Pheromone Blend Analysis and Cross-Attraction among Populations of Maruca vitrata from Asia and West Africa

Zoophytophagous predator sex pheromone and visual cues of opposing reflectance spectra lure predator and invasive prey

INTRODUCTION

In sub-Saharan Africa, the invasive South American leafminer Phthorimaea absoluta is the most damaging tomato pest. Females of the pest can reproduce both sexually and through parthenogenesis and lay their eggs on all tomato plant parts. The mirid predator Nesidiocoris tenuis, a biological control agent for the pest, is also a tomato pest when prey population is low. To date, however, no study has developed an eco-friendly solution that targets both the predator and its host in a tomato farming system.

OBJECTIVE

To develop a bio-based management system for both pest and predator based on the combined use of sexual communication in the predator and visual cues.

METHODS

We collected volatiles from both sexes of the Kenyan population of the predator N. tenuis and identified candidate sex pheromone components by coupled gas chromatography-mass spectrometry (GC-MS). We used electrophysiological assays to identify antennally-active odorants in the volatiles, followed by field trials with different pheromone-baited colored traps to validate the responses of both predator and prey. Thereafter, we compared the reflectance spectra of the colored traps with those of different tomato plant tissues.

RESULTS

Our results reveal an interplay between different sensory cues which in the predator-prey interaction may favor the predator. Antennae of both sexes of predator and prey detect the predator sex pheromone identified as 1-octanol and hexyl hexanoate. Unexpectedly, our field experiments led to the discovery of a lure for P. absoluta females, which were lured distinctly into a pheromone-baited trap whose reflectance spectrum mimicked that of ripe tomato fruit (long wavelength), an egg-laying site for females. Contrastingly, N. tenuis males were lured into baited white trap (short wavelength) when the predator is actively searching for prey.

CONCLUSION

Our results demonstrate the novel use of a predator sex pheromone and different visual cues to assess complex trophic interactions on tomatoes.

Functional Analysis of Pheromone Biosynthesis Activating Neuropeptide Receptor Isoforms in Maruca vitrata

Insect sex pheromones are volatile chemicals that induce mating behavior between conspecific individuals. In moths, sex pheromone biosynthesis is initiated when pheromone biosynthesis-activating neuropeptide (PBAN) synthesized in the suboesophageal ganglion binds to its receptor on the epithelial cell membrane of the pheromone gland. To investigate the function of PBAN receptor (PBANR), we identified two PBANR isoforms, MviPBANR-B and MviPBANR-C, in the pheromone glands of Maruca vitrata. These two genes belong to G protein-coupled receptors (GPCRs) and have differences in the C-terminus but share a 7-transmembrane region and GPCR family 1 signature. These isoforms were expressed in all developmental stages and adult tissues. MviPBANR-C had the highest expression level in pheromone glands among the examined tissues. Through in vitro heterologous expression in HeLa cell lines, only MviPBANR-C-transfected cells responded to MviPBAN (≥5 µM MviPBAN), inducing Ca²⁺ influx. Sex pheromone production and mating behavior were investigated using gas chromatography and a bioassay after MviPBANR-C suppression by RNA interference, which resulted in the major sex pheromone component, E10E12-16:Ald, being quantitatively reduced compared to the control, thereby decreasing the mating rate. Our findings indicate that MviPBANR-C is involved in the signal transduction of sex pheromone biosynthesis in M. vitrata and that the C-terminal tail plays an important role in its function.

Suppression of pheromone biosynthesis and mating behavior by RNA interference of pheromone gland-specific fatty acyl reductase in Maruca vitrata

In moths, various enzymes, such as fatty acid synthases, fatty acyl desaturases, and fatty acyl reductases (FARs), are involved in pheromone biosynthesis. In particular, pheromone gland-specific FAR (pgFAR) plays an important role in converting the functional group from carboxylic to alcohol during pheromone biosynthesis. A novel pgFAR of Maruca vitrata, Mvi-pgFAR, was identified through transcriptome sequencing of its pheromone gland. To investigate the involvement of Mvi-pgFAR in pheromone biosynthesis, Mvi-pgFAR was cloned from the pheromone gland and suppressed by RNA interference (RNAi). Mvi-pgFAR harbored several conserved motifs related to NAD(P)H-binding, N-glycosylation, and adenosine / guanosine triphosphate binding. Phylogenetic analysis revealed that Mvi-pgFAR with other lepidopteran pgFARs formed an independent clade. Mvi-pgFAR was specifically expressed only in the pheromone gland. Quantitative real-time polymerase chain reaction showed that the diurnal expression levels of Mvi-pgFAR in the pheromone gland were the highest at 2 h before the scotophase. After primarily confirming Mvi-pgFAR suppression by RNAi, (E,E)-10,12-hexadecadienal (E10E12-16:Ald), a major sex pheromone component, was quantified by gas chromatography. When Mvi-pgFAR was successfully suppressed, E10E12-16:Ald production was reduced by up to half of that of the control, and the mating rate was subsequently decreased. Our results demonstrate that Mvi-pgFAR downregulation can suppress mating behavior by changing the relative sex pheromone component ratio, suggesting that Mvi-pgFAR can be used as a novel control target.

Emergence of Maruca vitrata as a Major Pest of Food Legumes and Evolution of Management Practices in Asia and Africa

Legume pod borer, Maruca vitrata, has emerged as a major pest on food legumes in Asia and Africa. It is an oligophagous pest, feeding on over 70 species in Fabaceae. We examine the species complex in Asia, Africa, Oceania, and the Americas, with an emphasis on molecular taxonomy. Studies on pheromone production and perception suggest the existence of pheromone polymorphism, especially in Asia and Africa. No Maruca-resistant varieties are available in the major food legumes including cowpea, pigeonpea, mungbean, and yard-long bean. Legume growers use chemical pesticides indiscriminately, leading to the development of pesticide resistance. However recent developments in habitat management, classical biocontrol with more efficient parasitoids, biopesticides, and judicious use of insecticides pave the way for sustainable management of M. vitrata, which can reduce the pesticide misuse. Active engagement of the private sector and policy makers can increase the adoption of integrated pest management approaches in food legumes.

Effect of Pheromone-Mediated Mating Disruption on Pest Population Density of Maruca vitrata (Fabricius) (Crambidae: Lepidoptera)

Simple Summary

The legume pod borer is one of the most serious legume pests widely distributed in Asia, Africa, Australia, America, and Oceania. The use of synthetic pheromone lures has been developed as a more environmentally friendly alternative for its control. In this study, we evaluated the potential of the pheromone components as a mating disruption tool under laboratory and small-scale field conditions by identifying effective blends made out of single pheromone components or a different mix of them. The results from the laboratory experiment show that insects challenged with the blend ratio of 1:1:1 had lower fecundity and egg eclosion. A small-scale caged field experiment also showed a significantly disruption of normal mating with the above-mentioned ratio, leading to lower flower and pod damage, and higher mungbean yield. This study provides new evidence about the possibility to use pheromone components for mating disruption; however, more research is needed to determine appropriate ratios of pheromone blend to increase the effect of disruption. From an applied research perspective, more studies are needed to investigated the effectiveness of mating disruption strategy on different legume crops, dispenser types, release points in the field, and compatibility with conventional insecticides as part of an integrated pest management (IPM) combined approach.

Abstract

The legume pod borer (Maruca vitrata) is one of the most serious legume pests due to its wide host range and high damage potential. Pheromone components on M. vitrata have been previously identified, allowing research on more environmentally friendly IPM tools for its control. M. vitrata produces a three-component pheromone blend containing (E, E)-10,12-hexadecadienal (major), (E, E)-10,12-hexadecadienol (minor), (E)-10-hexadecenal (minor). This study focused on the efficacy of synthetic pheromone lures and their blend components for mating disruption in M. vitrata. Under laboratory conditions, the mating behavior of M. vitrata pairs was observed from 18:00 to 02:00 h in an interval of 20 min to assess the efficacy of different pheromone lures. The scotophase behavior results show that the complete pheromone blend (E, E)-10,12-hexadecadienal + (E, E)-10,12-hexadecadienol + (E)-10-hexadecenal with a blend ratio of 1:1:1 effectively disrupted mating. The impact on mating disruption was evident from the lower fecundity and egg hatch/eclosion. The same lures were evaluated in a small-scale caged field study. The results show that the pheromone blend of (E, E)-10,12-hexadecadienal + (E, E)-10,12-hexadecadienol + (E)-10-hexadecenal in a1:1:1 ratio significantly disrupted the normal mating, leading to lower flower and pod damage and higher mung bean yield.

Identification of Semiochemicals from Cowpea, Vigna unguiculata, for Low-input Management of the Legume Pod Borer, Maruca vitrata

Cowpea, Vigna unguiculata L. Walp. (Fabaceae), is one of the most important food legumes grown on the African continent, as it provides an affordable source of dietary protein. Yields of cowpea are significantly reduced through damage by legume pod-borer, Maruca vitrata (Lepidoptera: Crambidae), caterpillars to flowers, tender leaves and pods. Semiochemical-based strategies are considered as environmentally benign and affordable for pest management, particularly on smallholder farms. In this study, we investigated the importance of cowpea flower volatiles as host location cues for egg-laying M. vitrata, and herbivore-induced plant volatiles (HIPVs) as M. vitrata repellents and natural enemy (Apanteles taragamae and Phanerotoma syleptae parasitoid) attractants. In oviposition choice assays, M. vitrata laid more eggs on flowering cowpea plants than non-flowering plants. Coupled gas chromatography-electrophysiology (GC-EAG) analysis using the antennae of female M. vitrata and an extract of flower volatiles collected by dynamic headspace collection revealed the presence of five EAG-active components that were identified by coupled GC-mass spectrometry (GC-MS) analysis as benzaldehyde, benzyl alcohol, acetophenone, a vinylbenzaldehyde isomer and (E)-cinnamaldehyde. A synthetic blend of the identified compounds, prepared using 3-vinylbenzaldehyde, induced M. vitrata to lay as many eggs on non-flowering cowpea as on flowering plants. The moths also preferred laying eggs on intact plants compared to M. vitrata-infested plants. As the emission of EAG-active floral compounds was determined to be lower in the headspace of infested cowpea flowers, the role of HIPVs emitted by M. vitrata-damaged leaves was also investigated. Of the compounds induced by larval damage, (E)-DMNT, indole, n-hexyl acetate, 1-octen-3-ol and linalool were shown by GC-EAG to possess electrophysiological activity. A synthetic blend of the EAG-active compounds, using racemic 1-octen-3-ol and linalool, significantly reduced egg numbers on flowering cowpea. Larval and egg parasitoids, i.e. A. taragamae and Ph. syleptae, respectively, of M. vitrata both preferred the Y-tube olfactometer arm treated with synthetic (E)-DMNT, whereas preference for racemic linalool and (E)-nerolidol was dose-dependent in A. taragamae. Our results provide the platform for the development of future semiochemical-based pest management strategies against M. vitrata on smallholder farms in West Africa.

Pheromone-binding proteins based phylogenetics and phylogeography of Maruca spp. from Asia, Africa, Oceania, and South America

Variations in the functional response of legume pod borer (Maruca vitrata) populations to sex pheromone blends were observed in Asia and Africa. Hence, this study was carried out to understand the differences in pheromone-binding proteins (PBPs) among Maruca populations in Asia, Africa, Oceania, and South America. A de novo transcriptome assembly was adopted to sequence the entire transcribed mRNAs in M. vitrata from Taiwan. The raw-sequence data were assembled using homologous genes from related organisms in GenBank to detect M. vitrata PBPs (MvitPBPs). Sections of the cDNA of MvitPBP of different length were used to design primers to amplify the full-length cDNA of PBPs. All three PBP sequences comprised three exons interspersed by two introns. In total, 92 MvitPBP1 haplotypes, 77 MvitPBP2 haplotypes, and 64 MvitPBP3 haplotypes were identified in 105, 98, and 68 Maruca individuals, respectively. High pairwise F ST values (0.41-0.73) and phylogenetic analyses distinguished the putative Maruca species in South America from those occurring in rest of the world, and possibly two putative subspecies in Asia and Africa. The haplotype networks and Automatic Barcode Gap Discovery analyses also confirmed these results. The negative Tajima's D and Fu's F S values showed the recent demographic expansion of Maruca populations. Thus, this study confirmed the presence of different Maruca species and/or subspecies in different continents based on the diversity within PBP genes. Additional sampling and studies are suggested for Oceania and South America. The genetic differences among Maruca populations should be carefully considered while using sex pheromone lures and bio-control agents.

Key genes of the sex pheromone biosynthesis pathway in female moths are required for pheromone quality and possibly mediate olfactory plasticity in conspecific male moths in Spodoptera litura

Ninety sex pheromone biosynthesis genes in Spodoptera litura were identified in transcriptome data and were investigated and classified into the following five groups: fatty acid synthase, beta oxidase, fatty acyl-coenzyme A (CoA) reductase, desaturase and acetyl-CoA acetyltransferase. Fourteen female-specific genes were identified through semi-quantitative PCR, and 15 additional genes had expression levels that were 3- to 10-fold higher in females than in males. The majority of the genes had higher expression levels in the sex pheromone glands. Injection of double-stranded RNA (dsRNA) against nine selected genes showed that down-regulation of Desaturase 3 (Des3), Des5 or fatty acyl coenzyme A reductase 17 (FAR17) significantly changed the ratio of the four sex pheromone components (Z,E) -9,11-tetradecadienyl acetate (Z9E11-14:Ac), (Z,E)-9,12-Tetradecadienyl acetate(Z9E12-14:Ac), (Z)-9-tetradecenyl acetate (Z9-14:Ac), (E)-11-Tetradecenyl acetate(E11-14:Ac). These key genes were differentially expressed in female moths collected from different geographical regions. Furthermore, field bioassays demonstrated geographical variation in the olfactory profile of male moths in response to the different sex pheromone mixtures, which therefore indicates that a significant variation in the sex pheromone components exists in the natural population. Our results suggest that a change in the expression of these key genes, Des3, Des5 and FAR17, in the sex pheromone biosynthesis pathway could change the ratio of the sex pheromone components. We surmise that the differential expression levels of the key genes of the sex pheromone biosynthesis pathway may lead to differential ratios of the sex pheromones in the field. Our field trapping experiment suggested that the change of the ratio of the sex pheromone components may have been adapted by the olfactory system and possibly mediate olfactory plasticity in conspecific male moths.