Gut symbiotic bacteria are involved in nitrogen recycling in the tephritid fruit fly Bactrocera dorsalis

Insects and microbes: best friends from the nursery

Insects host microbes and interact with them throughout their life cycle. This microbiota is an important, if not essential, partner participating in many aspects of insect physiology. Recent omics studies have contributed to considerable advances in the current understanding of the molecular implications of microbiota during insect development. In this review, we present an overview of the current knowledge about the mechanisms underlying interactions between developing insects and their microbial companions. The microbiota is implicated in nutrition, both via compensating for metabolic pathways lacking in the host and via regulating host metabolism. Furthermore, the microbiota plays a protective role, enhancing the insect's tolerance to, or resistance against, various environmental stresses.

Two horizontally acquired bacterial genes steer the exceptionally efficient and flexible nitrogenous waste cycling in whiteflies

Nitrogen is an essential element for all life on earth. Nitrogen metabolism, including excretion, is essential for growth, development, and survival of plants and animals alike. Several nitrogen metabolic processes have been described, but the underlying molecular mechanisms are unclear. Here, we reveal a unique process of nitrogen metabolism in the whitefly Bemisia tabaci, a global pest. We show that it has acquired two bacterial uricolytic enzyme genes, B. tabaci urea carboxylase (BtUCA) and B. tabaci allophanate hydrolase (BtAtzF), through horizontal gene transfer. These genes operate in conjunction to not only coordinate an efficient way of metabolizing nitrogenous waste but also control B. tabaci's exceptionally flexible nitrogen recycling capacity. Its efficient nitrogen processing explains how this important pest can feed on a vast spectrum of plants. This finding provides insight into how the hijacking of microbial genes has allowed whiteflies to develop a highly economic and stable nitrogen metabolism network and offers clues for pest management strategies.

The Global Epidemic of Bactrocera Pests: Mixed-Species Invasions and Risk Assessment

Throughout the past century, the global spread of Bactrocera pests has continued to pose a significant threat to the commercial fruit and vegetable industry, resulting in substantial costs associated with both control measures and quarantine restrictions. The increasing volume of transcontinental trade has contributed to an escalating rate of Bactrocera pest introductions to new regions. To address the worldwide threat posed by this group of pests, we first provide an overview of Bactrocera. We then describe the global epidemic, including border interceptions, species diagnosis, population genetics, geographical expansion, and invasion tracing of Bactrocera pests. We further consider the literature concerning the invasion co-occurrences, life-history flexibility, risk assessment, bridgehead effects, and ongoing implications of invasion recurrences, as well as a case study of Bactrocera invasions of California. Finally, we call for global collaboration to effectively monitor, prevent, and control the ongoing spread of Bactrocera pests and to share experience and knowledge to combat it.

Gut microbiota facilitate adaptation of invasive moths to new host plants

Gut microbiota are important in the adaptation of phytophagous insects to their plant hosts. However, the interaction between gut microbiomes and pioneering populations of invasive insects during their adaptation to new hosts, particularly in the initial phases of invasion, has been less studied. We studied the contribution of the gut microbiome to host adaptation in the globally recognized invasive pest, Hyphantria cunea, as it expands its range into southern China. The southern population of H. cunea shows effective adaptation to Metasequoia glyptostroboides and exhibits greater larval survival on Metasequoia than the original population. Genome resequencing revealed no significant differences in functions related to host adaptation between the two populations. The compatibility between southern H. cunea populations and M. glyptostroboides revealed a correlation between the abundance of several gut bacteria genera (Bacteroides, Blautia, and Coprococcus) and H. cunea survival. Transplanting the larval gut microbiome from southern to northern populations enhanced the adaptability of the latter to the previously unsuitable plant M. glyptostroboides. This research provides evidence that the gut microbiome of pioneering populations can enhance the compatibility of invasive pests to new hosts and enable more rapid adaptation to new habitats.

Larval Rearing and Nutrition of the Polyphagous Tephritid Pest Anastrepha ludens on Artificial Diets with Calcium Alginate, Agar, or Carrageenan as Gelling Agents at Various Concentrations and across Extreme Larval Density Conditions

Research on larval rearing and nutrition of tephritid flies on artificial diets is key for the sterile insect technique. Here, we examined the effects of the type of gel (calcium alginate, agar, or carrageenan), at varying percentages in artificial diets for the polyphagous pest Anastrepha ludens, on the physicochemical and nutritional traits of the diets, and the effects of the type of gel, the gel content and the larval density (larvae/g of diet) used in production, quality parameters for mass-reared tephritids, diet removal (an indirect estimation of diet consumption), and nutritional traits of flies. Regardless of the gel content, calcium alginate diets were firmer and more resistant to penetration than the agar and carrageenan diets. The larval recovery, pupation, pupal weight, and flight ability of A. ludens were lower in calcium alginate diets than in agar and carrageenan diets. Diet removal was higher in calcium alginate diets; however, low levels of ammonium and high levels of uric acid in excretions from larvae on these diets suggest an alteration in protein metabolism. The firmness and penetration resistance characteristics of calcium alginate diets may have limited movement and feeding of larvae, but this could be overcome by the collective feeding of large groups of larvae. Our findings provide insights into the mechanism governing gel-diet rearing systems for A. ludens.

Altitude as a key environmental factor shaping microbial communities of tea green leafhoppers (Matsumurasca onukii)

IMPORTANCE

Host-associated microbial communities play an important role in the fitness of insect hosts. However, the factors shaping microbial communities in wild populations, including environmental factors and interactions among microbial species, remain largely unknown. The tea green leafhopper has a wide geographical distribution and is highly adaptable, providing a suitable model for studying the effect of ecological drivers on microbiomes. This is the first large-scale culture-independent study investigating the microbial communities of M. onukii sampled from different locations. Altitude as a key environmental factor may have shaped microbial communities of M. onukii by affecting the relative abundance of endosymbionts, especially Wolbachia. The results of this study, therefore, offer not only an in-depth view of the microbial diversity of this species but also an insight into the influence of environmental factors.

Improving the nutritional values of yellow mealworm Tenebrio molitor (Coleoptera: Tenebrionidae) larvae as an animal feed ingredient: a review

Yellow mealworm larvae (YML; Tenebrio molitor) are considered as a valuable insect species for animal feed due to their high nutritional values and ability to grow under different substrates and rearing conditions. Advances in the understanding of entomophagy and animal nutrition over the past decades have propelled research areas toward testing multiple aspects of YML to exploit them better as animal feed sources. This review aims to summarize various approaches that could be exploited to maximize the nutritional values of YML as an animal feed ingredient. In addition, YML has the potential to be used as an antimicrobial or bioactive agent to improve animal health and immune function in production animals. The dynamics of the nutritional profile of YML can be influenced by multiple factors and should be taken into account when attempting to optimize the nutrient contents of YML as an animal feed ingredient. Specifically, the use of novel land-based and aquatic feeding resources, probiotics, and the exploitation of larval gut microbiomes as novel strategies can assist to maximize the nutritional potential of YML. Selection of relevant feed supplies, optimization of ambient conditions, the introduction of novel genetic selection procedures, and implementation of effective post-harvest processing may be required in the future to commercialize mealworm production. Furthermore, the use of appropriate agricultural practices and technological improvements within the mealworm production sector should be aimed at achieving both economic and environmental sustainability. The issues highlighted in this review could pave the way for future approaches to improve the nutritional value of YML.

Bacterial biota composition in gut regions of black soldier fly larvae reared on industrial residual streams: revealing community dynamics along its intestinal tract

Some insect species have gained attention as efficient bioconverters of low-value organic substrates (i.e., residual streams) into high-value biomass. Black soldier fly (BSF) (Hermetia illucens) larvae are particularly interesting for bioconversion due to their ability to grow on a wide range of substrates, including low-value industrial residual streams. This is in part due to the plasticity of the gut microbiota of polyphagous insects, like BSF. Gut microbiota composition varies depending on rearing substrates, via a mechanism that might support the recruitment of microorganisms that facilitate digestion of a specific substrate. At the same time, specific microbial genera do persist on different substrates via unknown mechanisms. This study aimed to offer insights on this microbial plasticity by investigating how the composition of the bacterial community present in the gut of BSF larvae responds to two industrial residual streams: swill (a mixture of catering and supermarket leftovers) and distiller's dried grains with solubles. The bacterial biota composition of substrates, whole larvae at the beginning of the rearing period and at harvest, rearing residues, and larval gut regions were investigated through 16S rRNA gene sequencing. It was observed that both substrate and insect development influenced the bacterial composition of the whole larvae. Zooming in on the gut regions, there was a clear shift in community composition from a higher to a lower diversity between the anterior/middle midgut and the posterior midgut/hindgut, indicating a selective pressure occurring in the middle midgut region. Additionally, the abundance of the bacterial biota was always high in the hindgut, while its diversity was relatively low. Even more, the bacterial community in the hindgut was found to be relatively more conserved over the different substrates, harboring members of the BSF core microbiota. We postulate a potential role of the hindgut as a reservoir for insect-associated microbes. This warrants further research on that underexplored region of the intestinal tract. Overall, these findings contribute to our understanding of the bacterial biota structure and dynamics along the intestinal tract, which can aid microbiome engineering efforts to enhance larval performance on (industrial) residual streams.

The pivotal roles of gut microbiota in insect plant interactions for sustainable pest management

The gut microbiota serves as a critical "organ" in the life cycle of animals, particularly in the intricate interplay between herbivorous pests and plants. This review summarizes the pivotal functions of the gut microbiota in mediating the insect-plant interactions, encompassing their influence on host insects, modulation of plant physiology, and regulation of the third trophic level species within the ecological network. Given these significant functions, it is plausible to harness these interactions and their underlying mechanisms to develop novel eco-friendly pest control strategies. In this context, we also outline some emerging pest control methods based on the intestinal microbiota or bacteria-mediated interactions, such as symbiont-mediated RNAi and paratransgenesis, albeit these are still in their nascent stages and confront numerous challenges. Overall, both opportunities and challenges coexist in the exploration of the intestinal microbiota-mediated interactions between insect pests and plants, which will not only enrich the fundamental knowledge of plant-insect interactions but also facilitate the development of sustainable pest control strategies.

Gut bacterial population and community dynamics following adult emergence in pest tephritid fruit flies

Gut microbiota are important contributors to insect success. Host-microbe interactions are dynamic and can change as hosts age and/or encounter different environments. A turning point in these relationships the transition from immature to adult life stages, particularly for holometabolous insects where there is radical restructuring of the gut. Improved knowledge of population and community dynamics of gut microbiomes upon adult emergence inform drivers of community assembly and physiological aspects of host-microbe interactions. Here, we evaluated the bacterial communities of the pest tephritid species melon fly (Zeugodacus cucurbitae) and Medditeranean fruit fly (medfly, Ceratitis capitata) associated with the pupae life stage and timepoints immediately following adult eclosion. We used a combination of culturing to determine cultivatable bacterial titers, qPCR to determine 16S-rRNA SSU copy numbers, and 16S V4 sequencing to determine changes in communities. Both culturing and qPCR revealed that fly bacterial populations declined upon adult emergence by 10 to 100-fold followed by recovery within 24 h following eclosion. Titers reached ~ 107 CFUs (~ 108 16S rRNA copies) within a week post-emergence. We also observed concurrent changes in amplicon sequence variance (ASVs), where the ASV composition differed overtime for both melon fly and medfly adults at different timepoints. Medfly, in particular, had different microbiome compositions at each timepoint, indicating greater levels of variation before stabilization. These results demonstrate that tephritid microbiomes experience a period of flux following adult emergence, where both biomass and the makeup of the community undergoes dramatic shifts. The host-microbe dynamics we document suggest plasticity in the community and that there may be specific periods where the tephritid gut microbiome may be pliable to introduce and establish new microbial strains in the host.

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

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