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The expressions of antimicrobial peptides (AMPs) in the larvae of the black soldier fly, <i>Hermetia illucens</i>, were significantly increased by pathogen or stimulant induced innate immunity activation. We immunized <i>H. illucens</i> fifth instar larvae with five different <i>Lactobacillus</i> species, that is, <i>Lactobacillus acidophilus</i>, <i>L. brevis</i>, <i>L. casei</i>, <i>L. fermentum</i>, or <i>L. delbrueckii</i>, to induce the mass production of AMPs and selected optimal immune inducers. Antimicrobial activities in hemolymph and <i>H. illucens</i> larvae (HIL) extract were evaluated against three <i>salmonella</i> species (<i>Salmonella pullorum</i>, <i>Salmonella typhimurium</i>, and <i>Salmonella enteritidis</i>). Highest antimicrobial activity was shown by the hemolymph of HIL immunized by <i>L. casei</i> and its activity was closely linked with the inductions of cecropin 1 (HiCec1) and defensin 1 (HiDef1) gene expressions. Furthermore, antimicrobial activity in hemolymph was stable to heat and pH and the growth of three <i>Salmonella</i> species were dramatically suppressed by HIL hemolymph and extract after immunization with <i>L. casei</i>. The minimal inhibitory concentration (MICs) of <i>L. casei</i>-immunized HIL extract against <i>Staphylococcus aureus</i>, <i>Escherichia coli</i>, and <i>Salmonella</i> species ranged from 100~200 µg/100 µL and no cytotoxicity to CaCo-2 and L929 cells were observed in the concentration range 100~40,000 µg/100 µL. Taken together, the present investigation demonstrates that <i>L. casei</i>-immunized HIL extract is a powerful natural antibiotic and preservative that can prevent contamination by <i>Salmonella</i> species.