Taxonomy, Biology, Symbionts, Omics, and Management of Rhynchophorus Palm Weevils (Coleoptera: Curculionidae: Dryophthorinae).

Palm weevils, Rhynchophorus spp., are destructive pests of native, ornamental, and agricultural palm species. Of the 10 recognized species, two of the most injurious species, Rhynchophorus ferrugineus and Rhynchophorus palmarum, both of which have spread beyond their native range, are the best studied. Due to its greater global spread and damage to edible date industries in the Middle East, R. ferrugineus has received more research interest. Integrated pest management programs utilize traps baited with aggregation pheromone, removal of infested palms, and insecticides. However, weevil control is costly, development of resistance to insecticides is problematic, and program efficacy can be impaired because early detection of infestations is difficult. The genome of R. ferrugineus has been sequenced, and omics research is providing insight into pheromone communication and changes in volatile and metabolism profiles of weevil-infested palms. We outline how such developments could lead to new control strategies and early detection tools.

Omics in the Red Palm Weevil Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae): A Bridge to the Pest.

The red palm weevil (RPW), Rhynchophorus ferrugineus (Coleoptera: Curculionidae), is the most devastating pest of palm trees worldwide. Mitigation of the economic and biodiversity impact it causes is an international priority that could be greatly aided by a better understanding of its biology and genetics. Despite its relevance, the biology of the RPW remains poorly understood, and research on management strategies often focuses on outdated empirical methods that produce sub-optimal results. With the development of omics approaches in genetic research, new avenues for pest control are becoming increasingly feasible. For example, genetic engineering approaches become available once a species's target genes are well characterized in terms of their sequence, but also population variability, epistatic interactions, and more. In the last few years alone, there have been major advances in omics studies of the RPW. Multiple draft genomes are currently available, along with short and long-read transcriptomes, and metagenomes, which have facilitated the identification of genes of interest to the RPW scientific community. This review describes omics approaches previously applied to RPW research, highlights findings that could be impactful for pest management, and emphasizes future opportunities and challenges in this area of research.

Microsatellite Variation in the Most Devastating Beetle Pests (Coleoptera: Curculionidae) of Agricultural and Forest Crops.

Weevils, classified in the family Curculionidae (true weevils), constitute a group of phytophagous insects of which many species are considered significant pests of crops. Within this family, the red palm weevil (RPW), Rhynchophorus ferrugineus, has an integral role in destroying crops and has invaded all countries of the Middle East and many in North Africa, Southern Europe, Southeast Asia, Oceania, and the Caribbean Islands. Simple sequence repeats (SSRs), also termed microsatellites, have become the DNA marker technology most applied to study population structure, evolution, and genetic diversity. Although these markers have been widely examined in many mammalian and plant species, and draft genome assemblies are available for many species of true weevils, very little is yet known about SSRs in weevil genomes. Here we carried out a comparative analysis examining and comparing the relative abundance, relative density, and GC content of SSRs in previously sequenced draft genomes of nine true weevils, with an emphasis on R. ferrugineus. We also used Illumina paired-end sequencing to generate draft sequence for adult female RPW and characterized it in terms of perfect SSRs with 1-6 bp nucleotide motifs. Among weevil genomes, mono- to trinucleotide SSRs were the most frequent, and mono-, di-, and hexanucleotide SSRs exhibited the highest GC content. In these draft genomes, SSR number and genome size were significantly correlated. This work will aid our understanding of the genome architecture and evolution of Curculionidae weevils and facilitate exploring SSR molecular marker development in these species.

Complete mitochondrial genome of the longhorn date palm stem borer Jebusaea hammerschmidtii (Reiche, 1878).

The 15,619 bp mitochondrial genome of Jebusaea hammerschmidtii was assembled from short reads, annotated, and compared to the genomes of other longhorn beetles (Cerambycidae). Gene content was typical of animal mitochondrial genomes and contained 13 protein-coding, 22 tRNA, and 2 rRNA genes. Gene organization was identical to that of other longhorn beetles. Phylogenetic analysis placed J. hammerschmidtii within the subfamily Cerambycinae, and strongly supported the monophyly of the Cerambycinae, Lamiinae, and Prioninae subfamilies.

Haplotype-resolved genome assembly enables gene discovery in the red palm weevil Rhynchophorus ferrugineus.

The red palm weevil Rhynchophorus ferrugineus (Coleoptera: Curculionidae) is an economically-important invasive species that attacks multiple species of palm trees around the world. A better understanding of gene content and function in R. ferrugineus has the potential to inform pest control strategies and thereby mitigate economic and biodiversity losses caused by this species. Using 10x Genomics linked-read sequencing, we produced a haplotype-resolved diploid genome assembly for R. ferrugineus from a single heterozygous individual with modest sequencing coverage ([Formula: see text] 62x). Benchmarking against conserved single-copy Arthropod orthologs suggests both pseudo-haplotypes in our R. ferrugineus genome assembly are highly complete with respect to gene content, and do not suffer from haplotype-induced duplication artifacts present in a recently published hybrid assembly for this species. Annotation of the larger pseudo-haplotype in our assembly provides evidence for 23,413 protein-coding loci in R. ferrugineus, including over 13,000 predicted proteins annotated with Gene Ontology terms and over 6000 loci independently supported by high-quality Iso-Seq transcriptomic data. Our assembly also includes 95% of R. ferrugineus chemosensory, detoxification and neuropeptide-related transcripts identified previously using RNA-seq transcriptomic data, and provides a platform for the molecular analysis of these and other functionally-relevant genes that can help guide management of this widespread insect pest.

Detection of Phosphine Resistance in Field Populations of Four Key Stored-Grain Insect Pests in Pakistan.

In Pakistan, the control of stored-product insect pests mainly relies on the use of phosphine gas along with other control tactics. The aim of this study was to determine the level of phosphine resistance among ten differently located populations of the lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae), the granary weevil, Sitophilus granarius (L.) (Coleoptera: Curculionidae), the red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) and the khapra beetle, Trogoderma granarium Everts (Coleoptera: Dermestidae). Laboratory-susceptible populations of all insect species were also considered in the experiments. Concentration-response bioassays were conducted for each species. All of the tested populations (10 out of 10) of each species were found to be resistant to phosphine, but varied in their level of resistance. Probit analysis estimated LC50 at 2.85, 1.90, 2.54 and 2.01 ppm for laboratory-susceptible populations of R. dominica, S. granarius, T. castaneum and T. granarium, respectively. Against R. dominica, the highest and lowest resistance levels were observed in the Rahim Yar Khan (LC50 at 360.90 ppm) and Rawalpindi (LC50 at 210.98 ppm) populations, respectively. These resistant populations were 126.67- and 74.02-fold more resistant than the laboratory population. The Multan and Lahore populations of S. granarius exhibited the maximum (LC50 at 122.81 ppm) and minimum (LC50 at 45.96 ppm) resistance levels, respectively, i.e., they were 64.63- and 24.18-fold more resistant than the laboratory population. The Layyah population of T. castaneum showed the maximum resistance level (LC50 at 305.89 ppm) while the lowest was observed in the Lahore population (LC50 at 186.52 ppm), corresponding to 120.42- and 73.43-fold more resistant than the laboratory population, respectively. Regarding T. granarium, the Layyah population showed the maximum resistance level (LC50 at 169.99 ppm) while the Lahore population showed the minimum resistance (LC50 at 74.50 ppm), i.e., they were 84.57- and 37.06-fold more resistant than the laboratory population, respectively. Overall, R. dominica presented the highest resistance level, followed by T. castaneum, T. granarium and S. granarius. The current study suggests that the application of phosphine may not be an adequate control strategy for the management of the above tested insect pests in Pakistan.