High nitrogen in maize enriches gut microbiota conferring insecticide tolerance in lepidopteran pest Spodoptera litura

Role of Bacillus atrophaeus B1 in gut on nicotine tolerance of the fall armyworm

The fall armyworm (FAW), Spodoptera frugiperda is one of the most destructive polyphagous herbivores. Some detoxification genes have been proved to be involved in the adaptability to host plants in FAW, while the role of its gut microbiota on the responses of host switches, and their ability to adapt to new host plants remain poorly understood. Herein, we isolated five strains of nicotine-degrading bacteria from the gut of S. frugiperda larvae, among which Bacillus atrophaeus B1 exhibited the highest nicotine tolerance. This strain showed a minimum inhibitory concentration (MIC) value of 2 g/L and a nicotine degradation rate of 46.36 %. We sequenced the complete genome of B. atrophaeus B1 and 15 candidate genes were identified maybe related to nicotine degradation, among which GE003027, GE002849, GE002602, GE000220 and GE002708 had significantly higher expression when exposed to nicotine. Non-targeted metabolomics revealed 98 differentially accumulated metabolites (DAMs) under nicotine stress, which were 72 metabolites upregulated and 26 metabolites downregulated, and the pathways most affected involved xenobiotic biodegradation and metabolism, energy metabolism, and amino acid metabolism. B. atrophaeus B1 may accumulate 2-ketoglutaric acid and γ-aminobutyric acid during degradation of nicotine, which is non-toxic to S. frugiperda, and participated in the tricarboxylic acid (TCA) cycle. Additionally, 2-ketoglutaric acid and γ-aminobutyric acid were detected both in B. atrophaeus B1 and S. frugiperda treated with nicotine. Antibiotic treatment deprived most of the gut bacteria, followed by a decrease in tolerance of S. frugiperda to nicotine, and the nicotine degradation rate was significantly increased as expected after reinfection with B. atrophaeus B1. These findings provide new insights into the bacterial metabolism of nicotine degradation and offer a theoretical basis for understanding the rapid adaptability of S. frugiperda to various host plants.

The impact of the combined application of biochar and organic fertilizer on the growth and nutrient distribution in wheat under reduced chemical fertilizer conditions

Organic fertilizer can help replenish fertility in cropland and reduce the use of chemical fertilizers, with biochar is an important soil conditioner. Under the premise of chemical fertilizer reduction, whether the application of biochar and organic fertilizer affect the yield and nutrient absorption and utilization of wheat? In this experiment, 7 treatments were set up in a randomized field trial with each treatment repeated three times: (1) CK1: no fertilizer; (2) CK2: 100% inorganic fertilizer; (3) T1: recommended amount of biochar with 100% inorganic fertilizer; (4) T2: recommended amount of organic fertilizer with 80% inorganic fertilizer; (5) T3: recommended amount of organic fertilizer and biochar with 80% inorganic fertilizer; (6) T4: recommended amount of organic fertilizer with 60% inorganic fertilizer; and (7) T5: recommended amount of organic fertilizer and biochar with 60% inorganic fertilizer. The results of this study showed that biochar combined with organic fertilizer can reduce the amount of chemical fertilizer by 40%~20% while ensuring wheat yield. Combining the input and output, 80% inorganic fertilizer with biochar and organic fertilizer (T3) was recommended. Under this fertilization scheme, the wheat yield was 37.32% higher than that of 100% chemical fertilizer (CK2), and the photosynthetic capacity was 54.97% higher at seedling stage. At the tillering stage, the root nitrogen content of T3 was significantly higher than that of T2, T4 and T5, which was 21.44%, 54.63% and 60.16%, respectively. The nitrogen content of T3 was significantly higher than that of other treatments at maturity, and the nitrogen content of T3 was 4.38% higher than that of CK2. At heading stage, the nitrogen allocated to T3 leaves was 4.71% higher than CK2. Overall, the results of this study showed that the combination of biochar and organic fertilizer could effectively reduce the application of chemical fertilizer. The recommended fertilizer regimen was 80% inorganic fertilizer with biochar and organic fertilizer, under this scheme, wheat had stronger photosynthetic capacity and better nutrient absorption and distribution mechanism.

Widespread presence of gut bacterium Glutamicibacter ectropisis sp. nov. confers enhanced resistance to the pesticide bifenthrin in tea pests

The gut microbiota in Lepidopterans demonstrates variability and susceptibility to environmental influences, thereby presenting opportunities for the acquisition of novel bacterial strains. Ectropis grisescens (Warren), a notorious Lepidopteran pest, causes substantial damage to tea crops. Prolonged application usage of bifenthrin for the management of this pest has led to increased resistance. This study aims to investigate the relationship between the gut microbiota, as shaped by long-term pesticide use and the resistance of E. grisescenes. We employed high-throughput sequencing of the 16S rRNA gene to analyze the gut microbiota compositions in bifenthrin-resistant (BIF-R) and bifenthrin-sensitive (BIF-S) strains. Bifenthrin-degrading strains were isolated from the gut of BIF-R using selective media. The degradation efficiency and products of bifenthrin by the key strain were detected using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). The effect of the key strain on host resistance was verified in vivo. Finally, the distribution and abundance of the degrading bacterium, in conjunction with insect's pesticide resistance, were assessed in 22 distinct E. grisescens populations. Bifenthrin resistance was diminished in BIF-R following the removal of gut bacteria, a phenomenon not observed in BIF-S. Subsequent high-throughput amplicon sequencing revealed distinct structural differences in the gut microbiota between the two groups, notably an increased abundance of Glutamicibacter in BIF-R. A newly identified bacterial strain from BIF-R larvae, Glutamicibacter ectropisis (B1), demonstrated bifenthrin degradation efficiency and the main metabolite was 2,4-di-tert-butylphenol. Inoculation of B1 into BIF-S larvae conferred increased resistance to bifenthrin. Furthermore, we confirmed the prevalence of B1 in the gut of E. grisescens across 22 tea-growing areas in China. A positive correlation was observed between the absolute abundance of B1 and bifenthrin resistance in E. grisescens. This study represents the first identification of a novel gut bacterium, G. ectropisis, which mediates host resistance through the direct degradation of bifenthrin. This mechanism has been widely validated across 22 distinct populations.

The cultivable gut bacteria Enterococcus mundtii promotes early-instar larval growth of Conogethes punctiferalis via enhancing digestive enzyme activity

BACKGROUND

Gut bacteria are crucial in influencing insect development and even phenotypic plasticity. The yellow peach moth Conogethes punctiferalis, as a significant borer pest, has been the subject of limited reports regarding the structural and diversification changes in its gut microbiota during feeding, and their potential impacts on the growth and development of the host insects.

RESULTS

This study, employing 16S rRNA sequencing, demonstrates distinct shifts in the larvae gut microbiome of C. punctiferalis between different feeding stages, highlighting a pronounced diversity in the early-instar with Enterococcus as a predominant genus in laboratory populations. Through in vitro cultivation and sequencing, three bacterial strains - Micrococcus sp., Brevibacterium sp. and Enterococcus mundtii - were isolated and characterized. Bioassays revealed that E. mundtii-infused corn significantly boosts early-instar larval growth, enhancing both body length and weight. Quantitative PCR and spectrophotometry confirmed a higher abundance of E. mundtii in younger larvae, correlating with increased digestive enzyme activity and total protein levels.

CONCLUSION

This study reveals the heightened gut microbiota diversity in early instars of C. punctiferalis larvae, highlighting that Enterococcus represent a predominant bacteria in laboratory populations. In vitro cultivation and bioassays unequivocally demonstrate the significant role of the cultivable gut bacteria E. mundtii in promoting the growth of early-instar larva. These findings provide a solid theoretical foundation for advancing the comprehension of the intricate interactions between gut microbiota and insect hosts, as well as for the development of eco-friendly pest control technologies based on targeted manipulation of insect gut microbial communities. © 2024 Society of Chemical Industry.

Root inoculation with soil-borne microorganisms alters gut bacterial communities and performance of the leaf-chewer Spodoptera exigua

Soil-borne microorganisms can impact leaf-chewing insect fitness by modifying plant nutrition and defence. Whether the altered insect performance is linked to changes in microbial partners of caterpillars remains unclear. We investigated the effects of root inoculation with soil bacteria or fungi on the gut bacterial community and biomass of the folivore Spodoptera exigua. We also explored the potential correlation between both parameters. We performed herbivory bioassay using leaves of tomato plants (Solanum lycopersicum), measured caterpillar weight gain and characterized the gut bacterial communities via 16S rRNA gene metabarcoding. All soil microbes modified the gut bacterial communities, but the extent of these changes depended on the inoculated species. Rhizophagus irregularis and Bacillus amyloliquefaciens had opposite effects on S. exigua weight. While plant inoculation with the fungus influenced gut bacterial diversity, B. amyloliquefaciens also affected the community composition. A reduced abundance of two S. exigua enterococcal symbionts correlated with decreased insect biomass. Our results show that soil microorganisms can induce plant-mediated changes in the gut bacterial community of foliar-feeding caterpillars. We propose that the impact of these alterations on insect performance might rely on specific adaptations within the gut bacteria, rather than solely on the occurrence of changes.

Effects of Gut Bacteria on the Fitness of Rice Leaf Folder Cnaphalocrocis medinalis

The rice leaf folder Cnaphalocrocis medinalis is an important migratory pest in Asia. Although this pest possesses diverse bacterial communities in its gut, functions of these bacteria in modulating host fitness, including development durations, pupal weight, adult longevity, and fecundity, remain unknown. We isolated gut bacteria from field-collected C. medinalis larvae using a culture-dependent method and identified 15 bacterial isolates. Six of the isolates (Klebsiella aerogenes, Klebsiella pneumoniae, Enterobacter ludwigii, Enterobacter asburiae, Pantoea dispersa, and Pantoea ananatis) were newly discovered in C. medinalis. When larvae were orally inoculated with individual bacterial isolates, 15 isolates showed varying degrees of effects on C. medinalis fitness. Importantly, we found that 10 bacterial isolates induced significant larval mortality. Specifically, the inoculation of Pseudomonas mosselii, P. dispersa, Chryseobacterium culicis, P. ananatis, and Myroides odoratus caused high mortality ranging from 40.0% to 56.7%. However, reducing the entire gut bacterial community with antibiotic treatment negatively impacted C. medinalis fitness, while the reinoculation of a bacterial community to antibiotic-treated larvae recovered some of the adverse effects. In particular, control and bacterial community-inoculated C. medinalis laid approximately 37.6% more eggs than antibiotic-treated C. medinalis. This suggests that these bacteria affect their hosts differently when they are together as compared to alone. Our results reveal that C. medinalis harbors gut bacteria capable of both mutualistic and pathogenic interactions, suggesting their potential as biocontrol agents and indicating that targeting the gut bacterial community could be an effective strategy for controlling C. medinalis infestations.

High nitrogen application in maize enhances insecticide tolerance of the polyphagous herbivore Spodoptera litura by induction of detoxification enzymes and intensification of cuticle

Nitrogen (N) is one of the most intensively used fertilizers in cropping system and could exert a variety of bottom-up effects on the ecological fitness of herbivores. However, the effects of increased N inputs on insect pesticide tolerance have not been comprehensively understood. Bioassays showed that high N (HN) applied to maize plants significantly increased larval tolerance of Spodoptera litura to multiple insecticides. Activities of detoxification enzymes were significantly higher in the larvae fed on maize plants supplied with HN. RNA-seq analysis showed that numerous GST and cuticle-related genes were induced in the larvae fed on HN maize. RT-qPCR analysis further confirmed four GST genes and larval-specific cuticle gene LCP167. Furthermore, when injected with dsRNA specific to GSTe1, GSTs5, and LCP167, the mortality of larvae treated with methomyl was about 3-fold higher than that of dsGFP-injected larvae. Electron microscope observation showed that cuticle of the larvae fed on HN maize was thicker than the medium level of N. These findings suggest that increased application of N fertilizer enhances insecticide tolerance of lepidopteran pests via induction of detoxification enzymes and intensification of cuticle. Thus, overuse of N fertilizer may increase pest insecticide tolerance and usage of chemical insecticides.

Microbiome toxicology - bacterial activation and detoxification of insecticidal compounds

Insect gut bacteria have been implicated in a myriad of physiological processes from nutrient supplementation to pathogen protection. In fact, symbiont-mediated insecticide degradation has helped explain sudden control failure in the field to a range of active ingredients. The mechanisms behind the loss of susceptibility are varied based on host, symbiont, and insecticide identity. However, while some symbionts directly break down pesticides, others modulate endogenous host detoxification pathways or involve reciprocal degradation of insecticidal and bactericidal compounds both inspiring new questions and requiring the reexamination of past conclusions. Good steward of the chemical pesticide arsenal requires consideration of these ecological interactions from development to deployment.

Moderate nitrogen application facilitates Bt cotton growth and suppresses population expansion of aphids (Aphis gossypii) by altering plant physiological characteristics

Introduction

Excessive application of nitrogen fertilizer in cotton field causes soil and water pollution as well as significant increase of aphid population. Reasonable fertilization is an important approach to improve agricultural production efficiency and reduce agriculture-derived pollutions. This study was aimed to explore the effects of nitrogen fertilizer on the Bt cotton physiological characteristics and the growth and development of A. gossypii, a sap-sucking cotton pest.

Methods

Five different levels of Ca(NO3)2 (0.0 g/kg, 0.3 g/kg, 0.9 g/kg, 2.7 g/kg and 8.1 g/kg) were applied into vermiculite as nitrogen fertilizer in order to explore the effects of nitrogen fertilizer on the growth and development of Bt cotton and aphids.

Results

The results showed that the medium level of nitrogen fertilizer (0.9 g/kg) effectively facilitated the growth of Bt cotton plant and suppressed the population expansion of aphids, whereas high and extremely high nitrogen application (2.7 and 8.1 g/kg) significantly increased the population size of aphids. Both high and low nitrogen application benefited aphid growth in multiple aspects such as prolonging nymph period and adult lifespan, enhancing fecundity, and improving adult survival rate by elevating soluble sugar content in host Bt cotton plants. Cotton leaf Bt toxin content in medium nitrogen group (0.9 g/kg) was significantly higher than that in high (2.7 and 8.1 g/kg) and low (0.3 g/kg) nitrogen groups, but Bt toxin content in aphids was very low in all the nitrogen treatment groups, suggesting that medium level (0.9 g/kg) might be the optimal nitrogen fertilizer treatment level for promoting cotton seedling growth and inhibiting aphids.

Discussion

Overall, this study provides insight into trophic interaction among nitrogen fertilizer levels, Bt cotton, and cotton aphid, and reveals the multiple effects of nitrogen fertilizer levels on growth and development of cotton and aphids. Our findings will contribute to the optimization of the integrated management of Bt cotton and cotton aphids under nitrogen fertilization.

Toward an Integrated Understanding of the Lepidoptera Microbiome

Research over the past 30 years has led to a widespread acceptance that insects establish widespread and diverse associations with microorganisms. More recently, microbiome research has been accelerating in lepidopteran systems, leading to a greater understanding of both endosymbiont and gut microorganisms and how they contribute to integral aspects of the host. Lepidoptera are associated with a robust assemblage of microorganisms, some of which may be stable and routinely detected in larval and adult hosts, while others are ephemeral and transient. Certain microorganisms that populate Lepidoptera can contribute significantly to the hosts' performance and fitness, while others are inconsequential. We emphasize the context-dependent nature of the interactions between players. While our review discusses the contemporary literature, there are major avenues yet to be explored to determine both the fundamental aspects of host-microbe interactions and potential applications for the lepidopteran microbiome; we describe these avenues after our synthesis.

Lactic acid bacteria modulate the CncC pathway to enhance resistance to β-cypermethrin in the oriental fruit fly

The gut microbiota of insects has been shown to regulate host detoxification enzymes. However, the potential regulatory mechanisms involved remain unknown. Here, we report that gut bacteria increase insecticide resistance by activating the cap "n" collar isoform-C (CncC) pathway through enzymatically generated reactive oxygen species (ROS) in Bactrocera dorsalis. We demonstrated that Enterococcus casseliflavus and Lactococcus lactis, two lactic acid-producing bacteria, increase the resistance of B. dorsalis to β-cypermethrin by regulating cytochrome P450 (P450) enzymes and α-glutathione S-transferase (GST) activities. These gut symbionts also induced the expression of CncC and muscle aponeurosis fibromatosis. BdCncC knockdown led to a decrease in resistance caused by gut bacteria. Ingestion of the ROS scavenger vitamin C in resistant strain affected the expression of BdCncC/BdKeap1/BdMafK, resulting in reduced P450 and GST activity. Furthermore, feeding with E. casseliflavus or L. lactis showed that BdNOX5 increased ROS production, and BdNOX5 knockdown affected the expression of the BdCncC/BdMafK pathway and detoxification genes. Moreover, lactic acid feeding activated the ROS-associated regulation of P450 and GST activity. Collectively, our findings indicate that symbiotic gut bacteria modulate intestinal detoxification pathways by affecting physiological biochemistry, thus providing new insights into the involvement of insect gut microbes in the development of insecticide resistance.

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.

Diversity and Functional Roles of the Gut Microbiota in Lepidopteran Insects

Lepidopteran insects are one of the most widespread and speciose lineages on Earth, with many common pests and beneficial insect species. The evolutionary success of their diversification depends on the essential functions of gut microorganisms. This diverse gut microbiota of lepidopteran insects provides benefits in nutrition and reproductive regulation and plays an important role in the defence against pathogens, enhancing host immune homeostasis. In addition, gut symbionts have shown promising applications in the development of novel tools for biological control, biodegradation of waste, and blocking the transmission of insect-borne diseases. Even though most microbial symbionts are unculturable, the rapidly expanding catalogue of microbial genomes and the application of modern genetic techniques offer a viable alternative for studying these microbes. Here, we discuss the gut structure and microbial diversity of lepidopteran insects, as well as advances in the understanding of symbiotic relationships and interactions between hosts and symbionts. Furthermore, we provide an overview of the function of the gut microbiota, including in host nutrition and metabolism, immune defence, and potential mechanisms of detoxification. Due to the relevance of lepidopteran pests in agricultural production, it can be expected that the research on the interactions between lepidopteran insects and their gut microbiota will be used for biological pest control and protection of beneficial insects in the future.

Bt GS57 Interaction With Gut Microbiota Accelerates Spodoptera exigua Mortality

The Beet armyworm Spodoptera exigua (Lepidoptera: Noctuidae, Spodoptera) is an important global polyphagous pest. Pathogen infection could destroy the intestinal microbial homeostasis of insects, leading to the death of the host. However, the effect of the host intestinal microbial community on the insecticidal effect of Bacillus thuringiensis is rarely studied. In this study, the genome characteristics of Bt GS57 and the diversity and functions of the gut bacteria in S. exigua are investigated using crystal morphology, biological activity, and Illumina HiSeq high-throughput sequencing. The total size of the Bt GS57 genome is 6.17 Mbp with an average G + C content of 35.66%. Furthermore, the Bt GS57 genome contains six cry genes: cry1Ca, cry1Da, cry2Ab, cry9Ea, cry1Ia, and cry1Aa, and a vegetative insecticidal protein gene vip3Aa. The Bt GS57 strain can produce biconical crystals, mainly expressing 70 kDa and 130 kDa crystal proteins. The LC₅₀ value of the Bt GS57 strain against the S. exigua larvae was 0.339 mg mL^–1. Physiological and biochemical reactions showed that Bt GS57 belongs to B.t. var. thuringiensis. In addition, we found that B. thuringiensis can cause a dynamic change in the gut microbiota of S. exigua, with a significant reduction in bacterial diversity and a substantial increase in bacterial load. In turn, loss of gut microbiota significantly decreased the B. thuringiensis susceptibility of S. exigua larvae. Our findings reveal the vital contribution of the gut microbiota in B. thuringiensis-killing activity, providing new insights into the mechanisms of B. thuringiensis pathogenesis in insects.