EFFECT OF EXPOSURE OF THE SILKWORM, BOMBYX-MORI, TO HIGH-TEMPERATURE ON SURVIVAL RATE AND COCOON CHARACTERS

The silkworm, Bombyx mori, at various larval stages was exposed to hig h temperature (36 degrees C) for 48 h, and the effects on the survival rate and cocoon characters were investigated. The heat treatment in l arvae in each instar caused a reduction in the survival rate, and the heat susceptibility increased with age. Within the same instar (from I st to 4th instars) heat treatment of the larvae at early or later stag es resulted in a much higher survival rate than that of larvae treated at the middle stage. Results of heat experiment conducted at various times during the 5th instar showed that the larvae treated in the earl y half of the middle period and late stage before mounting were more s usceptible to heat which reduced their survival rates compared with th e larvae treated in the other periods. Heat treatment applied at vario us times in larvae also reduced the cocoon weight and cocoon shell wei ght, and the period of heat susceptibility was almost similar to that related to the reduction of the survival rate. When the cocoon weight and cocoon shell weight were reduced markedly by the heat treatment, t he cocoon shell ratio tended to show a high value. Heat treatment appl ied to the larvae in each instar (from Ist to 5th instars) prolonged o r shortened the duration of the feeding period by less than one day. W ithin the same instar, heat treatment at the early stage prolonged the duration of the feeding period, whereas the treatment at later stages shortened the feeding period. The results of the present study indica ted that the heat treatment affected the survival rate and cocoon char acters depending on the larval stage at which the treatment was applie d, and that the 5th instar larvae at the early stage were most suscept ible to the heat treatment. Accordingly, for testing the heat resistan ce of the silkworm, it is recommended to expose 5th instar larvae at t he early stage to high temperature (36 degrees C) for 48 h.