Population density during the ultimate larval instar stadium is one of the crucial issues for successful commercial cocoon crop of the mulberry silkworm, Bombyx mori L. Experiments were conducted with two commercial silkworm hybrids; one from the multivoltine x bivoltine hybrid, PM x CSR2 and the other from the bivoltine x bivoltine hybrid, CSR2 x CSR4 to examine the optimum population density requirements for commercial rearing during fifth instar larval period. Results clearly indicated that the fifth instar larval eating period of the silkworm is directly related to the fifth instar silkworm larval density while the fifth instar larval weight is related inversely. The two commercial silkworm hybrids studied differ in their optimum larval population density requirements during the fifth larval instar period for maximum fifth instar larval growth (weight) and minimum fifth instar larval duration. Based on all the population density regimes studied, the results revealed three fifth instar larval density zones; (1) un-economic fifth instar larval density zone, (2) optimum fifth instar larval density zone and (3) loss fifth instar larval density zone. For PM x CSR2, the optimum fifth instar larval density zone of 70 to 90 number of larvae/feet2 was more suited while that for CSR2 x CSR4, the fifth instar larval density zone was 60 to 70 number of larvae/feet2. Results were discussed on the basis of contemporary commercial silkworm rearing in India.
