Rate of spread of green muscardine disease in a healthy population of the silkworm, Bombyx mori L.

摘要

Silkworm diseases spread during the rearing through various means depending upon the pathogen. When large number of silkworms is reared in the rearing tray, as a routine practice and if some larvae are infected by disease, they extrude the pathogens by various means into the rearing environment to form the source of contamination and spread of disease. The pathogens are extruded by infected silkworms along with gut juice (most viral diseases and pebrine), faecal matter (pebrine and bacterial diseases), breakage of integument (nuclear polyhedrosis and septicemia diseases) and body surface (muscardine and aspergillosis diseases). The diseased larva/moth/other insects is a source of infection. The pathogens are stable, persist for longer periods in the rearing environment and with efficient mode of dispersal they contaminate the silkworm population leading to secondary infection during the rearing. Several authors have studied the rate of spread of different diseases in silkworm rearing. The spread of pebrine (Ichikawa, 1935; Ishihara and Fujiwara, 1965; Baig et al., 1988), nuclear polyhedrosis (Anonymous, 1990), infectious flacherie (Ishikawa and Miyajima, 1964), densonucleosis (Miyajima, 1979) and bacterial flacherie (Samson, 1987) in the silkworm population were studied by introduction of diseased larvae as carriers. High rate of multiplication and spread are the main characteristic of the fungal diseases in silkworm and white muscardine develops into an epizootic within a short period, if the conditions are congenial. Prabhakara et al. (1997) studied the rate of spread of white muscardine disease in a silkworm population after introduction of white muscardine larvae during all the larval instars. It was a general view that the green muscardine incidence was sporadic in India without causing any damage to the sericultural crops. However, no systematic study was available on this disease. Recently, Balavenkatasubbaiah et al. (2009) conducted one year survey covering all the three seasons in five sericultural areas of Karnataka, India and reported the prevalence of green muscardine disease through out the year along with white muscardine. The prevalence of green muscardine was more during rainy season followed by winter and summer. In the present study rate of spread of green muscardine disease in a healthy population of three popular commercial hybrids of silkworm viz., CSR2 * CSR4 (bivoltine hybrid), PM * CSR2 (multi * bivoltine hybrid) and bivoltine double hybrid (DH) i.e., (CSR6 * CSR26) * (CSR2 * CSR27) has been studied and discussed.