Synthesis of new-gene combinations by genetic manipulation is one of the powerful tools in exploiting the commercial qualities of plants and animals. The fundamental aim of silkworm breeding is to get robust and sturdy silkworm larvae for easy rearing and production of quality cocoon crop. Bivoltine breeds and hybrids excel in quality and productivity but do not display the crop stability. The main reason attributed to such instability is that the bivoltine breeds/hybrids suffer badly in adverse conditions. In the present study, an attempt was made to evaluate twenty-eight bivoltine hybrids raised by involving half (8×8) diallel set of bivoltine breeds including four hypersericigenous (PO 1 , PO 3 , ND 2 and ND 5) and four thermotolerents (Udhey-1, Udhey-3, Udhey-4 and Udhey-6) along with ruling bivoltine hybrid, SH 6 ×NB 4 D 2 in order to identify the thermotolerent, adaptable bi×bivoltine silkworm hybrids suited for subtropical climate. Observations were made on eight economically important traits namely, larval weight, crop weight, single cocoon weight, single shell weight, shell ratio, average filament length, non-breakable filament length and denier. The data was analyzed using Evaluation index (E.I.) method. Ten hybrid combinations recorded mean evaluation index (E.I.) values of >50 ranging from 51.13 to 63.35, whereas, control scored E.I. value of 49.22 only. Two hybrid combinations, ND 5 ×PO 1 (63.35) and ND 2 ×Udhey-6 (60.12) recorded Average E.I. value >60 for all the characters under subtropical conditions.

Silkworm breeds/hybrids which are reared over a series of environment exhibiting less variation are considered stable. Identification of high temperature tolerant bivoltine breeds/hybrids by screening for thermo tolerance in silkworm is an essential prerequisite for the development of thermo-tolerant bivoltine breeds/hybrids. One of the main aims of the breeders is to recommend breeds/hybrids that are stable under different environmental conditions and minimize the risk of falling below a certain yield level to farmers. In the present study, an attempt was made to evaluate and identify thermo-tolerant, adaptable bi×bivoltine silkworm hybrids suitable for rearing throughout the year, raised by involving seven productive breeds (CSR2, CSR3, CSR4, CSR5, CSR6, KA and NB 4 D 2) and two thermo-tolerant breeds (CSR18 and CSR19) at two different temperature regime (25±1 °C and 36±1 °C). The data was subjected to the estimation of heterosis in relation to mid parent value and heterobeltiosis in relation to better parent value. Observations made for four economically important traits namely; pupation percentage, single cocoon weight, single shell weight and shell ratio percentage revealed that at 25±1 °C (optimum environmental conditions) productive hybrid CSR3×CSR6 (heterosis over mid parent value (66.99) and heterobeltiosis over better parent value (50.62) results in better qualitative and quantitative traits as compared to other hybrids studied. Robust hybrid, CSR18×CSR19 showed better tolerance to high temperature i.e. at 36±1 °C and recorded heterosis for mid parent value of 55.03 and heterobeltiosis for better parent value of 32.40 when compared to productive hybrids.

In the present investigation, an attempt was made to study the effect of brushing time, silkworm hybrids and mulberry varieties for food dynamics and commercial trait expression. The experiment was laid out in factorial-completely randomized block design with eight treatments and three replications and the analysis of treatments was worked out by analysis of variance technique. The treatments comprised of two brushing time (early i.e. 1 st of March and late i.e. 20 th of March), two hybrids (PO 3 ×ND 5 and SH 6 ×NB 4 D 2) and two mulberry varieties (China white and Tr-10). The brushing time, silkworm hybrids and mulberry varieties influenced significantly most of the food dynamic and cocoon traits. Observations made for food dynamics and commercially important traits indicated that, food ingestion and digestion, conversion of ingested and digested food, approximate digestibility, production efficiency of cocoon shell, efficiency of cocoon production, percentage of cocoon shell weight, effective rate of rearing (by weight and by number), single cocoon weight, single shell weight, shell ratio percentage, fibroin and sericin content, total filament length, non-breakable filament length and renditta were significantly higher for early brushing time, PO 3 ×ND 5 and China white. Thus, only one commercial rearing is possible with brushing time of early i.e. 1 st of March for hybrid PO 3 ×ND 5 fed on China white mulberry variety for spring rearing in the field at farmer's level for better prospects of bivoltine sericulture in subtropical plains.