Diseases and Enemies of Silk Worm
1. Pebrine: It is the most important disease of silk worm and once infection starts it can wipe out all the worms. It is caused by a sporozoan Nosema bombycis. The full grown caterpillar is attacked and it turns pale brown or red with black spots, becomes inactive and eventually dies. The infection spreads to successive generations through eggs of diseased moths. Therefore, eggs from healthy moths alone should be taken for rearing worms. After a moth has laid eggs, she is crushed and the body fluid is examined under the microscope for Pebrine bodies and if she is found to carry the protozoan the eggs should be rejected and destroyed.
2. Muscardine: It is a fungal disease caused by Beauveria bassiana and transmitted by spores carried by wind. All stages of caterpillar are attacked. The body of the affected larva becomes soft but get hard and acquires green colour after its death.
3. Flacherie: It is a bacterial disease and the affected worms are infected by Bacillus bombysepticus. Caterpillars in their fourth instar are usually infected. Digestion in the affected caterpillar gets disturbed and it voids foul smelling semi-solid excreta and turns lethargic and soft. Regular feeding of the larvae and maintaining good hygienic conditions will prevent the disease.
4. Grasseric: The last instar caterpillars are affected. They become swollen and look like a bag of granules, the body fluid becomes thick and clouds and they die. It is caused by nuclear polyhedrosis virus.
All the above diseases can be checked by a) careful selection of disease free eggs b) feeding with the clean and limited quantity of leaves., tender chopped leaves to young caterpillars, young full leaves to grown up ones and never the coarse mature leaves to any c) maintenance of a low temperature in the rearing rooms and d) sanitation of the rearing materials and worms themselves.
A tachinid parasitoid, Sturmia sericariae parasitizes the silk worms.