Egg-Surface Bacteria Are Indirectly Associated with Oviposition Aversion in Bactrocera dorsalis

BdorOBP32 Perceiving β-Caryophyllene: A Molecular Target for Female Attractant Development in Bactrocera dorsalis

Bactrocera dorsalis is a destructive agricultural pest that attacks over 600 plant species. β-Caryophyllene is considered a potential compound for developing novel female attractants due to its attraction to B. dorsalis females. However, the unknown perception mechanism of β-caryophyllene has been the bottleneck of this process. Odorant binding proteins (OBPs) function to bind odorants and transport them to olfactory receptors. Here, behavioral assays revealed that β-caryophyllene strongly attracted mated, instead of virgin females. RT-qPCR confirmed BdorOBP32 up-regulation out of five OBPs in mated females compared to virgin females. Microscale thermophoresis (MST) results showed BdorOBP32 bind β-caryophyllene with relatively high affinity. Subsequently, CRISPR/Cas9 knockout of BdorOBP32 reduced electroantennograms responses and behavioral preferences to β-caryophyllene in mutants. Moreover, molecular docking and behavioral analysis identified a novel female attractant (α-angelica lactone) targeting BdorOBP32. These findings highlight BdorOBP32 plays critical roles in β-caryophyllene perception and offer new insights for developing novel olfactory behavior modulators.

Bacterial volatiles from aphid honeydew mediate ladybird beetles oviposition site choice

BACKGROUND

The cotton-melon aphid Aphis gossypii Glover is a destructive pest worldwide that causes substantial damage to diverse crops. The ladybird beetle Propylea japonica Thunberg is the dominant predatory natural enemy of A. gossypii. To date, the chemical cues of P. japonica associated with the selection of oviposition sites remain unclear.

RESULTS

Our results revealed that crude honeydew, but not sterilized honeydew, was strongly attractive to mated P. japonica. A total of eight bacterial strains were isolated from crude honeydew, with two (Acinetobacter sp. and Pseudomonas sp.) showing significant attractiveness. Volatiles from these bacteria were identified, and three compounds-DL-lactic acid, 4, 6-dimethyl-2-heptanone, and didodecyl phthalate-were found to significantly attract mated P. japonica in olfactometer assays. Further cage experiments confirmed that P. japonica preferred oviposition sites near these volatile substances.

CONCLUSION

The oviposition site selection by the ladybird beetle P. japonica was found to be influenced by volatiles produced by bacteria associated with cotton-melon aphid honeydew. These findings contribute to biologically based, environmentally friendly pest management strategies in agriculture. © 2025 Society of Chemical Industry.

Female oviposition decisions are influenced by the microbial environment

In ovipositing animals, egg placement decisions can be key determinants of offspring survival. One oviposition strategy reported across taxa is laying eggs in clusters. In some species, mothers provision eggs with diffusible defence compounds, such as antimicrobials, raising the possibility of public good benefits arising from egg clustering. Here we report that Drosophila melanogaster females frequently lay eggs in mixed-maternity clusters. We tested two hypotheses for potential drivers of this oviposition behaviour: (i) the microbial environment affects fecundity and egg placement in groups of females; (ii) eggs exhibit antimicrobial activity. The results partially supported the first hypothesis. Females reduced egg laying but did not alter egg clustering, on non-sterile substrates that had been naturally colonized with microbes from the environment. However, oviposition remained unaffected when the substrate community consisted of commensal (fly-associated) microbes. The second hypothesis was not supported. There was no evidence of antimicrobial activity, either in whole eggs or in soluble egg-surface material. In conclusion, while we found no behavioural or physiological evidence that egg clustering decisions are shaped by the opportunity to share antimicrobials, females are sensitive to their microbial environment and can adjust egg-laying rates accordingly.

Dynamics of Bactrocera dorsalis Resistance to Seven Insecticides in South China

Bactrocera dorsalis is a highly invasive and destructive pest distributed worldwide. Chemical insecticides remain the primary measure for their control; however, this species has already developed resistance to several insecticides. In recent years, there have been several reports of monitoring B. dorsalis resistance in China, but continuous monitoring results are lacking and do not even span a decade. In this study, we monitored the dynamics of resistance to seven insecticides among 11 geographically distinct Chinese populations of B. dorsalis (2010-2013; follow-up in 2023). The 11 populations were found to adapt rapidly to antibiotic insecticides (spinosad, emamectin benzoate, and avermectin), reaching high levels of insecticide resistance in several areas. Overall, a decreasing trend in resistance to organophosphorus insecticides (chlorpyrifos and trichlorfon) was observed, whereas pyrethroid (beta-cypermethrin and cyhalothrin) resistance trends were observed to both increase and decrease. The monitoring of field resistance among different B. dorsalis populations over the duration of this study is important for improving the efficiency and sustainability of agricultural pest management, and the results provide a scientific basis for the development of more effective resistance management strategies.

Olfactory preference of the litchi fruit borer for oviposition on two litchi varieties

BACKGROUND

The litchi fruit borer Conopomorpha sinensis Bradley is a major destructive pest of litchi and longan plants in China, India and South East Asia. Given its strong olfactory-based oviposition behaviour, interfering with the chemical communication between this insect pest and its host plant may serve as a potential control strategy. However, the chemical compounds associated with its egg-laying behaviour remain poorly understood.

RESULTS

In this study, we investigated the olfactory preference of female C. sinensis for oviposition on intact mature fruits of the Feizixiao (FZX) and Guiwei (GW) varieties. Results showed that female C. sinensis preferred to lay eggs on FZX compared with GW fruits, and this preference was olfactory-induced. In addition, we identified differences in the chemical composition of the volatile blend and proportions between FZX and GW fruits, with terpenes being the main volatile components contributing to this divergence. Compounds that induced electrophysiological activity in female borers were subsequently screened from FZX. d-Limonene exhibited the strongest oviposition attraction among four candidates. Furthermore, this compound served as a volatile olfactory cue for recognition and orientation in female C. sinensis.

CONCLUSION

The results of this study provide a deeper understanding of the olfactory preferences of female C. sinensis for oviposition on specific litchi varieties. © 2024 Society of Chemical Industry.

Plant growth-promoting rhizobacteria modulate induced corn defense against Spodoptera litura (Lepidoptera: Noctuidae)

Common cutworm, Spodoptera litura is an important pest of corn causing significant crop yield loss. Synthetic insecticides have mostly been used to combat this pest, raising human and environmental health concerns. Plant growth-promoting rhizobacteria (PGPR) could compensate for or augment the harmful effects of agrochemicals. Herein, we aimed to assess whether PGPR-induced defenses in corn plants impact the host-plant selection behavior of S. litura. Headspace volatile organic compounds were analyzed using gas chromatography-mass spectrometry. Larvae fed inoculated corn exhibited lower weights and relative growth rate than noninoculated plants. Under choice experiments, PGPR-treated plants significantly reduced percentage leaf damage area and oviposition rate compared to untreated plants. Volatile organic compound ratio emission varied significantly between control and PGPR treatments, which, in part, explains feeding and oviposition deterrence in PGPR-treated plants. The results demonstrate that PGPR inoculation can enhance corn resistance to S. litura, making it a promising candidate for crop protection strategies.

Shaping the Microbial Landscape: Parasitoid-Driven Modifications of Bactrocera dorsalis Microbiota

Koinobiont endoparasitoids regulate the physiology of their hosts through altering host immuno-metabolic responses, processes which function in tandem to shape the composition of the microbiota of these hosts. Here, we employed 16S rRNA and ITS amplicon sequencing to investigate whether parasitization by the parasitoid wasps, Diachasmimorpha longicaudata (Ashmaed) (Hymenoptera: Braconidae) and Psyttalia cosyrae (Wilkinson) (Hymenoptera: Braconidae), induces gut dysbiosis and differentially alter the gut microbial (bacteria and fungi) communities of an important horticultural pest, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae). We further investigated the composition of bacterial communities of adult D. longicaudata and P. cosyrae to ascertain whether the adult parasitoids and parasitized host larvae share microbial taxa through transmission. We demonstrated that parasitism by D. longicaudata induced significant gut perturbations, resulting in the colonization and increased relative abundance of pathogenic gut bacteria. Some pathogenic bacteria like Stenotrophomonas and Morganella were detected in both the guts of D. longicaudata-parasitized B. dorsalis larvae and adult D. longicaudata wasps, suggesting a horizontal transfer of microbes from the parasitoid to the host. The bacterial community of P. cosyrae adult wasps was dominated by Arsenophonus nasoniae, whereas that of D. longicaudata adults was dominated by Paucibater spp. and Pseudomonas spp. Parasitization by either parasitoid wasp was associated with an overall reduction in fungal diversity and evenness. These findings indicate that unlike P. cosyrae which is avirulent to B. dorsalis, parasitization by D. longicaudata induces shifts in the gut bacteriome of B. dorsalis larvae to a pathobiont-dominated community. This mechanism possibly enhances its virulence against the pest, further supporting its candidacy as an effective biocontrol agent of this frugivorous tephritid fruit fly pest.

Plasticity of the olfactory behaviors in Bactrocera dorsalis under various physiological states and environmental conditions

Insects rely heavily on their olfactory system for various behaviors, including foraging, mating, and oviposition. Numerous studies have demonstrated that insects can adjust their olfactory behaviors in response to different physiological states and environmental conditions. This flexibility allows them to perceive and process odorants according to different conditions. The Oriental fruit fly, Bactrocera dorsalis, is a highly destructive and invasive pest causing significant economic losses to fruit and vegetable crops worldwide. The olfactory behavior of B. dorsalis exhibits strong plasticity, resulting in its successful invasion. To enhance our understanding of B. dorsalis' olfactory behavior and explore potential strategies for behavior control, we have reviewed recent literature on its olfactory plasticity and potential molecular mechanisms.

Odorant Receptor BdorOR49b Mediates Oviposition and Attraction Behavior of Bactrocera dorsalis to Benzothiazole

The ability to recognize a host plant is crucial for insects to meet their nutritional needs and locate suitable sites for laying eggs. Bactrocera dorsalis is a highly destructive pest in fruit crops. Benzothiazole has been found to induce oviposition behavior in the gravid B. dorsalis. However, the ecological roles and the olfactory receptor responsible for benzothiazole are not yet fully understood. In this study, we found that adults were attracted to benzothiazole, which was an effective oviposition stimulant. In vitro experiments showed that BdorOR49b was narrowly tuned to benzothiazole. The electroantennogram results showed that knocking out BdorOR49b significantly reduced the antennal electrophysiological response to benzothiazole. Compared with wild-type flies, the attractiveness of benzothiazole to BdorOR49b knockout adult was significantly attenuated, and mutant females exhibited a severe decrease in oviposition behavior. Altogether, our work provides valuable insights into chemical communications and potential strategies for the control of this pest.

A classic screening marker does not affect antennal electrophysiology but strongly regulates reproductive behaviours in Bactrocera dorsalis

The key phenotype white eye (white) has been used for decades to selectively remove females before release in sterile insect technique programs and as an effective screening marker in genetic engineering. Bactrocera dorsalis is a representative tephritid pest causing damage to more than 150 fruit crops. Yet, the function of white in important biological processes remains unclear in B. dorsalis. In this study, the impacts of the white gene on electrophysiology and reproductive behaviour in B. dorsalis were tested. The results indicated that knocking out Bdwhite disrupted eye pigmentation in adults, consistent with previous reports. Bdwhite did not affect the antennal electrophysiology response to 63 chemical components with various structures. However, reproductive behaviours in both males and females were significantly reduced in Bdwhite-/- . Both pre-copulatory and copulation behaviours were significantly reduced in Bdwhite-/- , and the effect was male-specific. Mutant females significantly delayed their oviposition towards γ-octalactone, and the peak of oviposition behaviour towards orange juice was lost. These results show that Bdwhite might not be an ideal screening marker in functional gene research aiming to identify molecular targets for behaviour-modifying chemicals. Instead, owing to its strong effect on B. dorsalis sexual behaviours, the downstream genes regulated by Bdwhite or the genes from white-linked areas could be alternate molecular targets that promote the development of better behavioural modifying chemical-based pest management techniques.

An Antenna-Enriched Chemosensory Protein Plays Important Roles in the Perception of Host Plant Volatiles in Bactrocera dorsalis (Diptera: Tephritidae)

Olfaction plays indispensable roles in insect behavior such as host location, foraging, oviposition, and avoiding predators. Chemosensory proteins (CSPs) can discriminate the hydrophobic odorants and transfer them to the odorant receptors. Presently, CSPs have been identified in many insect species. However, their presence and functions remain unknown in Bactrocera dorsalis, a destructive and invasive insect pest in the fruit and vegetable industry. Here, we annotated eight CSP genes in the genome of B. dorsalis. The results of quantitative real-time polymerase chain reaction (RT-qPCR) showed that BdorCSP3 was highly expressed in the antennae. Molecular docking and in vitro binding assays showed that BdorCSP3 had a good binding ability to host volatiles methyl eugenol (ME, male-specific attractant) and β-caryophyllene (potential female attractant). Subsequently, CRISPR/Cas9 was used to generate BdorCSP3-/- mutants. Electroantennograms (EAGs) and behavioral assays revealed that male mutants significantly reduced the preference for ME, while female mutants lost their oviposition preference to β-caryophyllene. Our data indicated that BdorCSP3 played important roles in the perception of ME and β-caryophyllene. The results not only expanded our knowledge of the olfaction perception mechanism of insect CSPs but also provided a potential molecular target for the control of B. dorsalis.

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.

A female-specific odorant receptor mediates oviposition deterrence in the moth Helicoverpa armigera

Finding ideal oviposition sites is a task of vital importance for all female insects. To ensure optimal conditions for their progeny, females of herbivorous insects detect not only the odors of a relevant host plant but also chemicals released by eggs, named oviposition-deterring pheromones (ODPs). It is reported that such chemicals play critical roles in suppressing female oviposition behavior; however, the molecular mechanism underlying the detection of egg-derived ODPs remains elusive. Here, we have identified three specific fatty acid methyl esters from the surface of eggs of Helicoverpa armigera serving as ODPs-methyl oleate (C18:1ME), methyl palmitate (C16:0ME), and methyl stearate (C18:0ME). We demonstrated that these ODPs are detected by the receptor, HarmOR56, exclusively expressed in sensilla trichodea on female antennae. To assess the significance of this receptor, we disrupted HarmOR56 in H. armigera using CRISPR-Cas9 and found that mutant females did not respond to the ODPs, neither in behavioral nor in electrophysiological tests. We therefore conclude that HarmOR56 is indispensable for identifying the ODPs. This study explores, for the first time, how a female-specific odorant receptor detects chemicals from conspecific eggs. Our data elucidate the intriguing biological phenomenon of repulsion to conspecific eggs during oviposition and contribute new insight into a female-specific olfactory pathway linked to reproduction.

VOCs from fungi-infected apples attract and increase the oviposition of yellow peach moth Conogethes punctiferalis

BACKGROUND

Plant volatile organic compounds (VOCs) modified by plant-associated microbes can attract or repel the oviposition of herbivores. Here, we explored the effects of three different fungi on apples' VOCs and the cascading impacts on the oviposition preference of yellow peach moth [YPM, Conogethes punctiferalis (Guenée)].

RESULTS

Among Penicillium crustosum-infected apples (PCA), Rhizopus oryzae-infected apples (ROA), Colletotrichum gloeosporioides-infected apples (CGA) and healthy apples (HA), mated YPM females preferred to oviposit eggs on ROA and CGA, and showed significant attractiveness to VOCs from PCA, ROA, and CGA under laboratory conditions. The VOCs analyses showed that there were significant differences between fungi-infected apples (ROA, CGA) and control treatments (mechanically damaged apples (MDA), HA) in terms of the relative contents of 13 VOCs. The relative contents of ethyl 2-methylbutyrate, ethyl caprylate, estragole, ethyl hexanoate in ROA and CGA were higher than those in MDA. The relative content of isopropyl 2-methylbutyrate in ROA was significantly higher than those in HA and CGA. The relative contents of 2-methylbutyl acetate, butyl 2-methylbutyrate, hexyl 2-methylbutyrate, amyl hexanoate, hexyl hexanoate, (E, E)-α-farnesene in ROA and CGA were lower than those in HA. The relative content of hexyl acetate in ROA and CGA was significantly higher than that in MDA, but lower than that in HA. Additionally, 10 fungi-induced VOCs were detected in ROA and/or CGA. When 20 VOCs from ROA and/or CGA were tested as individuals or mixed blends in Y-tube olfactometer assays, mated YPM females preferred amyl 2-methylbutyrate, isoamyl 2-methylbutyrate, isopropyl 2-methylbutyrate, hexyl propionate (common VOCs in ROA, CGA, and HA), and heptacosane (a fungi-induced VOC in ROA), but no significant preferences were observed between individual compounds and mixed blends, except for hexyl propionate.

CONCLUSION

Different fungi infection increased the relative contents of common VOCs from healthy and fungi-infected apples, which ultimately resulted in the significant attractiveness for the oviposition of mated YPM females. This study clarified why fungi-infected apples were more attractive to YPMs than healthy apples and screened out the crucial VOCs for YPM oviposition. © 2023 Society of Chemical Industry.

MFS Transporter Bdorwp Does Not Affect Antennal Electrophysiology but Regulates Reproductive Behaviors in Bactrocera dorsalis

Developing behavioral modifying chemicals through molecular targets is a promising way to improve semiochemical-based technology for pest management. Identifying molecular targets that affect insect behavior largely relies on functional genetic techniques such as deletions, insertions, and substitutions. Selectable markers have thus been developed to increase the efficiency of screening for successful editing events. However, the effect of selectable markers on relevant phenotypic traits needs to be considered. In this study, we cloned the wp gene ofBactrocera dorsalis. Knocking out Bdorwp causes white pupae phenotypes. Reproductive behaviors in both males and females were strongly regulated by Bdorwp. Remarkably, Bdorwp did not affect the antennal electrophysiology response to 63 chemical components with various structures. It is recommended to indirectly apply Bdorwp as a selectable marker in functional gene research on behavioral modifying chemicals. Moreover, Bdorwp could also be a potential molecular target for developing new insecticides for tephritid species control.

Microbiota-mediated competition between Drosophila species

BACKGROUND

The influence of microbiota in ecological interactions, and in particular competition, is poorly known. We studied competition between two insect species, the invasive pest Drosophila suzukii and the model Drosophila melanogaster, whose larval ecological niches overlap in ripe, but not rotten, fruit.

RESULTS

We discovered D. suzukii females prevent costly interspecific larval competition by avoiding oviposition on substrates previously visited by D. melanogaster. More precisely, D. melanogaster association with gut bacteria of the genus Lactobacillus triggered D. suzukii avoidance. However, D. suzukii avoidance behavior is condition-dependent, and D. suzukii females that themselves carry D. melanogaster bacteria stop avoiding sites visited by D. melanogaster. The adaptive significance of avoiding cues from the competitor's microbiota was revealed by experimentally reproducing in-fruit larval competition: reduced survival of D. suzukii larvae only occurred if the competitor had its normal microbiota.

CONCLUSIONS

This study establishes microbiotas as potent mediators of interspecific competition and reveals a central role for context-dependent behaviors under bacterial influence. Video Abstract.

Contact Toxicity, Antifeedant Activity, and Oviposition Preference of Osthole against Agricultural Pests

Osthole, the dominant bioactive constituent in the Cnidium monnieri, has shown acute pesticidal activities. However, its detailed toxicity, antifeedant, and oviposition preference effects against agricultural pests have not been fully understood, limiting its practical use. This study aimed to investigate the contact toxicity, antifeedant activity, and oviposition preference of osthole against three agricultural pests (Tetranychus urticae, Myzus persicae, and Bactrocera dorsalis). Our results showed that the Cnidium monnieri (L.) Cusson (CMC) has a high osthole content of 11.4 mg/g. Osthole exhibited a higher level of acute toxicity against the T. urticae to four other coumarins found in CMC. It showed significant pesticidal activity against T. urticae and M. persicae first-instar nymphs and adults in a dose-dependent manner but not against B. dorsalis adults. Osthole exposure reduced the fecundity and prolonged the developmental time of the T. urticae and M. persicae. Leaf choice bioassays revealed potent antifeedant activity in the T. urticae and M. persicae. Furthermore, the female B. dorsalis showed a distinct preference for laying eggs in mango juice with 0.02 mg/mL osthole at 48 h, a preference that persisted at 96 h. These results provide valuable insights into the toxicity, repellent activity, and attractant activity of osthole, thereby providing valuable insights into its potential efficacy in pest control.

Egg-Associated Germs Induce Salicylate Defenses but Not Render Plant Against a Global Invasive Fruit Fly Effectively

Germs associated with insect eggs can profoundly mediate interactions between host plants and herbivores, with the potential to coordinate plant physiological reactions with cascading effects on insect fitness. An experimental system was established including the oriental fruit fly (OFF, Bactrocera dorsalis) and tomato to examine the functions of egg-associated germs in mediating plant-herbivore interactions. OFF feeding resulted in significantly increased tannins, flavonoids, amino acids, and salicylic acid in the host tomato. These defensive responses of tomato were induced by the egg-associated germs, including Lactococcus sp., Brevundimonas sp., and Vagococcus sp. Tannins and flavonoids had no significant feedback effects on the pupal weight of OFF, while pupal biomass was significantly decreased by tannins and flavonoids in the germ-free treatment. Metabolome analysis showed that OFF mainly induced metabolic changes in carboxylic acid derivatives. Phenylalanine significantly induced downstream metabolic changes associated with phenylpropanoid accumulation. Finally, we conclude that the effects of egg-associated germs played an important role in facilitating OFF population adaptation and growth by mediating plant defenses, which provides a new paradigm for exploring the interaction of plant-pest and implementing effective pest biocontrol.

Bioactive volatile compounds from Penicillium digitatum-infected apples: Oviposition attractants for yellow peach moth Conogethes punctiferalis (Lepidoptera: Crambidae)

Introduction

Plant-associated microbes critically shape the dynamics of plant-and insect-associated communities. In previous studies, we reported that the yellow peach moth Conogethes punctiferalis (YPM) preferred to Penicillium digitatum-infected apples (PDA) for oviposition. However, the underlying mechanisms remains unclear.

Methods

In the present study, the behavioral and physiological experiments were conducted to determine how P. digitatum affects the oviposition selection of mated YPM females via altering host plant volatile organic compounds (VOCs).

Results

Mated YPM females were attracted to and laid more eggs on PDA than on non-infected apples (NIA), mechanically damaged apples (MDA), and P. digitatum in potato dextrose agar medium (PPD) in the oviposition selection experiments. Four-arm olfactometer assays further confirmed that odors in PDA were responsible for the attractiveness of mated YPM females. Further analyses showed that 38 VOCs were collected and identified from all treatments by GC-MS, with five specific VOCs (methyl 2-methylbutyrate, styrene, methyl caproate, butyl caprylate, and n-tetradecane) emitting from PDA. A principal component analysis (PCA) based on the absolute contents of 38 VOCs revealed a clear separation of PDA from NIA, MDA, and PPD. Moreover, when P. digitatum-induced specific VOCs were added to apples in individual or synthetic blends, there was a significantly higher percentage of mated YPM females to apples with individual or synthetic blends consisting of methyl 2-methylbutyrate, butyl caprylate, or n-tetradecane in Y-tube olfactometer experiments, suggesting that these three specific VOCs acted as predominant olfactory signals for mated YPM females to PDA.

Discussion

Taken together, the microbe P. digitatum was an important driver of the interactions between YPMs and host plants by altering plant volatiles. These findings may form the basis for developing attractant baits for field trapping YPMs in the future.

Early infestation volatile biomarkers of fruit fly Bactrocera dorsalis (Hendel) ovipositional activity in mango (Mangifera indica L.)

Infestation of agricultural commodities by insect pests results in significant economic, import and export, food safety, and invasive insect introduction issues for growers, consumers, and inspectors. The Oriental fruit fly (Bactrocera dorsalis) is considered a highly invasive insect pest with populations reported in more than 60 countries, with prevalent distributions in Asia and Africa. B. dorsalis is phytophagous with a host range encompassing hundreds of fruits and vegetables. Damage to the fruit or vegetable is inflicted through oviposition and subsequent larval feeding resulting in spoilage. Early detection of insect pest infestations is a critical component for ensuring food safety as well as controlling introduction and spread of invasive insects. However, detection of ovipositional activity and early larval development is visually difficult, thus rapid and non-destructive detection often relies on odors associated with infestation. We investigated the odors of mangoes (Mangifera indica L.) infested with B. dorsalis and compared the volatile profiles of infested mangoes to non-infested and mechanically damaged mangoes 24 h post-infestation. GC-MS and multivariate analyses provided the identification of eleven compounds unique to infested mangoes compared to mechanically damaged or non-infested fruit. Results indicated compositional and quantitative differentiation of volatile profiles among treatments for detection of infested fruit at quality checks or points of commerce.

Group housing enhances mating and increases the sensitization of chemical cues in Bactrocera dorsalis

BACKGROUND

Changes in population density have profound impacts on mating behaviors in group-living animals. The plasticity of mating behavior enables insects to respond to social signals and adjust mating frequency in accordance with rival competition and reproductive opportunity.

RESULTS

In this study, we found that low levels of cis-vaccenyl acetate (cVA), a Drosophila pheromone, increased mating rates of Bactrocera dorsalis, but high concentrations of cVA inhibited mating, indicating a functional role of cVA in regulating mating behaviors in insect species other than Drosophila. Moreover, we demonstrated that group housing conditions had positive effects for B. dorsalis on their mating rates, responses toward cVA and cVA-mediated mating behaviors, which are dependent on the activity of c-AMP reponse element binding protein (CREB) binding protein (CBP).

CONCLUSIONS

Our data suggest that CBP-mediated plasticity in mating behavior and chemical recognition enables insects to adapt to different housing conditions and highlight the potential of cVA as an efficient agent in regulating mating behaviors in insect species other than Drosophila. © 2022 Society of Chemical Industry.

Yeast mediates the interspecific interaction between introduced Bactrocera dorsalis and indigenous Bactrocera minax

BACKGROUND

Host plant-microbe associations mediate interspecific interactions amongst herbivorous insects. However, this theory has rarely been ecologically verified in tephritid fruit flies. Research on this subject can not only help predict tephritid species invasion and occurrence patterns, but also develop potential novel lures for the control of the tephritid fruit fly pests. Recently, we observed mixed infestation of Bactrocera minax and Bactrocera dorsalis larvae in citrus orchards, which prompted us to explore the underlying mechanism.

RESULTS

Following oviposition by B. minax, the yeast Pichia kluyveri translocated to and proliferated inside the citrus fruit. The level of d-limonene released from citrus fruits containing P. kluyveri was 27 times higher than that released from healthy fruits. Mature B. dorsalis females were attracted to d-limonene and oviposited into fruits previously infested by B. minax. Furthermore, the interspecific interaction between B. dorsalis and B. minax within the same fruit significantly decreased the number of surviving larvae and pupal weight in B. dorsalis, but its effect on B. minax was weaker.

CONCLUSION

In the studied interspecific interaction, B. minax occupies the dominant position, implying ecological significance for this species in terms of consolidating its own niche and inhibiting the invasion of exotic species. To our best knowledge, this is the first report from both ecological and physiological perspectives on a symbiotic yeast mediating the interaction between B. minax and B. dorsalis through altering fruit volatiles. © 2022 Society of Chemical Industry.

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?

Exploration of Clove Bud (Syzygium aromaticum) Essential Oil as a Novel Attractant against Bactrocera dorsalis (Hendel) and Its Safety Evaluation

The oriental fruit fly Bactrocera dorsalis (Hendel) is a destructive polyphagous species that targets many economically important fruits and vegetables. The primary control of B. dorsalis relies mainly on the use of synthetic chemicals, and excessive use of these chemicals has adverse effects on both the environment and human health. Environmentally friendly management of pests involving plant essential oils is useful for controlling the populations of pests responsible for decreasing the yields and quality of crops. In the present study, we demonstrate that clove bud essential oil (CBEO) is strongly attractive to sexually mature males. Mature males responded to the CBEO differently throughout the day; the strongest response was elicited during the day and decreased at dusk. Virgin and mated mature males did not respond differently to CBEO. No obvious response behaviour to the CBEO was observed in two species of beneficial natural predator ladybirds. In addition, a cytotoxicity assessment demonstrated that CBEO is nontoxic to normal human and mouse cells. Based on our laboratory experiments, CBEO may serve as a promising, sustainable, and environmentally friendly attractant for B. dorsalis males; however, field experiments are needed to confirm this hypothesis.

Gut bacteria induce oviposition preference through ovipositor recognition in fruit fly

Gut bacteria play important roles in insect life cycle, and various routes can be used by insects to effectively transmit their gut bacteria. However, it is unclear if the gut bacteria can spread by actively attracting their insect hosts, and the recognition mechanisms of host insects are poorly understood. Here, we explore chemical interactions between Bactrocera dorsalis and its gut bacterium Citrobacter sp. (CF-BD). We found that CF-BD could affect the development of host ovaries and could be vertically transmitted via host oviposition. CF-BD could attract B. dorsalis to lay eggs by producing 3-hexenyl acetate (3-HA) in fruits that were hosts of B. dorsalis. Furthermore, we found that B. dorsalis could directly recognize CF-BD in fruits with their ovipositors in which olfactory genes were expressed to bind 3-HA. This work reports an important mechanism concerning the active spread of gut bacteria in their host insects.

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

Background

Nitrogen is considered the most limiting nutrient element for herbivorous insects. To alleviate nitrogen limitation, insects have evolved various symbiotically mediated strategies that enable them to colonize nitrogen-poor habitats or exploit nitrogen-poor diets. In frugivorous tephritid larvae developing in fruit pulp under nitrogen stress, it remains largely unknown how nitrogen is obtained and larval development is completed.

Results

In this study, we used metagenomics and metatranscriptomics sequencing technologies as well as in vitro verification tests to uncover the mechanism underlying the nitrogen exploitation in the larvae of Bactrocera dorsalis. Our results showed that nitrogenous waste recycling (NWR) could be successfully driven by symbiotic bacteria, including Enterobacterales, Lactobacillales, Orbales, Pseudomonadales, Flavobacteriales, and Bacteroidales. In this process, urea hydrolysis in the larval gut was mainly mediated by Morganella morganii and Klebsiella oxytoca. In addition, core bacteria mediated essential amino acid (arginine excluded) biosynthesis by ammonium assimilation and transamination.

Conclusions

Symbiotic bacteria contribute to nitrogen transformation in the larvae of B. dorsalis in fruit pulp. Our findings suggest that the pattern of NWR is more likely to be applied by B. dorsalis, and M. morganii, K. oxytoca, and other urease-positive strains play vital roles in hydrolysing nitrogenous waste and providing metabolizable nitrogen for B. dorsalis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01399-9.

Gut fungal community and its probiotic effect on Bactrocera dorsalis

The oriental fruit fly, Bactrocera dorsalis (Diptera: Tephritidae) is a destructive horticultural pest which causes considerable economic losses every year. A collection of microorganisms live within the B. dorsalis gut, and they are involved in its development, physiology, and behavior. However, knowledge regarding the composition and function of the gut mycobiota in B. dorsalis are still limited. Here, we comprehensively characterized the gut mycobiota in B. dorsalis across different developmental stages. High-throughput sequencing results showed a significant difference in fungal species abundance and diversity among different developmental stages of B. dorsalis. Quantitative polymerase chain reaction and culture-dependent methods showed that yeast species was the dominant group in the larval stage. We isolated 13 strains of yeast from the larval gut, and found that GF (germ-free) larvae mono-associated with strain Hanseniaspora uvarum developed faster than those mono-associated with other tested fungal strains. Supplementing the larval diet with H. uvarum fully rescued B. dorsalis development, shortened the larval developmental time, and increased adult wing lengths, as well as the body sizes and weights of both pupae and adults. Thus, our study highlights the close interactions between gut fungi, especially H. uvarum, and B. dorsalis. These findings can be applied to the sterile insect technique program to promote host development during mass insect rearing.

Dynamics of the intestinal bacterial community in black soldier fly larval guts and its influence on insect growth and development

Black soldier fly (BSF), Hermetia illucens (Diptera: Stratiomyidae), is a prominent insect for the bioconversion of various organic wastes. As a saprotrophic insect, the BSF inhabits microbe-rich environments. However, the influences of the intestinal microorganisms on BSF growth and development are not very clear. In this study, the dynamics of the intestinal bacterial community of BSF larvae (BSFL) were analyzed using pyrosequencing. Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria were the most prevalent bacterial phyla in the intestines of all larval instars. The dynamic changes in bacterial community compositions among different larval instars were striking at the genus level. Klebsiella, Clostridium, Providencia, and Dysgonomonas were the relatively most abundant bacteria in the 1st- to 4th-instar BSFL, respectively. Dysgonomonas and Providencia also dominated the 5th- and 6th-instar larvae, at ratios of 31.1% and 47.2%, respectively. In total, 148 bacterial strains affiliated with 20 genera were isolated on different media under aerobic and anaerobic conditions. Among them, 6 bacteria, BSF1-BSF6, were selected for further study. The inoculation of the 6 isolates independently into germ-free BSFL feeding on an artificial diet showed that all the bacteria, except BSF4, significantly promoted BSF growth and development compared with the germ-free control. Citrobacter, Dysgonomonas, Klebsiella, Ochrobactrum, and Providencia promoted BSF development significantly by increasing the weight gains of larvae and pupae, as well as increasing the prepupae and eclosion rates. In addition, Citrobacter, Klebsiella and Providencia shortened the BSF life cycle significantly. The results illustrate the promotive effects of intestinal bacteria on BSF growth and development.

Mutualism promotes insect fitness by fungal nutrient compensation and facilitates fungus propagation by mediating insect oviposition preference

Penicillium and Bactrocera dorsalis (oriental fruit fly, Hendel) are major pathogens and pests of citrus fruits, as both of them can cause detrimental losses in citrus production. However, their interaction in the cohabitation of citrus fruits remains elusive. In this study, we revealed a mutualistic relationship between Penicillium and B. dorsalis. We found that insect behaviors can facilitate the entry of fungal pathogens into fruits, and fungal pathogens promote the fitness of insects in return. More specifically, Penicillium could take advantage of the openings left by ovipositors of flies, and adult flies contaminated with Penicillium could spread the fungus to new sites. Moreover, the volatile emissions from fungi could attract gravid flies to the infected site for egg laying. The fungus and B. dorsalis were able to establish mutual interaction, as revealed by the presence of Penicillium DNA in intestinal tracts of flies throughout all larval stages. The fungal partner seemed to promote the emergence rate and shorten the emergence duration of the flies by providing pyridoxine, one of the B group vitamins. Different from previously reported scenarios of strong avoidance of Drosophila and attraction of Aedes aegypti toward Penicillium, our findings unveil a hitherto new paradigm of the mutualism between Penicillium and B. dorsalis, by which both insect and fungus earn benefits to facilitate their propagation.

Penicillium fungi mediate behavioral responses of the yellow peach moth, Conogethes punctiferalis (Guenée) to apple fruits via altering the emissions of host plant VOCs

Plant-associated microbes have been reported as important but overlooked drivers of plant-herbivorous insect interactions. Influence of plant-associated microbes on plant-insect interactions is diverse, including beneficial, detrimental, and neutral. Here, we determined the effects of three Penicillium fungi, including Penicillium citrinum, Penicillium sumatrense, and Penicillium digitatum, on the oviposition selection and behavior of the yellow peach moth (YPM), Conogethes punctiferalis (Guenée). Compared with fungi noninfected apples (NIA), mechanically damaged apples (MDA), and P. citrinum in potato dextrose agar medium (PC), the oviposition selection and four-arm olfactometer experiments both showed that mated YPM females preferred to P. citrinum-infected apples (PCA). For P. sumatrense or P. digitatum, we also found that mated YPM females preferred to P. sumatrense-infected apples (PSA) or P. digitatum-infected apples (PDA), respectively. Among three Penicillium fungi-infected apples, the selection rates including oviposition and olfactometer behavior of mated YPM females on PDA were both higher than those on PSA and PCA. Further analyses of host plant volatile organic compounds (VOCs) by GC-MS showed that the absolute contents of ethyl hexanoate and (Z, E)-α-farnesene in PCA, PSA, and PDA were all higher than those in NIA, and a total of 16 novel VOCs were detected in fungi-infected apples (PCA, PSA, and PDA), indicating that fungi infection changed the components and proportions of apple VOCs. Taken together, three Penicillium fungi play significant roles in mediating the host selection of YPMs via altering the emissions of VOCs. These findings will be beneficial for developing formulations for field trapping of YPMs in the future.

Preceding Phenological Events Rather than Climate Drive the Variations in Fruiting Phenology in the Desert Shrub Nitraria tangutorum

Fruit setting and ripening are crucial in the reproductive cycle of many desert plant species, but their response to precipitation changes is still unclear. To clarify the response patterns, a long-term in situ water addition experiment with five treatments, namely natural precipitation (control) plus an extra 25%, 50%, 75%, and 100% of the local mean annual precipitation (145 mm), was conducted in a temperate desert in northwestern China. A whole series of fruiting events including the onset, peak, and end of fruit setting and the onset, peak, and end of fruit ripening of a locally dominant shrub, Nitraria tangutorum, were observed from 2012 to 2018. The results show that (1) water addition treatments had no significant effects on all six fruiting events in almost all years, and the occurrence time of almost all fruiting events remained relatively stable compared with leaf phenology and flowering phenology after the water addition treatments; (2) the occurrence times of all fruiting events were not correlated to the amounts of water added in the treatments; (3) there are significant inter-annual variations in each fruiting event. However, neither temperature nor precipitation play key roles, but the preceding flowering events drive their inter-annual variation.

CRISPR/Cas9 mutagenesis abolishes odorant-binding protein BdorOBP56f-2 and impairs the perception of methyl eugenol in Bactrocera dorsalis (Hendel)

Olfaction underpins many insect behaviors, such as foraging, host location, mating, and predator avoidance. In the first step of insect olfaction, odorant-binding proteins (OBPs) bind hydrophobic odorants and transport them to odorant receptors. Methyl eugenol (ME) is a powerful attractant for mature males of the oriental fruit fly Bactrocera dorsalis (Hendel), one of the most destructive fruit pests. The underlying molecular mechanism is unclear, but there is in vitro evidence that BdorOBP56f-2 is involved in ME perception. We used microscale thermophoresis to confirm that BdorOBP56f-2 directly binds ME with strong affinity in vitro. We then used CRISPR/Cas9 to knock out the BdorOBP56f-2 gene, allowing us to establish a homozygous mutant B. dorsalis line. The electroantennogram response and behavioral attraction to ME were significantly reduced in the mutant, providing in vivo evidence that BdorOBP56f-2 is necessary for efficient ME perception. Our results offer insight into the molecular mechanism of ME perception in B. dorsalis and provide a theoretical basis for the functional analysis of other OBPs.

Insect Host Choice: Don't Put All the Eggs in One Basket

Unlike mammals, most insects have no chance to personally take care of their offspring. Insect mothers, therefore, carefully weigh egg-laying options to select an optimal site, which guarantees better survival and fitness for their progeny. A new study in oriental fruit flies reveals that gravid females rely on a bacteria-derived odour - β-caryophyllene - to avoid competition for their offspring.

Tephritid Fruit Fly Semiochemicals: Current Knowledge and Future Perspectives

The Dipteran family Tephritidae (true fruit flies) comprises more than 5000 species classified in 500 genera distributed worldwide. Tephritidae include devastating agricultural pests and highly invasive species whose spread is currently facilitated by globalization, international trade and human mobility. The ability to identify and exploit a wide range of host plants for oviposition, as well as effective and diversified reproductive strategies, are among the key features supporting tephritid biological success. Intraspecific communication involves the exchange of a complex set of sensory cues that are species- and sex-specific. Chemical signals, which are standing out in tephritid communication, comprise long-distance pheromones emitted by one or both sexes, cuticular hydrocarbons with limited volatility deposited on the surrounding substrate or on the insect body regulating medium- to short-distance communication, and host-marking compounds deposited on the fruit after oviposition. In this review, the current knowledge on tephritid chemical communication was analysed with a special emphasis on fruit fly pest species belonging to the Anastrepha, Bactrocera, Ceratitis, and Rhagoletis genera. The multidisciplinary approaches adopted for characterising tephritid semiochemicals, and the real-world applications and challenges for Integrated Pest Management (IPM) and biological control strategies are critically discussed. Future perspectives for targeted research on fruit fly chemical communication are highlighted.

Rectal bacteria produce sex pheromones in the male oriental fruit fly

Volatile molecules produced by the microbiota play a primary role in chemical communication between insects,¹ and direct production of pheromone components by the microbiota is one of the most obvious mechanisms.² Here, we investigated the production of male-borne sex pheromones of the oriental fruit fly, Bactrocera dorsalis. As observed in previous studies,^3,4 2,3,5-trimethylpyrazine (TMP) and 2,3,5,6-tetramethylpyrazine (TTMP) are sex pheromones produced in the male rectum. Mature virgin females are strongly attracted to TMP and TTMP. 16S rRNA sequencing results show that the rectal bacteria are dominated by Bacilli that harbor the pathway to produce TMP and TTMP.^5-8 The levels of Bacilli, TMP, and TTMP in the male rectum can be significantly decreased by feeding male flies with antibiotics. In vitro assays show that Bacillus species isolated from the male rectum can produce TMP and TTMP when provided with the substrates glucose and threonine, the levels of which are significantly higher in the rectum of mature males. These findings highlight the influence of microbial symbionts on insect pheromones and provide an example of direct bacterial production of pheromones in insects.

Recent Advancements in Studies on Chemosensory Mechanisms Underlying Detection of Semiochemicals in Dacini Fruit Flies of Economic Importance (Diptera: Tephritidae)

Dacini fruit flies mainly contain two genera, Bactrocera and Zeugodacus, and include many important pests of fruits and vegetables. Their life cycle is affected by various environmental cues. Among them, multiple characteristic semiochemicals have remarkable effects on their reproductive and host-finding behaviors. Notably, floral fragrances released from so-called fruit fly orchids strongly attract males of several Dacini fruit fly species. Focusing on the strong attraction of male flies to particular chemicals, natural and synthetic lures have been used for pest management. Thus, the perception of semiochemicals is important to understand environmental adaptation in Dacini fruit flies. Since next-generation sequencers are available, a large number of chemosensory-related genes have been identified in Dacini fruit flies, as well as other insects. Furthermore, recent studies have succeeded in the functional analyses of olfactory receptors in response to semiochemicals. Thus, characterization of molecular components required for chemoreception is under way. However, the mechanisms underlying chemoreception remain largely unknown. This paper reviews recent findings on peripheral mechanisms in the perception of odors in Dacini fruit flies, describing related studies in other dipteran species, mainly the model insect Drosophilamelanogaster. Based on the review, important themes for future research have also been discussed.