Field attractants for Pachnoda interrupta selected by means of GC-EAD and single sensillum screening

Electrophysiological and Behavioral Responses of Plutella xylostella (Lepidoptera: Plutellidae) to Volatiles from a Non-host Plant, Geranium, Pelargonium × hortorum (Geraniaceae)

The diamondback moth (DBM), Plutella xylostella (Lepidoptera: Plutellidae), is a notorious cruciferous vegetable pest globally. Mechanically damaged geranium (Pelargonium × hortorum) can strongly repel DBM, but specific plant bioactive compounds responsible for such effects have not been identified. The headspace volatiles from wounded geranium were analyzed using gas chromatography-electroantennographic detection and gas chromatography-mass spectrometry. The electrophysiological and behavioral responses of DBM females to these chemicals and two blends were investigated. The results showed that five components myrcene, γ-terpinene, linalool, camphor, and terpinen-4-ol in geranium plants mediated the repellence of DBM mated females. These substances significantly repelled the oviposition of females, and blend-2 (a mixture of the five components with a ratio 1:5:3:4:3) was most effective. The slow-release blend-2 had a repellent range of 0.9 m and a repellent longevity of 26 days. These five substances are promising behavioral regulators of the destructive moths and could be potential candidates for "push" components in plant-based "push-pull" strategies.

Structure and distribution of antennal sensilla in Pseudosymmachia flavescens (Brenske) (Coleoptera: Scarabaeidae: Melolonthinae)

Morphology, microstructure, and distribution of antennal sensilla were compared between female and male Pseudosymmachia flavescens (Brenske) (Coleoptera: Scarabaeidae: Melolonthinae). Lamellate antennae of P. flavescens were shown to have typical scape, pedicel, and flagellum segments. The flagellum consists of a four-segmented funicle and a three-segmented club. The lengths of their pedicel and funicle were found to be similar in females and males. Distinct sexual differences were observed in the length of lamellar segments. Nine types/subtypes of sensilla were identified on the antennae of both sexes, including Böhm sensilla, sensilla trichodea, sensilla basiconica (SB), and two subtypes for sensilla chaetica, sensilla coeloconica, and sensilla placodea each. Olfactory sensilla (e.g., SB and placodea) are mainly located on three lamellar segments of the antennal club. Variation was also seen in abundance of various types of antennal sensilla, with males possessing significantly more sensilla than females. Sensilla placodea were the most abundant, and their number in males was twice of that in females, showing a clear sexual dimorphism. The difference in the distribution of sensilla placodea might reflect their roles in sexual chemical communication.

Electrophysiological and Behavioral Responses of Holotrichia parallela to Volatiles from Peanut

Simple Summary

The dark black chafer, Holotrichia parallela, which is widely distributed all over the world, is an economically important pest in agriculture and forestry. In the north part of China, this beetle causes serious damage to the peanut plant. Much attention has been paid to olfactory perception of volatile compounds from supplemental nutrition hosts by H. parallela prior to sexual maturation. However, volatile compounds attractive to this beetle from the peanut plant have not been identified yet. In this study, we collected the volatile compounds from peanut seedlings by dynamic headspace adsorption and identified twelve electrophysiologically active compounds responsible for the attraction of H. parallela to the peanut. Among the eight chemically identified compounds, β-caryophyllene and hexanal significantly attracted both sexes of H. parallela when tested individually in the field. A blend of β-caryophyllene and hexanal at a ratio of 2:1 was most attractive to the beetles. The addition of the remaining compounds to the binary mixture did not increase the attractiveness. The findings of this study reveal that β-caryophyllene and hexanal can be potentially used for development of effective attractants for management of H. parallela.

Abstract

Holotrichia parallela (Coleoptera: Scarabaeidae: Melolonthinae) is a notorious pest of many crops, especially peanuts. In this study, volatiles from peanut plants were analyzed using both gas chromatographic-electroantennographic detection (GC-EAD) and gas chromatography/mass spectrometry (GC/MS) techniques, and tested for adult attraction with field trapping bioassays in Hebei Province, China. GC-EAD analyses indicated that H. parallela antennae strongly responded to twelve GC peaks, including eight identified compounds, (Z)-β-ocimene, hexanal, 6-methyl-5-hepten-2-one, nonanal, dihydromyrcenol, linalool, β-caryophyllene, methyl salicylate, and four unidentified compounds. When tested individually in field conditions from 24 to 31 July, 2020, β-caryophyllene and hexanal significantly attracted both sexes of H. parallela, whereas all other compounds were unattractive. A blend of β-caryophyllene and hexanal at a ratio of 2:1, close to the natural ratio of these two compounds from the intact peanut plant, was most attractive to the beetles. The remaining identified compounds, (Z)-β-ocimene, 6-methyl-5-hepten-2-one, nonanal, dihydromyrcenol, linalool, and methyl salicylate had no synergistic effects on H. parallela attraction when tested in combination with the blend of β-caryophyllene and hexanal. These results demonstrated that β-caryophyllene and hexanal in the volatiles from peanut plants have strong attraction to H. parallela. These two compounds have the potential to be used for monitoring H. parallela and its management programs.

Scent chemistry and pollinators in the holoparasitic plant Cynomorium songaricum (Cynomoriaceae)

Holoparasitic plants are interesting heterotrophic angiosperms. However, carrion- or faeces-mimicking is rarely described for such plants. There is no information on the pollination biology of Cynomoriaceae, despite the fact that these plants are rare and vulnerable. This is the first study to reveal pollination in a member of this family, Cynomorium songaricum, a root holoparasite with a distinctive and putrid floral odour. From 2016 to 2018, we studied the floral volatiles, floral visitors and pollinators, behavioural responses of visitors to floral volatiles, breeding system, flowering phenology and floral biology of two wild populations of C. songaricum in Alxa, Inner Mongolia, China. A total of 42 volatiles were identified in inflorescences of C. songaricum. Among these volatiles are compounds known as typical carrion scents, such as p-cresol, indole, dimethyl disulphide and 1-octen-3-ol. C. songaricum is pollinated by various Diptera, such as Musca domestica, M. stabulans (Muscidae), Delia setigera, D. platura (Anthomyiidae), Lucilia sericata, L. caesar (Calliphoridae), Wohlfahrtia indigens, Sarcophaga noverca, S. crassipalpis and Sarcophila meridionalis (Sarcophagidae). The inflorescence scent of C. songaricum attracted these pollinators. The plants significantly benefit from insect pollination, although wind can be a pollen vector in the absence of pollinators. C. songaricum is a cross-pollinated, self-incompatible plant. Our findings suggest that C. songaricum releases malodorous volatiles to attract Diptera to achieve pollination. This new example lays the foundation for further comparative studies in other members of this plant group and contributes to a better understanding of fly-pollinated, carrion mimicking plants.

Shared volatile organic compounds between camel metabolic products elicits strong Stomoxys calcitrans attraction

The sources of animal odours are highly diverse, yet their ecological importance, in host-vector communication, remains unexplored. Here, using the camel (host)-Stomoxys calcitrans (vector) interaction, we collected and analyzed the Volatile Organic Compounds (VOCs) of camels from four of its different odour sources: breath, body (skin), urine, and dung. On non-metric model multivariate analyses of VOCs we show that substantial chemo-diversity exists between metabolic products associated with an individual camel. VOCs from the four metabolic products were distinct and widely segregated. Next, we show electrophysiologically, that VOCs shared between metabolic products activated more Olfactory Sensory Neurons (OSNs) and elicited strong behavioural attractive responses from S. calcitrans under field conditions independent of geography. In our extended studies on house flies, the behavioural response to these VOCs appears to be conserved. Overall, our results establish that VOCs from a range of metabolic products determine host-vector ecological interactions and may provide a more rigorous approach for discovery of unique and more potent attractants.

Development of a Female-Targeted Lure for the Box Tree Moth Cydalima perspectalis (Lepidoptera: Crambidae): a Preliminary Report

The box tree moth, Cydalima perspectalis, is an invasive pest in Europe causing damage on Buxus species. In this study, we aimed to develop a "bisexual" lure to attract both female and male moths. Based on a previous screening bioassay we tested methyl salicylate, phenylacetaldehyde and eugenol as potential attractants in different combinations. The trapping results showed that both binary and ternary blends attracted male and female moths. Catches with these blends were comparable to catches with the synthetic pheromone. Subsequently we carried out single sensillum recordings, which proved the peripheral detection of the above-mentioned compounds on male and female antennae. To identify synergistic flower volatiles, which can be also attractive and can increase the trap capture, we collected flower headspace volatiles from 12 different flowering plant species. Several components of the floral scents evoked good responses from antennae of both females and males in gas chromatography-electroantennographic detection. The most active components were tentatively identified by gas chromatography coupled mass spectrometry as benzaldehyde, cis-ß-ocimene, (±)-linalool and phenethyl alcohol. These selected compounds in combination did not increase significantly the trap capture compared to the methyl salicylate- phenyacetaldehyde blend. Based on these results we discovered the first attractive blend, which was able to attract both adult male and female C. perspectalis in field conditions. These results will yield a good basis for the optimization and development of a practically usable bisexual lure against this invasive pest.

Egg-laying decisions based on olfactory cues enhance offspring fitness in Stomoxys calcitrans L. (Diptera: Muscidae)

Selection of oviposition substrate is critical in holometabolous insects. Female stable flies, Stomoxys calcitrans, locate and select vertebrate herbivore dung in which they lay their eggs. However, the preference for vertebrate herbivore dung by S. calcitrans females, its fitness consequences for offspring, and the semiochemicals used to locate and select oviposition substrates remain unclear. Using oviposition choice tests and life table bioassays we found that gravid female S. calcitrans prefer to oviposit on donkey and sheep dung, which also improves the performance of their offspring. GC-MS analysis followed by random forest classification identified β-citronellene and carvone as the most important predictive volatile organic compounds of donkey and sheep dung, respectively. In multiple choice oviposition bioassays, S. calcitrans laid more eggs in wet sand containing β-citronellene and carvone than in other treatments. The attractiveness of these compounds was confirmed in a field trial, with traps baited with β-citronellene and carvone catching more S. calcitrans. We conclude that gravid female S. calcitrans use semiochemical cues to choose oviposition substrates that maximise offspring fitness.

A Practical Technique for Electrophysiologically Recording from Lamellated Antenna of Scarab Beetle

Host-plant volatiles play a key role in finding mate and suitable host plants of phytophagous scarab beetles. Hence it is immensely important to collect and identify these volatiles. The gas chromatography coupled with electroantennographic detection (GC-EAD) technique has been used as a rapid and convenient tool for the identification of physiologically active components from plants. Here, we describe a practical method for electrophysiologically recording from lamellated antenna of scarab beetles. This method enables direct electroantennogram (EAG) recordings from antennal club without damage to the antenna in a similar manner to the conventional cut-tip EAG recording technique for clavate antenna. The headspace volatiles from walnut (Juglans regia L.) trees were collected with a Poropak-Q trap at dusk and then analyzed with GC-EAD. Those volatile compounds that elicited electrophysiological responses on the antennae of a scarab beetle, Metabolus flavescens Brenske (Coleoptera: Scarabeidae: Melolonthinae) were determined by means of gas chromatography-mass spectrometry (GC-MS). The lamella directly connected to the recording electrode was held apart from the other two lamellae on the antenna with a minuten pin and a disposable syringe needle. In order to improve electrical contact, a surfactant, Tween® 80, was used to lower the surface tension of Beadle-Ephrussi Ringer solution. This study demonstrated that addition of 0.05% Tween® 80 to the Beadle-Ephrussi Ringer solution suppressed baseline noise and assured significantly greater EAG response in general. Due to its simplicity and efficiency, this method may also be useful for studying the electrophysiology of other insect species having club-like antennae.

A SEM study of antennal sensilla in Maladera orientalis Motschulsky (Coleoptera: Scarabaeidae: Melolonthinae)

The external morphology of antenna and fine structure of its sensilla of Maladera orientalis were studied using scanning electron microscopy. The antenna consists of scape, pedicel, funicle and a club composed of three lamellae. Funicle has five, sometimes, four segments. Böhm bristles, three subtypes of sensilla chaetica, one subtype of sensilla trichodea, one subtype of sensilla coeloconica, four subtypes of sensilla basiconica, and one subtype of sensilla placodea were described. No sexual differences were found in number, type and distribution of sensilla on all antennomeres. The most notable feature of sensilla on antennae of M. orientalis is the presence of long and narrow strip-like sensilla placodea on both sides of all club lamellae. These porous sensilla placodea are slightly depressed below the lamella surface and tend to lie in parallel with the lamella axe. They are similar to sensilla placodea found in hymenopteran species but very different from those round plate-like sensilla placodea occurring on the lamellae of scarab beetles.

Pheromones and Other Semiochemicals for Monitoring Rare and Endangered Species

As global biodiversity declines, biodiversity and conservation have become ever more important research topics. Research in chemical ecology for conservation purposes has not adapted to address this need. During the last 10-15 years, only a few insect pheromones have been developed for biodiversity and conservation studies, including the identification and application of pheromones specifically for population monitoring. These investigations, supplemented with our knowledge from decades of studying pest insects, demonstrate that monitoring with pheromones and other semiochemicals can be applied widely for conservation of rare and threatened insects. Here, I summarize ongoing conservation research, and outline potential applications of chemical ecology and pheromone-based monitoring to studies of insect biodiversity and conservation research. Such applications include monitoring of insect population dynamics and distribution changes, including delineation of current ranges, the tracking of range expansions and contractions, and determination of their underlying causes. Sensitive and selective monitoring systems can further elucidate the importance of insect dispersal and landscape movements for conservation. Pheromone-based monitoring of indicator species will also be useful in identifying biodiversity hotspots, and in characterizing general changes in biodiversity in response to landscape, climatic, or other environmental changes.

Behavioral Evidence for Olfactory-Based Location of Honeybee Colonies by the Scarab Oplostomus haroldi

The Afro-tropical scarab Oplostomus haroldi (Witte) is a pest of honeybees in East Africa with little information available on its chemical ecology. Recently, we identified a female-produced contact sex pheromone, (Z)-9-pentacosene, from the cuticular lipids that attracted males. Here, we investigated the kairomonal basis of host location in O. haroldi. We used coupled gas chromatography/electroantennographic detection (GC/EAD) and GC/mass spectrometry to identify antennally-active compounds from volatiles collected from honeybee colonies. Antennae of both sexes of the beetle consistently detected seven components, which were identified as 3-hydroxy-2-butanone, 2,3-butanediol, butyl acetate, isopentyl acetate, butyl butyrate, hexyl acetate, and methyl benzoate. In olfactometer bioassays, both sexes responded to the full seven-component synthetic blend over solvent controls, but chose honeybee colony odors over the blend. These findings suggest that the seven compounds are components of a kairomone from honeybee colonies used by O. haroldi.

Sweet scents from good bacteria: Case studies on bacterial volatile compounds for plant growth and immunity

Beneficial bacteria produce diverse chemical compounds that affect the behavior of other organisms including plants. Bacterial volatile compounds (BVCs) contribute to triggering plant immunity and promoting plant growth. Previous studies investigated changes in plant physiology caused by in vitro application of the identified volatile compounds or the BVC-emitting bacteria. This review collates new information on BVC-mediated plant-bacteria airborne interactions, addresses unresolved questions about the biological relevance of BVCs, and summarizes data on recently identified BVCs that improve plant growth or protection. Recent explorations of bacterial metabolic engineering to alter BVC production using heterologous or endogenous genes are introduced. Molecular genetic approaches can expand the BVC repertoire of beneficial bacteria to target additional beneficial effects, or simply boost the production level of naturally occurring BVCs. The effects of direct BVC application in soil are reviewed and evaluated for potential large-scale field and agricultural applications. Our review of recent BVC data indicates that BVCs have great potential to serve as effective biostimulants and bioprotectants even under open-field conditions.

Smoke, pheromone and kairomone olfactory receptor neurons in males and females of the pine sawyer Monochamus galloprovincialis (Olivier) (Coleoptera: Cerambycidae)

The response of antennal olfactory receptor neurons (ORNs) of Monochamus galloprovincialis to several odourants was tested using single sensillum electrophysiology. Behaviourally active pheromone, and kairomone (host and sympatric bark beetle pheromone) odours were tested alongside smoke compounds released by burnt wood that are potentially attractive to the insect. The antennae bore several types of sensilla. Two plate areas in the proximal and distal ends of each antennal segment were covered with basiconic sensilla that responded to the odour stimuli. Sensilla basiconica contained one or two cells of different spike amplitude. The 32 male and 38 female ORNs tested responded with excitations or inhibitions to the different plant odours. In general the response of male and female receptors was very similar so they were pooled to perform a cluster analysis on ORN responses. Six ORNs were clearly specialised for pheromone reception. Responses to kairomone and smoke odours were less specific than those of pheromone, but a group of 9 cells was clearly excited by smoke compounds (mainly eugenol and 4-methyl 2-methoxyphenol), a group of 8 cells was very responsive to α-pinene, β-pinene and cis-verbenol, and a group of 14 cells responded to a wider range of compounds. The rest of the cells (47%) were either non-responsive or slightly inhibited by smoke compounds. Dose-response curves were obtained for several compounds. Different compounds induced significantly different latencies and these appeared to be unrelated to their boiling point.

Role of bacterial volatile compounds in bacterial biology

Bacterial interactions with neighboring microorganisms via production of small metabolites enable bacteria to respond and adapt to environmental changes. The study of intercellular interactions primarily focused on soluble metabolites, but bacteria also produce and release into their headspace a wide variety of volatile secondary metabolites, the ecological roles of which have generally been overlooked. However, bacterial volatile compounds are known to contribute to interkingdom interactions (plant, fungi and nematodes), and recent studies also identified their at-a-distance influence on bacterial behavior. The present review describes the biological roles of bacterial volatile compounds in inter- and intraspecies bacterial interactions, a new and yet unexplored research area, with potential clinical and industrial applications.

Semiochemistry of the Scarabaeoidea

The superfamily Scarabaeoidea comprises a large and diverse monophyletic group. Members share ancestral characteristics, but often exhibit considerable differences in their ecology, physiology, or mating strategies. A large number of species are regarded as pests of crop or amenity plants, while others are beneficial to humans and even may be extremely rare as a result of anthropogenic activities. A significant number of chemical ecology-based studies have been conducted with the Scarabaeoidea in order to characterize semiochemicals influencing their behavior, such as pheromones and plant-derived allelochemicals. These may be used either to control or preserve populations of the beetles, depending upon pest or beneficial status. This paper is a review of the role and identity of the semiochemicals of the Scarabaeoidea, with comments on possible future research and applied opportunities in the field of chemical ecology.

Two-way plant mediated interactions between root-associated microbes and insects: from ecology to mechanisms

Plants are members of complex communities and function as a link between above- and below-ground organisms. Associations between plants and soil-borne microbes commonly occur and have often been found beneficial for plant fitness. Root-associated microbes may trigger physiological changes in the host plant that influence interactions between plants and aboveground insects at several trophic levels. Aboveground, plants are under continuous attack by insect herbivores and mount multiple responses that also have systemic effects on belowground microbes. Until recently, both ecological and mechanistic studies have mostly focused on exploring these below- and above-ground interactions using simplified systems involving both single microbe and herbivore species, which is far from the naturally occurring interactions. Increasing the complexity of the systems studied is required to increase our understanding of microbe-plant-insect interactions and to gain more benefit from the use of non-pathogenic microbes in agriculture. In this review, we explore how colonization by either single non-pathogenic microbe species or a community of such microbes belowground affects plant growth and defense and how this affects the interactions of plants with aboveground insects at different trophic levels. Moreover, we review how plant responses to foliar herbivory by insects belonging to different feeding guilds affect interactions of plants with non-pathogenic soil-borne microbes. The role of phytohormones in coordinating plant growth, plant defenses against foliar herbivores while simultaneously establishing associations with non-pathogenic soil microbes is discussed.

Specificity and sensitivity of plant odor-detecting olfactory sensory neurons in Ctenarytaina eucalypti (Sternorrhyncha: Psyllidae)

The blue gum psyllid, Ctenarytaina eucalypti (Sternorrhyncha: Psyllidae), is an economic threat to Eucalyptus subgenus Symphyomyrtus plantations worldwide. To date, no generally applicable control method is available and the potential for semiochemical-based monitoring or control methods has not yet been investigated. Hence, we conducted the first study on the olfactory sense of C. eucalypti, investigating the specificity and sensitivity of its olfactory sensory neurons (OSNs) to host plant volatiles using single sensillum recordings (SSR). Synthetic compounds were selected from published identifications of Eucalyptus volatiles and after analysis of headspace collections from Eucalyptus cordata. The antenna of C. eucalypti carries four cavities containing olfactory sensilla (S1-S4). Our recordings revealed that each of these sensilla houses three OSNs that could be distinguished electrophysiologically based on spike amplitude differences (A, B, and C neuron with large, intermediate, and small amplitude, respectively). The A neuron in sensillum S1 responded primarily to β-caryophyllene and weaker to β-ocimene, whereas the accompanying B-neuron responded strongly and very specifically to linalool. Furthermore, the B-neuron in both S2 and S3 responded strongly to 1-hexanol, Z3-hexenol, and Z3-hexenyl acetate. OSNs in S4 responded only weakly to a few of the synthetic compounds. Response thresholds in strongly responding OSNs to putative key compounds were close to the 1ng dose on the filter paper and responses exhibited a phasic-tonic profile irrespective of compound dose. C. eucalypti may use the physiologically active compounds for long-range host finding. Future laboratory and field experiments will reveal whether plant volatiles can be used in the management and monitoring of C. eucalypti.

Floral scent in bird- and beetle-pollinated Protea species (Proteaceae): chemistry, emission rates and function

Evolutionary shifts between pollination systems are often accompanied by modifications of floral traits, including olfactory cues. We investigated the implications of a shift from passerine bird to beetle pollination in Protea for floral scent chemistry, and also explored the functional significance of Protea scent for pollinator attraction. Using headspace sampling and gas chromatography-mass spectrometry, we found distinct differences in the emission rates and chemical composition of floral scents between eight bird- and four beetle-pollinated species. The amount of scent emitted from inflorescences of beetle-pollinated species was, on average, about 10-fold greater than that of bird-pollinated species. Floral scent of bird-pollinated species consists mainly of small amounts of "green-leaf volatiles" and benzenoid compounds, including benzaldehyde, anisole and benzyl alcohol. The floral scent of beetle-pollinated species is dominated by emissions of linalool, a wide variety of other monoterpenes and the benzenoid methyl benzoate, which imparts a fruity odour to the human nose. The number of compounds recorded in the scent of beetle-pollinated species was, on average, greater than in bird-pollinated species (45 versus 29 compounds, respectively). Choice experiments using a Y-maze showed that a primary pollinator of Protea species, the cetoniine beetle Atrichelaphinis tigrina, strongly preferred the scent of inflorescences of the beetle-pollinated Protea simplex over those of the bird-pollinated sympatric congener, Protea roupelliae. This study shows that a shift from passerine bird- to insect-pollination can be associated with marked up-regulation and compositional changes in floral scent emissions.

Characterization of olfactory sensory neurons in the white clover seed weevil, Apion fulvipes (Coleoptera: Apionidae)

Seed-eating Apion weevils (Coleoptera: Apionidae) cause large economic losses in white and red clover seed production across Europe. Monitoring and control of clover weevils would be facilitated by semiochemical-based methods. Until now, however, nothing was known about physiological or behavioral responses to semiochemicals in this insect group. Here we analyzed the antenna of the white clover (Trifolium repens L.) specialist Apion fulvipes Geoffroy with scanning electron microscopy, and used single sensillum recordings with a set of 28 host compounds to characterize 18 classes of olfactory sensory neurons (OSNs). Nine of the OSN classes responded strongly to synthetic compounds with high abundance in clover leaves, flowers, or buds. Eight classes responded only weakly to the synthetic stimuli, whereas one collective class responded exclusively to volatiles released from a crushed clover leaf. The OSNs showed a remarkable degree of specificity, responding to only one or a few chemically related compounds. In addition, we recorded a marked difference in the temporal dynamics of responses between different neurons, compounds, and doses. The identified physiologically active compounds will be screened for behavioral activity, with the ultimate goal to develop an odor-based control strategy for this pest.

The key role of 4-methyl-5-vinylthiazole in the attraction of scarab beetle pollinators: a unique olfactory floral signal shared by Annonaceae and Araceae

Cyclocephaline scarabs are specialised scent-driven pollinators, implicated with the reproductive success of several Neotropical plant taxa. Night-blooming flowers pollinated by these beetles are thermogenic and release intense fragrances synchronized to pollinator activity. However, data on floral scent composition within such mutualistic interactions are scarce, and the identity of behaviorally active compounds involved is largely unknown. We performed GC-MS analyses of floral scents of four species of Annona (magnoliids, Annonaceae) and Caladium bicolor (monocots, Araceae), and demonstrated the chemical basis for the attraction of their effective pollinators. 4-Methyl-5-vinylthiazole, a nitrogen and sulphur-containing heterocyclic compound previously unreported in flowers, was found as a prominent constituent in all studied species. Field biotests confirmed that it is highly attractive to both male and female beetles of three species of the genus Cyclocephala, pollinators of the studied plant taxa. The origin of 4-methyl-5-vinylthiazole in plants might be associated with the metabolism of thiamine (vitamin B1), and we hypothesize that the presence of this compound in unrelated lineages of angiosperms is either linked to selective expression of a plesiomorphic biosynthetic pathway or to parallel evolution.

What reaches the antenna? How to calibrate odor flux and ligand-receptor affinities

Physiological studies on olfaction frequently ignore the airborne quantities of stimuli reaching the sensory organ. We used a gas chromatography-calibrated photoionization detector to estimate quantities released from standard Pasteur pipette stimulus cartridges during repeated puffing of 27 compounds and verified how lack of quantification could obscure olfactory sensory neuron (OSN) affinities. Chemical structure of the stimulus, solvent, dose, storage condition, puff interval, and puff number all influenced airborne quantities. A model including boiling point and lipophilicity, but excluding vapor pressure, predicted airborne quantities from stimuli in paraffin oil on filter paper. We recorded OSN responses of Drosophila melanogaster, Ips typographus, and Culex quinquefasciatus, to known quantities of airborne stimuli. These demonstrate that inferred OSN tuning width, ligand affinity, and classification can be confounded and require stimulus quantification. Additionally, proper dose-response analysis shows that Drosophila AB3A OSNs are not promiscuous, but highly specific for ethyl hexanoate, with other earlier proposed ligands 10- to 10 000-fold less potent. Finally, we reanalyzed published Drosophila OSN data (DoOR) and demonstrate substantial shifts in affinities after compensation for quantity and puff number. We conclude that consistent experimental protocols are necessary for correct OSN classification and present some simple rules that make calibration, even retroactively, readily possible.

Evolution of insect olfaction

Neuroethology utilizes a wide range of multidisciplinary approaches to decipher neural correlates of natural behaviors associated with an animal's ecological niche. By placing emphasis on comparative analyses of adaptive and evolutionary trends across species, a neuroethological perspective is uniquely suited to uncovering general organizational and biological principles that shape the function and anatomy of the nervous system. In this review, we focus on the application of neuroethological principles in the study of insect olfaction and discuss how ecological environment and other selective pressures influence the development of insect olfactory neurobiology, not only informing our understanding of olfactory evolution but also providing broader insights into sensory processing.

Electroantennogram responses of the tea slug moth, Iragoides fasciata to some plant volatiles associated with tea, Camellia sinensis

Electroantennogram responses to a wide range of plant volatile compounds that have been identified in tea plants Camellia sinensis L. (Ericales: Theaceae) were recorded from males and females of the tea slug moth, Iragoides fasciata Moore (Lepidoptera: Limacodidae). The responses to 26 compounds, belonging to several chemical classes, and two mixtures were evaluated. The results showed significantly different electroantennogram responses to the different chemicals, as well as significantly different responses according to gender. The green leaf volatile components elicited significantly greater responses in males. In general, the antennae of males were more sensitive, and responded more strongly, to most of the compounds. Responses to sesquiterpenoids were lower in both males and females. Dose-dependent response studies indicated differences in response between genders and concentrations, suggesting the existence of sexual dimorphism. Compounds belonging to the green leaf volatiles class appeared to be important clues in host-plant selection by this oligophagous species.

Fresh, dried or smoked? Repellent properties of volatiles emitted from ethnomedicinal plant leaves against malaria and yellow fever vectors in Ethiopia

BACKGROUND

In the search for plant-based mosquito repellents, volatile emanations were investigated from five plant species, Corymbia citriodora, Ocimum suave, Ocimum lamiifolium, Olea europaea and Ostostegia integrifolia, traditionally used in Ethiopia as protection against mosquitoes.

METHODS

The behaviour of two mosquitoes, the malaria vector Anopheles arabiensis and the arbovirus vector Aedes aegypti, was assessed towards volatiles collected from the headspace of fresh and dried leaves, and the smoke from burning the dried leaves in a two-choice landing bioassay and in the background of human odour.

RESULTS

Volatile extracts from the smoke of burning dried leaves were found to be more repellent than those from fresh leaves, which in turn were more repellent to mosquitoes than volatiles from dried leaves. Of all smoke and fresh volatile extracts, those from Co. citriodora (52-76%) and Oc. suave (58-68%) were found to be the most repellent, Os. integrifolia (29-56%) to be intermediate while Ol. europaea (23-40%) and Os. integrifolia (19-37%) were the least repellent. One volatile present in each of the fresh leaf extracts of Co. citriodora, Oc. suave and Os. integrifolia was ß-ocimene. The levels of ß-ocimene reflected the mosquito repellent activity of these three fresh leaf extracts. Female host-seeking mosquitoes responded dose-dependently to ß-ocimene, both physiologically and behaviourally, with a maximal behavioural repulsion at 14% ß-ocimene. ß-ocimene (14%) repels mosquitoes in our 6-minute landing assays comparable to the synthetic insect repellent N,N-diethyl-m-toluamide (10% DEET).

CONCLUSIONS

Volatiles in the smoke of burning as well as fresh leaves of Co. citriodora and Oc. suave have significant repellent properties against host seeking An. arabiensis and Ae. aegypti mosquitoes. ß-ocimene, present in the fresh leaf headspace of Co. citriodora, Oc. suave and Os. integrifolia, is a significantly effective volatile mosquito repellent in the laboratory. In addition to its repellent properties, ß-ocimene has long approved safe for use in food and cosmetics, making this volatile an intriguing compound to pursue in further tests in the laboratory and field to validate its mosquito repellent activity and potential for use in a commercial product. Also, the landing bioassay with humanised membranes is a potentially useful repellent screening technique that does not require the exposure of humans to the vectors, however further tests in parallel with conventional techniques are advised.

Massive production of butanediol during plant infection by phytopathogenic bacteria of the genera Dickeya and Pectobacterium

Plant pathogenic bacteria of the genera Dickeya and Pectobacterium are broad-host-range necrotrophs which cause soft-rot diseases in important crops. A metabolomic analysis, based on (13)C-NMR spectroscopy, was used to characterize the plant-bacteria interaction. Metabolic profiles revealed a decline in plant sugars and amino acids during infection and the concomitant appearance of a compound identified as 2,3-butanediol. Butanediol is the major metabolite found in macerated tissues of various host plants. It is accumulated during the symptomatic phase of the disease. Different species of Dickeya or Pectobacterium secrete high levels of butanediol during plant infection. Butanediol has been described as a signalling molecule involved in plant/bacterium interactions and, notably, able to induce plant systemic resistance. The bud genes, involved in butanediol production, are conserved in the phytopathogenic enterobacteria of the genera Dickeya, Pectobacterium, Erwinia, Pantoea and Brenneria. Inactivation of the bud genes of Dickeya dadantii revealed that the virulence of budA, budB and budR mutants was clearly reduced. The genes budA, budB and budC are highly expressed during plant infection. These data highlight the importance of butanediol metabolism in limiting acidification of the plant tissue during the development of the soft-rot disease caused by pectinolytic enterobacteria.

The 3-hydroxy-2-butanone pathway is required for Pectobacterium carotovorum pathogenesis

Pectobacterium species are necrotrophic bacterial pathogens that cause soft rot diseases in potatoes and several other crops worldwide. Gene expression data identified Pectobacterium carotovorum subsp. carotovorum budB, which encodes the α-acetolactate synthase enzyme in the 2,3-butanediol pathway, as more highly expressed in potato tubers than potato stems. This pathway is of interest because volatiles produced by the 2,3-butanediol pathway have been shown to act as plant growth promoting molecules, insect attractants, and, in other bacterial species, affect virulence and fitness. Disruption of the 2,3-butanediol pathway reduced virulence of P. c. subsp. carotovorum WPP14 on potato tubers and impaired alkalinization of growth medium and potato tubers under anaerobic conditions. Alkalinization of the milieu via this pathway may aid in plant cell maceration since Pectobacterium pectate lyases are most active at alkaline pH.

Conserved, highly specialized olfactory receptor neurons for food compounds in 2 congeneric scarab beetles, Pachnoda interrupta and Pachnoda marginata

Few studies have systematically addressed evolutionary changes in olfactory neuron assemblies, either by genetic drift or as an adaptation to specific odor environments. We have studied the sense of olfaction in 2 congeneric scarab beetles, Pachnoda interrupta Olivier and Pachnoda marginata Drury (Coleoptera: Scarabaeidae: Cetoniinae), which are both opportunistic polyphages, feeding mainly on fruit and flowers. The 2 species occur in dissimilar habitats: P. interrupta is found in dry savannah, and P. marginata in tropical parts of equatorial Africa. To study how these species may have adapted their sense of olfaction to their odor environments, we utilized single-unit electrophysiology on olfactory sensilla with a wide selection of food-related compounds. Despite the differences in habitat, we found that the species shared most of the physiological types of olfactory receptor neurons (ORNs) encountered, although their proportions frequently varied between the species. The high degree of conservation in olfaction between the species implies that a similar sensory strategy is efficient for food search in both habitats. However, shifts in proportions of receptor neuron classes, and slight shifts in response profiles and/or presence of some ORN classes unique to either species, may reflect adaptation to a different set of hosts.

Sorghum insect problems and management

Sorghum (Sorghum bicolor) has high levels of starch, sugar, and fiber and is one of the most important energy crops in the world. Insect damage is one of the challenges that impacts sorghum biomass production. There are at least 150 insect species that can infest sorghum varieties worldwide. These insects can complete several generations within a growing season, they target various parts of sorghum plants at developmental stages, and they cause significant biomass losses. Genetic research has revealed the existence of resistant genetics in sorghum and insect tolerant sorghum varieties have been identified. Various control methods have been developed, yet more effective management is needed for increasing sorghum biomass production. Although there are no transgenic sorghum products on the market yet, biotechnology has been recognized as an important tool for controlling insect pests and increasing sorghum production.

Peripheral modulation of pheromone response by inhibitory host compound in a beetle

We identified several compounds, by gas chromatographic-electroantennographic detection (GC-EAD), that were antennally active in the bark beetle Ips typographus and also abundant in beetle-attacked spruce trees. One of them, 1,8-cineole (Ci), strongly inhibited the attraction to pheromone in the field. Single-sensillum recordings (SSRs) previously showed olfactory receptor neurons (ORNs) on I. typographus antennae selectively responding to Ci. All Ci neurons were found within sensilla co-inhabited by a pheromone neuron responding to cis-verbenol (cV); however, in other sensilla, the cV neuron was paired with a neuron not responding to any test odorant. We hypothesized that the colocalization of ORNs had a functional and ecological relevance. We show by SSR that Ci inhibited spontaneous activity of the cV neuron only in sensilla in which the Ci neuron was also present. Using mixtures of cV and Ci, we further show that responses to low doses (1-10 ng) of cV were significantly reduced when the colocalized Ci neuron simultaneously responded to high doses (1-10 μg) of Ci. This indicated that the response of the Ci neuron, rather than ligand-receptor interactions in the cV neuron, caused the inhibition. Moreover, cV neurons paired with Ci neurons were more sensitive to cV alone than the ones paired with the non-responding ORN. Our observations question the traditional view that ORNs within a sensillum function as independent units. The colocalization of ORNs might sharpen adaptive responses to blends of semiochemicals with different ecological significance in the olfactory landscape.