Pathogenicity of nosema bombycis naegeli to silkworm, bombyx mori linnaeus and its management

Abstract

Pebrine, a chronic disease of silkworm, Bombyx mori L. caused by a highly virulent parasitic microsporidian, Nosema bombycis Naeg. was observed as a serious menace and an important production constriant of silk in the Kashmir Valley. The incidence of the disease at varying levels was observed on all the silkworm races in all the six districts of the valley. The prevailing silkworm rearing practices in the valley has a great influence upon the incidence of the disease. The common ways of infection were found as per oral and transovarial. The characteristic symptoms of the disease are poor appetite, poor growth, unequal body size, irregular moulting, change in colour (off-white, faint yellow, rustic brow), appearance of dark brown to black spots on the integument and whitish tumour like pustules along the length of distended silk glands. Infected pupae flabby and swollen, with blackish abdomen and infected moths are crippled, with deformed wings, distorted antenae and loose body scales. The pebrinised eggs are small in size and lighter in weight. First mortality was recorded on the 9th day of post inoculation. The larval weight was reduced by half and the total larval duration was prolonged by 9-11 days in case of infected batches of silkworm. The spore multiplication rate was found to increase with the advancing stage of infection and the developmental stage of B. mori. The sporulation rate varied among the individuals. The highest sporulation rate was recorded in CA2 x SF19 both at the pupal and moth stages. All the six chemicals, Bavistin, Topsin-M, Codrinal, Crodoxin-FM’ Malariaquine and Metrogyl proved efficacy against the disease. However, Malariaquine was more effective and showed the highest survival folowed by Bavistin, Codrinal, Topsin-M, Metrogyl and Croydoxin-F M’. The chemicals showed a significant increase in the weight of larvae, yield of cocoons and exhibited a positive influence upon all the economic trials and fecundity. The fast spreading nature of the disease through secondary contamination was observed by introducing pebrinised silkworms in healthy population. The extent of infection was in proportion to the number of infected larvae introduced. In the moth stage, the infection was found in the range of 30.00-73.33 per cent in the batches where 1 to 10 infected larvae were introduced. The larval duration was prolonged by 8-9 hours after mixing of larvae soon after the 3rd moult