An Investigation of the Performance and Relative Resistance to Grasserie Disease of 15 Hybrids of Silkworm (Bombyx mori), Reared under Infected and Non-infected Conditions

To compare the performance of fifteen hybrids of silkworm, Bombyx mori L. under natural rearing conditions on the farm level and determine their resistance to grasserie disease, an experiment was conducted in a completely randomized design (15 treatments and five replications) for either of non-infected or infected to NPV conditions. Treatments included hybrids Xinhang1?Koming1, Xinhang1?Koming2, Xinhang1?Y, Xinhang2?Koming1, Xinhang2? Koming2, Xinhang2?Y, Xinhang3? Koming1, Xinhang3?Koming2, Xinhang3?Y, 101433?Koming1, 101433? Koming2, 101433?Y along with three commercial hybrids of 31?32, 103?104 and 107?110. Mixed rearing was adopted in the rearing of larve in each race. In each race, larvae were divided into five groups (each including 250 larvae) at the beginning of the fourth instar. In order to make the environment infected with NPV, at the beginning of fourth instar, LD50 concentration of nuclear polyhedrosis virus (107 polyhydrate per milliliter) was extracted and fed to larvae orally and through the first meal. The traits assessed were single cocoon weight, single cocoon shell weight, single cocoon shell ratio, survived larvae number, survived pupae number and pupation rate. Infection to NPV at larval stage resulted in a significant (P<0.0001) decrease in single cocoon weight (5.57%), single cocoon shell weight (8.06%), single cocoon shell ratio (2.34%), survived larvae number (16.16%), survived pupae number (47.16%) as well as pupation rate (37.88%). Also, production characteristics are less affected by the disease. The results indicated that pathogenic effect of the virus occurs in both larval and pupal stages, with the disease peak occuring in pupal stage. Resistance as well as the performance of the hybrids produced through crossings of 1014433 by Chinese lines including Koming1, Koming2 and Y, were significantly higher and this hybrid could be used as a suitable replacement for the present commercial hybrids.