In the commercial mulberry silkworm, Bombyx mori L., the completion of final larval instar stadium is recognized as larval ripening stage, leading to initiation and complete metamorphosis of larval into pupal stage. At this juncture, the silkworm larvae wander and select a suitable site to construct the more awaited economically important silk cocoon, from which continuous silk thread of 800 to 1000 meters length is extracted which is called the queen of textile. Silkworm larval population density, in the final instar stadium is one of the critical factors, influencing successful and uniform commercial cocoon crop. Three fifth instar ‘larval population density zones’ (LPDZs) were identified viz., a. uneconomical larval population density zone (ULPDZ), b. optimum larval population density zone (OLPDZ) and c. loss larval population density zone (LLPDZ) to describe the fifth instar larval population density requirements of B. mori. Experiments were conducted with two commercial silkworm hybrids; multivoltine x bivoltine (PM x CSR2) and bivoltine x bivoltine (CSR2 x CSR4), reared under natural day photoperiod, LD 12: 12 to probe into the implications of the designated three LPDZs on three important larval ripening characteristics. Three larval ripening characteristics; a. fifth instar larval ripening patterns, b. fifth instar larval ripening magnitude (percentage) and c. fifth instar larval ripening period (duration) were considered emphasizing their significance in final instar silkworm rearing. Results irrevocably connoted that the three LPDZs studied influenced two silkworm hybrids differently and decisively. Further, all the three parameters studied were diversely affected too. Ripening patterns of two silkworm hybrids under ULPDZ and OLPDZ did not vary much among themselves while that between these two LPDZs and LLPDZ varied with highly significance. PM x CSR2 took two consecutive peaks (days) to complete larval ripening process while CSR2 x CSR4 took three consecutive peaks (3 days). Ripening patterns in PM x CSR2 and CSR2 x CSR4 exhibited a diurnal circadian rhythmicity with phase-locked to dawn. Further, LLPDZ induced a near arrhythmicity in both of the hybrids studied, leading to appearance of a near damp-out expression. Larval ripening magnitude and larval ripening period were differently reacted to LPDZs. Larval ripening magnitude showed inverse relationship to increased LPDZs. Thus, ULPDZ and OLPDZ registered high ripening magnitude, of course comparable, while ripening magnitude significantly decreased according to increase in LLPDZ (statistically significant). Opposing the ripening magnitude, ripening duration was directly related to increase in LPDZ. Thus, low larval ripening duration was observed for the first two larval population density zones (LPDZs); uneconomical larval population density zone (ULPDZ) and optimum larval population density zone (OLPDZ). The larval ripening period was high for loss larval population density zone (LLPDZ) compared to ULPDZ and OLPDZ and the differences were statistically significant. Results were discussed on the importance of identifying the larval population density zones in to three LPDZs (ULPDZ, OLPDZ and LLPDZ) and their applicability in commercial mulberry silkworm rearing. Finally, it is suggested that optimum larval population density zone (OLPDZ) is the most economic LPDZ for profitable commercial mulberry silkworm rearing.
