Vision-mediated exploitation of a novel host plant by a tephritid fruit fly

Phenological stage dependent sensory and behavioral responses of Zeugodacus cucurbitae (Coquillett) to cucurbit volatiles

Zeugodacus cucurbitae (Coquillett) flies significantly impact vegetable production in many tropical regions. This study aimed to identify physiologically and behaviorally relevant volatiles from host plants that could potentially be used for future monitoring and control of female Z. cucurbitae flies. Volatile organic compounds were collected from flower, immature fruit, and mature fruit stages of Cucumis sativus L., Cucurbita pepo L., and Cucurbita mixta L. in field conditions, and were analyzed using gas chromatography/mass spectrometry (GC-MS). A total of 81 compounds were identified from across the three species and their phenological stages. Volatilome diversity was higher within phenological stages of each species than between species. Electrophysiological responses of sexually mature Z. cucurbitae females to host volatiles were recorded using gas chromatography coupled electroantennogram detection (GC-EAD). Active compounds were then formulated into blends for behavioral assays conducted in a six-choice olfactometer. Synthetic blends based on physiologically active compounds from flower and immature fruit headspace attracted more females than blends derived from mature fruit and the paraffin oil control (P < 0.001). Some of the physiologically active compounds were found to be behaviorally redundant. The performance of these blends needs to be assessed under field conditions.

Specific transcription factors regulate the expression of Rh6 in Bactrocera minax and Bactrocera dorsalis (Diptera: Tephritidae)

Color vision is widely used by herbivorous insects to make host location. We have previously demonstrated that the long-wavelength-sensitive (LWS) opsin gene Rhodopsin 6 (Rh6) confers green preference in Bactrocera minax (a specialist) but yellow in B. dorsalis (a generalist). However, the transcriptional regulation underlying Rh6 expression and its association with color preference between these two sister species remains unclear. Here, we cloned the core promoter regions of BmRh6 and BdRh6, and identified the transcription factors (TFs) BmHmx in B. minax and BdPtx1 in B. dorsalis through bioinformatics and transcriptomic analysis. The functional impact of the two TFs on Rh6 transcription was validated using the dual luciferase reporter assays and yeast one-hybrid (Y1H) assays. RNA interference (RNAi)-mediated knockdown of the TFs resulted in significant downregulation of Rh6 expression. Furthermore, silencing of BmHmx eliminated the preference for green in B. minax, while knockdown of BdPtx1 in B. dorsalis led to the loss of yellow preference. Our results elucidate the mechanism underlying transcriptional regulation of Rh6 towards color preferences in tephritids, which also provide new insights into the links between host location and visual ecology in insects.

Population Dynamics of Galerucella birmanica and Its Aggregation Behavior in Brasenia schreberi Aquaculture System

The aquatic vegetable Brasenia schreberi Gmel. (Nymphaeales: Cabombaceae), widely cultivated in China, faces severe challenges from pest disturbances. With a field investigation, G. birmanica occurred for several generations in the Brasenia mono-cultivation system. The first visual peak on leaf surfaces appeared in July, with eggs, larvae, pupae and adults all being recorded. The highest number of flying adults were trapped in August in reverse to the absence of beetles on leaves, followed by an extraordinary high number of eggs in September. G. birmanica exhibited a conspicuous aggregation in distribution, with severely chewed areas having a G. birmanica abundance 21.6 times that of the non-chewed areas. Laboratory studies with GC-MS and two-choice tests revealed four volatiles significantly discrepant in contents released from chewed/intact leaves: cis-3-hexenyl acetate (917.33 ± 29.56 vs. 604.034 ± 23.24 ng, chewed vs. intact), 2-phenylethyl isothiocyanate (595.37 ± 28.42 vs. 356.00 ± 13.44 ng), undecane (771.44 ± 34.72 vs. 1003.28 ± 47.88 ng) and methyl salicylate (1079.84 ± 49.39 vs. 532.11 ± 18.23 ng); among them, 2-phenylethyl isothiocyanate could significantly attract G. birmanica adults. Our study suggests that G. birmanica can be a severe threat to mono-cultivated B. schreberi, whereas the damaged leaves may sacrifice themselves by attracting the beetles with an herbivore-induced volatile, thus protecting the whole plant population from pest disturbance.

Bycatch of common pollinators in pheromone baited traps for monitoring corn earworm (Lepidoptera: Noctuidae) in Missouri Industrial Hemp

BACKGROUND

Several traps are recommended for monitoring corn earworm, Helicoverpa zea (Boddie), an important pest of field, vegetable and fruit crops in the U.S. These traps, which are meant to capture H. zea in many cases end up capturing other insects. Helicoverpa zea has recently been found feeding on different seeds and dual-type (seed and fiber) hemp, Cannabis sativa L. (Rosales: Cannabaceae) causing serious damage. Limited work has been done on developing integrated pest management (IPM) for H. zea industrial hemp in Missouri.

METHODS

We evaluated the attractiveness of different traps with the aim of developing a monitoring system for the adult male H. zea in industrial hemp fields in two Missouri locations. In addition, we recorded other non-target insects in the traps to determine trap selectivity. Commercially available green, clear, tricolor bucket traps Heliothis Scentry and Scentry Delta 1X traps baited with H. zea sex pheromones were evaluated in 2021 and 2022.

RESULTS

Tricolor traps captured significantly more adult male H. zea than the other traps in both years. Non-target insects, mainly Xylocopa virginica (L.) (Hymenoptera: Apidae), Bombus spp. (Hymenoptera: Apidae), and Apis mellifera L. (Hymenoptera: Apidae) were also captured. The tricolor trap captured the most bees.

CONCLUSIONS

The presence of non-target species highlights the risk of using tricolor traps as a monitoring tool. This data provides information for planning the monitoring of corn earworm in industrial hemp farms in Missouri.

Role of mitochondrial complex I genes in host plant expansion of Bactrocera tau (Tephritidae: Diptera) by CRISPR/Cas9 system

Host expansion facilitates tephritid flies to expand their ranges. Unraveling the mechanisms of host expansion will help to efficiently control these pests. Our previous works showed mitochondrial complex I genes Ndufs1, Ndufs3, and Ndufa7 being upregulated during host expansion of Bactrocera tau (Walker), one of the highly hazardous species of tephritids. However, their roles in the host expansion of B. tau remain unknown. Here, using clustered regularly interspaced short palindromic repeats (CRISPR) / CRISPR-associated nuclease 9 (Cas9) editing system for the first time, a stable homozygous Ndufa7 strain (Btndufa7-/-), heterozygous Ndufs1 (Btndufs1+/-), and Ndufs3 strains (Btndufs3+/-) were obtained from F3 generation of B. tau, after gene knockout. Reduced sizes of larvae and pupae of the Ndufa7 knockout strain were first observed. Notably, the mean values of fitness estimation (pupal numbers, single-pupal weight and emergence rate) and Ndufa7 gene expression in the Ndufa7 knockout strain were slightly reduced on 2 native hosts (summer squash and cucumber), while it sharply decreased on the novel host banana and the potential host pitaya, compared with those of the wild-type strain. Furthermore, the Ndufa7 knockout strain did not survive on the novel host guava. These results suggested that Ndufa7 disturbs the survival on native hosts, expansion to novel hosts, and further expansion to potential hosts of B. tau. Homozygous lethality occurred after the knockout of Ndufs1 or Ndufs3, suggesting that these 2 genes play a role in the early development of B. tau. This study revealed that Ndufa7 is a target gene for the management of tephritids and opens a new avenue for pest control research.

Development of bait station to complement attract-and-kill agents of Zeugodacus tau (Diptera: Tephritidae)

Zeugodacus tau (Walker) (Diptera: Tephritidae) is an important agricultural pest currently managed primarily through the application of insecticides due to limited control strategies. Bait station devices are target specific and have emerged as a behaviorally based alternative to traditional insecticide sprays for managing Z. tau. In this study, we designed a bait station by integrating female-biased olfactory, visual, and gustatory elements, and a killing agent in a wax-matrix. Our results showed that the wax-matrix integrated with spinetoram showed the highest toxicity to immature and mature Z. tau females. Furthermore, the color and shape of the spinetoram bait station significantly influenced its attractiveness and toxicity to female Z. tau. Green sausage-shaped exteriors were the most effective color and shape examined. Subsequent experiments showed a length-dependent effect on mortality and visiting frequencies of Z. tau females when the bait stations were 9-13 cm long. The addition of the olfactory stimulus of 5% ammonium acetate to the bait station attracted a higher number of mature Z. tau females than the control. The killing efficacy of the weathered bait station was similar to that of the fresh station within an 8-week period (over 794 mm of rainfall). The bait station developed herein would provide new insight into the attract-and-kill strategy for Z. tau and alleviate the pressure of the actual management program for this pest.

Opsin mutants alter host plant selection by color vision in the nocturnal invasive pest Tuta absoluta

In insects, vision is crucial in finding host plants, but its role in nocturnal insects is largely unknown. Vision involves responses to specific spectra of photon wavelengths and opsins plays an important role in this process. Long-wavelength sensitive opsin (LW opsin) and blue-sensitive opsin (BL opsin) are main visual opsin proteins and play important in behavior regulation.We used CRISPR/Cas9 technology to mutate the long-wavelength-sensitive and blue wavelength-sensitive genes and explored the role of vision in the nocturnal invasive pest Tuta absoluta. Light wave experiments revealed that LW2(-/-) and BL(-/-) mutants showed abnormal wavelength tropism. Both LW2 and BL mutations affected the preference of T. absoluta for the green environment. Mutations in LW2 and BL are necessary to inhibit visual attraction. The elimination of LW2 and BL affected the preference of leaf moths for green plants, and mutations in both induced a preference in moths for white plants. Behavioral changes resulting from LW2(-/-) and BL(-/-) mutants were not affected by sense of smell, further supporting the regulatory role of vision in insect behavior. To the best of our knowledge, this is the first study to reveal that vision, not smell, plays an important role in the host-seeking behavior of nocturnal insects at night, of which LW2 and BL opsins are key regulatory factors. These study findings will drive the development of the "vision-ecology" theory.

Reproductive behavior of fruit flies: courtship, mating, and oviposition

Many species of the Tephritidae family are invasive and cause huge damage to agriculture and horticulture, owing to their reproductive characteristics. In this review, we have summarized the existing studies on the reproductive behavior of Tephritidae, particularly those regarding the genes and external factors that are associated with courtship, mating, and oviposition. Furthermore, we outline the issues that still need to be addressed in fruit fly reproduction research. The review highlights the implications for understanding the reproductive behavior of fruit flies and discusses methods for their integrated management and biological control. © 2023 Society of Chemical Industry.

Role of Genes in Regulating Host Plants Expansion in Tephritid Fruit Flies (Diptera) and Potential for RNAi-Based Control

Host plant expansion is an important survival strategy for tephritids as they expand their range. Successful host expansion requires tephritids to adapt to the chemical and nonchemical properties of a novel host fruit, such as fruit color, phenology, and phytochemicals. These plant properties trigger a series of processes in tephritids, with each process having its own genetic basis, which means that various genes are involved in regulating host plant expansion by tephritids. This review summarizes current knowledge on the categories and roles of genes involved in host plant expansion in several important tephritid species, including genes related to chemoreception (olfactory and gustation), vision, digestion, detoxification, development, ribosomal and energy metabolism. Chemoreception- and detoxification- and digestion-related genes are stimulated by volatile chemicals and secondary chemicals of different hosts, respectively, which are involved in the regulation of nervous signal transduction that triggers behavioral, physical, and chemical responses to the novel host fruit. Vision-, nerve-, and development-related genes and metabolism-associated genes are activated in response to nonchemical stimuli from different hosts, such as color and phenology, to regulate a comprehensive adaptation of the extending host for tephritids. The chemical and nonchemical signals of hosts activate ribosomal and energy-related genes that result in the basic regulation of many processes of host expansion, including detoxification and development. These genes do not regulate novel host use individually, but multiple genes regulate multilevel adaptation to novel host fruits via multiple mechanisms. These genes may also be potential target genes for RNAi-based control of tephritid pests.

Mechanisms of Novel Host Use by Bactrocera tau (Tephritid: Diptera) Revealed by RNA Transcriptomes

Use of novel plant hosts can facilitate the establishment and range expansion of herbivorous invasive species. However, the inherent mechanisms of novel host use are still unclear in many herbivorous species. Here, we examine mechanisms of novel host use in the invasive tephritid fruit fly Bactrocera tau (Walker)(Diptera: Tephritidae) by documenting changes in the RNA transcriptomes associated with a novel host. RNA transcripts of B. tau were obtained with high-throughput sequencing from samples continuously reared on two traditional Cucurbitaceae hosts and a novel host (banana). We found transcriptome variation was strongly associated with feeding on banana. Moreover, B. tau feeding on banana contained more differentially expressed genes (DEGs) and more annotated categories of DEGs in the Kyoto Encyclopedia of Genes and Genomes (KEGG) database with 1,595 DEGs and 21 major annotated pathways. The annotated categories of DEGs in individuals reared on banana differed with from those individuals feeding on other hosts and were enriched in oxidative phosphorylation, citrate cycle pathway, and four other carbohydrate pathways. For B. tau feeding on banana, the predominant numbers of upregulated genes in the mitochondrial NADH (56 on average) and a relatively higher numbers of upregulated genes (13 on average) were found in oxidative phosphorylation and the TCA pathway, respectively. Changes in RNA transcriptomes associated with novel host use, especially for genes related to energy and carbohydrate metabolism, help to explain how B. tau can be successful in use of novel hosts and may be useful in developing novel strategies for control of tephritid flies.

Asymmetric interactions of seed-predation network contribute to rare-species advantage

Although the asymmetry of species linkage within ecological networks is now well recognized, its effect on communities has scarcely been empirically investigated. Based on theory, we predicted that an asymmetric architecture of antagonistic plant-herbivore networks would emerge at the community level and that this asymmetry would negatively affect community-common plants more than rare ones. We tested this prediction by analyzing the architectural properties of an alpine plant and pre-dispersal seed-predator network and its effect on seed loss rate of plants in the Tibetan Plateau. This network showed an asymmetric architecture, where the common plant species (with a larger aboveground biomass per area) were infested by a higher number of predator species. Moreover, they asymmetrically interacted with specialized herbivores, presumably because of greater seed resource abundance. In turn, the asymmetric interactions led to a higher proportion of seed loss in the common plants at the species level. Our results suggest that asymmetric antagonistic networks may improve species coexistence by contributing to a mechanism of rare-species advantage.

Rh6 gene modulates the visual mechanism of host utilization in fruit fly Bactrocera minax

BACKGROUND

Vision plays a critical role in host location and oviposition behavior for herbivorous insects. However, the molecular mechanisms underlying visual regulation in host recognition and oviposition site selection in insects remains unknown. The aim of this study was to explore the key visual genes that are linked to the host plant location of the fruit fly, Bactrocera minax.

RESULTS

Using a host specialist fruit fly, B. minax, which lays eggs only into immature green citrus fruit, we undertook behavioral, transcriptomic, and RNAi research to identify the molecular basis for host fruit color recognition. In laboratory and field assays we found that adults prefer green over other colors, and this preference is significantly increased in sexually mature over immature flies. Furthermore, we identified that the Rh6 gene, responsible for green spectral sensitivity, has elevated expression in mature flies over immature flies. RNAi suppression of Rh6 eliminated the preference for green, resulting in a significant decrease in the number of eggs laid by B. minax in green unripe citrus.

CONCLUSION

These results show that the Rh6 gene modulates the visual mechanism of host utilization in B. minax, providing a genetic basis for visual host location in a non-model insect herbivore. © 2018 Society of Chemical Industry.

Effect of Physiological State on Female Melon Fly (Diptera: Tephritidae) Attraction to Host and Food Odor in the Field

Foraging behavior of wild female melon fly, Bactrocera (Zeugodacus) cucurbitae Coquillett, a worldwide pest of economically important cucurbit crops, was examined through mark and recapture studies in both wild (Kona: dominated by the invasive weed ivy gourd, Coccinea grandis [L.] Voigt [Cucurbitaceae]), and cultivated (Kapoho: dominated by papaya, Carica papaya L. [Caricaceae] orchards) habitats on Hawaii Island. In particular, the extent to which wild melon flies and color-marked F2 females responded to cucumber odor and Solulys yeast hydrolysate laced with ammonium acetate (1%, wt/vol) according to sexual maturity stage and degree of protein hunger was documented. Kona results indicated that more wild and color-marked F2 females responded to cucumber (Cucumis sativus L. [Cucurbitaceae]) odor than to protein odor with the exception of captured wild flies without eggs, which responded similarly to protein bait and cucumber odor. Results with captured wild females and color-marked F2 females in Kapoho suggested a significant preference for cucumber odor over protein odor regardless of whether or not they had eggs in their ovaries with the exception of protein-deprived color-marked F2 females, which responded to both odors in equal numbers. Implications of these new findings based on wild melon flies in natural habitats are discussed with respect to integrated pest management control strategies with protein bait sprays used in Hawaii. The possibility of adding cucurbit volatiles to protein-based baits is discussed.

Development of a Low-Cost and Effective Trapping Device for Apple Maggot Fly (Diptera: Tephritidae) Monitoring and Control in Mexican Commercial Hawthorn Groves

Few efforts have been made in Mexico to monitor Rhagoletis pomonella (Walsh) (Diptera: Tephritidae) in commercial hawthorn (Crataegus spp.) crops. Therefore, the main objectives of this study were to evaluate infestation levels of R. pomonella in feral and commercial Mexican hawthorn and to assess the efficacy of different trap-lure combinations to monitor the pest. Wild hawthorn was more infested than commercially grown hawthorn at the sample site. No differences among four commercial baits (Biolure, ammonium carbonate, CeraTrap, and Captor + borax) were detected when used in combination with a yellow sticky gel (SG) adherent trap under field conditions. However, liquid lures elicited a slightly higher, although not statistically different, capture. Cage experiments in the laboratory revealed that flies tended to land more often on the upper and middle than lower-bottom part of polyethylene (PET) bottle traps with color circles. Among red, orange, green, and yellow circles attached to a bottle trap, only yellow circles improved fly captures compared with a colorless trap. A PET bottle trap with a red circle over a yellow background captured more flies than a similar trap with yellow circles. An SG adherent yellow panel trap baited with ammonium carbonate was superior to the improved PET bottle trap (red over a yellow background) baited with different liquid proteins, but a higher proportion of females and no differences in fly detection were measured in PET traps baited with protein lures. These trials open the door for future research into development of a conventional nonadherent trap to monitor or control R. pomonella.