Identification and Behavioral Assays of Alarm Pheromone in the Vetch Aphid Megoura viciae

Discovery and properties of novel analogues of the aphid pheromones nepetalactone and nepetalactol

BACKGROUND

Pheromones have unique advantages for pest control. Current aphid pheromone research focuses on alarm and sex pheromones. However, practical applications are limited so far, as (E)-β-farnesene has only been investigated to a small extent as an alarm pheromone and only male aphids are targeted by sex pheromones. Previous literature reports electrophysiological responses and repellent behavior of asexual aphids to nepetalactone (1B), therefore our objective was to modify nepetalactone's structure to identify key fragments responsible for repellent effects, as guidance for subsequent modifications and further investigation.

RESULTS

In this study, seven derivatives were designed and synthesized based on nepetalactol (1A) and nepetalactone (1B) as lead compounds. Free-choice tests, conducted using cowpea aphids (Aphis craccivora), revealed that the lactone moiety was crucial for the repellent activity, and the removal of the carbonyl group eliminated the repelling effect. Compound (±)1I, an analogue of nepetalactone (1B), demonstrated a significantly higher repellent value than nepetalactone (1B) at three different concentrations, and even at 0.1 mg/mL it maintained a considerable repellent effect (26.5%). Electrostatic potential and density functional theory calculations supported the importance of the carbonyl group for the repellent effects.

CONCLUSION

The newly discovered para-pheromone (±)1I shows improved repellent effects and potential for development as a novel biological control agent. Based on our innovative findings, analogues with improved efficacy and properties can be designed and prepared. Our research contributes to understanding the effects of structural modifications on pheromone activity and properties, which is crucial for exploring novel pheromone-based products for crop protection. © 2024 Society of Chemical Industry.

Potential use of pinenes to improve localized insecticide injections targeting the western drywood termite (Blattodea: Kalotermitidae)

The western drywood termite, Incisitermes minor (Hagen), causes significant economic damage to wood structures in the United States of America, especially California. When infestation is not widespread, localized insecticide injections may be useful for remedial control. However, the extensive gallery structure of drywood termites and their tendency to aggregate at specific parts of the galleries can impact the efficacy of localized insecticide injection. Chemicals that attract termites from a distance may improve the localized insecticide injection by increasing the number of termites contacting the insecticide residues. Two volatile terpenes, α-pinene and β-pinene, commonly found in many coniferous timber trees, were applied to artificial termite galleries to determine if termites were attracted from their original aggregation site. Furthermore, we examined if adding these pinenes would improve the overall efficacy of some insecticide products for drywood termite control. Behavioral assay results showed that the treatment with pinenes increased the likelihood that drywood termites would leave their original aggregation site and contact the treated part of the gallery. When tested with the pesticide products applied in a small area away from the termite aggregation, β-pinene significantly accelerated the time course of mortality for the aqueous fipronil. The efficacy of disodium octaborate tetrahydrate dust was not influenced by the addition of pinenes. Implications for drywood termite management and future research directions are discussed.

Exploration of D-limonene as a sex pheromone for males of Bactrocera minax (Diptera: Tephritidae)

BACKGROUND

Bactrocera minax is a devastating pest of citrus fruits. However, there have been no effective control measures before. Few reports on the sex pheromones of B. minax are available.

RESULTS

In this study, nine of the volatile compounds in adult females were identified using headspace solid-phase microextraction (HS-SPME) in combination with gas chromatography-mass spectrometry (GC-MS). Among them, d-limonene, caprolactam, 2-Nitro-1H-imidazole, and creatinine could evoke antennal responses in males. Field bioassays showed that only d-limonene could lure male flies, with a relative lure rate of 78.18% in all tested samples, which was significantly higher than that of paraffin oil control, while all volatile compounds did not have any lure effective to female flies. Moreover, d-limonene was diluted with paraffin oil into differential concentrations, the lure effect on males was better at 100, 500, and 800 μL d-limonene mL-1 than pure d-limonene (1000 μL mL-1 ). The relative male lure rate of d-limonene at 100 μL mL-1 was 85.88%, which was significantly higher than that of food-baits (14.12%) on day 3. However, d-limonene was unattractive to female and male Bactrocera dorsalis and Zeugodacus tau. Further kinetic analysis showed that female adults released d-limonene around 15-day post eclosion. Electroantennography 1 results showed that 500 μL mL-1 d-limonene evoked the strongest responses to antennae of 10- to 25-day-old male flies.

CONCLUSION

Our results indicated that d-limonene could be a sex pheromone from female flies of B. minax, and it could be used as a male-specific sex attractant for B. minax. © 2023 Society of Chemical Industry.

Functional characterization of alkaline phosphatases involved alarm pheromone in the vetch aphid Megoura viciae

The alkaline phosphatases (ALPs) are highly promiscuous enzymes and have been extensively investigated in mammals for their medical significance, but their functional promiscuity is relatively poorly understood in insects. Here, we first identified four ALP genes (designated as MvALP1-4) in the vetch aphid Megoura viciae that contained one alkaline phosphatase site, three metal-binding sites, and varied other functional sites. Phylogenetic analysis, molecular docking and the spatiotemporal expression profiling of MvALP1-4 were very different, indicating a promiscuous functionality. We also found that MvALP4 involved the biosynthesis of aphid alarm pheromones (EβF) in vitro and in vivo. Finally, transcriptome analysis in the stimulated and unstimulated aphids supported the involvement of MvALPs in the biosynthesis of aphid alarm pheromones. Our study identified a multifunctional ALP involved terpene synthase enzyme activity in the aphid, which contributes to the understanding of the functional plasticity of ALPs in insects.

Rapid Identification of Aphid Species by Headspace GC-MS and Discriminant Analysis

Aphids are a ubiquitous group of pests in agriculture that cause serious losses. For sustainable aphid identification, it is necessary to develop a precise and fast aphid identification tool. A new simple chemotaxonomy approach to rapidly identify aphids was implemented. The method was calibrated in comparison to the established phylogenetic analysis. For chemotaxonomic analysis, aphids were crushed, their headspace compounds were collected through closed-loop stripping (CLS) and analysed using gas chromatography-mass spectrometry (GC-MS). GC-MS data were then subjected to a discriminant analysis using CAP12.exe software, which identified key biomarkers that distinguish aphid species. A dichotomous key taking into account the presence and absence of a set of species-specific biomarkers was derived from the discriminant analysis which enabled rapid and reliable identification of aphid species. As the method overcomes the limits of morphological identification, it works with aphids at all life stages and in both genders. Thus, our method enables entomologists to assign aphids to growth stages and identify the life history of the investigated aphids, i.e., the food plant(s) they fed on. Our experiments clearly showed that the method could be used as a software to automatically identify aphids.

Farnesyl/geranylgeranyl diphosphate synthases regulate the biosynthesis of alarm pheromone in a unique manner in the vetch aphid Megoura viciae

Farnesyl/geranylgeranyl diphosphate synthases (FPPS/GGPPS) as the short-chain prenyltransferases catalyse the formation of the acyclic precursors (E)-FPP and (E)-GGPP for isoprenoid biosynthesis. Here, we first cloned the cDNAs encoding FPPS and GGPPS in the vetch aphid Megoura viciae (designated as MvFPPS and MvGGPPS). They had an open reading frame of 1185 and 930 bp in length, encoding 395 and 309 amino acids, with a theoretical isoelectric point of 6.52 and 6.21, respectively. Sequence alignment and phylogenetic analysis showed that MvFPPS and MvGGPPS shared the conserved aspartate-rich motifs characterized by all prenyltransferases identified to date and were clustered with their homologues in two large clades. RNA interference (RNAi) combined with gas chromatography/mass spectrometry (GC-MS) analysis showed that both MvFPPS and MvGGPPS were involved in the biosynthesis of alarm pheromone. Spatiotemporal expression profiling showed that the expression of MvFPPS and MvGGPPS was significantly higher in embryos than in other tissues. RNAi and GC-MS performed specifically in embryos corroborated the function of MvFPPS and MvGGPPS. In vitro, enzymatic activity assay and product analysis demonstrated that MvFPPS could catalysed the formation of (E)-FPP using DMAPP or (E)-GPP as the allylic cosubstrates in the presence of IPP, while MvGGPPS could only use (E)-GPP or (E)-FPP as cosubstrates. Functional interaction analysis using RNAi revealed that MvGGPPS exerts unidirectional functional compensation for MvFPPS. Moreover, it can regulate the biosynthesis of alarm pheromone by imposing a negative feedback regulation on MvFPPS. Our study helps to understand the molecular regulatory mechanism of terpenoid biosynthesis in the aphid.

Functional analysis of odorant-binding proteins for the parasitic host location to implicate convergent evolution between the grain aphid and its parasitoid Aphidius gifuensis

(E)-β-farnesene (EBF) is a typical and ecologically important infochemical in tri-trophic level interactions among plant-aphid-natural enemies. However, the molecular mechanisms by which parasitoids recognize and utilize EBF are unclear. In this study, we functionally characterized 8 AgifOBPs in Aphidifus gifuensis, one dominant endo-parasitoid of wheat aphid as well as peach aphid in China. Among which, AgifOBP6 was the only OBP upregulated by various doses of EBF, and it showed a strong binding affinity to EBF in vitro. The lack of homology between AgifOBP6 and EBF-binding proteins from aphids or from other aphid natural enemies supported that this was a convergent evolution among insects from different orders driven by EBF. Molecular docking of AgifOBP6 with EBF revealed key interacting residues and hydrophobic forces as the main forces. AgifOBP6 is widely expressed among various antennal sensilla. Furthermore, two bioassays indicated that trace EBF may promote the biological control efficiency of A. gifuensis, especially on winged aphids. In summary, this study reveals an OBP (AgifOBP6) that may play a leading role in aphid alarm pheromone detection by parasitoids and offers a new perspective on aphid biological control by using EBF. These results will improve our understanding of tri-trophic level interactions among plant-aphid-natural enemies.

Plant volatile ligands for male-biased MmedOBP14 stimulate orientation behavior of the parasitoid wasp Microplitis mediator

As an important class of chemosensory-associated proteins, odorant binding proteins (OBPs) play a key role in the perception of olfactory signals for insects. Parasitoid wasp Microplitis mediator relies on its sensitive olfactory system to locate host larvae of Noctuidae and Geometridae. In the present study, MmedOBP14, a male-biased OBP in M. mediator, was functionally investigated. In fluorescence competitive binding assays, the recombinant MmedOBP14 showed strong binding abilities to five plant volatiles: β-ionone, 3,4-dimethylacetophenone, 4-ethylacetophenone, acetophenone and ocimene. Homology modeling and molecular docking results indicated that the binding sites of all five ligands were similar and concentrated in the binding pocket of MmedOBP14. Except acetophenone, the remaining four ligands at 1, 10 and 100 μg/μL caused strong antennal electrophysiological responses in adults M. mediator, and males showed more obvious EAG responses to most ligands than females. In behavioral trials, males were attracted by low concentrations of MmedOBP14 ligands, whereas high doses of β-ionone and acetophenone had a repellent effect on males. Moreover, 1 μg/μL of 3,4-dimethylacetophenone showed the strongest attractiveness to female wasps. These findings suggest that MmedOBP14 may play a more important role in the perception of plant volatiles for male wasps to locate habitat, supplement nutrition and search partners.

Kairomonal Effect of Aphid Alarm Pheromones and Analogs on the Parasitoid Diaeretiella rapae

Aphid alarm pheromones, as important semiochemicals, not only mediate behavioral response of aphids, but can also act as kairomones to attract their natural enemies. The sesquiterpene (E)-β-farnesene (EβF), the major alarm pheromone component of most aphid species, has been shown to have a kairomonal effect on the predators of aphids, but other alarm pheromone components, especially the monoterpenes and analogs, are rarely investigated. Here, two EβF analogs were successfully synthesized via the nucleophilic substitution reaction, and we then examined the kairomonal effects of four alarm pheromone components and two EβF analogs on the aphid parasitoid, Diaeretiella rapae. In olfactory bioassays, D. rapae females generally showed no significant behavioral response to these alarm pheromone components and analogs under low concentrations (0.1 μg/μL). Nevertheless, their olfactory response to these compounds gradually enhanced with increasing concentrations. Among the four pheromone components, EβF showed the highest attractive activity, but the parasitoid preferred blends over single compounds. Moreover, the response time decreased as the concentration increased. We confirmed the kairomonal effect of monoterpene alarm pheromone components and their blends, in addition to EβF, on the natural enemies of aphids. This is the first report that the blend of alarm pheromone components and their analogs has a stronger kairomonal effect than do the single components on the natural enemies of aphids. This study contributes to our understanding of the mechanisms involved in the regulation of parasitoid behaviors by kairomones and provides a promising opportunity for designing kairomones for the aphid parasitoid to mediate aphid populations in the field.

Functional characterization of two different decaprenyl diphosphate synthases in the vetch aphid Megoura viciae

Long-chain polyprenyl diphosphate synthases play a critical role in the formation of the prenyl side-chain of ubiquinones, but up to date, their functions have scarcely been characterized in insects. Here, we first cloned the complementary DNAs encoding the subunits of decaprenyl diphosphate synthase (DPPS) in the vetch aphid Megoura viciae, an important agricultural pest insect. The results showed that there existed three DPPS subunits, designated as MvDPPS1, MvDPPS2a, and MvDPPS2b, with an open reading frame of 1218, 1275, and 1290 bp, and a theoretical isoelectric point of 7.91, 6.63, and 9.62, respectively. The sequences of MvDPPS1s from different aphid species were nearly identical, while the sequences of MvDPPS2a and MvDPPS2b shared only moderate sequence similarity. Phylogenetic analysis clearly separated MvDPPS2a and MvDPPS2b, indicating a functional differentiation between them. Functional coexpression analysis in Escherichia coli showed that MvDPPS1 plus MvDPPS2a and MvDPPS1 plus MvDPPS2b, respectively, catalyzed the formation of the prenyl side-chain of the ubiquinone coenzyme Q10 (CoQ10). Interestingly, MvDPPS1 plus MvDPPS2b catalyzed the formation of the prenyl side-chain of a ubiquinone other than CoQ10. RNA interference-mediated knockdown of MvDPPS2a imposed no significant effect on MvDPPS2b, and vice versa, suggesting no compensatory action between them. In the end, we detected the product CoQ10 in the aphid, the first identification of CoQ10 in an insect species. Taken together, we characterized two functional DPPSs in M. viciae, one of which might be multifunctional. Our study helps to understand the functional plasticity of the terpenoid backbone biosynthesis pathway in insects.

Spatial Expression Analysis of Odorant Binding Proteins in Both Sexes of the Aphid Parasitoid Aphidius gifuensis and Their Ligand Binding Properties

In China, Aphidius gifuensis is one of the most common endoparasitoids of the green peach aphid Myzus persicae and grain aphid Sitobion miscanthi in the field. Insect odorant-binding proteins (OBPs) play vital roles in odor perception during feeding, host searching, mating and oviposition. In addition, some OBPs are involved in other physiological processes such as gustation and reproduction. In the present study, a comparative antennal transcriptomic analysis was applied between male and female A. gifuensis. The spatial expression patterns among antennae, heads, thoraxes, abdomens and legs of OBPs in both sexes were further profiled. Fifteen AgifOBPs were predicted, and 14 of them were identified by gene cloning, including 12 classic OBPs and 2 min-C OBPs. As expected, all OBPs were mainly expressed at high levels in antennae, heads or legs which are sensory organs and tissues. Finally, ligand binding properties of 2 OBPs (AgifOBP7 and AgifOBP9) were further evaluated. Female leg specifically expressed AgifOBP9 displays a broad and high binding property to aphid alarm pheromones, plant green volatiles and aphid sex pheromones (Ki < 10 μΜ). However, female leg specifically expressed AgifOBP7 displays poor affinity for all tested ligands except CAU-II-11 ((E)-3,7-dimethylocta-2,6-dien-1-yl-2-hydroxy-3-methoxybenzoate), a reported (E)-β-farnesene (EBF) analog with an exceptionally high binding affinity (Ki = 1.07 ± 0.08 μΜ). In summary, we reported the spatial expression pattern of the OBP repertoire in A. gifuensis, and further studied the binding properties of OBP7 and OBP9, which are mainly expressed in female legs, laying the foundation for the dissection of the contribution of OBPs to chemosensation in A. gifuensis.