RELATIVE TOLERANCE OF CERTAIN POLYVOLTINE BREEDS OF SILKWORM, BOMBYX MORI L. TO THE WHITE MUSCARDINE FUNGUS, BEAUVERIA BASSIANA (BALS.) VUILL.

摘要

Fifty polyoltine breeds of silkworm Bombyx mori L. were screened to find out their relative susceptibility/tolerance level against the white muscardine causing fungus, B. bassiana (Bals.) Vuill. Newly ecdysed third instar larvae (out of second moult) were topically inoculated by dipping them in the B. bassiana inoculum suspension of 1 x 10(5) conidia/ml concentration. After inoculation, larvae of most of the breeds succumbed to the pathogen during the course of rearing but variability existed in their susceptibility level with regard to day of initial mortality, mortality up to 5 days post inoculation and per cent of larvae out of third moult. Based on their survivability after third moult, tolerance index (TI) of the breeds were determined and were categorized into four groups viz., highly tolerant (above mean + SD), moderately tolerant (between mean and mean + SD), moderately susceptible (between mean and mean - SD) and highly susceptible (below mean - SD). Out of the 50 breeds, 5 breeds, viz., BL24, BL69, BL35, Madagascar and Kolar Gold were highly tolerant and 6 breeds viz., 96C, Hosa Mysore, Mysore Princess, MAD, GNP and MAR were highly susceptible. Twenty-five breeds were found moderately tolerant and 14 breeds were in the moderately susceptible category. Two highly tolerant (BL24 and BL69) and two highly susceptible breeds (MAR and GNP) and one moderately tolerant (Pure Mysore) breed were analysed for their LC50, LC90, LT50 and LT90 values. The LC50 ranged from 2.264 x 10(3) to 1.192 x 10(5) and LC50 values varied between 6.81 x 10(4) and 9.983 x 10(5). The median lethal time at 1 x 10(5) conidia/ml concentration was 7.98 days for BL 24 whereas, it was 4.85 days for MAR breed. Days to kill 90 % of the population in BL24 and BL69 breeds were 10.66 and 9.69 days, respectively while in MAR, it was 6.09 days and in GNP, 6.24 days. It took 5.09 days to kill 50 % and 7.03 days to cause 90 % mortality for B. bassiana in Pure Mysore breed.