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Nosema ceranae is an intracellular parasite that infects honey bee mid-gut epithelial cells. Previously, we identified microRNA-like small RNAs and found evidence for expression of the N. ceranae gene coding Dicer. The Dicer protein is critical for small regulatory RNA synthesis and RNA interference. In order to test the effects of Dicer on N. ceranae reproduction, we designed small interfering RNAs (siRNAs) to suppress Dicer gene expression. After inoculating honey bees with N. ceranae spores, siRNAs targeting the gene for Dicer were fed to individual honey bees for three days post infection at 24 h intervals (siRNA-Dicer group). Additional honey bees were fed with non-specific siRNAs (siRNA-scramble group). The third group of honey bees was inoculated with N. ceranae spores without any siRNA treatment (infection group). Overall, siRNA-Dicer significantly inhibited the spore production (ANOVA, P<0.05). The honey bees fed with siRNA-Dicer showed 60% less spores than the honey bees from infection and siRNA-scramble groups (adjusted P<0.05). The expression of Dicer was not detected at one day post infection (dpi) in the siRNA-Dicer group and the expression level of Dicer remained significantly suppressed at two dpi. The results suggest the gene Dicer is critical for N. ceranae reproduction and provide a new insight for this parasite control.