The hot climatic conditions of tropics prevailing particularly in summer are contributing to the poor performance of the bivoltine breeds and the most important aspect is that many quantitative characters such as viability and cocoon traits decline sharply when temperature is high. Hence, in a tropical country like India, it is very essential to develop bivoltine breeds/hybrids which can withstand the high temperature stress conditions. This has resulted in the development of CSR18×CSR19, compatible hybrid for rearing throughout the year by utilizing Japanese thermotolerant hybrids as breeding resource material. Though, the introduction of CSR18×CSR19 in the field during summer months had considerable impact, the productivity level and returns realized do not match that of other productive CSR hybrids. Therefore, the acceptance level of this hybrid with the farmers was not up to the expected level. This has necessitated the development of a temperature tolerant hybrid with better productivity traits than CSR18×CSR19. Though, it was a difficult task to break the negative correlation associated with survival and productivity traits, attempts on this line had resulted in the development of CSR46×CSR47, a temperature tolerant bivoltine hybrid with better productivity traits than CSR18×CSR19. However, though, these hybrids are tolerant to high temperature environments, they are not tolerant to many of the silkworm diseases. Keeping this in view, an attempt is made to develop silkworm hybrids tolerant to high temperature environments.

The nutritional efficiency of mulberry leaves consumed by silkworms, Bombyx mori L., is usually evaluated in terms of the proportion of cocoon shell weight to the amount of food ingested. The production efficiency of cocoon shell is generally used to identify the superiority of a mulberry variety for silkworm rearing. In this study the production efficiency of cocoon shell was used as an index for evaluating the nutritive value of different mulberry varieties of India. Among the varieties, V-1, having highest production efficiency of cocoon shell with less amount of food ingested and highest digestibility, is regarded as the best suitable variety with nutritive values ideal for silkworm rearing.

The reproductive performance of multivoltine breed 'Nistari' and bivoltine breeds as NB18, P5 and a bivoltine hybrid (NB18 x P5) of B. mori were studied in different seasons in terms of total number of eggs laid, unlaid and hatching efficiency of the laid eggs to critically assess the breeds / hybrid potentiality. The bivoltine hybrid showed better performance in all the seasons as compared to the breeds investigated, as expected due to possibility of hybrid vigour. Among the breeds, bivoltine P5 had a better egg laid performance in S1 (FebruaryMarch), S2 (May -June) and S3 (September -October) seasons but with an increase in the number of unlaid eggs. The hatching percentage, although didn't show any definitive trend, still suggested a marginal better performance in the hybrid. Most importantly, egg laying rhythmicity was studied by mating the female moth at 6.00 a.m. for 3½ hours, and subsequently allowing the laying of eggs both under BOD condition (25 ± 1°C and 85 ± 5% RH) and at ambient temperature (23 -28°C and 74.7 ± 5%) separately. Maximum (90%) egg laid occurred from 3.30 to 7.30 pm (i.e. 4 -6 h after decoupling). Similar laying patterns were observed when females where mated at 12 noon and decoupled at 3.30 pm, revealing that late photoperiod and early scotoperiod were favourable for egg laying in case of multivoltine breed and bivoltine prefers late photoperiod for egg laying.

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