Insight Into the Ultrastructure of Antennal Sensilla of Mythimna separata (Lepidoptera: Noctuidae)

Morphological characterization and distribution of antennal sensilla of Spodoptera frugiperda (Lepidoptera: Noctuidae) using scanning electron microscopy

The fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), is a globally significant agricultural pest, causing severe damage to corn production in China. Chemical odor-based trapping is a major approach for FAW control, making it essential to understand the FAW antennal sensillum types to enhance development of effective chemical odor attractants. In this study, we comprehensively examined the antennal sensilla types of FAW, identifying eight types and two subtypes, including Böhm's bristles, sensilla trichoidea, sensilla chaetica (I and II), sensilla coeloconica, sensilla styloconica, sensilla squamiformia (I and II), sensilla auricillica, and sensilla basiconica. Sensilla chaetica II, and sensilla squamiformia II are reported for the first time for FAW in this study. Detailed low-voltage field emission scanning electron microscope (LVSEM) images and descriptions are provided for each sensillum type. This study provides the morphological information to aid in conducting antennal sensillum neurophysiological tests on FAW. RESEARCH HIGHLIGHTS: The types of sensilla of fall armyworm were examined, identifying eight types and two subtypes, including Böhm's bristles, sensilla trichoidea, sensilla chaetica (I and II), sensilla coeloconica, sensilla styloconica, sensilla squamiformia (I and II), sensilla auricillica, and sensilla basiconica. Detailed low-voltage field emission scanning electron microscope images and descriptions were provided for each sensillum type.

Morphology and distribution of sensilla on antennae and mouthparts of the adult bruchid beetles, Bruchidius coreanus (Coleoptera: Bruchidae)

Bruchidius coreanus is a serious pest on Gleditsia sinensis Lam during seed storage, causing significant losses to their yield in southwest China. To gain insight into their behavioral mechanisms, the external morphology, ultrastructure, and distribution of sensilla on antennae, maxillary palps, and labial palps of both male and female B. coreanus were observed using a scanning electron microscope. The results revealed that both male and female adults had serrated antennae comprising a scape, a pedicel, and nine flagellomeres (F1-F9). There were eight types and seven subtypes of antenna sensilla observed in both sexes, including Böhm sensilla (BS), two subtypes of sensilla chaetica (SC1 and SC2), two subtypes of sensilla trichodea (ST1 and ST2), three subtypes of sensilla basiconica (SB1, SB2, and SB3), sensilla auricillica (SA), sensilla styloconicum (SS), capitate pegs (CP), and sensilla cavity (SCa). The average length of BS and ST (ST1 and ST2) showed significant differences between males and females. Furthermore, the number of SC (SC1 and SC2), ST1, and SCa differed significantly between the sexes. Four types of sensilla were found on the maxillary palps and labial palps, with the length of ST on these palps significantly differing between males and females. Additionally, SS on male labial palps was significantly longer than in females. The number of SC significantly differed between the male and female maxillary palps and labial palps, while ST and SS showed significant differences in the maxillary palps. These findings will contribute to further electrophysiological recording and behavioral research. RESEARCH HIGHLIGHTS: The external morphology and distribution of various sensilla on the antennae, maxillary palps, and labial palps of Bruchidius coreanus were described. Eight types and seven subtypes of antenna sensilla were observed on the antennae, while four types of sensilla were observed on the maxillary palps and labial palps. The capitate pegs were found exclusively on the antennae of female B. coreanus.

Morphological Characterization of the Antenna and Scent Patch of Three Danaus Species (Papilionoidea: Nymphalidae, Danainae)

The scent system of Danaus is important for the study of butterfly sexual communication and relevant investigations in biomimetics due to its involvement with mimicry. Using light, scanning, and transmission electron microscopy, the morphological characteristics of Danaus' antennae and scent patches of the scent system for three species, D. chrysippus, D. genutia, and D. plexippus, were investigated herein. Their apical clubs of the flagellums contain sensilla trichodea, sensilla chaetica, and sensilla coeloconica. The scent patch scales typically have a tree-like structure in its lumen at the nano-scale. Comparisons were made between the androconial scales and the other scales in scent patches. Rank sum tests showed significant differences in scent patch scales' characteristics between the species, as well as in the ultrastructure of antennal segments between species and sexes. Spearman's correlation tests showed significant correlations between the morphological characteristics of androconial scales in scent patches. Moreover, the antennal characteristics were significantly correlated. The morphological characteristics of the females' antennae were significantly correlated with those of the males' antennae and androconial scales. However, the significance and coefficient of these correlations were inconsistent across species and sexes. This study provides fundamental morphological information that helps in understanding the pheromone recognition system of Danaus.

Antennal excision reveals disparate olfactory expression patterns within castes in Reticulitermes aculabialis (Isoptera: Rhinotermitidae)

In lower termites, which exhibit a high degree of compound eye degradation or absence, antennae play a pivotal role in information acquisition. This comprehensive study investigates the olfactory system of Reticulitermes aculabialis, spanning five developmental stages and three castes. Initially, we characterize the structures and distribution of antennal sensilla across different developmental stages. Results demonstrate variations in sensilla types and distributions among stages, aligning with caste-specific division of labor and suggesting their involvement in environmental sensitivity detection, signal differentiation, and nestmate recognition. Subsequently, we explore the impact of antennal excision on olfactory gene expression in various caste categories through transcriptomics, homology analysis, and expression profiling. Findings reveal that olfactory genes expression is influenced by antennal excision, with outcomes varying according to caste and the extent of excision. Finally, utilizing fluorescence in situ hybridization, we precisely localize the expression sites of olfactory genes within the antennae. This research reveals the intricate and adaptable nature of the termite olfactory system, highlighting its significance in adapting to diverse ecological roles and demands of social living.

Flagellar sensilla of the hangingflies Bittacus planus Cheng and Bittacus sinicus Issiki (Mecoptera: Bittacidae)

Adult hangingflies are very sensitive to humidity, thereby frequently being used as ecological indicators to assess the degradation of the environment, especially forest ecosystems. Studies on the sense organs associated with hygro- and thermo-sensitivity, however, have been scant. Here, the ultramorphology and distribution of the flagellar sensilla were investigated in the adult hangingflies Bittacus planus Cheng, 1949 and Bittacus sinicus Issiki, 1931 using scanning electron microscopy. Four types of sensilla are identified in B. planus, including sensilla campaniformia, sensilla chaetica (SC1, SC2), sensilla basiconica (SB1, SB2), and sensilla coeloconica. In B. sinicus, sensilla chaetica III are present additionally from 7th to 14th flagellomeres. Abundant sensilla coeloconica are present on the flagella of Bittacus. Sensilla basiconica I are situated at the joints of flagellomeres. Sensilla campaniformia are densely distributed on the basal half of the first flagellomere, but scarcely on other flagellomeres. Sensilla chaetica II are present on the distal three flagellomeres. The size, abundance, and distribution of flagellar sensilla differ considerably between B. planus and B. sinicus. We infer that the abundance of sensilla coeloconica is likely associated with the hygro- and thermo-sensitivity in Bittacidae. Studies of flagellar sensilla can provide valuable information for subsequent electrophysiological, behavioral, biogeographical, and phylogenetic analyses of Bittacidae.

Ultrastructural and Descriptive Study on the Adult Body Surface of Heortia vitessoides (Lepidoptera: Crambidae)

Heortia vitessoides Moore, 1885 (Lepidoptera: Crambidae) is an economically important lepidopteran pest that caused severe damage to the plantation area of Aquilaria sinensis (Lour.) Gilg, 1825 (Thymelaeaceae), resulting in extensive defoliation of the trees during an epidemic. In this study, we used scanning electron microscopy (SEM) to analyze the external morphology and ultrastructure of sensilla on various body parts of H. vitessoides. Specifically, seven, four, four, and five types of sensilla were found, respectively, on the antennae, proboscis, labial palps, and legs. We described the types, distributions, and sexual dimorphism of these sensilla on antennae, and found that the number and size of sensilla differed significantly between males and females. This study provides crucial information for future investigations into the function of these sensilla in H. vitessoides.

ZmMYC2s play important roles in maize responses to simulated herbivory and jasmonate

Both herbivory and jasmonic acid (JA) activate the biosynthesis of defensive metabolites in maize, but the mechanism underlying this remains unclear. We generated maize mutants in which ZmMYC2a and ZmMYC2b, two transcription factor genes important in JA signaling, were individually or both knocked out. Genetic and biochemical analyses were used to elucidate the functions of ZmMYC2 proteins in the maize response to simulated herbivory and JA. Compared with the wild-type (WT) maize, the double mutant myc2ab was highly susceptible to insects, and the levels of benzoxazinoids and volatile terpenes, and the levels of their biosynthesis gene transcripts, were much lower in the mutants than in the WT maize after simulated insect feeding or JA treatment. Moreover, ZmMYC2a and ZmMYC2b played a redundant role in maize resistance to insects and JA signaling. Transcriptome and Cleavage Under Targets and Tagmentation-Sequencing (CUT&Tag-Seq) analysis indicated that ZmMYC2s physically targeted 60% of the JA-responsive genes, even though only 33% of these genes were transcriptionally ZmMYC2-dependent. Importantly, CUT&Tag-Seq and dual luciferase assays revealed that ZmMYC2s transactivate the benzoxazinoid and volatile terpene biosynthesis genes IGPS1/3, BX10/11/12/14, and TPS10/2/3/4/5/8 by directly binding to their promoters. Furthermore, several transcription factors physically targeted by ZmMYC2s were identified, and these are likely to function in the regulation of benzoxazinoid biosynthesis. This work reveals the transcriptional regulatory landscapes of both JA signaling and ZmMYC2s in maize and provides comprehensive mechanistic insight into how JA signaling modulates defenses in maize responses to herbivory through ZmMYC2s.

Morphology and location of sensilla in the antennae and ovipositor of Sirex noctilio (Hymenoptera: Siricidae)

The woodwasp Sirex noctilio Fabricius is a major quarantine pest that was reported in China in 2013 and mostly damages Pinus sylvestris var. mongolica. Reverse chemical ecology, which uses chemical lures to catch or block insects from mating is the classic way to control forestry pests. This indicates that insect sensilla play a crucial role in detecting external chemical and physical stimuli. Nonetheless, the categorization and distribution of sensilla on the antennae and ovipositor of S. noctilio are insufficiently specific. In this paper, scanning electron microscopy (SEM) was used to observe the ultrastructure of the sensilla of S. noctilio on the antenna and ovipositor. It was found that the types and distribution of sensilla on the antennae of S. noctilio male and female are consistent, and six types of sensilla are found: sensilla trichodea (ST), sensilla chaetica (SC), Böhm bristles (BB), sensilla basiconica (SB), sensilla ampullacea (SA), and contact chemoreceptors (CC). Besides, there are five types of sensilla on the female ovipositor. In addition to ST, SC and BB, two more types of sensilla are also found: sensilla cavity (SCa) and sensilla coeloconica (SCo). Through identification of the morphology and distribution of the sensilla, the functions of different sensilla in the mating and host selection mechanisms of S. noctilio are proposed, thereby establishing a foundation for S. noctilio chemical communication research.

Comparison of Morphological Characteristics of Antennae and Antennal Sensilla among Four Species of Bumblebees (Hymenoptera: Apidae)

Bumblebees, as pollinators, play an important role in maintaining natural and agricultural ecosystems. Antennae with sensilla of bumblebees as social insects have essential effects in foraging, nest searching, courting, and mating, and are different in species and sexes. Previous studies on the morphology of antennae and sensilla in bumblebees have been limited to a few species and a single caste. To better understand how bumblebees detect and receive the chemical signal from nectariferous plants and foraging behavior, the morphology of antennae with sensilla, including the antennal length, and type, distribution, and number of antennal sensilla in four species, Bombus atripes, Bombus breviceps, Bombus flavescens, and Bombus terrestris was compared by scanning electron microscopy (SEM) herein. The total antennal length of queens are the longest and workers are the shortest in three castes, and in four species the longest of the total antennal length among three castes all are in B. flavescens, which is significantly longer than other species (p < 0.05) and the length of the scape in queens and workers are both longer than males, significantly different in queens (p < 0.05), and not significantly different in workers (p > 0.05), and the length of flagellums in females are not always shorter than males, of which the length of flagellms in queens of B. flavescens are significantly longer than males (p < 0.05), and the length of pedicel and all flagellomeres varies among species and castes. A total of 13 major types of sensilla in total were observed, including trichodea sensilla (TS A-E), placodea sensilla (PS A-B), basiconica sensilla (BaS), coeloconica sensilla (COS A-B), chaetic sensilla (CS A-B), and Böhm sensilla (BS), of which chaetic sensilla B (CS B), only observed in females of B. atripes, was firstly reported in Apidae. Moreover, the number of all sensilla was the most in males, the least was in workers, the number of sensilla varies within castes and species. Furthermore, the morphological characteristics of antennae and the potential functions of sensilla are discussed.

Scanning electron microscopy study of the antennal sensilla of cob borer, Stenachroia elongella (Lepidoptera: Pyralidae)

Stenachroia elongella is an important pest of maize in Meghalaya, India. To understand the roles of sensory organs in the biological behaviors of this insect, we determined the structure and distribution of sensilla found on the antenna of both male and female S. elongella by scanning electron microscope. The antenna of both sexes is a filiform type, and the overall length of the female antenna is significantly longer than the male antenna. In both sexes, seven types and two subtypes of antennal sensilla are identified; sensilla trichodea, sensilla chaetica, sensilla styloconica, sensilla coeloconica, sensilla auricillica (I, II), sensilla squamiformia and Böhm sensilla. No sexual dimorphism on the specific occurrence of sensilla is observed. Probable functions are discussed based on their external morphology. These results would be helpful for further studies on TEM investigations, electrophysiological recordings, and behavioral studies. RESEARCH HIGHLIGHTS: Seven types and two subtypes of antennal sensilla were observed in both male and female Stenachroia elongella. Morphology and distribution of the antennal sensilla were discussed. Sexual dimorphism was observed on the overall length of the antenna where the female antenna is longer than the male antenna. No sexual dimorphism was observed on specific occurrences of antennal sensilla.

Ultrastructure Characteristics and Sexual Dimorphism of Antennal Sensilla in Tirathaba rufivena (Lepidoptera: Pyralidae)

Tirathaba rufivena Walker, a major insect pest of Areca catechu L., has severely threatened areca nut cultivation in Hainan, China. To improve our understanding of the communication mechanism in host plant seeking and mate-finding for T. rufivena, we described and further characterized the external morphology and internal sensilla structures using scanning electron microscopy and transmission electron microscopy in this study. The antennal morphology was similar between males and females, and there was no significant difference in length between the two sexes. In total, nine sensilla types were identified: sensilla trichodea (Str), sensilla chaetica (Sch), sensilla basiconica (Sba), sensilla auricillica (Sau), sensilla coeloconica (Sco), sensilla styloconica (Sst), Böhm sensilla (Bs), uniporous peg sensilla (Ups) and sensilla squamiformia (Ssq). Sexual dimorphism mainly occurs in variation in the length of Sba, Sch, Sco1 and Bs, and the abundance of Sba, Sau1 and Sau2. The Sba had larger size and numbers on female antennae than that on males, suggesting that these sensilla might have important roles in locating host plants. Both Sau1 and Sau2 were significantly more abundant in females and were probably associated with the detection of mates and host plant for oviposition. These data were important for ongoing studies on host plant seeking and mate-finding behavior in T. rufivena and provided a theoretical foundation to further studies of semiochemical control for this pest.

Morphological characterization and distribution of antennal sensilla of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) using scanning electron microscopy

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is a serious polyphagous pest of various field and horticultural crops. A complete knowledge on the morphological features of antennal sensory structures is essential for efficient semiochemical-based control methods. The external structure and distribution of antennal sensilla in male and female adults of H. armigera were investigated using scanning electron microscopy. Eight distinct morphological types of sensilla were identified in both sexes: sensilla trichodea, sensilla basiconica, sensilla auricillica, sensilla coeloconica (multiporous), sensilla chaetica (uniporous), sensilla styloconica, sensilla squamiformia, and Böhm sensilla (aporous) in varying numbers and distribution along the length of the antennae. Of these sensilla, the most widespread are sensilla trichodea and sensilla basiconica on the antennae of both sexes. Female antennae have comparatively greater number of sensilla trichodea than male antennae. Among eight types of sensilla, sensilla basiconica, auricillica, styloconica type II, squamiformia, and Böhm sensilla were identified and reported for the first time in H. armigera. Sexual dimorphism in H. armigera was mainly detected as the variations in sensilla shape, numbers, and distribution of each type of sensilla. The sexual difference was observed in the numbers of sensilla coeloconica, chaetica, styloconica, and squamiformia per flagellomere. The possible functions of these sensilla were discussed in view of previously reported lepidopteran insects. The findings provide fundamental information on the morphology and distribution of antennal sensory structures in H. armigera. It would be useful for further detailed studies on physiological and behavioral function of each sensillum type and helpful for formulating related pest control methods.

Functional Characterization of Sex Pheromone Neurons and Receptors in the Armyworm, Mythimna separata (Walker)

Pheromone receptors (PRs) of moths are expressed on the dendritic membrane of odorant receptor neurons (ORNs) housed in the long trichoid sensilla (TS) of antennae and are essential to sex pheromone reception. The function of peripheral neurons of Mythimna separata in recognizing sex pheromones is still unclear. In this study, electroantennogram recordings were performed from male and female antennae of M. separata, and showed that the major component of sex pheromones, (Z)-11-hexadecenal (Z11-16:Ald), evoked the strongest response of male antennae with significant differences between sexes. Single sensillum recording was used to record responses of neurons housed in TS of male M. separata. The results revealed four types of TS with three neurons housed in each type, based on profiles of responses to sex pheromone components and pheromone analogs. ORN-B of type-I TS was specifically tuned to the major sex pheromone component Z11-16:Ald; ORN-Bs in type-III and type-IV TSs were, respectively, activated by minor components (Z)-11-hexadecen-1-yl acetate (Z11-16:OAc) and hexadecenal (16:Ald); and ORNs in type-II TS were mainly activated by the sex pheromone analogs. We further cloned full-length sequences of six putative PR genes and an Orco gene. Functional characterization of PRs in the Xenopus oocyte system demonstrated that male antennae-biased MsepPR1 responded strongly to (Z)-9-tetradecenal (Z9-14:Ald), suggesting that MsepPR1 may be expressed in type-II TS. MsepPR6 was exclusively tuned to (Z)-9-tetradecen-1-yl acetate (Z9-14:OAc). MsepPR2 and MsepPR4 showed no responses to any tested components. Female antennae-biased MespPR5 was broadly tuned to Z9-14:Ald, Z9-14:OAc, Z11-16:Ald, and (Z)-11-hexadecen-1-ol (Z11-16:OH). Our results further enriched the sex pheromone recognition mechanism in the peripheral nervous system of moth M. separata.

Sensilla of the antenna and proboscis of Athetis lepigone (Möschler) (Lepidoptera: Noctuidae)

Sensory structures on the antennae and mouthparts of insects are associated with various activities, such as host location, feeding, attracting a mate, and identifying a suitable oviposition site. Athetis lepigone (Möschler) is an important polyphagous Eurasian pest with more than 30 species of host plants. The larvae target bud leaves, prop roots, and tender stems of many agricultural crops, but the feeding habits of the adults remain poorly known. Aiming to understand the feeding behavior of the species, we investigated the fine morphology of its antennae and proboscis using scanning electron microscopy. The antennae of both sexes are filiform, and bear eight types of sensilla: Böhm's bristles, sensilla squamiformia, trichodea, chaetica, basiconica, coeloconica, styloconica, and auricillica. Sensilla trichodea are the most abundant among these sensillum types. The proboscis consists of two elongated, interlocked maxillary galeae that enclose the food canal by dorsal and ventral legulae. The external galeal surface is covered with numerous triangular microtrichia on Zone 1 and abundant blunt microbumps on Zone 2. The surface of the food canal bears closely connected and smooth semicircular ridges, gradually tapering toward the proboscis tip. Three types of sensilla are noticeable on the proboscis: sensilla trichodea, basiconica, and styloconica. We briefly discuss the putative functional significance of the antennal and proboscis sensilla and, based on the specific structural modifications of the proboscis, predict a flower-visiting habit for A. lepigone.

Ultrastructure of Antennal Morphology and Sensilla of Teak Skeletonizer, Eutectona machaeralis Walker (Lepidoptera: Crambidae)

The leaf skeletonizer, Eutectona machaeralis (Walker) (Lepidoptera: Crambidae), is a severe insect pest of teak trees (Tectona grandis L.f.) in China. To provide some basic evidence for future semiochemical-based management strategies of E. machaeralis, the morphology, ultrastructure, and distribution of antennal sensilla of adults were observed under scanning and transmission electron microscopy. The shape and structure of antenna were similar between males and females, both being filiform. However, the antennal length of males was significantly longer than that of females. Eight morphological sensilla types were observed in both sexes: Böhm's bristles, sensilla trichodea, sensilla basiconica, sensilla chaetica, sensilla styloconica, sensilla coeloconica, sensilla auricillica, and sensilla squamiformia. Significant sexual dimorphism of the sensilla dimensions was found, especially in sensillar length. The putative and potential functions of the different sensilla types are discussed based on the fine structures of the cuticular walls and dendrites of the different sensilla types. We expect these results to help lay a solid foundation for future functional research and develop further investigations of E. machaeralis.

Olfactory coding of intra- and interspecific pheromonal messages by the male Mythimna separata in North China

Moths often use multi-component pheromones with fixed ratios to keep intraspecific communication and interspecific isolation. Unusually, the Oriental armyworm Mythimna separata in North China use only Z11-16:Ald as the essential component of its sex pheromone to find mates. To understand how this species keeps behavioral isolation from other species sharing Z11-16:Ald as a major pheromone component, we study the olfactory coding of intra- and interspecific pheromonal messages in the males of M. separata. Firstly, we functionally characterized the long trichoid sensilla in male antennae by single sensillum recording. Two types of sensilla were classified: the A type sensilla responded to Z11-16:Ald and Z9-14:Ald, and the B type sensilla mainly to Z9-14:Ald, and also to Z11-16:Ac, Z11-16:OH, and Z9-16:Ald. Next, we examined the glomerulus responses in the antennal lobes to these compounds by using in vivo optical imaging. The results showed that among the three subunits of the macroglomerular complex (MGC), Z11-16:Ald activated the cumulus, Z9-14:Ald activated the dorso-anterior and the cumulus, Z11-16:OH and Z11-16:Ac activated the dorso-anterior and dorso-posterior, respectively. However, Z9-16:Ald activated an ordinary glomerulus. Thirdly, we tested the behavioral responses of the males to these compounds in the wind tunnel. Addition of Z9-14:Ald at the ratio of 1:10 greatly reduced the attractiveness of Z11-16:Ald, addition of Z9-16:Ald or Z11-16:OH at the ratio of 1:1 also had behavioral antagonistic effects, while addition of Z11-16:Ac had no effect on the attractiveness of Z11-16:Ald. Finally, we used antennal transcriptome data and the Xenopus expression system to identify the receptor of Z9-14:Ald in M. separata. The Xenopus oocytes co-expressing MsepOR2 and MsepORco showed a strong response to Z9-14:Ald. Two-color fluorescence in situ hybridization validated that the cells expressing MsepOR2 and MsepOR3, tuned to Z9-14:Ald and Z11-16:Ald respectively, were localized in the different sensilla of male antennae. Comparing the sex pheromone communication channel of the related species, our results suggest that the conserved olfactory pathways for behavioral antagonists play a crucial role in behavioral isolation of noctuid species.

LC50 of lambda-cyhalothrin stimulates reproduction on the moth Mythimna separata (Walker)

Lambda-cyhalothrin has long been recommended as an effective insecticide to control the oriental armyworm, Mythimna separata (Walker), a notorious migratory pest of agricultural plants. Previous researches have suggested that survival, development, and reproduction of insects are influenced by sublethal concentrations of insecticides. However, the effects of sublethal concentrations of lambda-cyhalothrin on M. separata are less known. In this study, we determined the toxicity and effects of LC₂₀ and LC₅₀ concentration of lambda-cyhalothrin on development and reproduction of M. separata. Results indicate that LC₂₀ of lambda-cyhalothrin tends to decrease the life traits of M. separate, with a shortening larvae period of offspring and oviposition period, whereas LC₅₀ of lambda-cyhalothrin stimulates daily maximal fecundity and forwards the oviposition peak, suggesting a stimulation of reproduction by LC₅₀ of lambda-cyhalothrin. The M. separata population was increased by an LC₅₀ concentration of lambda-cyhalothrin, resulting in a net reproductive rate (R₀) and intrinsic rate of increase (r_m) significantly higher than that of the control. Transcripts of vitellogenin (MsVg) and vitellogenin receptor (MsVgR) genes were suppressed at day 1 after emergence of moth which developed from the larvae exposed to LC₂₀ and LC₅₀ of lambda-cyhalothrin, but were significant induced when the moth begin to lay eggs (day 4), with a more remarkable induction by LC₅₀ of lambda-cyhalothrin than those of LC₂₀ of lambda-cyhalothrin. Our results indicate that the observed stimulation of reproduction is therefore the results of up-regulation of MsVg and MsVgR by LC₅₀ of lambda-cyhalothrin.

Insecticidal Activity and Synergistic Combinations of Ten Different Bt Toxins against Mythimna separata (Walker)

The oriental armyworm (OAW), Mythimna separata (Walker), is a destructive pest of agricultural crops in Asia and Australia. Commercialized Bt crops have performed very well against their target pests; however, very few studies have been done on the susceptibility of OAW to Bt toxins in either sprays or expressed in Bt crops. In this work, we evaluated the toxicities of Cry1Ab, Cry1Ac, Cry1Ah, Cry1Fa, Cry2Aa, Cry2Ab, Cry1Ie, Vip3Aa19, Vip3Aa16, and Vip3Ca against OAW neonate larvae, as well as the interaction between Cry and Vip toxins. The results from bioassays revealed that LC50 (lethal concentration for 50% mortality) values ranged from 1.6 to 78.6 μg/g (toxin/diet) for those toxins. Among them, Vip3 proteins, along with Cry1A proteins and Cry2Aa, were the ones with the highest potency, with LC50 values ranging from 1.6 to 7.4 μg/g. Synergism between Cry and Vip toxins was observed, being high in the combination of Vip3Aa16 with Cry1 toxins, with synergetic factors ranging from 2.2 to 9.2. The Vip3Ca toxin did not show any synergistic effect with any of the toxins tested. These results can help in designing new combinations of pyramiding genes in Bt crops, as well as in recombinant bacteria, for the control of OAW as well as for resistance management programs.

A chemosensory protein MsepCSP5 involved in chemoreception of oriental armyworm Mythimna separata

Chemosensory proteins (CSPs) have been suggested to perform several functions in insects, including chemoreception. To find out whether MsepCSP5 identified from Mythimna separata shows potential physiological functions in olfaction, gene expression profiles, ligand-binding experiments, molecular docking, RNA interference, and behavioral test were performed. Results showed that MsepCSP5 was highly expressed in female antennae. MsepCSP5 showed high binding affinities to a wide range of host-related semiochemicals, and displayed that 26 out of 35 candidate volatiles were highly bound (Ki < 10 µM) at pH 5.0 rather than pH 7.4. The binding sites of MsepCSP5 to candidate volatiles were well predicted by three-dimensional structure modeling and molecular docking experiments. Pursuing further, biological activities of M. separata to highly bound compounds elicited strong behavioral responses, such as alcoholic compounds displayed strong attractiveness whereas terpenes showed repellency to M. separata. The transcript expression level of MsepCSP5 gene significantly decreased after injecting target dsRNAs, and resulted in non-significant preference responses of M. separata to semiochemicals, such as 3-pentanol and 1-octene-3-ol. In conclusion, MsepCSP5 may involve in semiochemical reception of M. separata.

Ultrastructure of antennal sensilla of three fruit borers (Lepidoptera: Crambidae or Tortricidae)

Three fruit borers Conogethes punctiferalis (Guenée) (Crambidae), Grapholita molesta Busck (Tortricidae), and Spilonota albicana Motschulsky (Tortricidae) are serious pests of fruit trees. In this study, their antennal morphology, types of sensilla, and distributions were observed by using SEM (Scanning Electron Microscope). Nine types of sensilla were found on the antennae of C. punctiferalis, while eight types of sensilla were presented on each of G. molesta and S. albicana. The sensilla trichodea with two subtypes were the most abundant sensilla among three fruit borers. Two subtypes of sensillum coeloconica (type I with spines and type II without spines) were observed on the antennae of C. punctiferalis and G. molesta. However, sensilla coeloconica (type I) were only found in S. albicana. Although the sensilla campaniformia were only found on the antennae of C. punctiferalis, our observations confirm sensilla campaniformia presence in the moths. In addition, the functions of these sensilla were discussed based on previously reported lepidopteran insects. As a result, our study may provide useful information for advanced electrophysiological and behavioral experiments to better understand the mechanisms related to pests control.

Central Projections of Antennal and Labial Palp Sensory Neurons in the Migratory Armyworm Mythimna separata

The oriental armyworm, Mythimna separata (Walker), is a polyphagous, migratory pest relying on olfactory cues to find mates, locate nectar, and guide long-distance flight behavior. In the present study, a combination of neuroanatomical techniques were utilized on this species, including backfills, confocal microscopy, and three-dimensional reconstructions, to trace the central projections of sensory neurons from the antenna and the labial pit organ, respectively. As previously shown, the axons of the labial sensory neurons project via the ipsilateral labial nerve and terminate in three main areas of the central nervous system: (1) the labial-palp pit organ glomerulus of each antennal lobe, (2) the gnathal ganglion, and (3) the prothoracic ganglion of the ventral nerve cord. Similarly, the antennal sensory axons project to multiple areas of the central nervous system. The ipsilateral antennal nerve targets mainly the antennal lobe, the antennal mechanosensory and motor center, and the prothoracic and mesothoracic ganglia. Specific staining experiments including dye application to each of the three antennal segments indicate that the antennal lobe receives input from flagellar olfactory neurons exclusively, while the antennal mechanosensory and motor center is innervated by mechanosensory neurons from the whole antenna, comprising the flagellum, pedicle, and scape. The terminals in the mechanosensory and motor center are organized in segregated zones relating to the origin of neurons. The flagellar mechanosensory axons target anterior zones, while the pedicular and scapal axons terminate in posterior zones. In the ventral nerve cord, the processes from the antennal sensory neurons terminate in the motor area of the thoracic ganglia, suggesting a close connection with motor neurons. Taken together, the numerous neuropils innervated by axons both from the antenna and labial palp indicate the multiple roles these sensory organs serve in insect behavior.

A cytochrome P450 gene plays a role in the recognition of sex pheromones in the tobacco cutworm, Spodoptera litura

Cytochrome P450 (P450 or CYP) genes are involved in fundamental physiological functions, and might be also associated with the olfactory recognition of sex pheromones in beetles and moths. A P450 gene, Spodoptera litura CYP4L4 (SlituCYP4L4), was cloned for the first time from the antennae of S. litura. SlituCYP4L4 was almost exclusively expressed in the adult stage and predominantly expressed in the adult antennae. In situ hybridization showed that SlituCYP4L4 localized mainly at the base of the long sensilla trichoidea, which responds to sex pheromone components. Pretreatment with an S. litura sex pheromone significantly reduced the expression levels of SlituCYP4L4, consistent with other genes involved in sex pheromone recognition. The expression level of SlituCYP4L4 was different in moths collected with different ratios of sex pheromone lures and collected in different geographical locations. After gene knockdown of SlituCYP4L4 in the antennae, the electroantennogram (EAG) responses of male and female moths to (9Z,11E)-tetradecadienyl acetate or (9Z,12E)-tetradecadienyl acetate were significantly decreased. In contrast, EAG responses to plant volatiles and sex pheromones of other moth species were not significantly influenced in these moths. SlituCYP4L4 was also expressed in the gustatory tissues and sensilla, which suggests that SlituCYP4L4 may have other functions in the chemosensory system. Our results have shown for the first time the function of a CYP gene with appendage-specific expression in insect sex pheromone recognition, especially in adult moths.

Sensory genes identification with head transcriptome of the migratory armyworm, Mythimna separata

Sensory system plays important roles in a wide array of insect’s behavior and physiological events, including the host landing and locating, feeding, flying, sex responding, mating and oviposition which happen independently and in sequence. The armyworm Mythimna separata (Lepidoptera: Noctuidae) of migratory insect is destructive for alimentarn crop and economic crop throughout the world. Here we present the high throughput sequencing of the head transcriptome and identify members of the major sensory genes which are crucial for armyworm’s success worldwide, including 8 opsins, 22 chemosensory proteins, 50 odorant binding proteins, 60 odorant receptors, 8 gustatory receptors, 24 ionotropic receptors, and 2 sensory neuron membrane proteins. It is worth noting that a duplication of the LW opsin gene exists in this insect. Several genes were clustered with functionally validated genes, such as Co-receptors of OR and IR, PBPs, PRs, CO2 GRs, bitter GRs and sweet GRs, were also identified. The transcriptome gene library provided the basis for further studies that elucidate the fundamental molecular mechanism of biology and control in M. separata. Our research exhibits the first comprehensive catalogue of the sensory genes fundamental for success and distribution in M. separata, which are potential novel targets for pest control strategies.

Transcriptome analysis of molecular mechanisms responsible for light-stress response in Mythimna separata (Walker)

Light is an important environmental signal for most insects. The Oriental Armyworm, Mythimna separata, is a serious pest of cereal crops worldwide, and is highly sensitive to light signals during its developmental and reproductive stages. However, molecular biological studies of its response to light stress are scarce, and related genomic information is not available. In this study, we sequenced and de novo assembled the transcriptomes of M. separata exposed to four different light conditions: dark, white light (WL), UV light (UVL) and yellow light (YL). A total of 46,327 unigenes with an average size of 571 base pairs (bp) were obtained, among which 24,344 (52.55%) matched to public databases. The numbers of genes differentially expressed between dark vs WL, dark vs UVL, dark vs YL, and UVL vs YL were 12,012, 12,950, 14,855, and 13,504, respectively. These results suggest that light exposure altered gene expression patterns in M. separata. Putative genes involved in phototransduction-fly, phototransduction, circadian rhythm-fly, olfactory transduction, and taste transduction were identified. This study thus identified a series of candidate genes and pathways potentially related to light stress in M. separata.

Ultrastructure of sensilla of antennae and ovipositor of Sitotroga cerealella (Lepidoptera: Gelechiidae), and location of female sex pheromone gland

The Angoumois grain moth, Sitotroga cerealella, is a serious pest of stored grains worldwide. Presently, the best effective control against the moth is to disrupt the sexual communication between sexes. Sexual communication in moths includes two processes in which females produce and release pheromones from the sex pheromone gland and males detect and respond to them with a relatively sophisticated olfactory system in their antennae. To better understand these processes, we studied the ultrastructure of antennal and ovipositor sensilla of S. cerealella and determined the location of the female sex pheromone gland. Seven types of antennal sensilla were identified on both sexes: sensilla trichodea, sensilla chaetica, sensilla coeloconica, sensilla styloconica, sensilla auricillica, sensilla squamiformia and Bӧhm bristles. Of these sensilla, the sensilla trichodea were significantly more abundant on male antennae than on those of females, suggesting that these sensilla may detect the sex pheromones. On the ovipositor, only sensilla chaetica of various lengths were found. The sexual gland was an eversible sac of glandular epithelium that was situated dorsally in the intersegmental membrane between the 8^th and 9^th abdominal segments. These results will lead to a better understanding of mate finding with sex pheromones for this worldwide pest species.

Age influences the olfactory profiles of the migratory oriental armyworm mythimna separate at the molecular level

Background

The oriental armyworm Mythimna separata (Walk) is a serious migratory pest; however, studies on its olfactory response and its underlying molecular mechanism are limited. To gain insights to the olfactory mechanism of migration, olfactory genes were identified using antennal transcriptome analysis. The olfactory response and the expression of olfactory genes for 1-day and 5-day-old moths were respectively investigated by EAG and RT-qPCR analyses.

Results

Putative 126 olfactory genes were identified in M. separata, which included 43 ORs, 13 GRs, 16 IRs, 37 OBPs, 14 CSPs, and 3 SNMPs. RPKM values of IR75d and 10 ORs were larger than co-receptors IR25a and ORco, and the RPKM value of PR2 was larger than that of other ORs. Expression of GR1 (sweet receptor) was higher than that of other GRs. Several sex pheromones activated evident EAG responses where the responses of 5-day-old male moths to the sex pheromones were significantly greater than those of female and 1-day old male moths. In accordance with the EAG response, 11 pheromone genes, including 6 PRs and 5 PBPs were identified in M. separate, and the expression levels of 7 pheromone genes in 5-day-old moths were significantly higher than those of females and 1-day-old moths. PR2 and PBP2 might be used in identifying Z11-16: Ald, which is the main sex pheromone component of M. separata. EAG responses to 16 plant volatiles and the expression levels of 43 olfactory genes in 1-day-old moths were significantly greater than that observed in the 5-day-old moths. Heptanal, Z6-nonenal, and benzaldehyde might be very important floral volatiles for host searching and recognized by several olfactory genes with high expression. Some plant volatiles might be important to male moths because the EAG response to 16 plant volatiles and the expression of 43 olfactory genes were significantly larger in males than in females.

Conclusions

The findings of the present study show the effect of adult age on olfactory responses and expression profile of olfactory genes in the migratory pest M. separate.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3427-2) contains supplementary material, which is available to authorized users.