Biochemical basis for the occurrence of microbial diseases in silkworm, Bombyx mori L. (Lepidoptera: Bombycidae)

Different types of pathogens, viz., protozoa, virus, fungus and bacteria are the major causes of the diseases to the larvae of silkworm, Bombyx mori L. The ubiquitous pebrine disease caused by Nosema bombycis takes a heavy toll of various silk producing larvae indicating its virulency and subsequent economic impact. Seventy percent of the silkworm diseases are due to viral infections. Viral diseases occur due to interruption in carbohydrate metabolism affecting trehalose levels in the blood. The loss due to different strains of bacteria is also more. Hypoproteinemia was also observed in the plasma of haemolymph of the grasserie infected B. mori larvae. This may be due to reduction of protein synthesis and release by fat body cells into the haemolymph. Bacterial diseases occur due to the variations in the alkaline phosphatase activity in the midgut of the larvae. Due to this the goblet cells of the midgut showed necrosis and its secretary activity was also affected significantly and this leads to acidic gut and hence, the worms die by acid digestion. 30–40% of the silkworm diseases are due to muscardine, a fungal disease which is much more prevalent in winter. Fungal diseases occur due to interruption in uric acid metabolism affecting haemolymph osmotic pressure, uric acid and ammonia levels in the blood. It was considered that oral infection of B. mori larvae after inoculation of Beauveria bassiana, occurred due to hyphal penetration into the body cavity from the intercellular spaces made in the midgut tissues, after their destruction by the toxins or substances secreted from the hyphae, and was not caused by direct hyphal invasion into the midgut tissues. Whereas fungal diseases are preventable as they attack the worms under unhygienic conditions where environmental parameters and good quality of mulberry leaf play a major role.