The infection may be observed at the third and fourth stadia of silkworm. In the beginning stage a big black spot is observed on the ventral side. The green-tubes of fungus develop into mycelia in the blood and bear cylindrical spores which are separated from mycelia and further form mycelia which bear cylindrical spores again. Thus, all the organs of silkworm are attacked by this fungus disturbing their normal functioning. As a result the diseased larvae do not moult and finally die.
- The rearing room, tools and utensils should be disinfected.
- The diseased larvae and their faeces should be piled into compost to kill the germs.
- The rearing bed should be kept dry as much as possible to prevent the germination of conidia of fungus.
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