Bacteria may contribute to distant species recognition in ant-aphid mutualistic relationships

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.

Semiochemicals Produced by Microbes in Mealybug Honeydew Attract Fire Ants

Semiochemicals produced by microbes in hemipteran honeydew play an important role in mediating the ant-hemipteran interaction. However, the specific volatile compounds and their origins remain unclear. Here, we found that honeydew produced by Phenacoccus solenopsis exhibited strong attractiveness to fire ant workers, but sterilized honeydew was much less attractive. Four compounds were identified from the honeydew volatiles, including 1-octen-3-ol, limonene, phenylethyl alcohol, and 2,4-ditert-butylphenol. All the identified compounds triggered strong electroantennography response of fire ants and showed repellence at relatively high concentrations while attractiveness at low concentrations to fire ant workers. Furthermore, six bacterial isolates were identified based on 16S rRNA sequencing, namely, Bacillus, Brachybacterium, Kocuria, Microbacterium, Pseudomonas, and Staphylococcus. Fire ants exhibited a strong preference for Brachybacterium, Kocuria, and Microbacterium, suggesting that these bacterial isolates are involved in the attracting effect of honeydew. Both limonene and phenylethyl alcohol were detected from Brachybacterium, while limonene was only detected from Kocuria, and phenylethyl alcohol was exclusively detected from Microbacterium. Reinoculation of these bacteria restored the attractiveness of honeydew to fire ants, and the active compounds, limonene and phenylethyl alcohol, were detectable in bacteria-reinoculated honeydew. Collectively, our results reveal the active compounds in hemipteran honeydew and their association with honeydew bacteria. The findings will contribute to the development of novel attractants for efficient monitoring of fire ants.

Symbiotic Bacteria Regulating Insect-Insect/Fungus/Virus Mutualism

Bacteria associated with insects potentially provide many beneficial services and have been well documented. Mutualism that relates to insects is widespread in ecosystems. However, the interrelation between "symbiotic bacteria" and "mutualism" has rarely been studied. We introduce three systems of mutualism that relate to insects (ants and honeydew-producing Hemiptera, fungus-growing insects and fungi, and plant persistent viruses and vector insects) and review the species of symbiotic bacteria in host insects, as well as their functions in host insects and the mechanisms underlying mutualism regulation. A deeper understanding of the molecular mechanisms and role of symbiotic bacteria, based on metagenomics, transcriptomics, proteomics, metabolomics, and microbiology, will be required for describing the entire interaction network.

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.

Honeydew Is a Food Source and a Contact Kairomone for Aphelinus mali

Many parasitoids need to feed on sugar sources at the adult stage. Although nectar has been proven to be a source of higher nutritional quality compared to honeydew excreted by phloem feeders, the latter can provide the necessary carbohydrates for parasitoids and increase their longevity, fecundity and host searching time. Honeydew is not only a trophic resource for parasitoids, but it can also constitute an olfactory stimulus involved in host searching. In this study, we combined longevity measurements in the laboratory, olfactometry and feeding history inference of individuals caught in the field to test the hypothesis that honeydew excreted by the aphid Eriosoma lanigerum could serve as a trophic resource for its parasitoid Aphelinus mali as well as a kairomone used by the parasitoid to discover its hosts. Results indicate that honeydew increased longevity of A. mali females if water was provided. Water could be necessary to feed on this food source because of its viscosity and its coating by wax. The presence of honeydew allowed longer stinging events by A. mali on E. lanigerum. However, no preference towards honeydew was observed, when given the choice. The role of honeydew excreted by E. lanigerum on A. mali feeding and searching behavior to increase its efficiency as a biological control agent is discussed.

Insect-Microorganism Interaction Has Implicates on Insect Olfactory Systems

Olfaction plays an essential role in various insect behaviors, including habitat selection, access to food, avoidance of predators, inter-species communication, aggregation, and reproduction. The olfactory process involves integrating multiple signals from external conditions and internal physiological states, including living environments, age, physiological conditions, and circadian rhythms. As microorganisms and insects form tight interactions, the behaviors of insects are constantly challenged by versatile microorganisms via olfactory cues. To better understand the microbial influences on insect behaviors via olfactory cues, this paper summarizes three different ways in which microorganisms modulate insect behaviors. Here, we deciphered three interesting aspects of microorganisms-contributed olfaction: (1) How do volatiles emitted by microorganisms affect the behaviors of insects? (2) How do microorganisms reshape the behaviors of insects by inducing changes in the synthesis of host volatiles? (3) How do symbiotic microorganisms act on insects by modulating behaviors?

Evolution of chemical interactions between ants and their mutualist partners

Mutualism is the reciprocal exploitation of interacting participants and is vulnerable to nonrewarding cheating. Ants are dominant insects in most terrestrial ecosystems, and some aphids and lycaenid butterfly species provide them with nutritional nectar rewards and employ ants as bodyguards. In this review, I discuss how chemical communication based on condition-dependent signaling and recognition plasticity regulate the payoff of interacting participants. I argue that the selfishness of both participants explains the signaling and communication among participants and contributes to the stability of these mutualisms. Uncovering the origin and maintenance of mutualistic association of ants will come from future research on ant collective behavior, the genetic and neural basis of cooperation, and a deeper understanding of the costs and benefits of these interactions.

Chemical communication in ant-hemipteran mutualism: potential implications for ant invasions

Ant-hemipteran mutualism is one of the most frequently observed food-for-protection associations in nature, and is recently found to contribute to the invasions of several of the most destructive invasive ants. Chemical communication underlies establishment and maintenance of such associations, in which a multitude of semiochemicals, such as pheromones, cuticular hydrocarbons, honeydew sugars and bacteria-produced honeydew volatiles mediate location, recognition, selection, learning of mutualistic partners. Here, we review what is known about the chemical communication between ants and honeydew-producing hemipterans, and discuss how invasive ants can rapidly recognize and establish a mutualistic relationship with the hemipterans with which they have never coevolved. We also highlight some future directions for a clearer understanding of the chemical communication in ant-hemipteran mutualism and its role in ant invasions.

Identification and application of bacterial volatiles to attract a generalist aphid parasitoid: from laboratory to greenhouse assays

BACKGROUND

Recent studies have shown that microorganisms emit volatile compounds that affect insect behaviour. However, it remains largely unclear whether microbes can be exploited as a source of attractants to improve biological control of insect pests. In this study, we used a combination of coupled gas chromatography-electroantennography (GC-EAG) and Y-tube olfactometer bioassays to identify attractive compounds in the volatile extracts of three bacterial strains that are associated with the habitat of the generalist aphid parasitoid Aphidius colemani, and to create mixtures of synthetic compounds to find attractive blends for A. colemani. Subsequently, the most attractive blend was evaluated in two-choice cage experiments under greenhouse conditions.

RESULTS

GC-EAG analysis revealed 20 compounds that were linked to behaviourally attractive bacterial strains. A mixture of two EAG-active compounds, styrene and benzaldehyde applied at a respective dose of 1 μg and 10 ng, was more attractive than the single compounds or the culture medium of the bacteria in Y-tube olfactometer bioassays. Application of this synthetic mixture under greenhouse conditions resulted in significant attraction of the parasitoids, and outperformed application of the bacterial culture medium.

CONCLUSION

Compounds isolated from bacterial blends were capable of attracting parasitoids both in laboratory and greenhouse assays, indicating that microbial cultures are an effective source of insect attractants. This opens new opportunities to attract and retain natural enemies of pest species and to enhance biological pest control.

Microbial volatile organic compounds in intra-kingdom and inter-kingdom interactions

Microorganisms produce and excrete a versatile array of metabolites with different physico-chemical properties and biological activities. However, the ability of microorganisms to release volatile compounds has only attracted research attention in the past decade. Recent research has revealed that microbial volatiles are chemically very diverse and have important roles in distant interactions and communication. Microbial volatiles can diffuse fast in both gas and water phases, and thus can mediate swift chemical interactions. As well as constitutively emitted volatiles, microorganisms can emit induced volatiles that are triggered by biological interactions or environmental cues. In this Review, we highlight recent discoveries concerning microbial volatile compounds and their roles in intra-kingdom microbial interactions and inter-kingdom interactions with plants and insects. Furthermore, we indicate the potential biotechnological applications of microbial volatiles and discuss challenges and perspectives in this emerging research field.

Bacterial Semiochemicals and Transkingdom Interactions with Insects and Plants

A peculiar feature of all living beings is their capability to communicate. With the discovery of the quorum sensing phenomenon in bioluminescent bacteria in the late 1960s, it became clear that intraspecies and interspecies communications and social behaviors also occur in simple microorganisms such as bacteria. However, at that time, it was difficult to imagine how such small organisms-invisible to the naked eye-could influence the behavior and wellbeing of the larger, more complex and visible organisms they colonize. Now that we know this information, the challenge is to identify the myriad of bacterial chemical signals and communication networks that regulate the life of what can be defined, in a whole, as a meta-organism. In this review, we described the transkingdom crosstalk between bacteria, insects, and plants from an ecological perspective, providing some paradigmatic examples. Second, we reviewed what is known about the genetic and biochemical bases of the bacterial chemical communication with other organisms and how explore the semiochemical potential of a bacterium can be explored. Finally, we illustrated how bacterial semiochemicals managing the transkingdom communication may be exploited from a biotechnological point of view.

Harnessing Insect-Microbe Chemical Communications To Control Insect Pests of Agricultural Systems

Insect pests cause serious economic, yield, and food safety problems to managed crops worldwide. Compounding these problems, insect pests often vector pathogenic or toxigenic microbes to plants. Previous work has considered plant-insect and plant-microbe interactions separately. Although insects are well-understood to use plant volatiles to locate hosts, microorganisms can produce distinct and abundant volatile compounds that in some cases strongly attract insects. In this paper, we focus on the microbial contribution to plant volatile blends, highlighting the compounds emitted and the potential for variation in microbial emission. We suggest that these aspects of microbial volatile emission may make these compounds ideal for use in agricultural applications, as they may be more specific or enhance methods currently used in insect control or monitoring. Our survey of microbial volatiles in insect-plant interactions suggests that these emissions not only signal host suitability but may indicate a distinctive time frame for optimal conditions for both insect and microbe. Exploitation of these host-specific microbe semiochemicals may provide important microbe- and host-based attractants and a basis for future plant-insect-microbe chemical ecology investigations.