Floral volatiles play a key role in specialized ant pollination

Chemical signals emitted by plants are crucial to understand the ecology and evolution of plant–animal interactions. Scent is an important component of floral phenotype and represents a decisive communication channel between plants and floral visitors. Floral volatiles promote attraction of mutualistic pollinators and, in some cases, serve to prevent flower visitation by antagonists such as ants. Despite ant visits to flowers have been suggested to be detrimental to plant fitness, in recent years there has been a growing recognition of the positive role of ants in pollination. Nevertheless, the question of whether floral volatiles mediate mutualisms between ants and ant-pollinated plants still remains largely unexplored. Here we review the documented cases of ant pollination and investigate the chemical composition of the floral scent in the ant-pollinated plant Cytinus hypocistis. By using chemical-electrophysiological analyses and field behavioural assays, we examine the importance of olfactory cues for ants, identify compounds that stimulate antennal responses, and evaluate whether these compounds elicit behavioural responses. Our findings reveal that floral scent plays a crucial role in this mutualistic ant–flower interaction, and that only ant species that provide pollination services and not others occurring in the habitat are efficiently attracted by floral volatiles. 4-oxoisophorone, (E)-cinnamaldehyde, and (E)-cinnamyl alcohol were the most abundant compounds in Cytinus flowers, and ant antennae responded to all of them. Four ant pollinator species were significantly attracted to volatiles emitted by Cytinus inflorescences as well as to synthetic mixtures and single antennal-active compounds. The small amount of available data so far suggest that there is broad interspecific variation in floral scent composition among ant-pollinated plants, which could reflect differential responses and olfactory preferences among different ant species. Many exciting discoveries will be made as we enter into further research on chemical communication between ants and plants.     

Forage quality and quantity affect red mason bees and honeybees differently in flowers of strawberry varieties

Nectar is a vital source of energy for bees and other pollinators and pollen represents the only source of protein in the diet of bees. Nectar and pollen quality and quantity can therefore affect foraging choices. Strawberry, Fragaria × ananassa (Rosaceae), is a flowering crop that requires insect pollination for the berries to develop optimally. The solitary red mason bee, Osmia bicornis L. (Hymenoptera: Megachilidae), occurs naturally but like the eusocial western honeybee, Apis mellifera mellifera L. (Hymenoptera: Apidae), it is also a commercially reared pollinator used in strawberry production. We hypothesized that strawberry nectar and pollen quality would affect the foraging choice of these two types of bees. In this study nectar and pollen quality is represented by various levels of sugar and protein content, respectively, as well as the number of open strawberry flowers in the experimental field, would affect the foraging choice of these two types of bees. Consistent with previous studies, we found significant and major differences between strawberry varieties in proportions of sucrose in the nectar sugar and in pollen viability – a proxy for pollen protein content. All measured parameters had a significant effect on red mason bee visitation frequency. Contrary to expectations, honeybee foraging behavior was only affected by the number of open flowers and not by any of the quality parameters measured. Our findings indicate that red mason bees were capable of assessing nectar and pollen quality and prioritize accordingly. The pattern observed indicates that individual red mason bees changed foraging focus between strawberry varieties depending on whether nectar or pollen was collected. Our results suggest that targeted breeding of varieties toward high levels of nectar sugar and sucrose concentrations and high pollen protein content may increase pollination success from red mason bees and possibly other solitary bees.     

Spatial and temporal patterns of floral scent emission in Dianthus inoxianus and electroantennographic responses of its hawkmoth pollinator

Scent emission is important in nocturnal pollination systems, and plant species pollinated by nocturnal insects often present characteristic odor compositions and temporal patterns of emission. We investigated the temporal (day/night; flower lifetime) and spatial (different flower parts, nectar) pattern of flower scent emission in nocturnally pollinated Dianthusinoxianus, and determined which compounds elicit physiological responses on the antennae of the sphingid pollinator Hyles livornica.The scent of D.inoxianus comprises 68 volatile compounds, but is dominated by aliphatic 2-ketones and sesquiterpenoids, which altogether make up 82% of collected volatiles. Several major and minor compounds elicit electrophysiological responses in the antennae of H. livornica. Total odor emission does not vary along day and night hours, and neither does along the life of the flower. However, the proportion of compounds eliciting physiological responses varies between day and night. All flower parts as well as nectar release volatiles. The scent of isolated flower parts is dominated by fatty acid derivatives, whereas nectar is dominated by benzenoids. Dissection (= damage) of flowers induced a ca. 20-fold increase in the rate of emission of EAD-active volatiles, especially aliphatic 2-ketones.We suggest that aliphatic 2-ketones might contribute to pollinator attraction in D. inoxianus, even though they have been attributed an insect repellent function in other plant species. We also hypothesize that the benzenoids in nectar may act as an honest signal (‘nectar guide’) for pollinators.     

Electrophysiological and behavioral responses of Asian and European honeybees to pear flower volatiles

Honeybee pollination behavior is influenced by flower volatiles, which honeybees sense via olfactory receptors. Honeybees are only weakly attracted to pear flowers. To investigate the potential reasons, we extracted and determined the floral volatile compounds from three pear cultivars (Su, Ya, and Xuehua) using headspace solid-phase micro-extraction (HS-SPME) and gas chromatography-mass spectrometry (GC–MS). The effects of pear flower volatiles on the Asian honeybee (Apis cerana cerana Fabricius) and the European honeybee (Apis mellifera ligustica Spinola) were determined by electroantennogram (EAG) assays and behavioral tests in a three-arm olfactometer. Among the 76 flower volatiles detected with GC–MS, 21 were found in all three pear cultivars, accounting for approximately 70% of the total volatile content. 3-Methyl-1-butanol and (+)-limonene volatiles had the highest relative content. Five compounds elicited strong EAG responses in both bee species: 2-methylbutyraldehyde, 1-nonanal, 6-methyl-5-hepten-2-one, 3-methyl-1-butanol, and (+)-limonene. Neither bee species showed positive taxis to these volatiles. In behavioral tests, A. mellifera ligustica showed a low preference for 6-methyl-5-hepten-2-one (20%, 400 µg/µL) and 2-phenethyl alcohol (16.7%, 400 µg/µL). Apis cerana cerana showed a low preference for 6-methyl-5-hepten-2-one (6.7%, 400 µg/µL) and 1-nonanal (10%, 400 µg/µL), whereas its preferences for 3-methyl-1-butanol (43.3%, 400 µg/µL) and α-farnesene (40%, 400 µg/µL) were similar to that for the control. Therefore, a lack of attractive volatile compounds could explain why honeybees are only weakly attracted to pear flowers. Therefore, to achieve acceptable pollination in pear orchards, we suggest using flower-scent sugar syrup feeding and a saturation pollination strategy.     

Pea plant volatiles guide host location behaviour in the pea moth

Identification of plant volatiles that attract mated insect females for oviposition can provide important information about plant–insect relationships that can be used to develop pest control strategies involving manipulation of the female host search. Our study represents a first step towards identifying volatiles that affect the host location behaviour of the pea moth Cydia nigricana. The behaviours of virgin and mated males and females were analysed in cage experiments testing a two-choice situation at close range and in wind tunnel experiments evaluating upwind orientation over a distance. In both experimental setups, flowering pea plants constituted the most attractive phenological stage for mated females, with 58 % landing on such plants in the wind tunnel. Testing headspace extracts of different phenological stages of pea and of detached pea buds and flowers in the wind tunnel, mated females showed the highest landing responses to volatiles during flower development (budding 42 % and flowering 56 %) and from detached buds (46 %) and flowers (66 %). Volatile compounds collected from the various phenological stages of pea were analysed by gas chromatography–mass spectrometry, and the antennal responses to these headspace collections were evaluated by gas chromatography–electroantennography. Ten antennally active compounds were identified, nine of which were present in the headspace extracts of the whole pea plants at all tested phenological stages and in detached buds and flowers. Overall, our results demonstrate a clear link between host plant phenology, the corresponding plant odour, and the behaviour of mated C. nigricana females.     

Red mason bees cannot compete with honey bees for floral resources in a cage experiment

Intensive beekeeping to mitigate crop pollination deficits and habitat loss may cause interspecific competition between bees. Studies show negative correlations between flower visitation of honey bees (Apis mellifera) and wild bees, but effects on the reproduction of wild bees were not proven. Likely reasons are that honey bees can hardly be excluded from controls and wild bee nests are generally difficult to detect in field experiments. The goal of this study was to investigate whether red mason bees (Osmia bicornis) compete with honey bees in cages in order to compare the reproduction of red mason bees under different honey bee densities. Three treatments were applied, each replicated in four cages of 18 m³ with 38 red mason bees in all treatments and 0, 100, and 300 honey bees per treatment with 10–20% being foragers. Within the cages, the flower visitation and interspecific displacements from flowers were observed. Niche breadths and resource overlaps of both bee species were calculated, and the reproduction of red mason bees was measured. Red mason bees visited fewer flowers when honey bees were present. Niche breadth of red mason bees decreased with increasing honey bee density while resource overlaps remained constant. The reproduction of red mason bees decreased in cages with honey bees. In conclusion, our experimental results show that in small and isolated flower patches, wild bees can temporarily suffer from competition with honey bees. Further research should aim to test for competition on small and isolated flower patches in real landscapes.     

Solitary bees – Potential vectors for gene flow from cultivated to wild strawberries

The genus Fragaria (Rosaceae) contains 24 plant species, including hybrid species such as the octoploid garden strawberry (F. × ananassa). Natural hybridization between Fragaria species has repeatedly been reported, and the potential future cultivation of genetically modified strawberries has made the study of hybridization potential between F. × ananassa and its wild relatives increasingly important. In Europe, F. × ananassa is the only octoploid species present, and the most likely candidate for hybridization is the common diploid woodland strawberry (F. vesca). To date, it is unknown whether pollinator spectra of the two Fragaria species overlap and thus might promote interspecific gene flow. We carried out a survey of flower visitors in northwestern Switzerland to identify major flower visitors of F. vesca and F. × ananassa. This survey indicated that wild bees are the most important shared flower visitors of F. × ananassa and F. vesca. Therefore, we studied flower choice behavior of the common wild bee Osmia bicornis in a greenhouse experiment. Osmia bicornis did not discriminate between F. × ananassa and F. vesca flowers. We conclude that wild bees are important shared flower visitors of both F. × ananassa and F. vesca and are potential vectors for gene flow between cultivated and wild strawberries.     

Variation in highbush blueberry floral volatile profiles as a function of pollination status, cultivar, time of day and flower part: implications for flower visitation by bees

Background and Aims

Studies of the effects of pollination on floral scent and bee visitation remain rare, particularly in agricultural crops. To fill this gap, the hypothesis that bee visitation to flowers decreases after pollination through reduced floral volatile emissions in highbush blueberries, Vaccinium corymbosum, was tested. Other sources of variation in floral emissions and the role of floral volatiles in bee attraction were also examined.

Methods

Pollinator visitation to blueberry flowers was manipulated by bagging all flowers within a bush (pollinator excluded) or leaving them unbagged (open pollinated), and then the effect on floral volatile emissions and future bee visitation were measured. Floral volatiles were also measured from different blueberry cultivars, times of the day and flower parts, and a study was conducted to test the attraction of bees to floral volatiles.

Key Results

Open-pollinated blueberry flowers had 32 % lower volatile emissions than pollinator-excluded flowers. In particular, cinnamyl alcohol, a major component of the floral blend that is emitted exclusively from petals, was emitted in lower quantities from open-pollinated flowers. Although, no differences in cinnamyl alcohol emissions were detected among three blueberry cultivars or at different times of day, some components of the blueberry floral blend were emitted in higher amounts from certain cultivars and at mid-day. Field observations showed that more bees visited bushes with pollinator-excluded flowers. Also, more honey bees were caught in traps baited with a synthetic blueberry floral blend than in unbaited traps.

Conclusions

Greater volatile emissions may help guide bees to unpollinated flowers, and thus increase plant fitness and bee energetic return when foraging in blueberries. Furthermore, the variation in volatile emissions from blueberry flowers depending on pollination status, plant cultivar and time of day suggests an adaptive role of floral signals in increasing pollination of flowers.     

Concurrent modulation of neuronal and behavioural olfactory responses to sex and host plant cues in a male moth

Mating has profound effects on animal physiology and behaviour, not only in females but also in males, which we show here for olfactory responses. In cotton leafworm moths, Spodoptera littoralis, odour-mediated attraction to sex pheromone and plant volatiles are modulated after mating, producing a behavioural response that matches the physiological condition of the male insect. Unmated males are attracted by upwind flight to sex pheromone released by calling females, as well as to volatiles of lilac flowers and green leaves of the host plant cotton, signalling adult food and mating sites, respectively. Mating temporarily abolishes male attraction to females and host plant odour, but does not diminish attraction to flowers. This behavioural modulation is correlated with a response modulation in the olfactory system, as shown by electro-physiological recordings from antennae and by functional imaging of the antennal lobe, using natural odours and synthetic compounds. An effect of mating on the olfactory responses to pheromone and cotton plant volatiles but not to lilac flowers indicates the presence of functionally independent neural circuits within the olfactory system. Our results indicate that these circuits interconnect and weigh perception of social and habitat odour signals to generate appropriate behavioural responses according to mating state.     

Generalized pollination system: Are floral traits adapted to different pollinators?

Flowers that are pollinated both during the day and at night could exhibit two different groups of pollinators and produce two different sets of attractants and rewards. We explored the patterns of emission of flower scents and production of nectar in the cactus Echinopsis chiloensis ssp. chiloensis, in relation to the patterns of activity of its diurnal and nocturnal pollinators. We measured frequency of flower visitors, analyzed floral scents, measured nectar production and sugar concentration, and performed pollination exclusion experiments. Bees were the main visitors at daytime and hawkmoths at nighttime. Diurnal scents were dominated by several compounds that can attract a wide range of pollinators, whereas nocturnal scents were less diverse and were dominated by (E)-nerolidol, a compound eliciting antennal responses in hawkmoths. Nectar volume and sugar concentration at night were similar to those recorded in hawkmoth-pollinated flowers. Daytime nectar volume was higher than those commonly found in bee-pollinated flowers, but similar to those found in flowers pollinated by several pollinators. Daytime sugar concentration was similar to those recorded in bee-pollinated flowers. Flowers of E. chiloensis ssp. chiloensis seem morphologically adapted to hawkmoth pollination, but diurnal and nocturnal pollinators contribute to similar extents to reproductive success. Additionally, diurnal and nocturnal pollinators showed a synergic effect on the product of fruit set and seed set. The results are discussed in terms of the linkage between floral traits and perception abilities and requirements of pollinators.     

A novel pollination mode,saprocantharophily, in Duguetia cadaverica (Annonaceae): A stinkhorn (Phallales) flower mimic

Duguetia cadaverica (Annonaceae), a small understory tree of humid primary forest from the Guianas to Pará state, Brazil, unites several unusual blossom and floral characters such as flagelliflory and putrid-smelling flowers, respectively. The few pollination studies conducted in the large genus Duguetia have shown that species are usually cantharophilous, pollinated by either small (mostly Nitidulidae) or large specialized dynastid (Scarabaeidae) beetles. Foul-smelling flowers are a novelty within the genus, and to better understand their significance, we undertook a study of the reproductive biology and flower scent chemistry of D. cadaverica. In a primary forest of French Guiana, we observed and measured morphology and phenology of trees and flowers; additionally, flower pollination chamber temperature was measured and insect visitors to flowers observed. Flower scent was collected in situ and later analyzed in the laboratory by GC–MS. Flowers are visited by small beetles of a single Pycnocnemus species (Nitidulidae), which are the only insects observed to enter the pollination chamber. Moreover, flowers evince a rhythm in sexual stage, scent emission and temperature, which finds correspondence in behavioral characters of the putative nitidulid pollinator, such as timing of entry and exit from the pollination chamber. Floral scent analysis revealed an unusual, previously undescribed combination of chemical odor classes. The earthy, rank flower scent contained 18 compounds, among them fatty acid derivatives, terpenoids and N- and S-bearing compounds. The most abundant volatiles were 1-octen-3-ol, 3-octanone, and (E)-2-octenol, which are characteristic earthy odors of fungi; additionally, there were sulfides and 4-methylpentanoic acid, which are molecules associated with carcass and cheese odors, respectively.     

Physiological state influences the antennal response of Anastrepha obliqua to male and host volatiles

The sexual and host‐related behaviours of the fruit fly Anastrepha obliqua Macquart (Diptera: Tephritidae) are mediated by volatile compounds. However, whether the physiological state of this species affects its antennal and behavioural responses to semiochemicals is unknown. The effects of age, mating status, diet and the topical application of methoprene, a Juvenile hormone analogue (JHA), on the antennal sensitivity of this tephritid fruit fly species to selected male [(Z)‐3‐nonenol] and host fruit volatiles (ethyl benzoate, ethyl hexanoate, ethyl butyrate and trans‐β‐ocimene) are investigated using electroantennography (EAG). Overall, (Z)‐3‐nonenol and ethyl benzoate elicit the highest EAG responses in both sexes. Flies of both sexes aged 1, 5 and 10 days old show higher EAG responses to the tested compounds compared with flies aged 20 days old. Virgin females and males show higher EAG responses to volatile compounds than mated flies. Females and males fed with sugar plus protein show higher antennal responses to volatiles compared with flies fed sugar or protein alone. Flies of both sexes treated with methoprene show higher antennal responses than flies treated with acetone (control). These results suggest that the peripheral olfactory system in A. obliqua is modulated by the physiological state of the flies.     

Analyses of volatiles produced by the African fruit fly species complex (Diptera,Tephritidae)

Ceratitis fasciventris, Ceratitis anonae and Ceratitis rosa are polyphagous agricultural pests originating from the African continent. The taxonomy of this group (the so-called Ceratitis FAR complex) is unclear. To clarify the taxonomic relationships, male and female-produced volatiles presumably involved in pre-mating communication were studied using comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC×GC-TOFMS) followed by multivariate analysis, and gas chromatography combined with electroantennographic detection (GC-EAD). GC×GC-TOFMS analyses revealed sex specific differences in produced volatiles. Male volatiles are complex mixtures that differ both qualitatively and quantitatively but share some common compounds. GC-EAD analyses of male volatiles revealed that the antennal sensitivities of females significantly differ in the studied species. No female volatiles elicited antennal responses in males. The results show clear species-specific differences in volatile production and provide complementary information for the distinct delimitation of the putative species by chemotaxonomic markers.     

Managed bumble bees increase flower visitation but not fruit weight in polytunnel strawberry crops

Animal-mediated pollination is essential for the production and quality of fruits and seeds of many crops consumed by humans. However, crop pollination services might be compromised when wild pollinators are scarce. Managed pollinators are commonly used in crops to supplement such services with the assumption that they will enhance crop yield. However, information on the spatiotemporal pollinator-dependence of crops is still limited. We assessed the contribution of commercial bumble bee colonies compared to the available pollinator community on strawberry (‘Fortuna’ variety) flower visitation and strawberry quality across a landscape gradient of agricultural intensification (i.e. polytunnel berry crop cover). We used colonies of bumble bees in winter and in spring, i.e. when few and most wild pollinators are in their flight period, respectively. The placement of colonies increased visits of bumble bees to strawberry flowers, especially in winter. The use of bumble bee colonies did not affect flower visitation by other insects, mainly honey bees, hoverflies and other Diptera. Flower visitation by both honey bees and wild insects did not vary between seasons and was unrelated to the landscape gradient of berry crop cover. Strawberries were of the highest quality (i.e. weight) when insect-mediated pollination was allowed, and their quality was positively related to wild flower visitors in winter but not in spring. However, increased visits to strawberry flowers by managed bumble bees and honey bees had no effect on strawberry weight. Our results suggest that the pollination services producing high quality strawberry fruits are provided by the flower visitor community present in the study region without the need to use managed bumble bees.     

Telipogon peruvianus (Orchidaceae) Flowers Elicit Pre-Mating Behaviour in Eudejeania (Tachinidae) Males for Pollination

Several neotropical orchid genera have been proposed as being sexually deceptive; however, this has been carefully tested in only a few cases. The genus Telipogon has long been assumed to be pollinated by male tachinid flies during pseudocopulatory events but no detailed confirmatory reports are available. Here, we have used an array of methods to elucidate the pollination mechanism in Telipogon peruvianus. The species presents flowers that have a mean floral longevity of 33 days and that are self-compatible, although spontaneous self-pollination does not occur. The flowers attract males of four tachinid species but only the males of an undescribed Eudejeania (Eudejeania aff. browni; Tachinidae) species are specific pollinators. Males visit the flowers during the first few hours of the day and the pollination success is very high (42% in one patch) compared with other sexually deceptive species. Female-seeking males are attracted to the flowers but do not attempt copulation with the flowers, as is usually described in sexually deceptive species. Nevertheless, morphological analysis and behavioural tests have shown an imperfect mimicry between flowers and females suggesting that the attractant stimulus is not based only on visual cues, as long thought. Challenging previous conclusions, our chemical analysis has confirmed that flowers of Telipogon release volatile compounds; however, the role of these volatiles in pollinator behaviour remains to be established. Pollinator behaviour and histological analyses indicate that Telipogon flowers possess scent-producing structures throughout the corolla. Our study provides the first confirmed case of (i) a sexually deceptive species in the Onciidinae, (ii) pollination by pre-copulatory behaviour and (iii) pollination by sexual deception involving tachinid flies.     

Chemical cues involved in the attraction of the oligolectic bee Hoplitis adunca to its host plant Echium vulgare

Host recognition is a key process in oligolectic bees but the mechanisms through which they find and recognize appropriate pollen host plant are not entirely clear. Hoplitis adunca is a monolectic bee collecting pollen only from Echium spp. (Boraginaceae). We aimed to test whether Echium vulgare floral scent plays a major role in the attraction of H. adunca females, and to identify components of E. vulgare scent that may be involved in this specific attraction. We used a combination of behavioral and chemical (GC/GC–MS, PTR-MS) analyses. In order to identify the chemical cues likely to be involved in the specific attraction of H. adunca, we compared the scent of fresh flowers, nectar, pollen, and whole plants of E. vulgare and Anchusa officinalis, another Boraginaceae, which does not attract H. adunca. H. adunca females were attracted to the scent of E. vulgare flowers when offered against a blank or against the scent of A. officinalis flowers. However, H. adunca females were not attracted to the scent of A. officinalis flowers when offered against a blank. The emission spectra of the two plant species differed markedly, as did the emission spectra of various flower components (pollen, nectar and whole flowers) within a species. Pollen presented a low volatile release, but emitted significantly higher amounts of mass 55 (butanal, 1,3-butadiene, or other volatiles of molecular mass 54), and mass 83 (hexanal, hexenols, hexenyl acetate, or other volatiles of molecular mass 82) in E. vulgare than in A. officinalis. Nectar produced a particular emission spectrum with high emission rates of masses 109 and 123. Mass 109 may likely correspond to 1,4-benzoquinone, a volatile specifically measured in E. vulgare in parallel studies to this one. The flower emission spectrum was mainly a combination of the pollen and the nectar scents, although it also contained additional volatile compounds such as those of mass 63 or mass 81. As for terpenes, E. vulgare emitted limonene, longicyclene, junipene, trans-caryophyllene and α-humulene, that were not detected in A. officinalis, and the most emitted monoterpenes were α-pinene, junipene and limonene whereas the most emitted terpenoid by A. officinalis was α-pinene. After identifying these chemical cues, olfactory/behavioural assays with specific volatiles and combinations of volatiles are necessary to understand the chemical interactions of the H. adunca-E. vulgare system.     

Differential electroantennogram response of females and males of two parasitoid species to host-related green leaf volatiles and inducible compounds

Parasitoids employ different types of host-related volatile signals for foraging and host-location. Host-related volatile signals can be plant-based, originate from the herbivore host or produced from an interaction between herbivores and their plant host. In order to investigate potential sex- and species-related differences in the antennal response of parasitoids to different host-related volatiles, we compared the electroantennogram (EAG) responses of both sexes of the specialist parasitoid, Microplitis croceipes (Cresson), and the generalist, Cotesia marginiventris (Cresson), to varying doses of selected plant-based host-related volatiles: two green leaf volatiles (cis-3-hexenol and hexanal) and three inducible compounds (cis-3-hexenyl acetate, linalool, and (E,E)-alpha-farnesene). Mating had no significant effect on EAG response. Females of both species showed significantly greater EAG responses than conspecific males to green leaf volatiles, which are released immediately after initiation of herbivore feeding damage. In contrast, males showed greater responses than conspecific females to inducible compounds released much later after initial damage. Cotesia marginiventris females and males showed greater EAG responses than counterpart M. croceipes to the tested compounds at various doses, suggesting that the generalist parasitoid shows greater antennal sensitivity than the specialist to the tested host-plant volatiles. These results are discussed in relation to the possible roles of green leaf volatiles and inducible compounds in the ecology of female and male parasitoids.     

Floral resource distribution and fitness consequences for two solitary bee species in agricultural landscapes

Floral resources are crucial for wild pollinators. Identifying the spatio-temporal floral resource use of wild pollinators and effects of resource distribution on their development might help to promote them and their pollination services to crops in agricultural landscapes.We established populations of Osmia cornuta and Osmia bicornis, two solitary wild bees, in 24 agricultural landscapes with varying floral resource availability. Based on their pollen use, we mapped the landscape-scale distribution of the visited plants, estimated pooled specific floral resource availabilities and measured its effects on reproductive output.Woody semi-natural habitats such as hedgerows provided the majority of pollen sources for both Osmia species. Pollen use differed strongly between the two species. The offspring of both Osmia increased with availability of pooled specific pollen resources. In accordance with their preferred pollen types, offspring of O. cornuta increased with increasing cover of trees and shrubs of the Rosaceae family, and that of O. bicornis with increasing cover of Papaver rhoeas, Ranunculus acris and Quercus spp. as well as with the proximity to oilseed rape. In spite of their specific responses to pollen resources, the offspring of both species decreased with the distance to forest. The floral resource availability did not significantly affect the proportion of adult females and the weight of the offspring. As forest does not appear to be a main foraging habitat for both species, the benefit of forest proximity indicates an additional role of forest in addition to food availability.Specific flowering plants and forests should thus be conserved and enhanced to maintain and support O. cornuta, O. bicornis and likely other wild bee populations in agricultural landscapes. The combined information of land cover and detailed floral resource availability gives a deeper understanding into population processes in agricultural landscapes.     

Repellent activity of Cymbopogon citratus and Tagetes minuta and their specific volatiles against Megalurothrips sjostedti

Cowpea is an important source of protein for people in Africa. However, the crop suffers major damage and yield losses due to bean flower thrips, Megalurothrips sjostedti Trybom (Thysanoptera: Thripidae). Although companion plants are known to reduce the damage caused by insect pests, the role of their volatiles in repelling pests from target plants has been the subject of few investigations. Here, we used the Y‐tube olfactometer experiments and chemical analyses to investigate the effect of volatiles from cowpea flowers and two companion plants; lemongrass, Cymbopogon citratus and Mexican marigold, Tagetes minuta on the olfactory responses of M. sjostedti. The results revealed that M. sjostedti males and females were repelled by the volatiles from freshly cut leaves of C. citratus. The combination of freshly cut leaves of C. citratus and cowpea flower was repellent to females but not to males. The female thrips, but not males, were repelled by the volatiles from the vegetative stage of T. minuta. Fifty‐four compounds were identified in the volatiles from two herbal plants. Among the major compounds, citral and a 4‐component blend comprised of dihydrotagetone, (Z)‐3‐hexenyl acetate, limonene and (Z)‐β‐ocimene repelled females but dihydrotagetone alone attracted females. While myrcene combined with cowpea flower volatiles enhanced the attraction of females M. sjostedti, when tested alone was not attractive. These results highlight the potential of volatiles from C. citratus and T. minuta to repel M. sjostedti females. The use of these plants as companion plants in a cowpea cropping system could reduce M. sjostedti infestation.     

Use of visual and olfactory cues of flowers of two brassicaceous species by insect pollinators

1. Pollinating insects exploit visual and olfactory cues associated with flower traits indicative of flower location and reward quality. Pollination can induce changes in these flower-associated cues, thereby influencing the behaviour of flower visitors. 2. This study investigated the main cues exploited by the syrphid fly Episyrphus balteatus and the butterfly Pieris brassicae when visiting flowers of Brassica nigra and Raphanus sativus plants. Whether pollen is used as a cue and whether pollination-induced changes affect flower volatile emission and the behavioural responses of the two pollinator species were also studied. 3. Pollinator preference was investigated by offering visual and olfactory cues individually as well as simultaneously in two-choice bioassays. Plant treatments included emasculation, hand-pollination and untreated control plants. The composition of flower volatiles from pollinated and unpollinated control plants was analysed by gas chromatography-mass spectrometry. 4. Both pollinators exhibited a strong bias for visual cues over olfactory cues. Neither pollinator used pollen as a cue. However, E. balteatus discriminated between newly opened and long-open flowers at short distance only when pollen was available. Flower visits by pollinators were influenced by pollination-induced changes in B. nigra but not R. sativus flowers. Pieris brassicae only responded to pollination-induced changes when visual and olfactory cues were offered simultaneously. The blend of volatiles emitted by B. nigra, but not R. sativus inflorescences was affected by pollination. 5. Collectively, the findings of this study show that different pollinators exploit different visual and olfactory traits when searching for flowers of two brassicaceous plant species.