Olfactory cues from plants infected by powdery mildew guide foraging by a mycophagous ladybird beetle

Small volatile lipophilic molecules induced belowground by aphid attack elicit a defensive response in neighbouring un-infested plants

In pioneering studies on plant-aphid interactions, we have observed that Vicia faba plants infested by aphids can transmit signals via the rhizosphere that induce aboveground defence in intact, neighbouring plants. The aphid parasitoid Aphidius ervi is significantly attracted towards intact broad bean plants grown in a hydroponic solution previously harbouring Acyrtosiphon pisum-infested plants. To identify the rhizosphere signal(s) possibly mediating this belowground plant-plant communication, root exudates were collected using Solid-Phase Extraction (SPE) from 10-day old A. pisum-infested and un-infested Vicia faba plants hydroponically grown. To verify the ability of these root exudates to trigger defence mechanisms against the aphids we added them to V. fabae plants grown in hydroponic solution, and tested these plants in the wind-tunnel bioassay to assess their attractiveness towards the aphids' parasitoids A. ervi. We identified three small volatile lipophilic molecules as plant defence elicitors: 1-octen-3-ol, sulcatone and sulcatol, in SPE extracts of A. pisum-infested broad bean plants. In wind tunnel assays, we recorded a significant increase in the attractiveness towards A. ervi of V. faba plants grown in hydroponic solution treated with these compounds, compared to plants grown in hydroponic treated with ethanol (control). Both 1-octen-3-ol and sulcatol have asymmetrically substituted carbon atoms at positions 3 and 2, respectively. Hence, we tested both their enantiomers alone or in mixture. We highlighted a synergistic effect on the level of attractiveness towards the parasitoid when testing the three compounds together in respect to the response recorded against them singly tested. These behavioural responses were supported by the characterization of headspace volatiles released by tested plants. These results shed new light on the mechanisms underlying plant-plant communication belowground and prompt the use of bio-derived semiochemicals for a sustainable protection of agricultural crops.

Fungivory: a new and complex ecological function of Doru luteipes (Scudder) (Dermaptera: Forficulidae)

We report the discovery that the earwig predator Doru luteipes (Scudder, 1876) (Dermaptera: Forficulidae) feed on Puccinia polysora Underw uredospore, the causal agent of Southern Rust of Corn (SRC), which is a primary disease affecting the maize crop in Brazil. We performed experiments in laboratory and greenhouse to test the effect of D. luteipes (1st/2nd and 3rd/4th instars, and adults) fungivory on the P. polysora uredospore concentration. All trials showed a significant reduction of the initial concentration of uredospore. There was a reduction in uredospore concentration with increase in number of D. luteipes feeding on them. We also tested the uredospore consumption by quantifying its percentage in the feces of D. luteipes. Nymphs of the 2nd, 4th instar and adults fed 88%, 85%, and 83.8% of the uredospore, respectively. For nymphs of the 3rd instar, the percentage of uredospore consumption (75.6%) was statistically significant compared with the other groups. In greenhouse experiment, at twenty-eight days after plant inoculation with 9.9 x 104 uredospores, the percentage of uredospore consumption was 81.7%. Our results confirmed the fungivory of D. luteipes on P. polysora uredospore. This is the first report of D. luteipes fungivory, which may play an important role in the biological control of P. polysora in corn.

Extreme diversification of floral volatiles within and among species of Lithophragma (Saxifragaceae)

A major challenge in evolutionary biology is to understand how complex traits of multiple functions have diversified and codiversified across interacting lineages and geographic ranges. We evaluate intra- and interspecific variation in floral scent, which is a complex trait of documented importance for mutualistic and antagonistic interactions between plants, pollinators, and herbivores. We performed a large-scale, phylogenetically structured study of an entire plant genus (Lithophragma, Saxifragaceae), of which several species are coevolving with specialized pollinating floral parasites of the moth genus Greya (Prodoxidae). We sampled 94 Lithophragma populations distributed across all 12 recognized Lithophragma species and subspecies, and four populations of related saxifragaceous species. Our results reveal an unusually high diversity of floral volatiles among populations, species, and clades within the genus. Moreover, we found unexpectedly major changes at each of these levels in the biosynthetic pathways used by local populations in their floral scents. Finally, we detected significant, but variable, genus- and species-level patterns of ecological convergence in the floral scent signal, including an impact of the presence and absence of two pollinating Greya moth species. We propose that one potential key to understanding floral scent variation in this hypervariable genus is its geographically diverse interactions with the obligate specialized Greya moths and, in some species and sites, more generalized copollinators.

Plant volatile emission depends on the species composition of the neighboring plant community

BACKGROUND

Plants grow in multi-species communities rather than monocultures. Yet most studies on the emission of volatile organic compounds (VOCs) from plants in response to insect herbivore feeding focus on one plant species. Whether the presence and identity of neighboring plants or plant community attributes, such as plant species richness and plant species composition, affect the herbivore-induced VOC emission of a focal plant is poorly understood.

METHODS

We established experimental plant communities in pots in the greenhouse where the focal plant species, red clover (Trifolium pratense), was grown in monoculture, in a two species mixture together with Geranium pratense or Dactylis glomerata, or in a mixture of all three species. We measured VOC emission of the focal plant and the entire plant community, with and without herbivory of Spodoptera littoralis caterpillars caged on one red clover individual within the communities.

RESULTS

Herbivory increased VOC emission from red clover, and increasing plant species richness changed emissions of red clover and also from the entire plant community. Neighbor identity strongly affected red clover emission, with highest emission rates for plants growing together with D. glomerata.

CONCLUSION

The results from this study indicate that the blend of VOCs perceived by host searching insects can be affected by plant-plant interactions.

Leaf rust infection reduces herbivore-induced volatile emission in black poplar and attracts a generalist herbivore

Plants release complex volatile blends after separate attack by herbivores and pathogens, which play many roles in interactions with other organisms. Large perennials are often attacked by multiple enemies, but the effect of combined attacks on volatile emission is rarely studied, particularly in trees. We infested Populus nigra trees with a pathogen, the rust fungus Melampsora larici-populina, and Lymantria dispar caterpillars alone and in combination. We investigated poplar volatile emission and its regulation, as well as the behavior of the caterpillars towards volatiles from rust-infected and uninfected trees. Both the rust fungus and the caterpillars alone induced volatile emission from poplar trees. However, the herbivore-induced volatile emission was significantly reduced when trees were under combined attack by the herbivore and the fungus. Herbivory induced terpene synthase transcripts as well as jasmonate concentrations, but these increases were suppressed when the tree was additionally infected with rust. Caterpillars preferred volatiles from rust-infected over uninfected trees. Our results suggest a defense hormone crosstalk upon combined herbivore-pathogen attack in poplar trees which results in lowered emission of herbivore-induced volatiles. This influences the preference of herbivores, and might have other far-reaching consequences for the insect and pathogen communities in natural poplar forests.

Priming of cowpea volatile emissions with defense inducers enhances the plant's attractiveness to parasitoids when attacked by caterpillars

BACKGROUND

The manipulation of herbivore-induced volatile organic compounds (HI-VOCs) via the application of the inducers benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH) and laminarin (β-1,3-glucan) is known to enhance the attractiveness of caterpillar-damaged cotton and maize plants to parasitoids. To test if this is also the case for legumes, we treated cowpea (Vigna unguiculata var. unguiculata) with these inducers and studied the effects on HI-VOC emissions and the attraction of three generalist endoparasitoids.

RESULTS

After the inducers had been applied and the plants subjected to either real or mimicked herbivory by Spodoptera littoralis caterpillars, females of the parasitoids Campoletis sonorensis and Microplitis rufiventris showed a strong preference for BTH-treated plants, whereas Cotesia females were strongly attracted to both BTH- and laminarin-treated plants with real or mimicked herbivory. Treated plants emitted more of certain HI-VOCs, but considerably less indole and linalool and less of several sesquiterpenes. Multivariate data analysis revealed that enhanced wasp attraction after treatment was correlated with high relative concentrations of nonanal, α-pinene, (E)-β-ocimene and (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), and with low relative concentrations of indole, (S)-linalool and (E)-β-farnesene. Inducer treatments had no significant effect on leaf consumption by the caterpillars.

CONCLUSION

Our findings confirm that treating cowpea plants with inducers can enhance their attractiveness to biological control agents. © 2017 Society of Chemical Industry.

Stronger diversity effects with increased environmental stress: A study of multitrophic interactions between oak, powdery mildew and ladybirds

Recent research has suggested that increasing neighbourhood tree species diversity may mitigate the impact of pests or pathogens by supporting the activities of their natural enemies and/or reducing the density of available hosts. In this study, we attempted to assess these mechanisms in a multitrophic study system of young oak (Quercus), oak powdery mildew (PM, caused by Erysiphe spp.) and a mycophagous ladybird (Psyllobora vigintiduopunctata). We assessed ladybird mycophagy on oak PM in function of different neighbourhood tree species compositions. We also evaluated whether these species interactions were modulated by environmental conditions as suggested by the Stress Gradient Hypothesis. We adopted a complementary approach of a field experiment where we monitored oak saplings subjected to a reduced rainfall gradient in a young planted forest consisting of different tree species mixtures, as well as a lab experiment where we independently evaluated the effect of different watering treatments on PM infections and ladybird mycophagy. In the field experiment, we found effects of neighbourhood tree species richness on ladybird mycophagy becoming more positive as the target trees received less water. This effect was only found as weather conditions grew drier. In the lab experiment, we found a preference of ladybirds to graze on infected leaves from trees that received less water. We discuss potential mechanisms that might explain this preference, such as emissions of volatile leaf chemicals. Our results are in line with the expectations of the Natural Enemies Hypothesis and support the hypothesis that biodiversity effects become stronger with increased environmental stress.

Floral scent of Joshua trees (Yucca brevifolia sensu lato): Divergence in scent profiles between species but breakdown of signal integrity in a narrow hybrid zone

PREMISE OF THE STUDY

The role of floral scent in facilitating reproductive isolation between closely related plants remains poorly understood. Yucca brevifolia and Yucca jaegeriana are pollinated by different moth species in allopatry, but in a narrow contact zone, pollinator-host specificity breaks down, resulting in hybridization between species. We explored the chemical basis for reproductive isolation and hybridization in these Joshua trees by characterizing the floral scent of each species in allopatry, analyzing scent profiles from trees in the contact zone, and matching these data with genotypic and phenotypic data.

METHODS

We analyzed floral volatiles using gas chromatography-mass spectrometry, tested for species divergence of scent profiles and classified trees in the contact zone as hybrid or either parental species. We used floral and vegetative morphological data and genotypic data to classify trees and analyzed whether certain trait combinations were more correlated than others with respect to assignment of trees and whether frequencies of classified tree types differed depending on which data set was used.

KEY RESULTS

The Joshua tree floral scent included oxygenated 8-carbon compounds not reported for other yuccas. The two species differed (P < 0.001) in scent profiles. In the contact zone, many hybrids were found, and phenotypic traits were generally weakly correlated, which may be explained by extensive gene flow between species or by exposure to different selection pressures.

CONCLUSIONS

Although the two Joshua tree species produce distinct floral scent profiles, it is insufficient to prevent attraction of associated pollinators to both hosts. Instead, floral morphology may be the key trait mediating gene flow between species.

Volatile organic compounds emitted by Trichoderma species mediate plant growth

Background

Many Trichoderma species are applied as biofungicides and biofertilizers to agricultural soils to enhance crop growth. These filamentous fungi have the ability to reduce plant diseases and promote plant growth and productivity through overlapping modes of action including induced systemic resistance, antibiosis, enhanced nutrient efficiency, and myco-parasitism. Trichoderma species are prolific producers of many small metabolites with antifungal, antibacterial, and anticancer properties. Volatile metabolites of Trichoderma also have the ability to induce resistance to plant pathogens leading to improved plant health. In this study, Arabidopsis plants were exposed to mixtures of volatile organic compounds (VOCs) emitted by growing cultures of Trichoderma from 20 strains, representing 11 different Trichoderma species.

Results

We identified nine Trichoderma strains that produced plant growth promoting VOCs. Exposure to mixtures of VOCs emitted by these strains increased plant biomass (37.1–41.6 %) and chlorophyll content (82.5–89.3 %). Trichoderma volatile-mediated changes in plant growth were strain- and species-specific. VOCs emitted by T. pseudokoningii (CBS 130756) were associated with the greatest Arabidopsis growth promotion. One strain, T. atroviride (CBS 01-209), in our screen decreased growth (50.5 %) and chlorophyll production (13.1 %). Similarly, tomatoes exposed to VOCs from T. viride (BBA 70239) showed a significant increase in plant biomass (>99 %), larger plant size, and significant development of lateral roots. We also observed that the tomato plant growths were dependent on the duration of the volatile exposure. A GC–MS analysis of VOCs from Trichoderma strains identified more than 141 unique compounds including several unknown sesquiterpenes, diterpenes, and tetraterpenes.

Conclusions

Plants grown in the presence of fungal VOCs emitted by different species and strains of Trichoderma exhibited a range of effects. This study demonstrates that the blend of volatiles produced by actively growing fungi and volatile exposure time in plant development both influence the outcome of volatile-mediated interactions. Only some of our growth promoting strains produced microbial VOCs known to enhance plant growth. Compounds such as 6-pentyl-2H-pyran-2-one were not common to all promoting strains. We found that biostimulatory strains tended to have a larger number of complex terpenes which may explain the variation in growth induced by different Trichoderma strains.

Attraction of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) to four varieties of Lathyrus sativus L. seed volatiles

Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) is an important stored grain pest of Lathyrus sativus L. (Leguminosae), commonly known as khesari, in India, Bangladesh and Ethiopia. Volatiles were collected from four varieties, i.e., Bio L 212 Ratan, Nirmal B-1, WBK-14-7 and WBK-13-1 of uninfested khesari seeds, and subsequently identified and quantified by gas chromatography mass spectrometry and gas chromatography flame ionization detector analyses, respectively. A total of 23 volatiles were identified in the four varieties of khesari seeds. In Bio L 212 Ratan and WBK-13-1 seeds, nonanal was the most abundant followed by farnesyl acetone; whereas farnesyl acetone was predominant followed by nonanal in Nirmal B-1 and WBK-14-7 khesari seeds. The olfactory responses of female C. maculatus toward volatile blends from four varieties of khesari seeds, and individual synthetic compounds and their combinations were examined through Y-shaped glass tube olfactometer bioassays. Callosobruchus maculatus showed significant preference for the whole volatile blends from Bio L 212 Ratan seeds compared to whole volatile blends from other three varieties. The insect exhibited attraction to five individual synthetic compounds, 3-octanone, 3-octanol, linalool oxide, 1-octanol and nonanal. A synthetic blend of 448, 390, 1182, 659 and 8114 ng/20 μl methylene chloride of 3-octanone, 3-octanol, linalool oxide, 1-octanol and nonanal, respectively, was most attractive to C. maculatus, and this combination might be used for insect pest management program such as baited traps.

Epichloë endophytes alter inducible indirect defences in host grasses

Epichloë endophytes are common symbionts living asymptomatically in pooid grasses and may provide chemical defences against herbivorous insects. While the mechanisms underlying these fungal defences have been well studied, it remains unknown whether endophyte presence affects the host's own defences. We addressed this issue by examining variation in the impact of Epichloë on constitutive and herbivore-induced emissions of volatile organic compounds (VOC), a well-known indirect plant defence, between two grass species, Schedonorus phoenix (ex. Festuca arundinacea; tall fescue) and Festuca pratensis (meadow fescue). We found that feeding by a generalist aphid species, Rhopalosiphum padi, induced VOC emissions by uninfected plants of both grass species but to varying extents, while mechanical wounding failed to do so in both species after one day of damage. Interestingly, regardless of damage treatment, Epichloë uncinata-infected F. pratensis emitted significantly lower quantities of VOCs than their uninfected counterparts. In contrast, Epichloë coenophiala-infected S. phoenix did not differ from their uninfected counterparts in constitutive VOC emissions but tended to increase VOC emissions under intense aphid feeding. A multivariate analysis showed that endophyte status imposed stronger differences in VOC profiles of F. pratensis than damage treatment, while the reverse was true for S. phoenix. Additionally, both endophytes inhibited R. padi population growth as measured by aphid dry biomass, with the inhibition appearing greater in E. uncinata-infected F. pratensis. Our results suggest, not only that Epichloë endophytes may play important roles in mediating host VOC responses to herbivory, but also that the magnitude and direction of such responses may vary with the identity of the Epichloë–grass symbiosis. Whether Epichloë-mediated host VOC responses will eventually translate into effects on higher trophic levels merits future investigation.

Development of a female attractant for the click beetle pest Agriotes brevis

BACKGROUND

Traps suitable for catching female Agriotes click beetles may provide better reconnaissance than pheromone-baited traps which catch only males, thereby contributing to more efficient crop protection. The basis for this study came from (i) observations of female Agriotes brevis (Candeze) aggregating beneath foliage of Medicago sativa (L.) and Lolium italicum (A. Br.) placed on plastic sheets on bare soil, and (ii) field tests demonstrating attraction of females to traps baited with foliage from these plants. The aim was to identify and field test volatile compounds from M. sativa and L. italicum leaves.

RESULTS

A number of electrophysiologically active chemicals were identified from headspace extracts of M. sativa and L. italicum. Three different synthetic blends of the identified compounds, comprising four, seven and nine components, were field tested. The four- and nine-component blends caught more female A. brevis than unbaited traps, with the proportion of females not differing between blends.

CONCLUSION

The plant-derived blends were shown to catch female A. brevis under field conditions when applied in traps. Of these, the four-component blend, given its relatively simple composition [(Z)-3-hexenyl acetate:methyl benzoate:(Z)-3-hexen-1-ol:methyl salicylate 300:5:30:30 mg bait(-1)], may be a suitable 'standard' blend for bait optimisation.

Functional diversity among sensory receptors in a Drosophila olfactory circuit

The ability of an animal to detect, discriminate, and respond to odors depends on the function of its olfactory receptor neurons (ORNs), which in turn depends ultimately on odorant receptors. To understand the diverse mechanisms used by an animal in olfactory coding and computation, it is essential to understand the functional diversity of its odor receptors. The larval olfactory system of Drosophila melanogaster contains 21 ORNs and a comparable number of odorant receptors whose properties have been examined in only a limited way. We systematically screened them with a panel of ∼500 odorants, yielding >10,000 receptor-odorant combinations. We identify for each of 19 receptors an odorant that excites it strongly. The responses elicited by each of these odorants are analyzed in detail. The odorants elicited little cross-activation of other receptors at the test concentration; thus, low concentrations of many of these odorants in nature may be signaled by a single ORN. The receptors differed dramatically in sensitivity to their cognate odorants. The responses showed diverse temporal dynamics, with some odorants eliciting supersustained responses. An intriguing question in the field concerns the roles of different ORNs and receptors in driving behavior. We found that the cognate odorants elicited behavioral responses that varied across a broad range. Some odorants elicited strong physiological responses but weak behavioral responses or weak physiological responses but strong behavioral responses.

Quantitative comparative analysis of the bio-active and toxic constituents of leaves and spikes of Schizonepeta tenuifolia at different harvesting times

A GC-MS-Selected Ion Monitoring (SIM) detection method was developed for simultaneous determination of four monoterpenes: (−)-menthone, (+)-pulegone, (−)-limonene and (+)-menthofuran as the main bio-active and toxic constituents, and four other main compounds in the volatile oils of Schizonepeta tenuifolia (ST) leaves and spikes at different harvesting times. The results showed that the method was simple, sensitive and reproducible, and that harvesting time was a possible key factor in influencing the quality of ST leaves, but not its spikes. The research might be helpful for determining the harvesting time of ST samples and establishing a validated method for the quality control of ST volatile oil and other relative products.