Abstract
Holotrichia oblita (Coleoptera: Scarabaeidae) and some other scarab beetles are the main soil-dwelling pests in China. Bacillus thuringiensis (Bt) and Beauveria bassiana (Bb) are entomopathogens that have been used as biocontrol agents of various pests. However, scarab larvae especially H. oblita exhibited strong adaptability to these pathogens. Compared to other scarabs, H. oblita could form a specific soil egg case (SEC) structure surrounding its eggs, and young larvae complete the initial development process inside this structure. In this study, we investigated the role of SEC structure and microorganisms from SEC and egg surface in pathogen adaptability. 16S rRNA gene analysis revealed low bacterial richness and high community unevenness in egg surface, with Proteobacteria, Firmicutes, Bacteroidetes and Fusobacteria dominating. In terms of OTUs composition analysis, the data show that the egg surface contains a large number of unique bacteria, indicating that the egg bacterial community may be derived from maternal transmission. Furthermore, we found that all culturable bacteria isolated from egg surface possessed antimicrobial activity against both Bt and Bb. The Pseudomonas bacteria with a significantly higher abundance in egg surface showed strong Bt- and Bb antagonistic ability. In conclusion, this study demonstrated a unique and antimicrobial bacterial community of H. oblita egg surface, which may contribute to its adaptability. Furthermore, the specific SEC structure surrounding the H. oblita eggs will provide a stable microenvironment for the eggs and egg surface bacteria, which probably provides more advantages for H. oblita adaptation ability.
Highlights
Holotrichia oblita (Coleoptera: Scarabaeidae) and some other scarab beetles are the main soil-dwelling pests in China, which cause significant economic losses in agriculture, horticulture, and forestry
All the effective sequences were grouped at 97% DNA sequence similarity, and an average of 3545, 3261 and 311 operational taxonomic units (OTUs) were obtained from bulk soil (B), soil egg case (C) and egg surface (E) samples, respectively (Table 1)
Our institute has been focused on the biological properties and efficient control strategies of scarab pests for many years, and previous data indicate that scarab larvae exhibit strong adaptability to pathogens
Summary
Holotrichia oblita (Coleoptera: Scarabaeidae) and some other scarab beetles are the main soil-dwelling pests in China, which cause significant economic losses in agriculture, horticulture, and forestry. Bt cell can proliferate in the rhizosphere of plants where the scarabs lay their eggs, which means that more insecticidal proteins and virulence factors are produced and exposed to the eggs and larvae[14]. Scarab adults usually lay their eggs in the soil near their preferred host plants, where the newly hatched larvae are exposed to the rich soil microflora including these entomopathogenic microorganisms. We find that H. oblita egg is coated with a unique soil egg case (SEC) structure, which is different from two other scarab pests H. parallela and Anomala corpulenta (see Fig. 1). In this work, to understand the role of SEC structure and egg bacteria in pathogen adaptability, we analyzed the microbial community structure of H. oblita egg surface and SEC structure respectively using 16S rRNA gene sequencing; we tested the antimicrobial potential of the cultivable isolates from egg surface and SEC against scarab-specific Bt and Bb strains. The specific SEC structure provides protection for the eggs and hatchlings of H. oblita, which is beneficial for better adaptation ability of H. oblita
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