The structure of the cereal leaf beetle (Oulema melanopus) microbiome depends on the insect's developmental stage, host plant, and origin

Characterisation of the bacteriomes harboured by major wireworm pest species in the Canadian Prairies

Nearly all insects harbour bacterial communities that can have a profound effect on their life history, including regulating and shaping host metabolism, development, immunity and fitness. The bacteriomes of several coleopterans have been described; however, very little has been reported for wireworms. These long-lived larvae of click beetles (Coleoptera: Elateridae) are major agricultural pests of a variety of crops grown in the Canadian Prairies. Consequently, the goal of this study was to characterise the bacteriomes of five of the most significant pest species within the region: Limonius californicus, Hypnoidus abbreviatus, H. bicolor, Aeolus mellillus and Dalopius spp. To do this, we collected larvae from southern Manitoba fields (pre-seeding) and carried out 16S rRNA sequencing on individual specimens. Our results indicate wireworms have diverse and taxon-rich bacterial communities, with over 400 genera identified predominately from the phyla Proteobacteria, Actinobacteriota, Bacteroidota and Firmicutes. However, each species had nine or fewer genera comprising >80% of their bacteriome. Network analyses revealed some community structuring consistent among species, which may culminate in shaping/regulating host biology. Moreover, the microbial signatures were influenced by both ontogeny (early vs. late stage larvae) and reproductive strategy (sexual vs. parthenogenetic), with a myriad of other factors likely contributing to bacterial diversity that are impossible to resolve from our study. Overall, this metagenomics study represents the first to characterise the bacteriomes of wireworms in the Canadian Prairies and the findings could assist in the development of sustainable management strategies for these important agricultural pests.

Influence of the Chemical Properties of Cereal Grains on the Structure and Metabolism of the Bacteriome of Rhyzopertha dominica (F.) and Its Development: A Cause-Effect Analysis

Rhyzopertha dominica causes significant economic losses in stored cereals. Insects' digestive tract microbiome is crucial for their development, metabolism, resistance, and digestion. This work aimed to test whether the different chemical properties of different wheat and barley grain cultivars cause disturbances in insect foraging and rearrangements of the structure of the R. dominica microbiome. The results indicated that grain cultivars significantly influence the microbiome, metabolism, and insect foraging. Most observed traits and microbiome structures were not correlated at the species level, as confirmed by ANOSIM (p = 0.441). However, the PLS-PM analysis revealed significant patterns within barley cultivars. The study found associations between C18:2 fatty acids, entomopathogenic bacteria, an impaired nitrogen cycle, lysine production of bacterial origin, and insect feeding. The antioxidant effects also showed trends towards impacting the microbiome and insect development. The findings suggest that manipulating grain chemical properties (increasing C18:2 and antioxidant levels) can influence the R. dominica microbiome, disrupting their foraging behaviours and adaptation to storage environments. This research supports the potential for breeding resistant cereals, offering an effective pest control strategy and reducing pesticide use in food production.

Natural plant disease suppressiveness in soils extends to insect pest control

BACKGROUND

Since the 1980s, soils in a 22-km2 area near Lake Neuchâtel in Switzerland have been recognized for their innate ability to suppress the black root rot plant disease caused by the fungal pathogen Thielaviopsis basicola. However, the efficacy of natural disease suppressive soils against insect pests has not been studied.

RESULTS

We demonstrate that natural soil suppressiveness also protects plants from the leaf-feeding pest insect Oulema melanopus. Plants grown in the most suppressive soil have a reduced stress response to Oulema feeding, reflected by dampened levels of herbivore defense-related phytohormones and benzoxazinoids. Enhanced salicylate levels in insect-free plants indicate defense-priming operating in this soil. The rhizosphere microbiome of suppressive soils contained a higher proportion of plant-beneficial bacteria, coinciding with their microbiome networks being highly tolerant to the destabilizing impact of insect exposure observed in the rhizosphere of plants grown in the conducive soils. We suggest that presence of plant-beneficial bacteria in the suppressive soils along with priming, conferred plant resistance to the insect pest, manifesting also in the onset of insect microbiome dysbiosis by the displacement of the insect endosymbionts.

CONCLUSIONS

Our results show that an intricate soil-plant-insect feedback, relying on a stress tolerant microbiome network with the presence of plant-beneficial bacteria and plant priming, extends natural soil suppressiveness from soilborne diseases to insect pests. Video Abstract.

Bacterial communities varied in different Coccinella transversoguttata populations located in Tibetan plateau

Coccinella transversoguttata is an important predatory beetle in Asia and America. Currently, few studies have investigated C. transversoguttata in China especially in the Tibetan plateau. In this study, full-length 16 s rRNA sequencing and qPCR experiment were performed on eight C. transversoguttata populations collected from Tibet to analyze their bacterial communities and bacteria abundance. In summary, our results revealed the microbial compositions, diversities and bacterial titers in the bacterial communities in C. transversoguttata populations in the Tibetan plateau. In future, there is a need to explore the differences in microbiota among various C. transversoguttata populations collected from different locations. These results add to our understanding of the complex bacterial communities of C. transversoguttata and their utilization as potential biocontrol factors.

Cereal leaf beetle-associated bacteria enhance the survival of their host upon insecticide treatments and respond differently to insecticides with different modes of action

The cereal leaf beetle (CLB, Oulema melanopus) is one of the major cereal pests. The effect of insecticides belonging to different chemical classes, with different mechanisms of action and the active substances' concentrations on the CLB bacterial microbiome, was investigated. Targeted metagenomic analysis of the V3-V4 regions of the 16S ribosomal gene was used to determine the composition of the CLB bacterial microbiome. Each of the insecticides caused a decrease in the abundance of bacteria of the genus Pantoea, and an increase in the abundance of bacteria of the genus Stenotrophomonas, Acinetobacter, compared to untreated insects. After cypermethrin application, a decrease in the relative abundance of bacteria of the genus Pseudomonas was noted. The dominant bacterial genera in cypermethrin-treated larvae were Lactococcus, Pantoea, while in insects exposed to chlorpyrifos or flonicamid it was Pseudomonas. Insecticide-treated larvae were characterized, on average, by higher biodiversity and richness of bacterial genera, compared to untreated insects. The depletion of CLB-associated bacteria resulted in a decrease in larval survival, especially after cypermethrin and chlorpyrifos treatments. The use of a metagenome-based functional prediction approach revealed a higher predicted function of bacterial acetyl-CoA C-acetyltransferase in flonicamid and chlorpyrifos-treated larvae and tRNA dimethyltransferase in cypermethrin-treated insects than in untreated insects.

Concentration-dependent effect of plant secondary metabolites on bacterial and fungal microbiomes in caterpillar guts

IMPORTANCE

The caterpillar gut is an excellent model system for studying host-microbiome interactions, as it represents an extreme environment for microbial life that usually has low diversity and considerable variability in community composition. Our study design combines feeding caterpillars on a natural and artificial diet with controlled levels of plant secondary metabolites and uses metabarcoding and quantitative PCR to simultaneously profile bacterial and fungal assemblages, which has never been performed. Moreover, we focus on multiple caterpillar species and consider diet breadth. Contrary to many previous studies, our study suggested the functional importance of certain microbial taxa, especially bacteria, and confirmed the previously proposed lower importance of fungi for caterpillar holobiont. Our study revealed the lack of differences between monophagous and polyphagous species in the responses of microbial assemblages to plant secondary metabolites, suggesting the limited role of the microbiome in the plasticity of the herbivore diet.

Does Leptinotarsa decemlineata larval survival after pesticide treatment depend on microbiome composition?

BACKGROUND

The microbiomes of some arthropods are believed to eliminate pesticides by chemical degradation or stimulation of the host immune system. The Colorado potato beetle (CPB; Leptinotarsa decemlineata) is an important agricultural pest with known resistance to used pesticides. We sought to analyze microbiome composition in CPB larvae from different sites and to identify the effect of pesticides on the microbiome of surviving and dead larvae after chlorpyrifos treatment in laboratory. Changes in the Lactococcus lactis community in larvae treated with chlorpyrifos and fed by potato leaves with L. lactis cover were studied by manipulative experiment. The microbiome was characterized by sequencing the 16S RNA gene.

RESULTS

The microbiome of L. decemlineata larvae is composed of a few operational taxonomic units (OTUs) (Enterobacteriaceae, Pseudocitrobacter, Acinetobacter, Pseudomonas, L. lactis, Enterococcus, Burkholderia and Spiroplasma leptinotarsae). The microbiome varied among the samples from eight sites and showed differences in profiles between surviving and dead larvae. The survival of larvae after chlorpyrifos treatment was correlated with a higher proportion of L. lactis sequences in the microbiome. The S. leptinotarsa profile also increased in the surviving larvae, but this OTU was not present in all sampling sites. In manipulative experiments, larvae treated with L. lactis had five-fold lower mortality rates than untreated larvae.

CONCLUSION

These results indicate that the microbiome of larvae is formed from a few bacterial taxa depending on the sampling site. A member of the gut microbiome, L. lactis, is believed to help overcome the toxic effects of chlorpyrifos in the larval gut. © 2023 Society of Chemical Industry.

Insect Gut Bacteria Promoting the Growth of Tomato Plants (Solanum lycopersicum L.)

We investigated gut bacteria from three insect species for the presence of plant growth properties (PGP). Out of 146 bacterial strains obtained from 20 adult specimens of Scolytidae sp., 50 specimens of Oulema melanopus, and 150 specimens of Diabrotica virgifera, we selected 11 strains displaying the following: PGP, phosphate solubility, production of cellulase, siderophore, lipase, protease, and hydrogen cyanide. The strains were tested for growth promotion ability on tomato (Lycopersicon esculentum) plants. Each strain was tested individually, and all strains were tested together as a bacterial consortium. Tomato fruit yield was compared with the negative control. The plants treated with bacterial consortium showed a significant increase in fruit yield, in both number of fruits (+41%) and weight of fruits (+44%). The second highest yield was obtained for treatment with Serratia liquefaciens Dv032 strain, where the number and weight of yielded fruits increased by 35% and 30%, respectively. All selected 11 strains were obtained from Western Corn Rootworm (WCR), Diabrotica virgifera. The consortium comprised: Ewingella americana, Lactococcus garvieae, L. lactis, Pseudomonas putida, Serratia liquefaciens, and S. plymuthica. To our knowledge, this is the first successful application of D. virgifera gut bacteria for tomato plant growth stimulation that has been described.

The Impact of Oulema melanopus—Associated Bacteria on the Wheat Defense Response to the Feeding of Their Insect Hosts

Wheat production is threatened by the destructive effects of numerous pests, including Oulema melanopus (cereal leaf beetle, CLB). Both adults and larvae of CLB damage grain crops, but the target of insecticide treatments are the larvae. Insect-associated bacteria are important for many of the insects’ life processes and may also modulate plant defense responses to feeding of their insect host. The aim of our study was to elucidate the early wheat plants’ reaction to this herbivore feeding and to disclose the CLB-associated bacteria modulation of the wheat-insect interactions. Transcriptome analyses were performed for the leaves wounded mechanically and by feeding of the CLB larvae as well as for the distal leaves to study both, the plant’s local and systemic response. Comparative transcriptome analysis indicated that 24 h after the plant treatment, a much larger number of up-regulated DEGs in damaged leaves was noted, especially those on which larvae were fed. It may suggest that at the analysed time point, the local response was stronger than the systemic one. In the leaves on which larvae with natural bacterial flora were fed (local response), the number of up- and down-regulated differentially expressed genes (DEGs) was 7136 and 7411, respectively, in comparison to the dataset obtained for the leaves wounded by larvae with a reduced number of bacteria. In the distal leaves, 3015 up- and 2372 down-regulated DEGs were noted. CLB-associated bacteria were found to affect many aspects of the physiology of wheat plants, especially in wounded leaves, including the expression of genes related to primary metabolism, phytohormone signaling and photosynthesis. We also observed that CLB-associated bacteria mitigated numerous anti-herbivore processes and pathways associated with the synthesis of metabolites and proteins, potentially harmful to the insects. The bacteria also reversed the expression of some genes involved, inter alia, in the phosphorylation of proteins, oxidative stress, cell wall organization, and biogenesis. Understanding the role of CLB-associated bacteria in the plant’s defense response will be important to the fields of pest control and herbivore and its host ecology and evolution.