Morphologic Characters of the Rostrum in Two Weevils, Eucryptorrhynchus scrobiculatus Motschulsky and E. brandti Harold (Coleoptera: Curculionidae: Cryptorrhychinae)

(1) Eucryptorrhynchus scrobiculatus and E. brandti (Coleoptera: Curculionidae: Cryptorrhychinae) are both pests of Ailanthus altissima, found in China. During ovipositing, gravid females of the two weevils need to excavate a cavity in the oviposition substrate with their rostrum, while their oviposition sites are different. (2) In this study, to explore the boring mechanism of E. scrobiculatus and E. brandti during ovipositing, the morphologic characters of the rostra of two weevils were studied in detail by scanning electron microscopy and micro-CT. (3) Their rostra appear similar, but the rostrum surface of E. scrobiculatus is rougher than that of E. brandti; their fine structures of rostrum and sensilla distribution are similar, but the sensilla twig basiconica 3 is distributed at the apex of labial palpus in E. brandti females, while not at the apex of labial palpus in E. scrobiculatus females; their rostra are hollow and their cuticle thickness is constantly changing, but the proportion of the whole rostrum tube cuticle in E. scrobiculatus is significantly larger than that of E. brandti. The above structural differences make E. scrobiculatus more conducive to oviposition in the soil and E. brandti more conducive to oviposition in the trunk of A. altissima. (4) Overall, this study not only plays an important role in exploring the excavating mechanism during the oviposition of the two weevils, but also provides new insights into the coexistence of two weevil species on the same host A. altissima.

Structural comparison of the rostra of two species of weevils coexisting on Ailanthus altissima: the response to ecological demands of egg deposition

Background

Elongated rostra play an important role in the egg-laying of weevils, and its emergence plays a key role in the adaptive radiation of weevils. Eucryptorrhynchus scrobiculatus Motschulsky and E. brandti Harold co-occur on the same only host Ailanthus altissima, while their oviposition sites are different. In order to understand the adaptation between the rostra of the two weevils and their oviposition sites, the structural differentiation of the rostra in E. scrobiculatus and E. brandti was compared.

Results

The present study reveals that: (1) The rostra length of E. scrobiculatus and E. brandti was found to be correlated with body size, larger weevils have a correspondingly longer rostrum. The increase of rostra length may be a byproduct of larger weevils. (2) There were significant differences in the external shape of the two rostra, especially the shape of the mandibles of the mouthparts at the apex of the rostra used to excavate an oviposition cavity. (3) There was no difference in the size of the abductor muscles that control the extension of the mandibles, but there were significant differences in the size of the adductor muscles that control the contraction of the mandibles.

Conclusions

These structural differences reflect the functional potential ovipositional tactics of rostra, which is considered to be a response to the ecological demands of egg deposition, and also provide new insights into the coexistence of two weevil species in the same host A. altissima.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01824-7.

Comparison and Functional Analysis of Chemosensory Protein Genes From Eucryptorrhynchus scrobiculatus Motschulsky and Eucryptorrhynchus brandti Harold

The tree-of-heaven root weevil (Eucryptorrhynchus scrobiculatus) and the tree-of-heaven trunk weevil (Eucryptorrhynchus brandti) are closely related species that monophagously feed on the same host plant, the Ailanthus altissima (Mill) Swingle, at different locations. However, the mechanisms of how they select different parts of the host tree are unclear. As chemosensory systems play important roles in host location and oviposition, we screened candidate chemosensory protein genes from the transcriptomes of the two weevils at different developmental stages. In this study, we identified 12 candidate chemosensory proteins (CSPs) of E. scrobiculatus and E. brandti, three EscrCSPs, and one EbraCSPs, respectively, were newly identified. The qRT-PCR results showed that EscrCSP7/8a/9 and EbraCSP7/8/9 were significantly expressed in adult antennae, while EscrCSP8a and EbraCSP8 shared low sequence identity, suggesting that they may respond to different odorant molecule binding. Additionally, EbraCSP6 and EscrCSP6 were mainly expressed in antennae and proboscises and likely participate in the process of chemoreception. The binding simulation of nine volatile compounds of the host plant to EscrCSP8a and EbraCSP8 indicated that (1R)-(+)-alpha-pinene, (-)-beta-caryophyllene, and beta-elemen have higher binding affinities with EscrCSP8a and lower affinities with EbraCSP8. In addition, there were seven, two, and one EbraCSPs mainly expressed in pupae, larvae, and eggs, respectively, indicating possible developmental-related roles in E. brandti. We screened out several olfactory-related possible CSP genes in E. brandti and E. scrobiculatus and simulated the binding model of CSPs with different compounds, providing a basis for explaining the niche differentiation of the two weevils.

A novel adhesive trunk trap net for trapping Eucryptorrhynchus brandti (Coleoptera: Curculionidae)

BACKGROUND

Eucryptorrhynchus brandti (Harold) is a destructive wood-boring pest of tree of heaven, Ailanthus altissima, in China. At present, the management of E. brandti relies exclusively on frequent applications of synthetic insecticides. Environmentally friendly alternatives to the use of synthetic insecticides would be beneficial. A trunk trap net (TTN), an adhesive trap (AT), and an adhesive trunk trap net (ATTN) were evaluated in their ability to capture E. brandti.

RESULTS

Significantly greater laboratory weevil recapture rates were found using the ATTN (78%) than using the TTN (8%) and AT (0%). In total, 45% of marked weevils were captured by the ATTN, an eight-fold increase in catch rates using the TTN in mark-release-recapture field trials in 2017. No marked weevils were captured by the AT. Field trials in 2017 and 2018 showed that the ATTN captured six to seven times more wild weevils than using the TTN.

CONCLUSION

Adhesive Trunk Trap nets (ATTNs) were more effective in capturing E. brandti adults than Trunk Trap nets (TTNs). These results support the use of the ATTN as an eco-friendly tool for the integrated management of E. brandti. © 2019 Society of Chemical Industry.

Oviposition Behavior and Distribution of Eucryptorrhynchus scrobiculatus and E. brandti (Coleoptera: Curculionidae) on Ailanthus altissima (Mill.)

(1) Eucryptorrhynchus scrobiculatus Motschulsky (Coleoptera: Curculionidae: Cryptorrhychinae) is a major quarantine forest pest in China. It often co-occurs with E. brandti (Coleoptera: Curculionidae: Cryptorrhychinae) on a single host Ailanthus altissima (Mill.) Swingle (tree of heaven). (2) In this study, to explain the coexistence of the two weevils on a single host, we investigated the oviposition behavior of E. scrobiculatus and oviposition sites of E. scrobiculatus and E. brandti under afield and laboratory conditions. (3) The characteristic behaviors of E. scrobiculatus females prior to oviposition included searching, locating, excavation, turning, locating the oviposition cavity, egg deposition, and hiding. (4) The oviposition sites used by E. scrobiculatus and E. brandti differed. Eucryptorrhynchus scrobiculatus females laid eggs in the soil near A. altissima and compound leaf petioles, while E. brandti females laid eggs in A. altissima trunks. The eggs in compound leaf petioles did not hatch in the field. (5) Eucryptorrhynchus scrobiculatus and E. brandti utilized different oviposition sites and these differences in habitat use may reduce the competition for resources between species during the larval period, thus facilitating their coexistence on A. altissima.

Identification and Comparison of Chemosensory Genes in the Antennal Transcriptomes of Eucryptorrhynchus scrobiculatus and E. brandti Fed on Ailanthus altissima

The key to the coexistence of two or more species on the same host is ecological niche separation. Adult Eucryptorrhynchus scrobiculatus and E. brandti both feed on the tree of heaven, Ailanthus altissima, but on different sections of the plant. Olfaction plays a vital role in foraging for food resources. Chemosensory genes on the antennae, the main organ for insect olfaction, might explain their feeding differentiation. In the present study, we identified 130 and 129 putative chemosensory genes in E. scrobiculatus and E. brandti, respectively, by antennal transcriptome sequencing, including 31 odorant-binding proteins (OBPs), 11 chemosensory proteins (CSPs), 49 odorant receptors (ORs), 17 ionotropic receptors (IRs), 19 gustatory receptors (GRs), and three sensory neuron membrane proteins (SNMPs) in E. scrobiculatus and 28 OBPs, 11 CSPs, 45 ORs, 25 IRs, 17 GRs, and three SNMPs in E. brandti. We inferred that EscrOBP8 (EscrPBP1), EscrOBP24 (EscrPBP2) and EbraOBP8 (EbraPBP1), EbraOBP24 (EbraPBP2) were putative PBPs by the phylogenetic analysis. We identified species-specific OR transcripts (10 EscrORs and 8 EbraORs) with potential roles in the recognition of specific volatiles of A. altissima. In addition to conserved “antennal IRs,” we also found several “divergent IRs” orthologues in E. scrobiculatus and E. brandti, such as EscrIR16, EbraIR19, and EbraIR20. Compared with other chemosensory genes, GRs between E. scrobiculatus and E. brandti shared lower amino acid identities, which could explain the different feeding habits of the species. We examined OBP expression patterns in various tissues and sexes. Although amino acid sequence similarities were high between EscrOBPs and EbraOBPs, the homologous OBPs showed different tissue expression pattern between two weevils. Our systematic comparison of chemosensory genes in E. scrobiculatus and E. brandti provides a foundation for studies of olfaction and olfactory differentiation in the two weevils as well as a theoretical basis for studying species differentiation.

The nearly complete mitochondrial genome of a snout weevil, Eucryptorrhynchus brandti (Coleoptera: Curculionidae)

We report the nearly complete mitochondrial genome of a snout weevil, Eucryptorrhynchus brandti (Coleoptera: Curculionidae). The 16,919 bp long genome consists of 13 protein-coding genes, 2 ribosomal RNAs, 22 transfer RNAs and a partial control region. A phylogenetic tree has been built using the 13 protein-coding genes of 11 related species from Coleoptera. Our results would contribute to further study of phylogeny in Coleoptera.

The complete mitogenome of Eucryptorrhynchus brandti (Harold) (Insecta: Coleoptera: Curculionidae)

The complete mitochondrial genome of Eucryptorrhynchus brandti (Harold) (Insecta: Coleoptera: Curculionidae) were reconstructed from whole-genome Illumina Hiseq 2000 sequencing data with an average coverage of 1406.7X. The circular genome is 15,122 bp in length, consisting of 13 protein-coding genes (PCGs), 21 transfer RNAs (tRNAs), two ribosomal RNAs (rRNAs) and one D-loop or control region. The tRNA-Ile gene was not found in the mitochondrial genome, as is identical to two other curculionidae species, i.e. Sphenophorus sp. (GU176342) and Naupactus xanthographus (GU176345). All PCGs initiated with ATN codons, except for the ND1 started with TTG. Two PCGs (COI and ND4) have an incomplete stop codon T. Two PCGs (ND4L and ND1) harbor the stop codon TAG, while all other PCGs terminated with the TAA codon. The nucleotide composition is highly asymmetric (38.7% A, 14.4% C, 9.2% G and 37.8% T) with an overall AT content of 76.5%.