Selected Thermotolerant Bivoltine Hybrids of Bombyx mori L. Exhibit Desirable Heterosis for Quantitative Traits under Beauveria bassiana Infection

Four thermotolerant bivoltine silkworm parents viz., B1, B4, B6 and B8 and their 12 hybrids viz., B1×B4, B1×B6, B1×B8, B4×B1, B4×B6, B4×B8, B6×B1, B6×B4, B6×B8, B8×B1, B8×B4 and B8×B6 were used in the experiments. One set of the parents and their hybrids were inoculated with Beauveria bassiana spore suspension (9.04 × 104 spores / ml), on the first day of the fifth instar before first feed and another set were exposed to high temperature (36 ± 1o C temp. & 85 ± 5 % RH) and their responses were studied in terms of heterotic expression of quantitative traits. Significantly highest positive relative heterosis under normal condition was recorded in B1×B4 hybrid for five characters (fifth instar larval weight, cocoon yield by weight, single cocoon weight, shell weight and filament length) and under muscardine inoculation significantly highest positive relative heterosis was recorded in the hybrid B4×B6 for ten characters (ERR, cocoon yield by number, fifth instar larval weight, cocoon yield by weight, single cocoon weight, shell weight, shell ratio, pupal weight, filament length and filament weight). Heterosis scoring over all the traits under muscardine inoculation revealed that the hybrid B4 × B6 scored high overall status of heterotic effects across the characters. Thus, B1 and B8 among parents and B1 × B8 and B4 × B6 among hybrids could be selected for dual stress tolerance against high temperature and muscardine disease.