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The aim of this study was to identify the production capacity of six commercial silkworm Bombyx mori L. (Lepidoptera: Bombycidae) lines (numbers 31, 32, 103, 104, 153, and 154), which were contaminated with Nosema bombycis Nageli spores (103 or 105 per ml), the fungus responsible for pebrine (pepper disease) in silkworms. Effects of the two fungal Microsporidia concentrations were tested on silkworm feeding on either treated (with 1.0% Lactobacillus rhamnosus probiotics or 0.5% Echinacea purpurea plant extract) or non‐treated (control) mulberry leaves (Moraceae). Traits included larval weight, cocoon weight, cocoon shell weight, pupal rate, silk shell percentage, total cocoon number, cocoon yield per 10 000 larvae, larval mortality, and incidence of infection in moth. Results showed that lines, feed supplements, and level of Microsporidia spore infection had significant effects on most estimated parameters. Both concentrations of Microsporidia spores led to a decrease in the silkworm performance and an increase in mortality. Supplementation of mulberry leaves with probiotics or plant extract resulted in improvement of economic traits in B. mori. Commercial line 153 performed the best in most of the studied traits and was highlighted as the optimum line for breeding in Iranian silkworm industry. Also, line 104 was the most resistant line to N. bombycis. This information will be helpful for selection of the best line for breeders according to the rearing conditions and desirable economic traits. [ABSTRACT FROM AUTHOR]