STUDY ON THE TOLERANCE OF DOUBLE AND FOUR-WAY SILKWORM, BOMBYX MORI L. HYBRIDS TO ADVERSE REARING CONDITIONS

Abstract

The study has been carried out at Sericulture and Agriculture Experiment Station /SAES/, Vratsa during the period 2010 – 2012 with six double hybrids and four four-way silkworm hybrids with comparatively high productivity. The hybrids studied are created at SAES - Vratsa, except for the Japanеsе hybrid Shunrei x Shogetsu. The following provocative regime for larval rearing during the 4th and 5th instars was used: air temperature 28 – 31°С, relative humidity 50 – 60%, rearing space and feeding amount, decreased with 50% and very reduced ventilation. It has been found out that due to the higher temperature under provocative regime, the total larval duration and 5th instar duration are shorter, compared with the standard rearing conditions. It has been found that the highest pupation rate under provocative regime larval rearing manifested the double hybrid VB 1 x HB 2 whose both parental breeds have blood of tropical polyvoltine breed Bonde 517. The studies found that the provocative larval rearing regime percent of cocoon without alive pupae is high and the fresh cocoon yield by a box of eggs was on average 80% lower, compared with standard larval rearing regime. These data prove that provocative regime used indeed gives rise to a lot of stress in the body of the larvae and increased mortality, and therefore could be successfully used in the testing of various breeds and hybrids in respect of their tolerance to adverse conditions of larval rearing. This study shows that a remarkably high tolerance to adverse rearing conditions perform the double VB 1 x HB 2 and four-way hybrid SN 1x VB 1 x MAGI 2 x HB 2. They are also characterized with comparatively high productivity at optimal rearing conditions and thus are recommended for growing in summer and autumn seasons as well as in rearing rooms where it is not possible to control the temperature rise.