Variations in Endosymbiont Infection Between Buprofezin-Resistant and Susceptible Strains of Laodelphax striatellus (Fallén)

Mobile Resistance Elements: Symbionts That Modify Insect Host Resistance

Mounting evidence indicates that symbionts play a beneficial role through secondary metabolic compounds and various chemical processes in host adaptation to adversity, particularly in herbivorous insects whose survival is severely threatened by insecticides or secondary metabolite stress. Despite extensive research on insect symbionts, the spread of these beneficial symbionts and the correlation with host phenotypes limit our ability to predict and manage the adaptive capabilities of insect populations in changing environments. In this review, we propose the concept of "Mobile Resistance Elements (MRE)" to describe the dynamic and adaptable nature of resistance-related symbionts that can be transmitted between insect hosts. These elements encompass both the symbionts themselves and the associated traits they confer to their hosts, such as enhanced resilience to environmental stressors, toxins, and pathogens. The mobility of these resistance traits, facilitated through various transmission modes─including vertical and horizontal pathways─allows susceptible insect populations to acquire beneficial symbionts and their associated resistance phenotypes. By weaving together the threads of how symbionts shape host adaptability and survival strategies, this concept underscores the potential for symbionts to act as agents of rapid adaptation, enabling pest populations to thrive in changing environments and presenting both challenges and opportunities for pest management strategies.

Differences in the abundance and diversity of endosymbiotic bacteria drive host resistance of Philodromus cespitum, a dominant spider of central European orchards, to selected insecticides

The ability of tissue endosymbionts to degrade and detoxify agrochemicals is increasingly recognized as a mechanism supporting the survival of arthropods in agroecosystems. Therefore, tissue endosymbionts have the potential to drive insecticide resistance in agrobiont spiders, i.e., in major generalist predators and pest control agents within agroecosystems. We hypothesized that the abundance and diversity of the endosymbiotic bacteria of Philodromus cespitum, a philodromid spider dominating central European apple orchards, vary with regard to differences in predation capacity and drive host insecticide resistance. We provisioned P. cespitum with diets of varying protein and lipid content and topically exposed them to field-relevant doses of commonly used insecticides, namely Mospilan (acetamiprid), Movento (spirotetramat), Gondola (sulfoxaflor), Decis (deltamethrin), Coragen (chlorantraniliprole), and Benevia (cyantraniliprole). The analyses were based on 16S rDNA profiles from lysates of the cephalothorax and legs of the tested spiders. The application of Benevia, Mospilan, and Movento was partially lethal. The spiders that were resistant to the treatments with Benevia, Mospilan, or Movento were associated with the increased relative abundance of Mycoplasmatota by more than one order of magnitude. Additionally, the abundance of other bacteria differed in Mospilan-resistant and Mospilan-sensitive individuals. In contrast, the diet regimens were not associated with any major differences in the microbiome diversity nor the diversity of endosymbionts. Philodromus cespitum hosts assemblages with unexpectedly high beta diversity of endosymbionts. The OTU identified as the alpha proteobacterium endosymbiont of Coelostomidia zealandica was an obligate endosymbiont of the analyzed P. cespitum population. Wolbachia, Rickettsia, and Spiroplasma endosymbionts were also highly prevalent and differed in their responses to the applied treatments. In conclusion, differences in the abundance and diversity of endosymbiotic bacteria drove the resistance of the spider host to selected insecticides.

Study on the gut symbiotic microbiota in long- and short-winged brown planthopper, Nilaparvata lugens (Stål)

The brown planthopper (BPH), Nilaparvata lugens (Stål), is one of the most important rice pests in Asia rice regions. BPH has monophagy, migration, rapid reproduction and strong environmental adaptability, and its control is a major problem in pest management. Adult BPH exhibit wing dimorphism, and the symbiotic microbiota enriched in the gut can provide energy for wing flight muscles as a source of nutrition. In order to study the diversity of symbiotic microbiota in different winged BPHs, this paper takes female BPH as the research object. It was found that the number of symbiotic microbiota of different winged BPHs would change at different development stages. Then, based on the 16S rRNA and ITS sequences, a metagenomic library was constructed, combined with fluorescent quantitative PCR and high-throughput sequencing, the dominant symbiotic microbiota flora in the gut of different winged BPHs was found, and the community structure and composition of symbiotic microbiota in different winged BPHs were further determined. Together, our results preliminarily revealed that symbiotic microbiota in the gut of BPHs have certain effects on wing morphology, and understanding the mechanisms underlying wing morph differentiation will clarify how nutritional factors or environmental cues alter or regulate physiological and metabolic pathways. These findings also establish a theoretical basis for subsequent explorations into BPH-symbiont interplay.

Association Between Wolbachia Infection and Susceptibility to Deltamethrin Insecticide in Phlebotomus papatasi (Diptera: Psychodidae), the Main Vector of Zoonotic Cutaneous Leishmaniasis

Background: Phlebotomus papatasi (Diptera: Psychodidae) is the main vector of zoonotic cutaneous leishmaniasis. Wolbachia is a symbiotic alphaproteobacteria of arthropods that can be involved in susceptibility or resistance. This study aimed to investigate the relationship between Wolbachia and Deltamethrin susceptibility/resistance in Ph. papatasi. Deltamethrin filter papers (0.00002%) were used to test sand fly field collected from southern Iran. After the test, PCR amplification of the Wolbachia surface protein gene (wsp) was used to measure Wolbachia infection rate in the killed, surviving, and control groups. Result: The rates of infection by Wolbachia strain (wPap, super group A) differed between killed (susceptible) and surviving (resistant) Ph. papatasi specimens. The rate of Wolbachia infection in susceptible individuals was more than twice (2.3) (39% vs. 17%) in resistant individuals with the same genetic background. This difference was highly significant (p < 0.001), indicating a positive association between Wolbachia infection and susceptibility to Deltamethrin. In addition, the results showed that Deltamethrin can act as a PCR inhibitor during detection of Wolbachia in Ph. papatasi. Conclusion: Results of this study show that Wolbachia is associated with Deltamethrin susceptibility level in Ph. papatasi. Also, as Deltamethrin has been identified as a PCR inhibitor, great care must be taken in interpreting Wolbachia infection status in infected populations. The results of this study may provide information for a better understanding of the host-symbiont relationship, as well as application of host symbiosis in pest management.

Symbiotic bacteria confer insecticide resistance by metabolizing buprofezin in the brown planthopper, Nilaparvata lugens (Stål)

Buprofezin, a chitin synthesis inhibitor, is widely used to control several economically important insect crop pests. However, the overuse of buprofezin has led to the evolution of resistance and exposed off-target organisms present in agri-environments to this compound. As many as six different strains of bacteria isolated from these environments have been shown to degrade buprofezin. However, whether insects can acquire these buprofezin-degrading bacteria from soil and enhance their own resistance to buprofezin remains unknown. Here we show that field strains of the brown planthopper, Nilaparvata lugens, have acquired a symbiotic bacteria, occurring naturally in soil and water, that provides them with resistance to buprofezin. We isolated a symbiotic bacterium, Serratia marcescens (Bup_Serratia), from buprofezin-resistant N. lugens and showed it has the capacity to degrade buprofezin. Buprofezin-susceptible N. lugens inoculated with Bup_Serratia became resistant to buprofezin, while antibiotic-treated N. lugens became susceptible to this insecticide, confirming the important role of Bup_Serratia in resistance. Sequencing of the Bup_Serratia genome identified a suite of candidate genes involved in the degradation of buprofezin, that were upregulated upon exposure to buprofezin. Our findings demonstrate that S. marcescens, an opportunistic pathogen of humans, can metabolize the insecticide buprofezin and form a mutualistic relationship with N. lugens to enhance host resistance to buprofezin. These results provide new insight into the mechanisms underlying insecticide resistance and the interactions between bacteria, insects and insecticides in the environment. From an applied perspective they also have implications for the control of highly damaging crop pests.

Risk assessment of predatory lady beetle Propylea japonica's multi-generational exposure to three non-insecticidal agrochemicals

The effects of non-insecticidal agrochemicals on pest natural predators remain largely unexplored except bees and silkworm. The herbicide quizalofop-p-ethyl (QpE), fungicide thiophanate-methyl (TM), and plant growth regulator mepiquat chloride (MC) have been extensively applied as non-insecticidal agrochemicals. Here, we systematically evaluated multiple effects of these 3 non-insecticidal agrochemicals on three generations of Propylea japonica, an important agroforestry predatory beetle, including the effects on its development, reproduction, enterobacteria, and transcriptomic response. The results showed that QpE exhibited a hormetic effect on P. japonica, thus significantly increasing the survival rate of generation 2 (F2) females, generation 3 (F3) females, and F3 males and body weight of F3 males. However, three successive generations exposed to TM and MC had no significant effect on longevity, body weight, survival rate, pre-oviposition period, and fecundity of P. japonica. Additionally, we investigated the effects of MC, TM, and QpE exposure on gene expression and gut bacterial community of F3 P. japonica. Under MC, TM, and QpE exposure, the overwhelming genes of P. japonica (99.90 %, 99.45 %, and 99.7 %) remained unaffected, respectively. Under TM and MC exposure, differentially expressed genes (DEGs) were not significantly enriched in any KEGG pathway, indicating TM and MC did not significantly affect functions of P. japonica, but under QpE exposure, the expression levels of drug metabolism-related genes were down-regulated. Although QpE treatment did not affect gut dominant bacterial community composition, it significantly increased relative abundances of detoxification metabolism-related bacteria such as Wolbachia, Pseudomonas and Burkholderia in P. japonica. However, TM and MC had no significant effect on the gut bacterial community composition and relative abundance in P. japonica. This study revealed for the first time the mechanism by which P. japonica might compensate for gene downregulation-induced detoxification metabolism decline through altering symbiotic bacteria under QpE exposure. Our findings provide reference for the rational application of non-insecticidal agrochemicals.

Bacterial Endosymbionts Identified From Leafhopper (Hemiptera: Cicadellidae) Vectors of Phytoplasmas

Insects often harbor bacterial endosymbionts that provide them with nutritional benefit or with protection against natural enemies, plant defenses, insecticides, and abiotic stresses. Certain endosymbionts may also alter acquisition and transmission of plant pathogens by insect vectors. We identified bacterial endosymbionts from four leafhopper vectors (Hemiptera: Cicadellidae) of 'Candidatus Phytoplasma' species by direct sequencing 16S rDNA and confirmed endosymbiont presence and identity by species-specific conventional PCR. We examined three vectors of Ca. Phytoplasma pruni, causal agent of cherry X-disease [Colladonus geminatus (Van Duzee), Colladonus montanus reductus (Van Duzee), Euscelidius variegatus (Kirschbaum)] - and a vector of Ca. Phytoplasma trifolii, the causal agent of potato purple top disease [Circulifer tenellus (Baker)]. Direct sequencing of 16S identified the two obligate endosymbionts of leafhoppers, 'Ca. Sulcia' and 'Ca. Nasuia', which are known to produce essential amino acids lacking in the leafhoppers' phloem sap diet. About 57% of C. geminatus also harbored endosymbiotic Rickettsia. We identified 'Ca. Yamatotoia cicadellidicola' in Euscelidius variegatus, providing just the second host record for this endosymbiont. Circulifer tenellus harbored the facultative endosymbiont Wolbachia, although the average infection rate was only 13% and all males were Wolbachia-uninfected. A significantly greater percentage of Wolbachia-infected Ci. tenellus adults than uninfected adults carried Ca. P. trifolii, suggesting that Wolbachia may increase this insect's ability to tolerate or acquire this pathogen. Results of our study provide a foundation for continued work on interactions between leafhoppers, bacterial endosymbionts, and phytoplasma.

Ampicillin enhanced the resistance of Myzus persicae to imidacloprid and cyantraniliprole

BACKGROUND

Recent studies have shown that symbionts are involved in regulating insecticide detoxification in insects. However, there are few studies on the relationship between the symbionts found in Myzus persicae and the mechanism underlying host detoxification of insecticides. In this study, antibiotic ampicillin treatment was used to investigate the possible relationship between symbiotic bacteria and the detoxification of insecticides in the host, M. persicae.

RESULTS

Bioassays showed that ampicillin significantly reduced the susceptibilities of M. persicae to imidacloprid and cyantraniliprole. Synergistic bioassays and RNAi assays showed that the susceptibilities of M. persicae to imidacloprid and cyantraniliprole were related to metabolic detoxification enzyme activities and the expression level of the cytochrome P450 gene, CYP6CY3. Also, treatment to a combination of ampicillin and enzyme inhibitors or dsCYP6CY3 showed that the negative effect of ampicillin on the susceptibility of M. persicae was effectively inhibited bydetoxification enzyme inhibitors and dsCYP6CY3. Additionally, ampicillin treatment resulted in significant increases in the activities of multifunctional oxidases and esterases, the expression level of CYP6CY3 and fitness of M. persicae. Further, ampicillin significantly reduced the total bacterial abundance and changed symbiont diversity in M. persicae. The abundance of Pseudomonadaceae decreased significantly, while the abundance of Rhodococcus and Buchnera increased significantly.

CONCLUSION

Our study showed that ampicillin enhanced the resistance levels to imidacloprid and cyantraniliprole of M. persicae, which might be related to the selective elimination of symbiotic bacteria, the upregulated activities of detoxification enzymes and the increased fitness. © 2022 Society of Chemical Industry.

Contribution of insect gut microbiota and their associated enzymes in insect physiology and biodegradation of pesticides

Synthetic pesticides are extensively and injudiciously applied to control agriculture and household pests worldwide. Due to their high use, their toxic residues have enormously increased in the agroecosystem in the past several years. They have caused many severe threats to non-target organisms, including humans. Therefore, the complete removal of toxic compounds is gaining wide attention to protect the ecosystem and the diversity of living organisms. Several methods, such as physical, chemical and biological, are applied to degrade compounds, but as compared to other methods, biological methods are considered more efficient, fast, eco-friendly and less expensive. In particular, employing microbial species and their purified enzymes makes the degradation of toxic pollutants more accessible and converts them into non-toxic products by several metabolic pathways. The digestive tract of insects is usually known as a superior organ that provides a nutrient-rich environment to hundreds of microbial species that perform a pivotal role in various physiological and ecological functions. There is a direct relationship between pesticides and insect pests: pesticides reduce the growth of insect species and alter the phyla located in the gut microbiome. In comparison, the insect gut microbiota tries to degrade toxic compounds by changing their toxicity, increasing the production and regulation of a diverse range of enzymes. These enzymes breakdown into their derivatives, and microbial species utilize them as a sole source of carbon, sulfur and energy. The resistance of pesticides (carbamates, pyrethroids, organophosphates, organochlorines, and neonicotinoids) in insect species is developed by metabolic mechanisms, regulation of enzymes and the expression of various microbial detoxifying genes in insect guts. This review summarizes the toxic effects of agrochemicals on humans, animals, birds and beneficial arthropods. It explores the preferential role of insect gut microbial species in the degradation process and the resistance mechanism of several pesticides in insect species. Additionally, various metabolic pathways have been systematically discussed to better understand the degradation of xenobiotics by insect gut microbial species.

Unraveling the Basis of Neonicotinoid Resistance in Whitefly Species Complex: Role of Endosymbiotic Bacteria and Insecticide Resistance Genes

Bemisia tabaci (whitefly) is one of the most detrimental agricultural insect pests and vectors of many plant viruses distributed worldwide. Knowledge of the distribution patterns and insecticide resistance of this cryptic species is crucial for its management. In this study, genetic variation of mitochondrial cytochrome oxidase subunit 1 (MtCoI) gene of B. tabaci was analyzed followed by a study of the infection profile of various endosymbionts in 26 whitefly populations collected from West Bengal, India. Phylogenetic analysis revealed Asia I as the major cryptic species (65.38%), followed by Asia II 5, China 3, and Asia II 7, which were diversified into 20 different haplotypes. In addition to the primary endosymbiont (C. poriera), each of the four whitefly species showed a variable population of three secondary endosymbionts, majorly Arsenophonus with the highest infection rate (73.07%), followed by Wolbachia and Rickettsia. Further phylogenetic analyses revealed the presence of two subgroups of Arsenophonus, viz., A1 and A2, and one each in Wolbachia (W1) and Rickettsia (R3). Resistance to thiamethoxam, imidacloprid, and acetamiprid insecticides was analyzed for a clear picture of pesticide resistance status. The highest susceptibility was noted toward thiamethoxam (LC₅₀ = 5.36 mg/L), followed by imidacloprid and acetamiprid. The whitefly population from Purulia and Hooghly districts bearing Asia II 7 and Asia II 5 cryptic species, respectively, shows maximum resistance. The differences in mean relative titer of four symbiotic bacteria among field populations varied considerably; however, a significant positive linear correlation was observed between the resistance level and relative titer of Arsenophonus and Wolbachia in the case of imidacloprid and thiamethoxam, while only Wolbachia was found in case of acetamiprid. Expression analysis demonstrated differential upregulation of insecticide resistance genes with Purulia and Hooghly populations showing maximally upregulated P450 genes. Moreover, thiamethoxam and imidacloprid resistance ratio (RR) showed a significant correlation with CYP6CM1, CYP6DZ7, and CYP4C64 genes, while acetamiprid RR correlated with CYP6CX1, CYP6DW2, CYP6DZ7, and CYP4C64 genes. Taken together, these findings suggested that P450 mono-oxygenase and symbiotic bacteria together affected whitefly resistance to neonicotinoids. Hence, a symbiont-oriented management programme could be a better alternative to control or delay resistance development in whitefly and can be used for pesticide clean-up in an agricultural field.

Bacterial symbionts influence host susceptibility to fenitrothion and imidacloprid in the obligate hematophagous bed bug, Cimex hemipterus

The use of insecticides remains important in managing pest insects. Over the years, many insects manifested physiological and behavioral modifications resulting in reduced efficacy of insecticides targeted against them. Emerging evidence suggests that bacterial symbionts could modulate susceptibility of host insects against insecticides. Here, we explore the influence of host microbiota in affecting the susceptibility of insect host against different insecticides in the blood-sucking bed bug, Cimex hemipterus. Rifampicin antibiotic treatment resulted in increased susceptibility to fenitrothion and imidacloprid, but not against deltamethrin. Meanwhile, the host fitness parameters measured in the present study were not significantly affected by rifampicin treatment, suggesting the role of bacterial symbionts influencing susceptibility against the insecticides. 16S metagenomics sequencing revealed a drastic shift in the composition of several bacterial taxa following rifampicin treatment. The highly abundant Alphaproteobacteria (Wolbachia > 90%) and Gammaproteobacteria (Yersinia > 6%) in control bed bugs were significantly suppressed and replaced by Actinobacteria, Bacilli, and Betaproteobacteria in the rifampicin treated F1 bed bugs, suggesting possibilities of Wolbachia mediating insecticide susceptibility in C. hemipterus. However, no significant changes in the total esterase, GST, and P450 activities were observed following rifampicin treatment, indicating yet unknown bacterial mechanisms explaining the observed phenomena. Re-inoculation of microbial content from control individuals regained the tolerance of rifampicin treated bed bugs to imidacloprid and fenitrothion. This study provides a foundation for a symbiont-mediated mechanism in influencing insecticide susceptibility that was previously unknown to bed bugs.

Plant secondary metabolite and temperature determine the prevalence of Arsenophonus endosymbionts in aphid populations

Transmission rate and role in hosts contribute to the prevalence of an endosymbiont. However, factors affecting transmission and role of facultative endosymbionts are still not well understood. Here, we illustrated that host plants and environmental temperatures affected the transmission, relative abundance, and role of Arsenophonus in the cotton aphid Aphis gossypii. The transmission rate of this endosymbiont from mother aphids to offspring was relatively lower. High temperatures impeded the transmission, and infection rates declined as aphids were exposed to 30 °C. Contents of amino acids and secondary metabolites were remarkable different among host plants. Aphids feeding on zucchini leaves containing a higher titer of amino acids and lower secondary metabolites harbored a relatively lower abundance of Arsenophonus. Concentrations of an amino acid and a plant secondary metabolite, cucurbitacin B, in aphid diet were not associated with Arsenophonus abundance. However, gossypol, another plant secondary metabolite, was strongly related with the abundance. Arsenophonus imparted a fitness benefit to aphids, and the benefit was dependent on host plants and gossypol concentration. In sum, plant secondary metabolite and environmental temperature affect transmission, relative abundance, and role of Arsenophonus, which determine the endosymbiont prevalence in aphid populations. This article is protected by copyright. All rights reserved.

Effect of Neonicotinoids on Bacterial Symbionts and Insecticide-Resistant Gene in Whitefly, Bemisia tabaci

The silverleaf whitefly, Bemisia tabaci (Gennadius, Hemiptera: Aleyrodidae), is a major threat to field and horticultural crops worldwide. Persistent use of insecticides for the management of this pest is a lingering problem. In the present study, the status of sensitivity of B. tabaci to two neonicotinoids, imidacloprid and thiamethoxam, was evaluated. The expression pattern of two cytochrome P450 (cyp) genes and changes in the relative amount of symbionts in insecticide-treated B. tabaci were also assessed. Quantitative PCR (qPCR) studies indicate that the CYP6CM1 and CYP6CX1 genes were always expressed higher in imidacloprid-treated whitefly, suggesting a correlation between gene expression and the insect's ability to detoxify toxic compounds such as insecticides. In addition, the thiamethoxam-treated population harbored higher Portiera and lower Rickettsia titers, whereas the imidacloprid-treated population harbored more Rickettsia at different time intervals. Interestingly, we also examined that an increase in exposure to both the insecticides resulted in a reduction in the mutualistic partners from their insect host. These differential responses of endosymbionts to insecticide exposure imply the complex interactions among the symbionts inside the host insect. The results also provide a deeper understanding of the molecular mechanism of resistance development that might be useful for formulating effective management strategies to control B. tabaci by manipulating symbionts and detoxifying genes.

Three antimicrobials alter gut microbial communities and causing different mortality of brown planthopper, Nilaparvata lugens Stål

The symbionts in the gut of brown planthopper play an important role in the nutrition utilization and growth of their host, Nilaparvata lugens Stål (Hemiptera: Delphacidae). Controlling the BPH infection on rice by inhibiting the symbionts using antimicrobials is feasible. However, the impact of antimicrobials on the microbiome in the gut has not been fully elucidated. In this study, we found the mortality reached 35.5%, 33.1% and 19.4%, when BPHs were exposed to toyocamycin, tebuconazole, and zhongshengmycin, respectively. Significant differences were found between the structures of gut microbial communities in adult BPHs treated with different antimicrobials and water. The antimicrobials reduced the fungal diversity by reducing the non-dominant fungi abundance, and increased bacterial diversity by inhibiting the dominant bacteria Acinetobacter in the gut. The diversification of taxonomic groups in gut depended on the different selective stress of antimicrobials. For the microbial absolute abundance, the total microbial gut community abundance decreased under antimicrobial exposure, but the absolute abundance of Serratia significantly increased in the antimicrobial treatment group. Overall, our study enriched the knowledge of microbiomes in the gut of BPH under the antimicrobial treatment and provided guidelines to enhance the pest management effect of BPH by using antimicrobials.

Host-Plant Induced Shifts in Microbial Community Structure in Small Brown Planthopper, Laodelphax striatellus (Homoptera: Delphacidae)

Microbiome associated with insects play vital roles in host ecology and physiology. The small brown planthopper (SBPH), Laodelphax striatellus, is a polyphagous insect pest that caused enormous damage to a wide range of cereal crops. Previous studies have assessed the effects of environmental factors, such as antibiotics, insecticide, and geographical habitat on the bacterial composition of SBPH. However, the influence of host plants on the microbial community in SBPH still unclear. Here, we characterized and compared the microbial community in three SBPH populations feeding on rice, barley, and wheat, respectively, using high-throughput amplicon sequencing. Our observations revealed that the microbiome harbored by SBPH was abundant and diverse. Ten phyla comprising 141 genera of bacteria were annotated, and four fungal phyla consisting of 47 genera were assigned. The bacteria belonging to the phylum Proteobacteria were the most prevalent and the fungi with the highest abundance were from the order Hypocreales. Comparative analysis showed that host plants could significantly induce structural changes of SBPH microbiome. Significant differences in abundance were observed in two main bacterial orders (Rickettsiales and Rhodospirillales) and three fungal classes (Sordariomycetes, an unclassified class in Ascomycota and Eurotiomycetes) among three host-adapted SBPH populations. Our results could broaden our understanding of interactions among SBPH, its microbial associates and host plants, and also represented the basis of future SBPH biological management.

Relationship between Wolbachia infection in Culex quinquefasciatus and its resistance to insecticide

Many studies have been done on the various factors affecting resistance to insecticides. The relationship between Wolbachia bacteria and resistance to insecticides is one of the factors that has attracted a lot of attentions. Wolbachia are obligatory intracellular endosymbionts that naturally occur in a wide range of arthropods and nematodes, including the mosquito Culex quinquefasciatus. Initially, the presence of bacteria was proved by molecular assays. Then the resistance level of this species was evaluated in adults against DDT 4.0% and deltamethrin 0.05% using the standard WHO guideline. After elimination of Wolbachia by tetracycline and its proof by molecular assays, the susceptibility tests were conducted again on uninfected line. Finally, the two lines were compared in terms of responding to insecticides. The findings indicated that there is no significant correlation between susceptibility of two lines in response to DDT 4.0% while they represented a significant correlation for deltamethrin (P =0.00). We propose that Wolbachia bacteria increase the susceptibility to deltamethrin but they show neutral effect on DDT susceptibility in Cx. quinquefasciatus. However, more studies on other vectors and insecticides still need to be done.

Interaction of Arsenophonus with Wolbachia in Nilaparvata lugens

Background

Co-infection of endosymbionts in the same host is ubiquitous, and the interactions of the most common symbiont Wolbachia with other symbionts, including Spiroplasma, in invertebrate organisms have received increasing attention. However, the interactions between Wolbachia and Arsenophonus, another widely distributed symbiont in nature, are poorly understood. We tested the co-infection of Wolbachia and Arsenophonus in different populations of Nilaparvata lugens and investigated whether co-infection affected the population size of the symbionts in their host.

Results

A significant difference was observed in the co-infection incidence of Wolbachia and Arsenophonus among 5 populations of N. lugens from China, with nearly half of the individuals in the Zhenjiang population harbouring the two symbionts simultaneously, and the rate of occurrence was significantly higher than that of the other 4 populations. The Arsenophonus density in the superinfection line was significantly higher only in the Maanshan population compared with that of the single-infection line. Differences in the density of Wolbachia and Arsenophonus were found in all the tested double-infection lines, and the dominant symbiont species varied with the population only in the Nanjing population, with Arsenophonus the overall dominant symbiont.

Conclusions

Wolbachia and Arsenophonus could coexist in N. lugens, and the co-infection incidence varied with the geographic populations. Antagonistic interactions were not observed between Arsenophonus and Wolbachia, and the latter was the dominant symbiont in most populations.

Background-dependent Wolbachia-mediated insecticide resistance in Laodelphax striatellus

Although facultative endosymbionts are now known to protect insect hosts against pathogens and parasitoids, the effects of endosymbionts on insecticide resistance are still unclear. Here we show that Wolbachia are associated with increased resistance to the commonly used insecticide, buprofezin, in the small brown planthopper (Laodelphax striatellus) in some genetic backgrounds while having no effect in other backgrounds. In three Wolbachia-infected lines from experimental buprofezin-resistant strains and one line from a buprofezin-susceptible line established from Chuxiong, Yunnan province, China, susceptibility to buprofezin increased after removal of Wolbachia. An increase in susceptibility was also evident in a Wolbachia-infected line established from a field population in Rugao, Jiangsu province. However, no increase was evident in two field populations from Nanjing and Fengxian, Jiangsu province, China. When Wolbachia was introgressed into different genetic backgrounds, followed by Wolbachia removal, the data pointed to Wolbachia effects that depend on the nuclear background as well as on the Wolbachia strain. However, there was no relationship between Wolbachia density and the component of buprofezin resistance associated with the symbiont. The results suggest that Wolbachia effects associated with chemical resistance are complex and unpredictable, but also that they can be substantial.

Population Dynamics of Wolbachia in Laodelphax striatellus (Fallén) Under Successive Stress of Antibiotics

Wolbachia are the most common symbionts in arthropods; antibiotic treatment for eliminating the symbionts from their host is necessary to investigate the functions. Tetracycline antibiotics are widely used to remove endosymbiont Wolbachia from insect hosts. However, very little has been known on the effects of tetracycline on population size of Wolbachia in small brown planthopper (SBPH), Laodelphax striatellus (Fallén), an important insect pest of rice in Asia. Here, we investigated the dynamics of Wolbachia population density in females and males of L. striatellus by real-time fluorescent quantitative PCR method. The Wolbachia density in females and males of L. striatellus all declined sharply after treatment with 2 mg/mL tetracycline for one generation, and continued to decrease to a level which could not be detected by both qPCR and diagnostic PCR after treated for another generation, then maintained at 0 in the following three generations with continuous antibiotic treatment. Wolbachia infection did not recover in L. striatellus after stopping tetracycline treatment for ten generations. This is the first report to precisely monitor the population dynamics of Wolbachia in L. striatellus during successive tetracycline treatment and after that. The results provide a useful method for evaluating the efficiency of artificial operation of endosymbionts.

Influences of two coexisting endosymbionts, CI-inducing Wolbachia and male-killing Spiroplasma, on the performance of their host Laodelphax striatellus (Hemiptera: Delphacidae)

The small brown planthopper Laodelphax striatellus (Hemiptera: Delphacidae) is reported to have the endosymbiont Wolbachia, which shows a strong cytoplasmic incompatibility (CI) between infected males and uninfected females. In the 2000s, female-biased L. striatellus populations were found in Taiwan, and this sex ratio distortion was the result of male-killing induced by the infection of another endosymbiont, Spiroplasma. Spiroplasma infection is considered to negatively affect both L. striatellus and Wolbachia because the male-killing halves the offspring of L. striatellus and hinders the spread of Wolbachia infection via CI. Spiroplasma could have traits that increase the fitness of infected L. striatellus and/or coexisting organisms because the coinfection rates of Wolbachia and Spiroplasma were rather high in some areas. In this study, we investigated the influences of the infection of these two endosymbionts on the development, reproduction, and insecticide resistance of L. striatellus in the laboratory. Our results show that the single-infection state of Spiroplasma had a negative influence on the fertility of L. striatellus, while the double-infection state had no significant influence. At late nymphal and adult stages, the abundance of Spiroplasma was lower in the double-infection state than in the single-infection state. In the double-infection state, the reduction of Spiroplasma density may be caused by competition between the two endosymbionts, and the negative influence of Spiroplasma on the fertility of host may be relieved. The resistance of L. striatellus to four insecticides was compared among different infection states of endosymbionts, but Spiroplasma infection did not contribute to increase insecticide resistance. Because positive influences of Spiroplasma infection were not found in terms of the development, reproduction, and insecticide resistance of L. striatellus, other factors improving the fitness of Spiroplasma-infected L. striatellus may be related to the high frequency of double infection in some L. striatellus populations.

First Evidence of Intracellular Bacteria Cardinium in Thermophilic Mite Microzetorchestes emeryi (Acari: Oribatida): Molecular Screening of Bacterial Endosymbiont Species

We undertook the issue of the distribution of intracellular bacteria among Oribatida (Acari). Six genera of bacteria were detected by PCR and Sanger DNA sequencing: Wolbachia, Cardinium, Rickettsia, Spiroplasma, Arsenophonus, and Hamiltonella. Our research, for the first time, revealed the presence of Cardinium in Microzetorchestes emeryi in two subpopulations separated from each other by 300 m. The percentages of infected animals were the same in both subpopulations-ca. 20%. The identity of 16S rDNA sequences of Cardinium between these two subpopulations of M. emeryi was 97%. Phylogenetic analysis showed that the Cardinium in M. emeryi was clustered into the group A. The occurrence of M. emeryi in Poland has not been reported before and our report is the first one. Cardinium maybe help the thermophilic M. emeryi to adapt to low temperatures in the Central Europe.

Importance of endosymbionts Wolbachia and Rickettsia in insect resistance development

Endosymbionts play important roles in protecting hosts from environmental stress, such as natural enemies, heat, and toxins. Many insects are infected with the facultative nonessential endosymbionts Wolbachia and Rickettsia, which are the crux in this review, although other relevant symbiont genera will also be treated. Insecticide resistance of hosts can be related to infections with Wolbachia and Rickettsia. These endosymbionts commonly increase host susceptibility to chemical insecticides, but cases of increased resistance also exist. The symbiont-mediated insecticide resistance/susceptibility varies with species of insect, species of symbiont, and chemical compound. Changes in insecticide resistance levels of insects can be associated with fluctuations in population density of endosymbionts. Effects of endosymbionts on host fitness, metabolism, immune system, and gene expression may determine how endosymbionts influence insecticide resistance. A clearer understanding of these interactions can improve our knowledge about drivers of decreasing insecticide resistance.