| Summary: | <i>Propylea japonica</i> (Thunberg) (Coleoptera: Coccinellidae) is a dominant natural enemy of insect pests in farmland ecosystems. It also serves as an important non-target insect for environmental safety evaluations of transgenic crops. Widespread planting of transgenic crops may result in direct or indirect exposure of <i>P. japonica</i> to recombinant <i>Bacillus thuringiensis</i> (Bt) protein, which may in turn affect the biological performance of this natural enemy by affecting the <i>P. japonica</i> microflora. However, the effects of Bt proteins (such as Cry1B) on the <i>P. japonica</i> microbiota are currently unclear. Here, we used a high-throughput sequencing method to investigate differences in the <i>P. japonica</i> microbiota resulting from treatment with Cry1B compared to a sucrose control. The results demonstrated that the <i>P. japonica</i> microbiome was dominated by Firmicutes at the phylum level and by Staphylococcus at the genus level. Within-sample (α) diversity indices demonstrated a high degree of consistency between the microbial communities of <i>P. japonica</i> treated with the sucrose control and those treated with 0.25 or 0.5 mg/mL Cry1B. Furthermore, there were no significant differences in the abundance of any taxa after treatment with 0.25 mg/mL Cry1B for 24 or 48 h, and treatment with 0.5 mg/mL Cry1B for 24 or 48 h led to changes only in <i>Staphylococcus</i>, a member of the phylum Firmicutes. Treatment with a high Cry1B concentration (1.0 mg/mL) for 24 or 48 h caused significant changes in the abundance of specific taxa (e.g., <i>Gemmatimonades</i>, <i>Patescibacteria</i>, <i>Thauera</i>, and <i>Microbacterium</i>). However, compared with the control, most taxa remained unchanged. The statistically significant differences may have been due to the stimulatory effects of treatment with a high concentration of Cry1B. Overall, the results showed that Cry1B protein could alter endophytic bacterial community abundance, but not composition, in <i>P. japonica</i>. The effects of Bt proteins on endophytes and other parameters in non-target insects require further study. This study provides data support for the safety evaluation of transgenic plants.
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