Floral nectar and honeydew microbial diversity and their role in biocontrol of insect pests and pollination

Impact of Nectar Composition and Nectar Yeasts on Volatile Emissions and Parasitoid Behavior

Nectar yeasts can significantly influence the scent of floral nectar and therefore the foraging behavior of flower-visiting insects. While these effects likely depend on nectar chemistry and yeast species, their joint impact on nectar volatile profiles and associated insect responses remain poorly understood. Here, we used four synthetic nectar types varying in sugar and amino acid concentration and two specialist nectar yeasts (Metschnikowia gruessii and Metschnikowia reukaufii) to investigate how nectar composition and yeast species affect volatile profiles and the olfactory responses of the generalist aphid parasitoid Aphidius ervi. Olfactometer assays showed that A. ervi females significantly preferred fermented nectars with high amino acid-low sugar content (HL) and low amino acid-high sugar (LH) content, regardless being fermented by M. gruessii or M. reukaufii, over non-inoculated nectars. This effect was not observed for nectars with low amino acid-low sugar (LL) and high amino acid-high sugar (HH) content. Moreover, LL nectar fermented with M. gruessii became even repellent to the parasitoids. GC-MS analysis of volatile organic compounds (VOCs) revealed that VOC profiles of fermented nectars depended significantly on nectar type (i.e., chemical composition), yeast species, and their interaction. Whereas propyl acetate, isobutyl acetate, styrene, α-guaiene and pentyl-octanoate were associated with the LH fermented nectars, ethyl acetate and E-methyl isoeugenol were mainly associated with the HL fermented nectars, suggesting possible involvement in A. ervi attraction to these nectars. In contrast, isopropyl-hexadecanoate was associated with the non-attractive or repellent LL fermented nectars. Altogether, our results indicate that nectar composition has a strong impact on nectar scent when fermented by specialist nectar yeasts and subsequently on insect foraging behavior.

Specialized metabolites present in Camellia reticulata nectar inhibit the growth of nectar-inhabiting microorganisms

Plant specialized metabolites are species-specific compounds that help plants adapt and survive in constantly changing ecological environments. Nectar contains various specialized metabolites, essential for maintaining nectar homeostasis. In this study, we employed high-performance liquid chromatography (HPLC) to compare the sugar composition between spoilage nectar and natural nectar, with further analysis of variations in color, odor, pH, and hydrogen peroxide (H₂O₂) content. Microbial strains in Camellia reticulata nectar were isolated and identified using the spread plate method coupled with DNA sequencing. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was implemented to characterize metabolite differences between spoilage and natural nectars. Subsequent in vitro experiments were conducted to validate the effects of screened nectar metabolites on the isolated microbial strains. The results showed that some C. reticulata nectar could spoil and deteriorate, which disrupted nectar homeostasis and significantly reduced the pollination efficiency by pollinators. Spoilage nectar had significant differences in color, odor, sugar composition, pH, and H2O2 content compared to natural nectar. The number of microbial species and quantity in spoilage nectar were much higher. The H2O2 content in natural nectar could reach (55.5 ± 1.80) μM, while it was undetectable in spoilage nectar. A total of 15 distinct microbial strains and 364 differential metabolites were isolated and identified from two types of nectar. In vitro experiments demonstrated that H2O2 could inhibit all the bacteria in C. reticulata nectar except Serratia liquefaciens. 12-Methyltetradecanoic Acid inhibited Bacillus subtilis, Curtobacterium flaccumfaciens, and Rothia terrae, and Myristic Acid only inhibited Rothia terrae. The nectar metabolites screened in this study had no effect on the nectar specialist yeast Metschnikowia reukaufii. In conclusion, the findings of this study revealed that C. reticulata nectar regulates the growth of microorganisms through its metabolites to maintain nectar homeostasis and prevent spoilage. This study improves the understanding of the physiological mechanisms of C. reticulata in maintaining nectar homeostasis and provides theoretical support for controlling nectar diseases and sustaining the reproductive fitness of C. reticulata. Future research could focus on further exploring the complex interactions between different metabolites in C. reticulata nectar and a wider range of microorganisms. Moreover, the development of practical applications based on these findings, such as the development of natural preservatives for nectar-related products or the optimization of pollination efficiency in C. reticulata cultivation, could be an important area for future exploration.

Tiny but mighty? Overview of a decade of research on nectar bacteria

An emerging focus of research at the intersection of botany, zoology, and microbiology is the study of floral nectar as a microbial habitat, referred to as the nectar microbiome, which can alter plant-pollinator interactions. Studies on these microbial communities have primarily focused on yeasts, and it was only about a decade ago that bacteria began to be studied as widespread inhabitants of floral nectar. This review aims to give an overview of the current knowledge on nectar bacteria, with emphasis on evolutionary origin, dispersal mode, effects on nectar chemistry and plant-animal interactions, community assembly, agricultural applications, and their use as model systems in ecological research. We further outline gaps in our understanding of the ecological significance of these microorganisms, their response to environmental changes, and the potential cascading effects.

Identifying mosquito plant hosts from ingested nectar secondary metabolites

Establishing how plants contribute food and refuge to insects can be challenging for small species that are difficult to observe in their natural habitat, such as disease vectoring mosquitoes. Currently indirect methods of plant-host identification rely on DNA sequencing of ingested plant material but are often unsuccessful for small insects that feed primarily on plant sugars or have little contact with plant cells. Here we developed an innovative approach to determine species-specific phytophagy by detecting taxon-specific plant secondary metabolites (PSMs) in nectar. Two mosquito species were exposed to three PSMs, each present in the nectar of a known plant host, firstly from dosed sucrose solutions and secondly from flowers. Both experiments yielded high rates of PSM detection in mosquitoes using liquid chromatography-mass spectrometry (LC-MS). PSMs were consistently detected in mosquitoes up to 8 h post-ingestion. In experiments consisting of two or three plant species, multiple PSMs from different host plants could be detected. These positive results demonstrate that PSMs could be useful indicators of insect plant-hosts selection in the wild. With expanded knowledge of nectar-based PSMs across a landscape, improved knowledge of plant-host relationships could be achieved where direct observations in their natural habitat are lacking. Increasing understanding of vector insect ecology will have an important role in tackling vector-borne disease.

Floral Resource Integration: Enhancing Biocontrol of Tuta absoluta Within Sustainable IPM Frameworks

The tomato leaf miner, Tuta absoluta, is a pest threatening global tomato production. This pest's adaptability and resistance to chemical insecticides have necessitated integrated pest management (IPM) strategies prioritizing sustainable alternatives. This review explores the role of biological control agents (BCAs) in managing T. absoluta populations, emphasizing the integration of floral resources to enhance their efficacy. Predatory mirids such as Macrolophus pygmaeus and Nesidiocoris tenuis and parasitoids such as N. artynes and Trichogramma spp. are pivotal in pest suppression; however, their performance depends on nutritional and habitat support. Floral resources provide essential sugars and proteins, improving the longevity, fecundity, and predation efficiency of these BCAs. This review synthesizes case studies highlighting the benefits of selected flowering plants, such as Lobularia maritima and Fagopyrum esculentum, in supporting predator and parasitoid populations while minimizing advantages to T. absoluta. Mechanisms such as nectar quality, floral accessibility, and spatial-temporal resource availability are explored in detail. Additionally, the challenges of selective floral attraction, microbial impacts on nectar composition, and the unintended support of non-target organisms are discussed. This review proposes targeted floral management strategies to optimize BCA performance within IPM systems by integrating ecological and chemical insights. This approach offers a pathway toward reducing chemical pesticide reliance, fostering sustainable agriculture, and mitigating the economic impacts of T. absoluta infestations.

Demethylation Inhibitor Fungicides Have a Significantly Detrimental Impact on Population Growth and Composition of Nectar Microbial Communities

Demethylation inhibitor (DMI) fungicides are a mainstay of modern agriculture due to their widespread use for crop protection against plant-pathogenic fungi. However, DMI residues can disperse and persist in the environment, potentially affecting non-target fungi. Previous research has demonstrated that DMIs and other fungicides inhibit yeast growth in floral nectar microbial communities and decrease fungal richness and diversity of exposed flowers with no apparent effect on bacteria. Nevertheless, the effect of DMIs on the population growth of different species of nectar inhabitants and the dynamics of these microbial communities remains understudied. To address these issues, in this study we created synthetic microbial communities including yeasts (Metschnikowia reukaufii and Metschnikowia pulcherrima) and bacteria (Rosenbergiella epipactidis and Comamonas sp.) and propagated them in culture media containing different DMIs (imazalil, propiconazole, and prothioconazole) at different doses or no fungicide. Our results showed that DMIs have a significant impact on some of the most common microbial inhabitants of floral nectar by favoring the growth of bacteria over yeasts. Furthermore, habitat generalists such as M. pulcherrima and Comamonas sp. were more impacted by the presence of fungicides than the nectar specialists M. reukaufii and R. epipactidis, especially upon dispersal across habitat patches. Future research should determine if the patterns observed in the present study hold true for other species of nectar microbes and explore the interaction between growth limitation due to fungicide presence, dispersal limitation, and other mechanisms involved in community assembly in floral nectar.