A Rapid Diagnostic Technique to Discriminate between Two Pests of Palms, Brontispa longissima and Octodonta nipae (Coleoptera: Chrysomelidae), for Quarantine Applications

A chromosome-level genome assembly of the Brontispa longissima

Brontispa longissima is a highly destructive pest that affects coconut and ornamental palm plants. It is widely distributed across Southeast and East Asia and the Pacific region, causing production losses of up to 50-70%. While control methods and ecological phenomena have been the primary focus of research, there is a significant lack of studies on the molecular mechanisms underlying these ecological phenomena. The absence of a reference genome has also hindered the development of new molecular-targeted control technologies. In this study, we conducted a karyotype analysis of B. longissima and assembled the first high-quality chromosome-level genome. The assembled genome is 582.24 Mb in size, with a scaffold N50 size of 63.81 Mb, consisting of 10 chromosomes and a GC content of 33.71%. The BUSCO assessment indicated a completeness estimate of 98.1%. A total of 23,051 protein-coding genes were predicted. Our study provides a valuable genomic resource for understanding the mechanisms of adaptive evolution and facilitates the development of new molecular-targeted control methods for B. longissima.

A chromosome-scale genome assembly of the nipa palm hispid beetle Octodonta nipae

Nipa palm hispid beetle (Octodonta nipae) is an insect species that is native to Malaysia but has spread to southern China and beyond, seriously threatening palm production. A lack of high-quality genome resources has hindered understanding of the insect's invasive characteristics and ecological adaptations. Here, we combined Illumina short read, PacBio long-read, and high-throughput chromosome conformation capture (Hi-C) sequencing technologies to generate a high-quality, chromosome-scale genome assembly of nipa palm hispid beetle. The genome assembly was 1.31 Gb in size, consisting of nine chromosomes. The contig and scaffold N50 values were 1.022 Mb and 148.6 Mb, respectively. The genome assembly completeness was estimated at 99.1%. Annotation revealed 16,305 protein-coding genes and 62.16% repeat sequences. This high-quality genome assembly is a valuable resource that will contribute to understanding of the genetic factors underlying the invasive characteristics of nipa palm hispid beetle, ultimately promoting development of efficient control policies.

The Entomopathogenic Nematodes H. bacteriophora and S. carpocapsae Inhibit the Activation of proPO System of the Nipa Palm Hispid Octodonta nipae (Coleoptera: Chrysomelidae)

Entomopathogenic nematodes are biocontrol agents of invasive insect pests in soil and cryptic habitats. Nipa palm hispid, Octodonta nipae, is a pest of palm trees in Sothern China. To address its increasing damage, environmentally friendly control methods are required. This study aimed to test efficacy of Heterorhabditis bacteriophora and Steinernema carpocapsae on O. nipae and investigated the influence of secondary metabolites, nematodes, and their isolated cuticles on the activation of O. nipae’s prophenoloxidase system using qPCR analysis. Our data revealed that O. nipae were less susceptible to H. bacteriophora than S. carpocapsae and penetrations of infective juveniles were higher with S. carpocapsae treatment than H. bacteriophora. Moreover, expression levels of the serine protease P56, prophenoloxidase activation factor 1, PPO and serine protease inhibitor 28 upon S. carpocapsae and H. bacteriophora infections were generally downregulated at all times. However, upon heating, the cuticles lost their inhibitory effects and resulted in upregulation of the PPO gene. Similarly, the addition of arachidonic acid reversed the process and resulted in the upregulation of the PPO gene compared to the control. Further work is needed to identify toxic substances secreted by these EPNs to evade O. nipae’s immune system.

Exploring the Role of Relish on Antimicrobial Peptide Expressions (AMPs) Upon Nematode-Bacteria Complex Challenge in the Nipa Palm Hispid Beetle, Octodonta nipae Maulik (Coleoptera: Chrysomelidae)

The humoral immune responses of the nipa palm hispid beetle Octodonta nipae involves the inducible expression of the genes coding for antimicrobial peptides (AMPs) which are mediated by immune deficiency signaling pathways. In insects, the nuclear factor-κB (NF-κB) transcription factor, Relish, has been shown to regulate AMP gene expressions upon microbial infections. Here, we dissect the expression patterns of some AMPs in O. nipae during infections by entomopathogenic nematodes (EPNs) and their symbionts, before and after Relish knock down. Our results indicate that, prior to gene silencing, the AMPs attacin C1, attacin C2, and defensin 2B were especially expressed to great extents in the insects challenged with the nematodes Steinernema carpocapsae and Heterorhabditis bacteriophora as well as with their respective symbionts Xenorhabdus nematophila and Photorhabdus luminescens. The study also established the partial sequence of OnRelish/NF-κB p110 subunit in O. nipae, with an open reading frame coding for a protein with 102 amino acid residues. A typical Death domain-containing protein was detected (as seen in Drosophila) at the C-terminus of the protein. Phylogenetic analysis revealed that in O. nipae, Relish is clustered with registered Relish/NF-κB p110 proteins from other species of insect especially Leptinotarsa decemlineata from the same order Coleoptera. Injection of OnRelish dsRNA remarkably brought down the expression of OnRelish and also reduced the magnitude of transcription of attacin C1 and defensin 2B upon S. carpocapsae and H. bacteriophora and their symbionts infections. Altogether, our data unveil the expression pattern of OnRelish as well as that of some AMP genes it influences during immune responses of O. nipae against EPNs and their symbionts.

Areca Palm Necrotic Ringspot Virus, Classified Within a Recently Proposed Genus Arepavirus of the Family Potyviridae, Is Associated With Necrotic Ringspot Disease in Areca Palm

Areca palm (Areca catechu), one of the two most important commercial crops in Hainan, China, has been severely damaged by a variety of pathogens and insects. Here, we report a new disease, tentatively referred to as areca palm necrotic ringspot disease (ANRSD), which is highly epidemic in the main growing regions in Hainan. Transmission electron microscopy observation and small RNA deep sequencing revealed the existence of a viral agent of the family Potyviridae in a diseased areca palm plant (XC1). The virus was tentatively named areca palm necrotic ringspot virus (ANRSV). Subsequently, the positive-sense single-stranded genome of ANRSV isolate XC1 was completely determined. The genome annotation revealed the existence of two cysteine proteinases in tandem (HC-Pro1 and HC-Pro2) in the genomic 5' terminus of ANRSV. Sequence comparison and phylogenetic analysis suggested the taxonomic classification of ANRSV into the recently proposed genus Arepavirus in the family Potyviridae. Given the close relationship of ANRSV with another newly reported arepavirus (areca palm necrotic spindle-spot virus), the exact taxonomic status of ANRSV needs to be further investigated. In this study, a reverse transcription polymerase chain reaction assay for ANRSV-specific detection was developed and a close association between ANRSV and ANRSD was found.

The Endosymbiotic Wolbachia and Host COI Gene Enables to Distinguish Between Two Invasive Palm Pests; Coconut Leaf Beetle, Brontispa longissima and Hispid Leaf Beetle, Octodonta nipae

To elucidate taxonomic eminence of identical pest species is essential for many ecological and conservation studies. Without proficient skills, accurate molecular identification and characterization are laborious and time-consuming. The coconut leaf beetle, Brontispa longissima (Gestro) (Coleoptera: Chrysomelidae), is biologically and morphologically identical to hispid leaf beetle, Octodonta nipae (Maulik) (Coleoptera: Chrysomelidae), and is known as the most harming nuisances of palm cultivation worldwide. The present examination was to establish Wolbachia genotyping analysis along with host cytochrome oxidase subunit I (COI) gene for accurate identification between these individuals of the same family (Chrysomelidae). Here, we have cloned and sequenced a gene coding Wolbachia surface protein (wsp) and COI gene regions amplified from both species by polymerase chain reaction. The nucleotide sequences were directly determined (≈600 bp for wsp and ≈804 bp for COI) and aligned using the multiple alignment algorithms in the ESPript3 package and the MEGA5 program. Comparative sequence analysis indicated that the representative of wsp and COI sequences from these two beetles were highly variable. To ensure this bacterial variation, multilocus sequence typing (MLST) of bacterial genes was conducted, and the results vindicated the same trend of variations. Furthermore, the phylogenetic analysis also indicates that B. longissima and O. nipae being the two different species harbors two distinct Wolbachia Hertig and Burt (Rickettsiales: Anaplamataceae) supergroups B and A, respectively. The present outcomes quickly discriminate between these two species. Considering its simplicity and cost-effectiveness, it can be used as a diagnostic tool for discriminating such invasive species particularly B. longissima and O. nipae which has overlapping morphologic characters.

Entomopathogenic nematode Steinernema carpocapsae surpasses the cellular immune responses of the hispid beetle, Octodonta nipae (Coleoptera: Chrysomelidae)

The Nipa palm hispid, Octodonta nipae (Maulik) is an important invasive pest of palm trees particularly in Southern China. How this beetle interacts with invading pathogens via its immune system remains to be dissected. Steinernema carpocapsae is a pathogenic nematode that attacks a number of insects of economic importance. The present study systematically investigates the cellular immune responses of O. nipae against S. carpocapsae infection using combined immunological, biochemical and transcriptomics approaches. Our data reveal that S. carpocapsae efficiently resists being encapsulated and melanized within the host's hemolymph and most of the nematodes were observed moving freely in the hemolymph even at 24 h post incubation. Consistently, isolated cuticles from the parasite also withstand encapsulation by the O. nipae hemocytes at all-time points. However, significant encapsulation and melanization of the isolated cuticles were recorded following heat treatment of the cuticles. The host's phenoloxidase activity was found to be slightly suppressed due to S. carpocapsae infection. Furthermore, the expression levels of some antimicrobial peptide (AMP) genes were significantly up-regulated in the S. carpocapsae-challenged O. nipae. Taken together, our data suggest that S. carpocapsae modulates and surpasses the O. nipae immune responses and hence can serve as an excellent biological control agent of the pest.

Combination of label-free quantitative proteomics and transcriptomics reveals intraspecific venom variation between the two strains of Tetrastichus brontispae, a parasitoid of two invasive beetles

The venom apparatus is a conserved organ in parasitoids that shows adaptations correlated with life-style diversification. Combining transcriptomics and label-free quantitative proteomics, here we explored the venom apparatus components of the endoparasitoid Tetrastichus brontispae (Eulophidae), and provide a comparison of the venom apparatus proteomes between its two closely related strains, T. brontispae-Octodonta nipae (Tb-On) and T. brontispae-Brontispa longissima (Tb-Bl). Tb-Bl targets the B. longissima pupa as its habitual host. However, Tb-On is an experimental derivative of Tb-Bl, which has been exposed to the O. nipae pupa as host consecutively for over 40 generation. Results showed that approximately 1505 venom proteins were identified in the T. brontispae venom apparatus. The extracts contained novel venom proteins, such as 4-coumarate-CoA ligase 4. A comparative venom proteome analysis revealed that significant quantitative and qualitative differences in venom composition exist between the two strains; although the most abundant venom proteins were shared between them. The differentially produced proteins were mainly enriched in fatty acid biosynthesis and melanotic encapsulation response. Six of these enriched proteins presented increased levels in Tb-On, and this result was validated by parallel reaction monitoring (PRM) analysis. Overall, our data reveal that venom composition can evolve quickly and respond to host selection.

Effects of ovarian fluid, venom and egg surface characteristics of Tetrastichus brontispae (Hymenoptera: Eulophidae) on the immune response of Octodonta nipae (Coleoptera: Chrysomelidae)

Although the importance of parasitoids as biocontrol agents has long been recognized, systematic studies of the physiological mechanisms are scarce, especially in those parasitoids that are able to successfully invade their hosts by activating host immune responses. This study explored this phenomenon by investigating the effects of ovarian fluid, venom and egg surface characteristics of Tetrastichus brontispae (Hymenoptera: Eulophidae) on host immunity. The results showed that the injection of venom alone induced higher phenoloxidase activity, while a mixture of ovarian plus venom fluids provoked higher granulocyte and plasmatocyte spreading ratios, highlighting the role that egg surface characteristics may play in successful parasitism. After thorough investigation, the presence of a hemomucin homologue was documented on the egg surface (which was named Tetrastichus brontispae adipocyte plasma membrane associated protein-like, TbAPMAP-like), while the absence of polydnaviruses, fibrous layers and virus-like filaments was confirmed. The higher encapsulation index of eggs incubated with TbAPMAP-like polyclonal antibody demonstrated the protection of the protein against encapsulation. These results contribute to our understanding of the mechanisms used by endoparasitoids to evade encapsulation during the early parasitism stage while enriching our knowledge of local active regulatory mechanisms. It is likely that this is the first study to determine the egg protective properties of TbAPMAP-like in host-parasite systems.

Development of Microsatellite Markers for the Nipa Palm Hispid Beetle, Octodonta nipae (Maulik)

The nipa palm hispid beetle, Octodonta nipae (Maulik) (Coleoptera: Chrysomelidae), is an important invasive pest on palm plants in southern China. Based on existing transcriptome data, polymorphism simple sequence repeat (SSR) loci were identified. In total, 1274 SSR loci were identified from 49919 unigenes. The majority of them contained mononucleotide, dinucleotide, and trinucleotide motifs (43.56%, 26.14%, and 28.18%), in which A/T (41.21%) and AT/TA (15.86%) were the most abundant motifs. 104 pairs of the SSR primers produced amplification bands of expected sizes in O. nipae, 80 pairs of SSR primers were tested randomly for polymorphism, 9 loci of them were validated to be polymorphic markers, and the number of alleles ranged from 2 to 3, with an average of 2.56 per locus. The population of Zhangzhou and Fuzhou was analyzed by the 9 loci (On1-On9). These SSR transcriptome data can provide invaluable resource for SSR development, population genetics research, invasion and expansion mechanism, paternity testing, and other research on O. nipae and its related species.

Genomic evaluations of Wolbachia and mtDNA in the population of coconut hispine beetle, Brontispa longissima (Coleoptera: Chrysomelidae)

Wolbachia pipientis is a diverse, ubiquitous and most prevalent intracellular bacterial group of alpha-Proteobacteria that is concerned with many biological processes in arthropods. The coconut hispine beetle (CHB), Brontispa longissima (Gestro) is an economically important pest of palm cultivation worldwide. In the present study, we comprehensively surveyed the Wolbachia-infection prevalence and mitochondrial DNA (mtDNA) polymorphism in CHB from five different geographical locations, including China's Mainland and Taiwan, Vietnam, Thailand, Malaysia and Indonesia. A total of 540 sequences were screened in this study through three different genes, i.e., cytochrome oxidase subunit I (COI), Wolbachia outer surface protein (wsp) and multilocus sequencing type (MLST) genes. The COI genetic divergence ranges from 0.08% to 0.67%, and likewise, a significant genetic diversity (π = 0.00082; P = 0.049) was noted within and between all analyzed samples. In the meantime, ten different haplotypes (H) were characterized (haplotype diversity = 0.4379) from 21 different locations, and among them, H6 (46 individuals) have shown a maximum number of population clusters than others. Subsequently, Wolbachia-prevalence results indicated that all tested specimens of CHB were found positive (100%), which suggested that CHB was naturally infected with Wolbachia. Wolbachia sequence results (wsp gene) revealed a high level of nucleotide diversity (π = 0.00047) under Tajima's D test (P = 0.049). Meanwhile, the same trend of nucleotide diversity (π = 0.00041) was observed in Wolbachia concatenated MLST locus. Furthermore, phylogenetic analysis (wsp and concatenated MLST genes) revealed that all collected samples of CHB attributed to same Wolbachia B-supergroup. Our results strongly suggest that Wolbachia bacteria and mtDNA were highly concordant with each other and Wolbachia can affect the genetic structure and diversity within the CHB populations.

Descriptions of immature stages of Octodonta nipae (Maulik) (Coleoptera, Chrysomelidae, Cassidinae, Cryptonychini)

Octodonta nipae (Maulik, 1921), a hispid that damages several species of palm trees, was introduced accidently into China in 2001. The egg, larva, prepupa and pupa of O. nipae are illustrated and described in detail and compared with another invasive species, Brontispa longissima (Gestro, 1885); the scanning electron micrographs of the head capsule, antenna, maxilla, labium and lateral scoli are provided, as well as photos of body of all larval instars and pupa. It is the second description of immature stages in the genus Octodonta Chapuis.

A novel bacterial symbiont association in the hispid beetle, Octodonta nipae (Coleoptera: Chrysomelidae), their dynamics and phylogeny

The hispid leaf beetle, Octodonta nipae (Maulik), (Coleoptera: Chrysomelidae), is a devastating pest of palm cultivation worldwide. Endosymbiotic bacteria in the genus Wolbachia are arguably one of the most abundant bacterial group associated with arthropods. Owing to its critical effects on host reproduction, Wolbachia has garnered much attention as a prospective future tool for insect pest management. However, their association, infection dynamics, and functionality remain unknown in this insect pest. Here, we diagnosis for the first time, the infection prevalence, and occurrence of Wolbachia in O. nipae. Experimental evidence by the exploration of wsp gene vindicate that O. nipae is naturally infected with bacterial symbiont of genus Wolbachia, showing a complete maternal inheritance with shared a common Wolbachia strain (wNip). Moreover, MLST (gatB, fbpA, coxA, ftsZ, and hcpA) analysis enabled the detections of new sequence type (ST-484), suggesting a particular genotypic association of O. nipae and Wolbachia. Subsequently, quantitative real-time PCR (qPCR) assay demonstrated variable infection density across different life stages (eggs, larvae, pupae and adult male and female), body parts (head, thorax, abdomen), and tissues (ovaries, testes, and guts). Infection density was higher in egg and female adult stage, as well as abdomen and reproductive tissues as compared to other samples. Interestingly, Wolbachia harbored dominantly in a female than the male adult, while, no significant differences were observed between male and female body parts and tissues. Phylogeny of Wolbachia infection associated with O. nipae rectified from all tested life stages were unique and fall within the same monophyletic supergroup-A of Wolbachia clades. The infection density of symbiont is among the valuable tool to understand their biological influence on hosts, and this latest discovery would facilitate the future investigations to understand the host-symbiont complications and its prospective role as a microbiological agent to reduce pest populations.

An entomopathogenic bacterium strain, Bacillus thuringiensis, as a biological control agent against the red palm weevil, Rhynchophorus ferrugineus (Coleoptera: Curculionidae)

BACKGROUND

The red palm weevil (RPW), Rhynchophorus ferrugineus, is an invasive wood-boring insect that damages palms and sugarcane. Bacillus thuringiensis (Bt) is an entomopathogenic bacterium which has been modified into various strains and widely used in pest management. The aim of this study was to evaluate the susceptibility of RPW to the HA strain of Bt.

RESULTS

Five concentrations of Bt bioassays were used on RPW eggs, second instars and fourth instars. Average egg hatching rates exceeded 85% using Bt suspensions or distilled water. Hatch times were extended significantly using higher Bt concentrations. For second instar larvae, the LC₅₀ was 4.92 × 10⁹ CFU mL^-1 15 d after feeding; the LT₅₀ values decreased with each higher concentration. The corrected mortality of second instars increased significantly with increased concentrations after 15 d, ranging from 16.97% to 94.32%. Significant differences occurred in the boring activity of fourth instars when dipped in Bt suspensions or crawling on treated sugarcane. Bacterial infection in dead larvae was confirmed using molecular techniques.

CONCLUSION

Our results indicated that Bt can be used in RPW control as a potential biological control agent and can effectively reduce palm trees damage. © 2016 Society of Chemical Industry.