Dominant Egg Surface Bacteria of Holotrichia Oblita (Coleoptera: Scarabaeidae) Inhibit The Multiplication of Bacillus Thuringiensis and Beauveria Bassiana

Abstract Many species of scarabs (Coleoptera: Scarabaeidae) are important pests. Bacillus thuringiensis (Bt) and Beauveria bassiana (Bb) are entomopathogens that have been used as biocontrol agents of various pests. However, neither provides adequate control of scarab pests in the field. In this study, we used Holotrichia oblita, an important scarab pest in China, as a model to explore the reason of difficult to control. Compared to other scarabs, H. oblita could form a unique egg cocoon structure, in which the young larvae complete the initial development process. We analyzed the microbiome of eggs surface, egg cocoon and bulk soil respectively, and investigated the role of these microorganisms in pathogen infection. 16S rRNA gene analysis revealed low bacterial richness in egg surface with Proteobacteria, Firmicutes, Bacteroidetes and Fusobacteria dominating in terms of OTUs number, and indicated that the microbial community structure varied among egg and soil samples. Some dominant species of the egg surface have been reported to exhibit antimicrobial activity, which may contribute to the low susceptibility of young larvae to entomopathogens. The further confrontation culture analysis verified this hypothesis. We found that 11 of 28 cultivable isolates possessed antimicrobial activity against both Bt and Bb, and all isolates from egg surface samples showed antagonistic ability. Four isolates belonging to Pseudomonas showed strong Bt- and weak Bb-antagonistic ability, and this genus significantly increased in egg surface samples. This study demonstrated that H. oblita egg has a unique microbial community structure, containing microorganisms with antimicrobial activity against entomopathogens. These microorganisms protect the newly hatched larvae from pathogen infection.