SILKWORM DISEASES

Protozoan disease:

Pebrine: This dreaded disease is also known as pepper disease or corpuscle disease. The name pebrine was given to the disease in 1860 by De Quatrefages because the black spots that appear on the diseased silkworms look like pepper grains.

Causal agent and infection: The disease causing organism is the spore of Nosema bombycis belonging to the family Nosematidae.

Symptoms: Pebrinized eggs easily become detached from the egg cards or substratum on which they are laid as they lack the adhesive gum. The eggs are laid one on top of another in lumps instead of closely side by side

General preventive measures: Surface sterilization of disease‑free layings should be carried out by dipping the egg cards in a 2 percent formalin solution for a few minutes and then washing in running water. Other general preventive measures are maintenance of strict sanitation, hygienic rearing, frequent and careful inspection of stocks for signs of infection, destruction of diseased material, and disinfection of rearing rooms and appliances.

Disinfectants and disinfection: The most commonly used disinfectants after the rearing is over are formalin and bleaching powder. Bleaching powder is used for disinfection of rearing rooms which cannot be made air‑tight. A 2 percent solution is strong enough for disinfection and is the simplest and the most economical germicide. This can be prepared by mixing one part of, 40 percent formalin with 19 parts of water.

Rearing resistant races: The resistance to pebrine differs according to the race. It has been claimed that the Japanese species of silkworms have a higher resistance than most other races; that European races are susceptible; and that Chinese and certain indigenous Indian species are comparatively resistant. Search for a totally resistant race has been unsuccessful so far.

Bacterial diseases:

Flacherie: In early literature on insect pathology, the word "flacherie", which is one of the most general and common of the descriptive terms, was used to describe the flaccid condition seen in the silkworm, Bombyx mori, ailing from all sorts of dysenteries. Even now this term is widely used to describe many different diseases.

Symptoms: The symptoms are not always uniform and vary according to the time of occurrence the kind of bacteria multiplying in the digestive organ, the race of the silkworm and other conditions. The general symptoms are: the larva loses appetite, becomes sluggish and grows slowly.

Control: (1) The first and foremost requisite is to raise healthy and strong silkworms since the primary cause is the weakness of the silkworms. (2) Proper incubation of eggs: the silkworms become weak if the eggs are incubated at higher temperatures or greater dryness than approximately 60 percent of relative humidity. Temperature and humidity should be maintained at 220 ‑ 250C and 8o ‑ 85 percent respectively. (3) Selection of suitable race: it is essential to select a race that is sturdy and resistant to adverse conditions for summer and autumn rearing. (4) Feeding of good quality leaf: the health and development of the silkworm is closely related to the quality and quantity of the leaf fed.

Septicemia:

Causal agent: Penetration and multiplication of certain kinds of bacteria in the haemolymph cause septicemia. The principal pathogenic bacteria are large and small bacilli, streptococci, staphylococci, etc. Infection is through an injury or wound in the skin.

Symptoms: They vary depending on the kind of bacteria. When a silkworm is infected with more than one kind of bacteria, the symptoms are determined by the predominantly propagated one. As the disease advances, the pro‑legs lose the capacity to clasp and the worm dies. The body of diseased larvae does not differ much in appearance to that of a healthy larva until it dies, but when the larvae vomit fluid, the body shrinks.

Control: Irrespective of the health of the silkworm the disease is transmitted mainly through an injury or wound. Hence the infected silkworms should be treated as for any other infectious disease. The affected silkworms should be isolated from the healthy ones as soon as possible and destroyed by burning or burying deep in the soil. General disinfection of rearing rooms and rearing appliances with a 2 percent formalin must be carried out after rearing is over.

Virus diseases: Virus diseases found in silkworms are grasserie or nuclear polyhedrosis, cytoplasmic polyhedrosis, infectious flacherie and gattine.

Grasserie: This disease is also known as jaundice or nuclear polyhedrosis. Jaundice indicates the yellowish color of the diseased insect.

Causal agent: Among the various amicrobial factors once attributed to its cause were such things as poor nourishment, unsuitable leaves, uneven temperature, damp air, poor ventilation and excessive moisture.

Infection: It is believed that the disease begins with the digestion of infectious material polyhedra or free virus) into the alimentary tract of the insect.

Symptoms: As the disease advances, appetite decreases and skin tension is lost. Usually five to seven days after infection, the intersegmental membranes of the body become swollen and the skin becomes shiny.

Control: Rearing the larva under hygienic conditions, avoiding unsuitable leaves, proper ventilation and spacing, timely picking out of and destroying of affected worms are some of the precautionary measures that can be followed.

Fungal diseases:

Muscardine or calcino: The term muscardine originated from the Italian word “moscardino” meaning a musc comfit because the bodies of the insects infected with the fungi are transformed into white mummified specimens resembling in appearance comfits or bonbons. The Italian name Itcalcinoll for muscardine refers to the chalky or limelike appearance of the dead silkworm infected with white, green, yellow or red muscardines.

White muscardine:

Causal agent: The causal organism of white muscardine in silkworms is Beuveria bassiana (Bals) Vuill. This fungus belongs to fungi imperfecti. Conidia are round or globular, or, rarely, oval; cylindrical conidia are also found. The conidia appear chalky white collectively but colorless individually. The conidium at the time of germination swells and gives out one or two germ tubes.

Infection and life cycles: Infection mainly takes place through the skin due to body contamination. Per os infection or infection through the tracheal opening may also occur but infrequently. Infection begins soon after the insect's body is contaminated with the conidia of the fungus.

Symptoms: The progress of the disease in the infected larva is very rapid. The larva loses appetite and becomes inactive. Specks of an oozing oily substance may appear on the skin when the disease advances.

Prevention and control: There are two types of control measures: (a) control measures taken during the course of rearing; (b) control measures taken in between two rearing or seasons.

(a)  Control measures taken during the course of rearing: Infected larvae must be removed and burnt prior to the appearance of the conidia ' A convenient method for separating infected larvae from healthy larvae is to use a net. The healthy larvae will crawl to the fresh leaves in the net while the infected ones, which will be inactive, will remain with the litter. Litter should be removed as frequently as possible, and burnt. Muscardine develops rapidly in hot and humid weather. At such times it is better to have good ventilation and low moisture in the rearing room.

(b) Control measures taken between two rearings or seasons: If any infection was observed in the previous rearing disinfection of the rearing rooms and equipment should be carried out immediately after the rearing, instead of waiting for the next rearing time, as this will prevent the conidia spreading. Formalin is most effective since the vapor is penetrating as well as fungicidal. The rearing rooms and all the rearing equipment should be sprayed with a 3 5 percent formalin solution in a closed room. This stronger solution is better since a 12 percent solution which is good enough for routine disinfection, is not strong enough to cope with an active outbreak.

Layings prepared from a rearing where the disease was present should be surface sterilized in a 1 ‑ 2 percent formalin solution for a few minutes immediately after oviposition is complete.