Microbial metabolites as engines of behavioral variation across animals

The microbiome, especially that present in the gut, has emerged as a key modulator of animal behavior. However, the extent of its influence across species and behavioral repertoires, as well as the underlying mechanisms, remains poorly understood. Increasing evidence suggests that microbial metabolites play an important role in driving behavioral variation. In this review, we synthesize findings from vertebrates to invertebrates, spanning both model and non-model organisms, to define key groups of microbial-derived metabolites involved in modulating seven distinct behaviors: nutrition, olfaction, circadian rhythms, reproduction, locomotion, aggression, and social interactions. We discuss how these microbial metabolites interact with host chemosensory systems, neurotransmitter signaling, and epigenetic modifications to shape behavior. Additionally, we highlight critical gaps in mechanistic understanding, including the need to map additional host receptors and signaling pathways, as well as the untapped potential of microbial biosynthetic gene clusters as sources for novel bioactive compounds. Advancing these areas will enhance understanding of the microbiome's role in behavioral modulation and open new avenues for microbiome-based interventions for behavioral disorders.

Effects of gut symbiotic bacteria, Caballeronia insecticola, on reproductive capacity and mating behaviors of insect host, Riptortus pedestris (Hemiptera: Alydidae)

This study addresses how gut symbiont, Caballeronia insecticola, could change reproductive capacity, mating behaviors, and copulation success of host insect, Riptortus pedestris (Fabricius) (Hemiptera: Alydidae). First, we evaluated symbiotic effects on the female reproductive capacity with varying numbers of males available to a female. Overall, symbiotic females displayed on average a 1.8-fold increase in egg production compared to aposymbiotic individuals. However, eggs from symbiotic females were on average 42% less viable, compared to those from the aposymbiotic, when paired with single male. The decrease in the hatchability was alleviated to 12% when paired with 3 males. Consequently, this yielded significant increase in the number of viable offspring by symbiotic females when multiple males were available. Second, we evaluated symbiotic effects on male morphometric characteristics including hind legs used as weapon, and found significant increases in hind leg sizes associated with symbiosis. Finally, we investigated mating behaviors between a female and 2 males of different symbiotic status. Symbiotic females displayed on average a 1.4-fold increase in the number of copulations compared to the aposymbiotic. From both female types, however, no significant difference was observed in their mate choice and copulation success rate between aposymbiotic and symbiotic males. However, symbiotic females exhibited on average 17% reduction in copulation duration compared to the aposymbiotic. Copulation failure was caused more frequently by female's rejection than by intruder male's disruption for both female types. Our study demonstrates that symbiotic females benefit from the symbiosis increasing their reproductive capacity and copulation frequency.

Investigating the role of chemical ecology in plant-pathogen, vector, and secondary consumer interactions and their consequences for integrated pest management

Phytopathogens modify chemical communication between host plants and herbivorous vectors of those pathogens. These chemical cues often attract vectors to sources of inoculum and facilitate the further spread of the pathogens. Recent investigations have demonstrated that secondary consumers also respond to the same pathogen-induced cues that affect the behavior of vectors. Therefore, efforts to manipulate the behavior of natural enemies to improve biological control may yield unpredictable outcomes since coincident volatiles are induced by herbivory and pathogen attacks. We suggest that case-specific analyses of the costs and benefits of these multitrophic interactions are required to translate biological findings into integrated pest management practices.

Origin and function of beneficial bacterial symbioses in insects

Beneficial bacterial symbionts are widespread in insects and affect the fitness of their hosts by contributing to nutrition, digestion, detoxification, communication or protection from abiotic stressors or natural enemies. Decades of research have formed our understanding of the identity, localization and functional benefits of insect symbionts, and the increasing availability of genome sequences spanning a diversity of pathogens and beneficial bacteria now enables comparative approaches of their metabolic features and their phylogenetic affiliations, shedding new light on the origin and function of beneficial symbioses in insects. In this Review, we explore the symbionts' metabolic traits that can provide benefits to insect hosts and discuss the evolutionary paths to the formation of host-beneficial symbiotic associations. Phylogenetic analyses and molecular studies reveal that extracellular symbioses colonizing cuticular organs or the digestive tract evolved from a broad diversity of bacterial partners, whereas intracellular beneficial symbionts appear to be restricted to a limited number of lineages within the Gram-negative bacteria and probably originated from parasitic ancestors. To unravel the general principles underlying host-symbiont interactions and recapitulate the early evolutionary steps leading towards beneficial symbioses, future efforts should aim to establish more symbiotic systems that are amenable to genetic manipulation and experimental evolution.

Shift patterns of internal bacterial communities across five life stages of laboratory-maintained Eremobelba eharai (Acari: Oribatida: Eremobelbidae)

Microbial communities play a crucial role in the physiological characteristics, adaptability, and reproductive success of arthropods. However, the patterns and functions of microbial variation across different life stages of mites remain poorly understood. In this study, we used high throughput Illumina sequencing of the 16S rRNA gene to investigate the composition and functional potential of bacterial communities in five life stages (larva, protonymph, deutonymph, tritonymph, and adult) of the oribatid mite Eremobelba eharai. We found significant differences in bacterial diversity and community structure across the different life stages despite being fed the same diet. The bacterial diversity was highest at the protonymph stage and lowest at the tritonymph stage. Beta diversity analysis indicated distinct bacterial community compositions among the different life stages. Bacteroidota, Proteobacteria, and Firmicutes dominated the bacterial communities throughout the host's life cycle. Key bacterial genera, such as Bacillus, Streptomyces, Achromobacter, and Tsukamurella, showed significant differences in abundance across the different life stages. Predicted functional profiles revealed substantial changes in metabolic pathways, which may reflect changes in the nutritional needs of E. eharai during its developmental process. PICRUSt prediction results also showed that in most KEGG pathways, the larval and adult stages consistently maintain similar relative abundances of bacteria. Different stages such as the deutonymph and adult stages show consistent differences in the "biosynthesis of other secondary metabolites" and "glycan biosynthesis and metabolism" pathways. This study provides new insights into the dynamic changes of bacterial communities within oribatid mites and lays a foundation for further research on the interactions between oribatid mites and microorganisms.

Molecular module for glucose production influences sex pheromone synthesis in Bactrocera dorsalis

Some insects have evolved beneficial relationships with intestinal microbes for sex pheromone production to communicate with conspecifics effectively. However, it is not clear whether the sex pheromone synthesis activity of intestinal microbes can be controlled by the host, and the molecular mechanisms need to be further unraveled. In this study, we find that rectal gland Bacillus species of male Bactrocera dorsalis specifically produce sex pheromones in the evening, which is significantly associated with glucose levels. In vitro Bacillus culture assays show that glucose levels significantly influence the amount of sex pheromone produced. Comparative rectal gland transcriptome analysis reveals that the expressions of the alpha-galactosidase gene (GLA), a Bactrocera dorsalis transcription factor (BDTF), and a pigment-dispersing factor (PDF) are responsible for producing glucose. Our findings reveal that the PDF-BDTF-GLA module influences the intestinal-microbe-produced sex pheromone by regulating glucose levels and advance our understanding of interactions between insects and their intestinal microbes.

Evidence for a gut microbial community conferring adaptability to diet quality and temperature stressors in phytophagous insects: the melon fruit fly Zeugodacus cucurbitae (Diptera: Tephritidae) as a case study

BACKGROUND

The high invasiveness of phytophagous insects is related to their adaptability to various environments, that can be influenced by their associated microbial community. Microbial symbionts are known to play a key role in the biology, ecology, and evolution of phytophagous insects, but their abundance and diversity are suggested to be influenced by environmental stressors. In this work, using 16 S rRNA metabarcoding we aim to verify (1) if laboratory rearing affects microbial symbiont communities of Zeugodacus cucurbitae females, a cosmopolitan pest of cucurbitaceous crops (2) if temperature, diet quality, and antibiotic treatments affect microbial symbiont communities of both laboratory and wild populations, and (3) if changes in microbial symbiont communities due to temperature, diet and antibiotic affect longevity and fecundity of Z. cucurbitae.

RESULTS

The results showed that microbial diversity, particularly the β-diversity was significantly affected by insect origin, temperature, diet quality, and antibiotic treatment. The alteration of gut microbial symbionts, specifically Enterobacteriaceae, was associated with low fecundity and longevity of Z. cucurbitae females feeding on optimal diet only. Fecundity reduction in antibiotic treated females was more pronounced when flies were fed on a poor diet without protein.

CONCLUSIONS

our study proves the relationship between gut microbiome and host fitness under thermal and diet fluctuation highlighting the importance of microbial community in the adaptation of Z. cucurbitae to environmental stress.

CLINICAL TRIAL NUMBER

Not applicable.

Microbial interactions shaping host attractiveness: insights into dynamic behavioral relationships

Insects discern the presence of hosts (host plants) by integrating chemosensory, gustatory, and visual cues, with olfaction playing a pivotal role in this process. Among these factors, volatile signals produced by host-associated microbial communities significantly affect insect attraction. Microorganisms are widely and abundantly found on the surfaces of humans, plants, and insects. Notably, these microorganisms can metabolize compounds from the host surface and regulate the production of characteristic volatiles, which may guide the use of host microorganisms to modulate insect behavior. Essentially, the attraction of hosts to insects is intricately linked to the presence of their symbiotic microorganisms. This review underscores the critical role of microorganisms in shaping the dynamics of attractiveness between insects and their hosts.

Alteration in microbes changed the contents of oviposition-deterrent pheromones on the Spodoptera litura egg surface

Microorganisms symbiotic with insects, whether permanently or temporarily, play a crucial role in the nutrition, development, reproduction, defence, and metamorphosis regulation. In some Lepidoptera, oviposition-deterrent pheromones (ODPs) on egg surface were used by pregnant females to modify the behaviour of conspecifics to avoid excessive competition for limited resources. In this study, we constructed four different Spodoptera litura groups, including, OH, OA, SH, and OA, which either feed on different hosts or grow in different environments. The 16S rDNA libraries of microbes from the egg surface of the four groups were constructed and sequenced. According to alpha and beta diversity indices, the microbes in environments and diets considerably influenced the richness, diversity, and community compositions of the microbiota on egg surfaces. The quantity of the main ODP components and the corresponding oviposition-deterrent activity among four groups were significantly differed among the four groups. The result of this study revealed that altering of microbes in environments or diets considerably changed the contents of ODP and oviposition-deterrent activity. As ODPs impart oviposition-deterrent activity towards closely related species, the findings of this study suggest that we should pay more attention to the role of symbiotic microorganisms in changing the ability of insects, especially sympatric species, to occupy the optimal niche when developing novel pest-control strategies.

Insect Lipid Metabolism in the Presence of Symbiotic and Pathogenic Viruses and Bacteria

Insects, like most animals, have intimate interactions with microorganisms that can influence the insect host's lipid metabolism. In this chapter, we describe what is known so far about the role prokaryotic microorganisms play in insect lipid metabolism. We start exploring microbe-insect lipid interactions focusing on endosymbionts, and more specifically the gut microbiota that has been predominantly studied in Drosophila melanogaster. We then move on to an overview of the work done on the common and well-studied endosymbiont Wolbachia pipientis, also in interaction with other microbes. Taking a slightly different angle, we then look at the effect of human pathogens, including dengue and other viruses, on the lipids of mosquito vectors. We extend the work on human pathogens and include interactions with the endosymbiont Wolbachia that was identified as a natural tool to reduce the spread of mosquito-borne diseases. Research on lipid metabolism of plant disease vectors is up and coming and we end this chapter by highlighting current knowledge in that field.

Diet affects reproductive development and microbiota composition in honey bees

BACKGROUND

Gut microbes are important to the health and fitness of many animals. Many factors have been shown to affect gut microbial communities including diet, lifestyle, and age. Most animals have very complex physiologies, lifestyles, and microbiomes, making it virtually impossible to disentangle what factors have the largest impact on microbiota composition. Honeybees are an excellent model to study host-microbe interactions due to their relatively simple gut microbiota, experimental tractability, and eusociality. Worker honey bees have distinct gut microbiota from their queen mothers despite being close genetic relatives and living in the same environment. Queens and workers differ in numerous ways including development, physiology, pheromone production, diet, and behavior. In the prolonged absence of a queen or Queen Mandibular Pheromones (QMP), some but not all workers will develop ovaries and become "queen-like". Using this inducible developmental change, we aimed to determine if diet and/or reproductive development impacts the gut microbiota of honey bee workers.

RESULTS

Microbiota-depleted newly emerged workers were inoculated with a mixture of queen and worker gut homogenates and reared under four conditions varying in diet and pheromone exposure. Three weeks post-emergence, workers were evaluated for ovary development and their gut microbiota communities were characterized. The proportion of workers with developed ovaries was increased in the absence of QMP but also when fed a queen diet (royal jelly). Overall, we found that diet, rather than reproductive development or pheromone exposure, led to more "queen-like" microbiota in workers. However, we revealed that diet alone cannot explain the microbiota composition of workers.

CONCLUSION

The hypothesis that reproductive development explains microbiota differences between queens and workers was rejected. We found evidence that diet is one of the main drivers of differences between the gut microbial community compositions of queens and workers but cannot fully explain the distinct microbiota of queens. Thus, we predict that behavioral and other physiological differences dictate microbiota composition in workers and queens. Our findings not only contribute to our understanding of the factors affecting the honey bee microbiota, which is important for bee health, but also illustrate the versatility and benefits of utilizing honeybees as a model system to study host-microbe interactions.

The Sexual Dimorphism in Rectum and Protein Digestion Pathway Influence Sex Pheromone Synthesis in Male Bactrocera Dorsalis

Sexual dimorphism is a crucial aspect of mating and reproduction in many animals, yet the molecular mechanisms remain unclear. In Bactrocera dorsalis, sex pheromones trimethylpyrazine (TMP) and tetramethylpyrazine (TTMP) are specifically synthesized by Bacillus strains in the male rectum. In the female rectum, Bacillus strains are found, but TMP and TTMP are not, indicating sexually dimorphic differences in sex pheromone synthesis. Our anatomical observations and precursor measurements revealed significant differences in rectal structure and ammonium levels between sexes.  In vitro and in vivo experiments reveal that ammonium is vital for sex pheromone synthesis in rectal Bacillus strains. Comparative transcriptome analysis identified ammonium-producing genes (carboxypeptidase B and peptide transporter) in the protein digestion pathway that show much higher expression in the male rectum than in the female rectum. Knocking down the expression of either carboxypeptidase B (or inhibiting enzyme activity) or peptide transporter decreases rectal ammonium levels significantly, resulting in the failure of sex pheromone synthesis in the male rectum. This study provides insights into the presence of sexual dimorphism in internal organs and their functionalities in male-specific sex pheromone synthesis and has significant implications for understanding the molecular mechanisms underlying sex pheromone synthesis by symbionts in insects.

Pinewood nematode induced changes in the assembly process of gallery microbiomes benefit its vector beetle's development

Microbiomes play crucial roles in insect adaptation, especially under stress such as pathogen invasion. Yet, how beneficial microbiomes assemble remains unclear. The wood-boring beetle Monochamus alternatus, a major pest and vector of the pine wilt disease (PWD) nematode, offers a unique model. We conducted controlled experiments using amplicon sequencing (16S rRNA and ITS) within galleries where beetles and microbes interact. PWD significantly altered bacterial and fungal communities, suggesting distinct assembly processes. Deterministic factors like priority effects, host selection, and microbial interactions shaped microbiome composition, distinguishing healthy from PWN-infected galleries. Actinobacteria, Firmicutes, and Ophiostomataceae emerged as potentially beneficial, aiding beetle's development and pathogen resistance. This study unveils how nematode-induced changes in gallery microbiomes influence beetle's development, shedding light on microbiome assembly amid insect-pathogen interactions. Insights gleaned enhance understanding of PWD spread and suggest novel management strategies via microbiome manipulation.IMPORTANCEThis study explores the assembly process of gallery microbiomes associated with a wood-boring beetles, Monochamus alternatus, a vector of the pine wilt disease (PWD). By conducting controlled comparison experiments and employing amplicon approaches, the study reveals significant changes in taxonomic composition and functional adaptation of bacterial and fungal communities induced by PWD. It identifies deterministic processes, including priority effects, host selection, and microbial interactions, as major drivers in microbiome assembly. Additionally, the study highlights the presence of potentially beneficial microbes such as Actinobacteria, Firmicutes, and Ophiostomataceae, which could enhance beetle development and resistance to pathogens. These findings shed light on the intricate interplay among insects, microbiomes, and pathogens, contributing to a deeper understanding of PWD prevalence and suggesting innovative management strategies through microbiome manipulation.

Microbiota and the volatile profile of avian nests are associated with each other and with the intensity of parasitism

Bacteria have been suggested as being partially responsible for avian nest odours and, thus, volatiles from their metabolism could influence the intensity of selection pressures due to parasites detecting olfactory cues of their hosts. Here, we tested this hypothesis by exploring intraspecific and interspecific variability in microbial environments, volatile profiles and intensity of ectoparasitism by Carnus hemapterus in the nests of 10 avian species. As expected, we found that (i) alpha and beta diversity of microbial and volatile profiles were associated with each other. Moreover, (ii) alpha diversity of bacteria and volatiles of the nest environment, as well as some particular bacteria and volatiles, was associated with the intensity of parasitism at early and late stages of the nestling period. Finally, (iii) alpha diversity of the nest microbiota, as well as some particular bacteria and volatiles, was correlated with fledging success. When considering them together, the results support the expected links between the microbial environment and nest odours in different bird species, and between the microbial environment and both ectoparasitism intensity and fledging success. Relative abundances of particular volatiles and bacteria predicted ectoparasitism and/or fledging success. Future research should prioritise experimental approaches directed to determine the role of bacteria and volatiles in the outcomes of host-ectoparasite interactions.

Wolbachia strain diversity in a complex group of sympatric cryptic parasitoid wasp species

BACKGROUND

Maternally-inherited symbionts can induce pre-mating and/or post-mating reproductive isolation between sympatric host lineages, and speciation, by modifying host reproductive phenotypes. The large parasitoid wasp genus Cotesia (Braconidae) includes a diversity of cryptic species, each specialized in parasitizing one to few related Lepidoptera host species. Here, we characterized the infection status of an assemblage of 21 Cotesia species from 15 countries by several microbial symbionts, as a first step toward investigating whether symbionts may provide a barrier to gene flow between these parasitoid host lineages.

RESULTS

The symbiotic microbes Arsenophonus, Cardinium, Microsporidium and Spiroplasma were not detected in the Cotesia wasps. However, the endosymbiotic bacterium Wolbachia was present in at least eight Cotesia species, and hence we concentrated on it upon screening additional DNA extracts and SRAs from NCBI. Some of the closely related Cotesia species carry similar Wolbachia strains, but most Wolbachia strains showed patterns of horizontal transfer between phylogenetically distant host lineages.

CONCLUSIONS

The lack of co-phylogenetic signal between Wolbachia and Cotesia suggests that the symbiont and hosts have not coevolved to an extent that would drive species divergence between the Cotesia host lineages. However, as the most common facultative symbiont of Cotesia species, Wolbachia may still function as a key-player in the biology of the parasitoid wasps. Its precise role in the evolution of this complex clade of cryptic species remains to be experimentally investigated.

Pine Response to Sawfly Pheromones: Effects on Sawfly's Oviposition and Larval Growth

Insect pheromones have been intensively studied with respect to their role in insect communication. However, scarce knowledge is available on the impact of pheromones on plant responses, and how these in turn affect herbivorous insects. A previous study showed that exposure of pine (Pinus sylvestris) to the sex pheromones of the pine sawfly Diprion pini results in enhanced defenses against the eggs of this sawfly; the egg survival rate on pheromone-exposed pine needles was lower than that on unexposed pine. The long-lasting common evolutionary history of D. pini and P. sylvestris suggests that D. pini has developed counter-adaptations to these pine responses. Here, we investigated by behavioral assays how D. pini copes with the defenses of pheromone-exposed pine. The sawfly females did not discriminate between the odor of pheromone-exposed and unexposed pine. However, when they had the chance to contact the trees, more unexposed than pheromone-exposed trees received eggs. The exposure of pine to the pheromones did not affect the performance of larvae and their pupation success. Our findings indicate that the effects that responses of pine to D. pini sex pheromones exert on the sawfly eggs and sawfly oviposition behavior do not extend to effects on the larvae.

Volatiles of symbiotic bacterial origin explain ectoparasitism and fledging success of hoopoes

BACKGROUND

Some parasites use olfactory cues to detect their hosts and, since bacterial symbionts are partially responsible for animal odours, they could influence host parasitism. By autoclaving nest materials of hoopoe (Upupa epops) nests before reproduction started, we explored the hypothetical links between host-associated bacteria, volatiles and parasitism. During the nestling stage, we (i) estimated the level of ectoparasitism by chewing lice (Suborder Mallophaga) in adult hoopoe females and by Carnus haemapterus flies in nestlings, and (ii) characterized microbial communities and volatile profiles of nest environments (nest material and nest cavity, respectively) and uropygial secretions.

RESULTS

Experimental nests had less diverse bacterial communities and more diverse volatile profiles than control nests, while occupants experienced lower intensity of parasitism in experimental than in control nests. The experiment also affected beta diversity of the microbial communities of nest material and of the volatiles of the nestling uropygial secretions. Moreover, microbial communities of uropygial secretions and of nest materials covaried with their volatile profiles, while the volatile profile of the bird secretions explained nest volatile profile. Finally, a subset of the volatiles and bacteria detected in the nest material and uropygial secretions were associated with the ectoparasitism intensity of both adult females and nestlings, and with fledging success.

CONCLUSIONS

These results show that a component of animal odours is linked with the microbial communities of the host and its reproductive environment, and emphasize that the associations between bacteria, ectoparasitism and reproductive success are partially mediated by volatiles of bacterial origin. Future work should focus on mechanisms underlying the detected patterns.

Analysis of gut microbiota of ladybug beetle (Harmonia axyridis) after feeding on different artificial diets

BACKGROUND

Harmonia axyridis is an effective natural enemy insect to a variety of phloem-sucking pests and Lepidopteran larvae, such as aphids, scabies, and phylloxera, while its industrial production is limited due to unmature artificial diet. Insect intestinal microbiota affect host development and reproduction. The aim of this study is to understand intestinal microbiota composition of H. axyridis and screen effective probiotics on artificial diet. Considering the role of the components and composition of the diet on the structure and composition of the intestinal microbiome, four kinds of diets were set up: (1) aphid; (2) basic diet; (3) basic diet + glucose; (4) basic diet + trehalose. The gut microbiota of H. axyridis was detected after feeding on different diets.

RESULTS

Results showed that the gut microbiota between artificial diet group and aphid groups were far apart, while the basic and glucose groups were clearly clustered. Besides, the glucose group and trehalose group had one unique phylum, Cryptophyta and Candidatus Saccharibacteria, respectively. The highest abundance of Proteobacteria was found in the aphid diet. The highest abundance of Firmicutes was found in the basic diet. However, the addition of glucose or trehalose alleviated the change. In addition, the relative abundance of Enterobacter, Klebsiella, Enterobacteriaceae_unclassified, Enterobacteriales_unclassified and Serratia in the aphid group was higher than other groups. Moreover, the function of gut genes in each group also showed clear differences.

CONCLUSION

These results have offered a strong link between artificial diets and gut microbes, and also have provided a theoretical basis for the screening of synergistic probiotics in artificial diet.

Dynamic Gut Microbiota of Apolygus lucorum Across Different Life Stages Reveals Potential Pathogenic Bacteria for Facilitating the Pest Management

Insect's gut microbiota has diverse effects on their fitness, and a comprehensive understanding of gut microbiota functions requires analyzing its diversity. Apolygus lucorum is a highly destructive pest that threatens many economically important crops in China. This study investigated the gut microbiota of A. lucorum across its life cycle using both culture-dependent and culture-independent methods. A total of 87 gut bacterial isolates were identified, belonging to 4 phyla, 27 families, and 45 genera, while Miseq sequencing detected 91 amplicon sequence variants (ASVs) assigned to 5 phyla, 28 families, and 39 genera. Proteobacteria and Firmicutes were the predominant phyla, with Staphylococcus and Serratia as the major genera. There were significant differences in the relative abundance of these genera between the nymph and adult stages. Staphylococcus was significantly more abundant in nymphs than it in adults, while Serratia was significantly more abundant in sexually mature adults than in other developmental stages. Notably, Serratia is a common opportunistic pathogen in many insects. Injecting the gut-dominant isolate Serratia marcescens verified its high pathogenicity. Additionally, immune indicators of the bug at different developmental stages supported the hypothesis that Serratia is a pathogen of A. lucorum. This study provides a foundation for understanding the role of gut bacteria in the life history of A. lucorum and developing new pest control strategies based on microbes.

Pathogen-Mediated Alterations of Insect Chemical Communication: From Pheromones to Behavior

Pathogens can influence the physiology and behavior of both animal and plant hosts in a manner that promotes their own transmission and dispersal. Recent research focusing on insects has revealed that these manipulations can extend to the production of pheromones, which are pivotal in chemical communication. This review provides an overview of the current state of research and available data concerning the impacts of bacterial, viral, fungal, and eukaryotic pathogens on chemical communication across different insect orders. While our understanding of the influence of pathogenic bacteria on host chemical profiles is still limited, viral infections have been shown to induce behavioral changes in the host, such as altered pheromone production, olfaction, and locomotion. Entomopathogenic fungi affect host chemical communication by manipulating cuticular hydrocarbons and pheromone production, while various eukaryotic parasites have been observed to influence insect behavior by affecting the production of pheromones and other chemical cues. The effects induced by these infections are explored in the context of the evolutionary advantages they confer to the pathogen. The molecular mechanisms governing the observed pathogen-mediated behavioral changes, as well as the dynamic and mutually influential relationships between the pathogen and its host, are still poorly understood. A deeper comprehension of these mechanisms will prove invaluable in identifying novel targets in the perspective of practical applications aimed at controlling detrimental insect species.

Viral Chemotaxis of Paramecium Bursaria Altered by Algal Endosymbionts

Chemotaxis is widespread across many taxa and often aids resource acquisition or predator avoidance. Species interactions can modify the degree of movement facilitated by chemotaxis. In this study, we investigated the influence of symbionts on Paramecium bursaria's chemotactic behavior toward chloroviruses. To achieve this, we performed choice experiments using chlorovirus and control candidate attractors (virus stabilization buffer and pond water). We quantified the movement of Paramecia grown with or without algal and viral symbionts toward each attractor. All Paramecia showed some chemotaxis toward viruses, but cells without algae and viruses showed the most movement toward viruses. Thus, the endosymbiotic algae (zoochlorellae) appeared to alter the movement of Paramecia toward chloroviruses, but it was not clear that ectosymbiotic viruses (chlorovirus) also had this effect. The change in behavior was consistent with a change in swimming speed, but a change in attraction remains possible. The potential costs and benefits of chemotactic movement toward chloroviruses for either the Paramecia hosts or its symbionts remain unclear.

Impact of intraspecific variation in insect microbiomes on host phenotype and evolution

Microbes can be an important source of phenotypic plasticity in insects. Insect physiology, behaviour, and ecology are influenced by individual variation in the microbial communities held within the insect gut, reproductive organs, bacteriome, and other tissues. It is becoming increasingly clear how important the insect microbiome is for insect fitness, expansion into novel ecological niches, and novel environments. These investigations have garnered heightened interest recently, yet a comprehensive understanding of how intraspecific variation in the assembly and function of these insect-associated microbial communities can shape the plasticity of insects is still lacking. Most research focuses on the core microbiome associated with a species of interest and ignores intraspecific variation. We argue that microbiome variation among insects can be an important driver of evolution, and we provide examples showing how such variation can influence fitness and health of insects, insect invasions, their persistence in new environments, and their responses to global environmental changes. A and B are two stages of an individual or a population of the same species. The drivers lead to a shift in the insect associated microbial community, which has consequences for the host. The complex interplay of those consequences affects insect adaptation and evolution and influences insect population resilience or invasion.

Plasticity of aggregation pheromones in insects

Pheromone plasticity is widely observed in insects and enhances their survival, adaptation, and reproductive success. Aggregation pheromones, which cause notable individual aggregation and consequently impact agriculture and human life, are renowned for their special function. Here, we present a review of research progress regarding pheromone plasticity in three typical aggregative insects: locusts, bark beetles, and cockroaches. These insects are major pest species with considerable impacts on the social economy and public health. Numerous studies have demonstrated the plasticity of aggregation pheromones in different populations of these insect species. Although pheromone chemicals and compositions vary across the three groups, the plasticity of aggregation pheromones is significantly impacted by population density, location, food resources, and gut symbiotic microorganisms, indicating the complexity of pheromone plasticity regulated by multiple factors. Finally, we discuss the potential application of pheromone plasticity in basic research and pest management.

Influences of Microbial Symbionts on Chemoreception of Their Insect Hosts

Chemical communication is widespread among insects and exploited to adjust their behavior, such as food and habitat seeking and preferences, recruitment, defense, and mate attraction. Recently, many studies have revealed that microbial symbionts could regulate host chemical communication by affecting the synthesis and perception of insect semiochemicals. In this paper, we review recent studies of the influence of microbial symbionts on insect chemoreception. Microbial symbionts may influence insect sensitivity to semiochemicals by regulating the synthesis of odorant-binding proteins or chemosensory proteins and olfactory or gustatory receptors and regulating host neurotransmission, thereby adjusting insect behavior. The manipulation of insect chemosensory behavior by microbial symbionts is conducive to their proliferation and dispersal and provides the impetus for insects to change their feeding habits and aggregation and dispersal behavior, which contributes to population differentiation in insects. Future research is necessary to reveal the material and information exchange between both partners to improve our comprehension of the evolution of chemoreception in insects. Manipulating insect chemoreception physiology by inoculating them with microbes could be utilized as a potential approach to managing insect populations.

Impact of transgenerational host switch on gut bacterial assemblage in generalist pest, Spodoptera littoralis (Lepidoptera: Noctuidae)

Diet composition is vital in shaping gut microbial assemblage in many insects. Minimal knowledge is available about the influence of transgenerational diet transition on gut microbial community structure and function in polyphagous pests. This study investigated transgenerational diet-induced changes in Spodoptera littoralis larval gut bacteriome using 16S ribosomal sequencing. Our data revealed that 88% of bacterial populations in the S. littoralis larval gut comprise Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes. The first diet transition experiment from an artificial diet (F0) to a plant diet (F1), cabbage and cotton, caused an alteration of bacterial communities in the S. littoralis larval gut. The second transgenerational diet switch, where F1 larvae feed on the same plant in the F2 generation, displayed a significant variation suggesting further restructuring of the microbial communities in the Spodoptera larval gut. F1 larvae were also challenged with the plant diet transition at the F2 generation (cabbage to cotton or cotton to cabbage). After feeding on different plant diets, the microbial assemblage of F2 larvae pointed to considerable differences from other F2 larvae that continued on the same diet. Our results showed that S. littoralis larval gut bacteriome responds rapidly and inexplicably to different diet changes. Further experiments must be conducted to determine the developmental and ecological consequences of such changes. Nevertheless, this study improves our perception of the impact of transgenerational diet switches on the resident gut bacteriome in S. littoralis larvae and could facilitate future research to understand the importance of symbiosis in lepidopteran generalists better.

Colonization Resistance of Symbionts in Their Insect Hosts

The symbiotic microbiome is critical in promoting insect resistance against colonization by exogenous microorganisms. The mechanisms by which symbionts contribute to the host's immune capacity is referred to as colonization resistance. Symbionts can protect insects from exogenous pathogens through a variety of mechanisms, including upregulating the expression of host immune-related genes, producing antimicrobial substances, and competitively excluding pathogens. Concordantly, insects have evolved fine-tuned regulatory mechanisms to avoid overactive immune responses against symbionts or specialized cells to harbor symbionts. Alternatively, some symbionts have evolved special adaptations, such as the formation of biofilms to increase their tolerance to host immune responses. Here, we provide a review of the mechanisms about colonization resistance of symbionts in their insect hosts. Adaptations of symbionts and their insect hosts that may maintain such symbiotic relationships, and the significance of such relationships in the coevolution of symbiotic systems are also discussed to provide insights into the in-depth study of the contribution of symbionts to host physiology and behavior.

Symbiotic Bacteria Modulate Lymantria dispar Immunity by Altering Community Proportions after Infection with LdMNPV

The symbiotic bacteria-insect interaction is considered to be associated with immunity and drug resistance. However, the wide variety of insect species and habitats is thought to have a significant impact on the symbiotic community, leading to disparate results. Here, we demonstrated that symbiotic bacteria regulated the immune response by changing the proportion of the Gram-positive and the Gram-negative bacterial community in Lymantria dispar (L. dispar) after infection with its viral pathogen, L. dispar Nucleopolyhedrovirus (LdMNPV). After oral infection, the immune deficiency pathway was activated immediately, and the expression of Relish was up-regulated to promote the secretion of antimicrobial peptides. Meanwhile, the abundance of the Gram-negative bacterial community increased at the same time. Moreover, the Toll pathway was not regulated in the same way as the Imd pathway was after infection. However, the change in the Toll pathway's expression remained positively correlated to the abundance of Gram-positive bacteria. This finding implied that the ratio of Gram-negative to Gram-positive bacteria in the LdMNPV infected larvae had an effect on the immune response. Our findings revealed that the immune regulation of L. dispar was regulated by the relative abundance of its symbiotic bacteria at different infection times with LdMNPV, which provides a new way to understand symbiotic bacteria-insect interactions.

The effect of inactivation of aldehyde dehydrogenase on pheromone production by a gut bacterium of an invasive bark beetle, Dendroctonus valens

Semiochemical-based management strategies are important for controlling bark beetles, such as invasive Red Turpentine Beetle (Denroctonus valens), the causal agent for mass mortality of pine trees (Pinus spp.) in China. It has been previously shown that the pheromone verbenone regulates the attack density of this beetle in a dose-dependent manner and that the gut bacteria of D. valens are involved in verbenone production. However, molecular functional verification of the role of gut bacteria in the pheromone production of D. valens is still lacking. To better understand the molecular function of gut bacterial verbenone production, we chose a facultative anaerobic gut bacterium (Enterobacter xiangfangensis) of D. valens based on its strong ability to convert cis-verbenol to verbenone, as shown in our previous study, and investigated its transcriptomics in the presence or absence of cis-verbenol under anaerobic conditions (simulating the anoxic environment in the beetle's gut). Based on this transcriptome analysis, aldehyde dehydrogenase (ALDH1) was identified as a putative key gene responsible for verbenone production and was knocked-down by homologous recombination to obtain a mutant E. xiangfangensis strain. Our results show that these mutants had significantly decreased the ability to convert the monoterpene precursor to verbenone compared with the wild-type bacteria, indicating that ALDH1 is primarily responsible for verbenone conversion for this bacterium species. These findings provide further mechanistic evidence of bacterially mediated pheromone production by D. valens, add new perspective for functional studies of gut bacteria in general, and may aid the development of new gene silencing-based pest management strategies.

A Review of the Host Plant Location and Recognition Mechanisms of Asian Longhorn Beetle

The Asian longhorn beetle (ALB), Anoplophora glabripennis Motschulsky, is a polyphagous xylophage with dozens of reported host tree species. However, the mechanisms by which individuals locate and recognize host plants are still unknown. We summarize the current knowledge of the host plant list, host kairomones, odorant-binding proteins (OBPs) and microbial symbionts of this beetle and their practical applications, and finally discuss the host localization and recognition mechanisms. A total of 209 species (or cultivars) were reported as ALB host plants, including 101 species of higher sensitivity; host kairomones were preferentially bound to ALB recombinant OBPs, including cis-3-hexen-1-ol, δ-3-carene, nonanal, linalool, and β-caryophyllene. In addition, microbial symbionts may help ALB degrade their host. Complementarity of tree species with different levels of resistance may reduce damage, but trapping effectiveness for adults was limited using a combination of host kairomones and sex pheromones in the field. Therefore, we discuss host location behavior from a new perspective and show that multiple cues are used by ALB to locate and recognize host plants. Further research into host resistance mechanisms and visual signal recognition, and the interaction of sex pheromone synthesis, symbiont microbiota, and host plants may help reveal the host recognition mechanisms of ALBs.

The sex pheromone heptacosane enhances the mating competitiveness of sterile Aedes aegypti males

BACKGROUND

Aedes aegypti is a vector that transmits various viral diseases, including dengue and Zika. The radiation-based sterile insect technique (SIT) has a limited effect on mosquito control because of the difficulty in irradiating males without reducing their mating competitiveness. In this study, the insect sex pheromone heptacosane was applied to Ae. aegypti males to investigate whether it could enhance the mating competitiveness of irradiated males.

METHODS

Heptacosane was smeared on the abdomens of Ae. aegypti males that were allowed to mate with untreated virgin females. The insemination rate was used to assess the attractiveness of heptacosane-treated males to females. The pupae were irradiated with different doses of X-rays and γ-rays, and the emergence, survival time, egg number, and hatch rate were detected to find the optimal dose of X-ray and γ-ray radiation. The males irradiated at the optimal dose were smeared with heptacosane, released in different ratios with untreated males, and mated with females. The effect of heptacosane on the mating competitiveness of irradiated mosquitoes was then evaluated by the hatch rate, induced sterility, and mating competitiveness index.

RESULTS

Applying heptacosane to Ae. aegypti males significantly increased the insemination rate of females by 20%. Pupal radiation did not affect egg number but significantly reduced survival time and hatch rate. The emergence of the pupae was not affected by X-ray radiation but was affected by γ-ray radiation. Pupae exposed to 60 Gy X-rays and 40 Gy γ-rays were selected for subsequent experiments. After 60 Gy X-ray irradiation or 40 Gy γ-ray irradiation, the average hatch rate was less than 0.1%, and the average survival time was more than 15 days. Moreover, at the same release ratio, the hatch rate of the irradiated group perfumed with heptacosane was lower than that of the group without heptacosane. Conversely, the male sterility and male mating competitiveness index were significantly increased due to the use of heptacosane.

CONCLUSIONS

The sex pheromone heptacosane enhanced the interaction between Ae. aegypti males and females. Perfuming males irradiated by X-rays or γ-rays with heptacosane led to a significant increase in mating competitiveness. This study provided a new idea for improving the application effect of SIT.

How has horizontal gene transfer shaped the evolution of insect genomes?

As the most diverse group of animals on Earth, insects are key organisms in ecosystems. Horizontal gene transfer (HGT) refers to the transfer of genetic material between species by non-reproductive means. HGT is a major evolutionary force in prokaryotic genome evolution, but its importance in different eukaryotic groups, such as insects, has only recently begun to be understood. Genomic data from hundreds of insect species have enabled the detection of large numbers of HGT events and the elucidation of the functions of some of these foreign genes. Although quantification of the extent of HGT in insects broadens our understanding of its role in insect evolution, the scope of its influence and underlying mechanism(s) of its occurrence remain open questions for the field.

Identifying the Gut Virome of Diaphorina citri from Florida Groves

Asian citrus psyllid (Diaphorina citri) transmits the bacterial pathogen Candidatus Liberibacter asiaticus (CLas), the putative causative agent of citrus Huanglongbing disease (HLB). Insect-specific viruses can act against insects as their natural enemies, and recently, several D. citri-associated viruses were discovered. The insect gut plays an important role as not only a pool for diverse microbes but also as a physical barrier to prevent the spread of pathogens such as CLas. However, there is little evidence of the presence of D. citri-associated viruses in the gut and of the interaction between them and CLas. Here, we dissected psyllid guts collected from five growing regions in Florida, and the gut virome was analyzed by high throughput sequencing. Four insect viruses, including D. citri-associated C virus (DcACV), D. citri densovirus (DcDV), D. citri reovirus (DcRV), and D. citri flavi-like virus (DcFLV), were identified, and their presence in the gut, including an additional D. citri cimodo-like virus (DcCLV), were confirmed with PCR-based assays. Microscopic analysis showed that DcFLV infection leads to morphological abnormalities in the nuclear structure in the infected psyllid gut cells. The complex and diverse composition of microbiota in the psyllid gut suggests a possible interaction and dynamics between CLas and the D. citri-associated viruses. Our study identified various D. citri-associated viruses that localized in the psyllid gut and provided more information that helps to evaluate the potential vectors for manipulating CLas in the psyllid gut.

Volatiles of fungal cultivars act as cues for host-selection in the fungus-farming ambrosia beetle Xylosandrus germanus

Many wood-boring insects use aggregation pheromones during mass colonization of host trees. Bark beetles (Curculionidae: Scolytinae) are a model system, but much less is known about the role of semiochemicals during host selection by ambrosia beetles. As an ecological clade within the bark beetles, ambrosia beetles are obligately dependent on fungal mutualists for their sole source of nutrition. Mass colonization of trees growing in horticultural settings by exotic ambrosia beetles can occur, but aggregation cues have remained enigmatic. To elucidate this mechanism, we first characterized the fungal associates of the exotic, mass-aggregating ambrosia beetle Xylosandrus germanus in Southern Germany. Still-air olfactometer bioassays documented the attraction of X. germanus to its primary nutritional mutualist Ambrosiella grosmanniae and to a lesser extent another common fungal isolate (Acremonium sp.). During two-choice bioassays, X. germanus was preferentially attracted to branch sections (i.e., bolts) that were either pre-colonized by conspecifics or pre-inoculated with A. grosmanniae. Subsequent analyses identified microbial volatile organic compounds (MVOCs) that could potentially function as aggregation pheromones for X. germanus. To our knowledge, this is the first evidence for fungal volatiles as attractive cues during host selection by X. germanus. Adaptive benefits of responding to fungal cues associated with an infestation of conspecifics could be a function of locating a suitable substrate for cultivating fungal symbionts and/or increasing the likelihood of mating opportunities with the flightless males. However, this requires solutions for evolutionary conflict arising due to potential mixing of vertically transmitted and horizontally acquired symbiont strains, which are discussed.

Diversity of endosymbionts in camellia spiny whitefly, Aleurocanthus camelliae (Hemiptera: Aleyrodidae), estimated by 16S rRNA analysis and their biological implications

Camellia spiny whitefly, Aleurocanthus camelliae (Hemiptera: Aleyrodidae), is a major pest in tea, which poses a serious threat to tea production. Similar to many insects, various bacterial symbioses inside A. camelliae may participate in the reproduction, metabolism, and detoxification of the host. However, few reports included research on the microbial composition and influence on A. camelliae growth. We first applied high-throughput sequencing of the V4 region in the 16S rRNA of symbiotic bacteria to study its component and effect on the biological trait of A. camelliae by comparing it with the antibiotic treatment group. The population parameters, survival rate, and fecundity rate of A. camelliae were also analyzed using the age-stage two-sex life table. Our results demonstrated that phylum Proteobacteria (higher than 96.15%) dominated the whole life cycle of A. camelliae. It unveiled the presence of Candidatus Portiera (primary endosymbiont) (67.15-73.33%), Arsenophonus (5.58-22.89%), Wolbachia (4.53-11.58%), Rickettsia (0.75-2.59%), and Pseudomonas (0.99-1.88%) genus. Antibiotic treatment caused a significant decrease in the endosymbiont, which negatively affected the host's biological properties and life process. For example, 1.5% rifampicin treatment caused a longer preadult stage in the offspring generation (55.92 d) compared to the control (49.75d) and a lower survival rate (0.36) than the control (0.60). The decreased intrinsic rate of increase (r), net reproductive rate (R ₀), and prolonged mean generation time (T) were signs of all disadvantageous effects associated with symbiotic reduction. Our findings confirmed the composition and richness of symbiotic bacteria in larva and adult of A. camelliae by an Illumina NovaSeq 6000 analysis and their influence on the development of the host by demographic research. Together, the results suggested that symbiotic bacteria play an important role in manipulating the biological development of their hosts, which might help us for developing new pest control agents and technologies for better management of A. camelliae.

Wolbachia causes cytoplasmic incompatibility but not male-killing in a grain pest beetle

The endosymbiotic Wolbachia is one of the most common intracellular bacteria known in arthropods and nematodes. Its ability for reproductive manipulation can cause unequal inheritance to male and female offspring, allowing the manipulator to spread, but potentially also impact the evolutionary dynamics of infected hosts. Estimated to be present in up to 66% of insect species, little is known about the phenotypic impact of Wolbachia within the order Coleoptera. Here, we describe the reproductive manipulation by the Wolbachia strain wSur harboured by the sawtoothed grain beetle Oryzaephilus surinamensis (Coleoptera, Silvanidae), through a combination of genomics approaches and bioassays. The Wolbachia strain wSur belongs to supergroup B that contains well-described reproductive manipulators of insects and encodes a pair of cytoplasmic incompatibility factor (cif) genes, as well as multiple homologues of the WO-mediated killing (wmk) gene. A phylogenetic comparison with wmk homologues of wMel of Drosophila melanogaster identified 18 wmk copies in wSur, including one that is closely related to the wMel male-killing homologue. However, further analysis of this particular wmk gene revealed an eight-nucleotide deletion leading to a stop-codon and subsequent reading frame shift midsequence, probably rendering it nonfunctional. Concordantly, utilizing a Wolbachia-deprived O. surinamensis population and controlled mating pairs of wSur-infected and noninfected partners, we found no experimental evidence for male-killing. However, a significant ~50% reduction of hatching rates in hybrid crosses of uninfected females with infected males indicates that wSur is causing cytoplasmic incompatibility. Thus, Wolbachia also represents an important determinant of host fitness in Coleoptera.

Optimization and field evaluation of sex-pheromone of potato tuber moth, Phthorimaea operculella Zeller (Lepidoptera: Gelechiidae)

BACKGROUND

The potato tuber moth, Phthorimaea operculella is a key pest of potato and tobacco in China. We compared the sex pheromone titers released by P. operculella female, and the electroantennogram (EAG) responses of male antennae to pheromone compounds under laboratory conditions. Then, the optimal sex pheromone ratio was screened in the field.

RESULTS

The P. operculella sex pheromone feeding on potato or tobacco was extracted by solvent-extraction method. Main sex pheromone compounds including E4, Z7-13: AC (PTM1) and E4, Z7, Z10-13: AC (PTM2) were found in gland. The titer and relative ratio of P. operculella sex pheromone compounds secreted by female adult moths differ between host populations, while the relative EAG responses trend of P. operculella males to the pheromone components were the same when directly stimulated. In field trials, PTM1:PTM2 = 8:1-4:1 and PTM1:PTM2 = 1:4-1:7 was the most attractive ratio to P. operculella males in tobacco and potato fields, respectively. During 2019-2020, after 8 weeks of mass trapping for each year, the sex pheromone could reduce the population of P. operculella in the potato and tobacco fields.

CONCLUSIONS

In this study, fixed property and quantity analyzing methods were adopted to compare sex pheromones from feeding on potato versus tobacco. EAG tests then were carried out on P. operculella males. Finally, we optimized the ratio of two sex pheromone compounds in potato and examined how this influenced field trapping. By further improving the parameters of sex pheromone application in the field, we demonstrate that deployment of sex pheromones can provide effective control of P. operculella. © 2021 Society of Chemical Industry.

The Role of Insect Symbiotic Bacteria in Metabolizing Phytochemicals and Agrochemicals

Simple Summary

To counter plant chemical defenses and exposure to agrochemicals, herbivorous insects have developed several adaptive strategies to guard against the ingested detrimental substances, including enhancing detoxifying enzyme activities, avoidance behavior, amino acid mutation of target sites, and lower penetration through a thicker cuticle. Insect microbiota play important roles in many aspects of insect biology and physiology. To better understand the role of insect symbiotic bacteria in metabolizing these detrimental substances, we summarize the research progress on the function of insect bacteria in metabolizing phytochemicals and agrochemicals, and describe their future potential application in pest management and protection of beneficial insects.

Abstract

The diversity and high adaptability of insects are heavily associated with their symbiotic microbes, which include bacteria, fungi, viruses, protozoa, and archaea. These microbes play important roles in many aspects of the biology and physiology of insects, such as helping the host insects with food digestion, nutrition absorption, strengthening immunity and confronting plant defenses. To maintain normal development and population reproduction, herbivorous insects have developed strategies to detoxify the substances to which they may be exposed in the living habitat, such as the detoxifying enzymes carboxylesterase, glutathione-S-transferases (GSTs), and cytochrome P450 monooxygenases (CYP450s). Additionally, insect symbiotic bacteria can act as an important factor to modulate the adaptability of insects to the exposed detrimental substances. This review summarizes the current research progress on the role of insect symbiotic bacteria in metabolizing phytochemicals and agrochemicals (insecticides and herbicides). Given the importance of insect microbiota, more functional symbiotic bacteria that modulate the adaptability of insects to the detrimental substances to which they are exposed should be identified, and the underlying mechanisms should also be further studied, facilitating the development of microbial-resource-based pest control approaches or protective methods for beneficial insects.

Bacterial Isolates Derived from Nest Soil Affect the Attraction and Digging Behavior of Workers of the Red Imported Fire Ant, Solenopsis invicta Buren

Simple Summary

Populations of the red imported fire ant (Solenopsis invicta) are found throughout the southern United States. Because these invasive ants sting and are highly territorial, they are hazardous to people and livestock and are detrimental to native ant populations. Control of this species generally relies on insecticidal baits that attract and kill the ant. The aim of our study was to determine if bacteria cultured from S. invicta nest soils affected worker ant behaviors and whether the bacteria were attractive or repellent to the ants. Bacterial isolates cultured from nest soils were used in binary choice bioassays that tested for effects of bacterial species and bacterial concentrations on worker ant digging and residing preferences. Arthrobacter woluwensis (Actinobacteria) attracted worker ants while bacteria identified as Firmicutes generally repelled ants. This study provides a basis for the identification of new biologically derived compounds that can be used to alter behaviors of the red imported fire ant and be implemented in novel control strategies.

Abstract

Populations of monogyne and polygyne red imported fire ants (RIFA), Solenopsis invicta Buren, are distributed throughout the southern United States. This ant species is hazardous to farm animals and workers, damages infrastructure, and depletes native arthropod populations. Colony expansion is affected by several biotic factors, but the effects of soil microbes on ant behavior related to soil excavation within nest sites have not been investigated. Consequently, we cultured bacteria from RIFA nest soils. The effects of individual bacterial isolates and bacterial cell densities on the choice of digging site as well as digging activity of monogyne and polygyne RIFA worker ants were evaluated in two-choice bioassays. Based on phylogenetic analysis, 17 isolates were selected and tested initially at 5 × 10⁸ cells/mL and 20 workers per assay. Firmicutes (Bacillus, Paenibacillus, Brevibacillus) repelled the ants, but Arthrobacter woluwensis strongly attracted ants. Subsequently, the six isolates having the greatest positive or negative effects on ant behavior were evaluated at a lower bacterial cell and worker ant densities. Ant responses to these bacteria generally decreased as cell densities declined to 5 × 10⁶ cells/mL. Observations of ant behavior during a three-hour, two-choice bioassay revealed that ants generally visited both control and bacteria-treated sand prior to making a digging site choice. Our research results indicate that soil bacteria may mediate ant nest expansion or relocation and foraging tunnel construction. Identification of bacterial metabolites that affect RIFA digging behavior merits additional research because these compounds may provide a basis for novel management strategies that repel RIFA away from sensitive infrastructure or attract fire ants to insecticidal baits.

Cu-Catalyzed Dual C-O Bonds Cleavage of Cyclic Ethers with Carboxylic Acids, NaI, and TMSCF3 to Give Iodoalkyl Ester

Herein, by dual C-O bond cleavage of cyclic ethers with Cu catalysis, we eventually led to the development of a selective three-component coupling of commercially available chemicals, carboxylic acids, ethers, and halogens to synthesize more than 70 iodoalkyl esters in the presence of TMSCF₃. This allows for the concise synthesis of highly functionalized iodoalkyl esters directly. And the synthetic insect pheromones were also disclosed.

A review of physiological resistance to insecticide stress in Nilaparvata lugens

Insecticides are widely used in agriculture as effective means to control pests. However, pests have not been completely mitigated with the increased use of insecticides. Instead, many side effects have arisen, especially the '3Rs' (resistance, resurgence, and residue). The brown planthopper, Nilaparvata lugens, is one of the most threatening rice pests. The main insecticides for controlling N. lugens belong to organochlorine, organophosphorus, carbamate, neonicotinoid and pyrethroid groups. However, metabolic enzymes, including cytochrome P450s, esterases, glutathione-S-transferases, and ATP-binding cassette transporters, effectively promote the detoxification of insecticides. Besides, mutations of neurological target sites, such as acetylcholinesterase, nicotinic acetylcholine, γ-aminobutyric acid receptor, and ryanodine receptor, result in insensitivity to insecticides. Here, we review the physiological metabolic resistance in N. lugens under insecticide stress to provide a theoretical basis for identifying and developing more effective and harmless insecticides.

The microbiome of wild and mass-reared new world screwworm, Cochliomyia hominivorax

Insect population control through continual releases of large numbers of sterile insects, called sterile insect technique (SIT), is only possible if one can mass-rear large quantities of healthy insects. Adaptation of insect stocks to rearing conditions and artificial feeding systems can have a multitude of negative effects such as inbreeding depression, reduced compatibility with wild strains, unintentional selection for traits that lower fitness after release, and an altered microbiome. Changes to insect microbiomes can have many effects on insects ranging from a reduction in sex pheromones or reduced fitness. Thus understanding these systems is important for mass rearing and the performance of the sterile insect control programs. In this study we explored the microbiome of the New World screwworm, Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae) an economically important parasite of warm-blooded animals. Samples from myiases in cows and wild adults were compared to and mass-reared flies used by the SIT program. Significant differences were observed between these treatments, with wild captured flies having a significantly more diverse microbial composition. Bacteria known to stimulate oviposition were found in both wild and mass-reared flies. Two bacteria of veterinary importance were abundant in wild flies, suggesting screwworm is a potential vector of these diseases. Overall, this study provides the screwworm eradication program a platform to continue exploring the effects associated bacteria have on screwworm fitness.

Characterization of the reproductive tract bacterial microbiota of virgin, mated, and blood-fed Aedes aegypti and Aedes albopictus females

Background

Aedes aegypti and Ae. albopictus are vectors of numerous arboviruses that adversely affect human health. In mosquito vectors of disease, the bacterial microbiota influence several physiological processes, including fertility and vector competence, making manipulation of the bacterial community a promising method to control mosquito vectors. In this study, we describe the reproductive tract tissue microbiota of lab-reared virgin Ae. aegypti and Ae. albopictus males, and virgin, mated, and mated + blood-fed females of each species, comparing the bacterial composition found there to the well-described gut microbiota.

Methods

We performed metabarcoding of the 16S rRNA isolated from the gut, upper reproductive tract (URT; testes or ovaries), and lower reproductive tract (LRT; males: seminal vesicles and accessory glands; females: oviduct, spermathecae, and bursa) for each species, and evaluated the influence of host species, tissue, nutritional status, and reproductive status on microbiota composition. Finally, based on the identified taxonomic profiles of the tissues assessed, bacterial metabolic pathway abundance was predicted.

Results

The community structure of the reproductive tract is unique compared to the gut. Asaia is the most prevalent OTU in the LRTs of both Ae. aegypti and Ae. albopictus. In the URT, we observed differences between species, with Wolbachia OTUs being dominant in the Ae. albopictus URT, while Enterobacter and Serratia were dominant in Ae. aegypti URT. Host species and tissue were the best predictors of the community composition compared to reproductive status (i.e., virgin or mated) and nutritional status (i.e., sugar or blood-fed). The predicted functional profile shows changes in the abundance of specific microbial pathways that are associated with mating and blood-feeding, like energy production in mated tissues and siderophore synthesis in blood-fed female tissues.

Conclusions

Aedes aegypti and Ae. albopictus have distinct differences in the composition of microbiota found in the reproductive tract. The distribution of the bacterial taxonomic groups indicates that some bacteria have tissue-specific tropism for reproductive tract tissue, such as Asaia and Wolbachia. No significant differences in the taxonomic composition were observed in the reproductive tract between virgin, mated, and mated + blood-fed females, but changes in the abundance of specific metabolic pathways were found in the predicted microbial functional profiles in mated and blood-fed females.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-05093-7.

Biosynthesis, evolution and ecology of microbial terpenoids

Covering: through June 2021Terpenoids are the largest class of natural products recognised to date. While mostly known to humans as bioactive plant metabolites and part of essential oils, structurally diverse terpenoids are increasingly reported to be produced by microorganisms. For many of the compounds biological functions are yet unknown, but during the past years significant insights have been obtained for the role of terpenoids in microbial chemical ecology. Their functions include stress alleviation, maintenance of cell membrane integrity, photoprotection, attraction or repulsion of organisms, host growth promotion and defense. In this review we discuss the current knowledge of the biosynthesis and evolution of microbial terpenoids, and their ecological and biological roles in aquatic and terrestrial environments. Perspectives on their biotechnological applications, knowledge gaps and questions for future studies are discussed.

Regulation of host phenotypic plasticity by gut symbiont communities in the eastern subterranean termite (Reticulitermes flavipes)

Termites are eusocial insects that host a range of prokaryotic and eukaryotic gut symbionts and can differentiate into a range of caste phenotypes. Soldier caste differentiation from termite workers follows two successive molts (worker-presoldier-soldier) that are driven at the endocrine level by juvenile hormone (JH). Although physiological and eusocial mechanisms tied to JH signaling have been studied, the role of gut symbionts in the caste differentiation process is poorly understood. Here, we used the JH analog methoprene in combination with the antibiotic kanamycin to manipulate caste differentiation and gut bacterial loads in Reticulitermes flavipes termites via four bioassay treatments: kanamycin, methoprene, kanamycin+methoprene, and an untreated (negative) control. Bioassay results demonstrated a significantly higher number of presoldiers in the methoprene treatment, highest mortality in kanamycin+methoprene treatment, and significantly reduced protist numbers in all treatments except the untreated control. Bacterial 16S rRNA gene sequencing provided alpha and beta diversity results that mirrored bioassay findings. From ANCOM analysis, we found that several bacterial genera were differentially abundant among treatments. Finally, follow-up experiments showed that if methoprene and kanamycin or untreated termites are placed together, zero or rescued presoldier initiation, respectively, occurs. These findings reveal that endogenous JH selects for symbiont compositions required to successfully complete presoldier differentiation. However, if the gut is voided before the influx of JH, it cannot select for the necessary symbionts that are crucial for molting. Based on these results, we are able to provide a novel example of linkages between gut microbial communities and host phenotypic plasticity.

Cuticular Hydrocarbon Trails Released by Host Larvae Lose their Kairomonal Activity for Parasitoids by Solidification

Successful host search by parasitic wasps is often mediated by host-associated chemical cues. The ectoparasitoid Holepyris sylvanidis is known to follow chemical trails released by host larvae of the confused flour beetle, Tribolium confusum, for short-range host location. Although the hexane-extractable trails consist of stable, long-chain cuticular hydrocarbons (CHCs) with low volatility, the kairomonal activity of a trail is lost two days after release. Here, we studied whether this loss of kairomonal activity is due to changes in the chemical trail composition induced by microbial activity. We chemically analyzed trails consisting of hexane extracts of T. confusum larvae after different time intervals past deposition under sterile and non-sterile conditions. GC-MS analyses revealed that the qualitative and quantitative pattern of the long-chain CHCs of larval trails did not significantly change over time, neither under non-sterile nor sterile conditions. Hence, our results show that the loss of kairomonal activity of host trails is not due to microbially induced changes of the CHC pattern of a trail. Interestingly, the kairomonal activity of trails consisting of host larval CHC extracts was recoverable after two days by applying hexane to them. After hexane evaporation, the parasitoids followed the reactivated host trails as they followed freshly laid ones. Cryo-scanning electron microscopy showed that the trails gradually formed filament-shaped microstructures within two days. This self-assemblage of CHCs was reversible by hexane application. Our study suggests that the long-chain CHCs of a host trail slowly undergo solidification by a self-assembling process, which reduces the accessibility of CHCs to the parasitoid’s receptors as such that the trail is no longer eliciting trail-following behavior.

On the move: sloths and their epibionts as model mobile ecosystems

Sloths are unusual mobile ecosystems, containing a high diversity of epibionts living and growing in their fur as they climb slowly through the canopies of tropical forests. These epibionts include poorly studied algae, arthropods, fungi, and bacteria, making sloths likely reservoirs of unexplored biodiversity. This review aims to identify gaps and eliminate misconceptions in our knowledge of sloths and their epibionts, and to identify key questions to stimulate future research into the functions and roles of sloths within a broader ecological and evolutionary context. This review also seeks to position the sloth fur ecosystem as a model for addressing fundamental questions in metacommunity and movement ecology. The conceptual and evidence-based foundation of this review aims to serve as a guide for future hypothesis-driven research into sloths, their microbiota, sloth health and conservation, and the coevolution of symbioses in general.

Diversity and Functions of Yeast Communities Associated with Insects

Following the concept of the holobiont, insect-microbiota interactions play an important role in insect biology. Many examples of host-associated microorganisms have been reported to drastically influence insect biological processes such as development, physiology, nutrition, survival, immunity, or even vector competence. While a huge number of studies on insect-associated microbiota have focused on bacteria, other microbial partners including fungi have been comparatively neglected. Yeasts, which establish mostly commensal or symbiotic relationships with their host, can dominate the mycobiota of certain insects. This review presents key advances and progress in the research field highlighting the diversity of yeast communities associated with insects, as well as their impact on insect life-history traits, immunity, and behavior.

Structure and Dynamics of the Gut Bacterial Community Across the Developmental Stages of the Coffee Berry Borer, Hypothenemus hampei

The coffee berry borer (CBB); Hypothenemus hampei (Coleoptera: Curculionidae), is widely recognized as the major insect pest of coffee crops. Like many other arthropods, CBB harbors numerous bacteria species that may have important physiological roles in host nutrition, detoxification, immunity and protection. To date, the structure and dynamics of the gut-associated bacterial community across the CBB life cycle is not yet well understood. A better understanding of the complex relationship between CBB and its bacterial companions may provide new opportunities for insect control. In the current investigation, we analyzed the diversity and abundance of gut microbiota across the CBB developmental stages under field conditions by using high-throughput Illumina sequencing of the 16S ribosomal RNA gene. Overall, 15 bacterial phyla, 38 classes, 61 orders, 101 families and 177 genera were identified across all life stages, including egg, larva 1, larva 2, pupa, and adults (female and male). Proteobacteria and Firmicutes phyla dominated the microbiota along the entire insect life cycle. Among the 177 genera, the 10 most abundant were members of Ochrobactrum (15.1%), Pantoea (6.6%), Erwinia (5.7%), Lactobacillus (4.3%), Acinetobacter (3.4%), Stenotrophomonas (3.1%), Akkermansia (3.0%), Agrobacterium (2.9%), Curtobacterium (2.7%), and Clostridium (2.7%). We found that the overall bacterial composition is diverse, variable within each life stage and appears to vary across development. About 20% of the identified OTUs were shared across all life stages, from which 28 OTUs were consistently found in all life stage replicates. Among these OTUs there are members of genera Pantoea, Erwinia, Agrobacterium, Ochrobactrum, Pseudomonas, Acinetobacter, Brachybacterium, Sphingomonas and Methylobacterium, which can be considered as the gut-associated core microbiota of H. hampei. Our findings bring additional data to enrich the understanding of gut microbiota in CBB and its possible use for development of insect control strategies.