Background: Mulberry silkworm (Bombyx mori L.) is one of the best-studied insect models, regularly used as a type specimen for thermal tolerance experiments on insects. Still, the upper lethal limit of this lepidopteran has never been explored extensively using any sort of conventional lethal assay method. The present study deals with the employment of different lethal assay protocols for the study of survivorship of three different breeds of mulberry silkworm (B. mori) exposed to different temperatures (30-50 degrees C) and durations of stress (1-3 h) on different days (day 2, 4, and 6) of the fifth instar stage for formulating an extensive upper lethal temperature (ULT50 and ULT25) index. Results: Among treatment temperatures 30 degrees C, 35 degrees C, and 40 degrees C had a significant (p = < 0.0001) impact on the high-temperature survival rate of the silkworm. Among duration-1 h and 2 h influenced the survival rate significantly (p = < 0.0001). Plunging, one-way ramping, and two-way ramping assay methods seemed to exert a non-significant (Wald chi(2) = 3.253, p = 0.197) influence on silkworm survival. F1 hybrid was found to exhibit the highest survivorship across different temperatures, followed by the multivoltine Nistari plain and then by the bivoltine breeds. In F1 hybrid silkworms, the upper lethal temperatures ULT50, varied within the range of 37 degrees C to 44 degrees C and ULT25 varied within the range of 40-47 degrees C. The mean upper lethal limit-ULT0 for all three breeds of mulberry silkworm, across all experimental groups, was computed to be similar to 49 degrees C. Conclusions: Ultimately the overall thermal tolerance of mulberry silkworm exhibited a significant inter-breed variation based on the heterogeneous thermal plasticity of the three different breeds. The outcome of the present study in the form of upper lethal temperature ranges of the breeds under consideration can form the basis of future thermal stress experiments in mulberry silkworms.
