Herbivory by a Phloem-Feeding Insect Inhibits Floral Volatile Production

Citation: Pareja M, Qvarfordt E, Webster B, Mayon P, Pickett J, et al. ( Herbivory by a Phloem-Feeding Insect Inhibits Floral Volatile Production Martin Pareja 0 Erika Qvarfordt 0 Ben Webster 0 Patrick Mayon 0 John Pickett 0 Michael Birkett 0 Robert Glinwood 0 Juergen Kroymann, French National Centre for Scientific Research, Universite Paris-Sud, France 0 1 Departmento de Entomologia, Universidade Federal de Lavras, Lavras, Brazil, 2 Department of Ecology, Swedish University of Agricultural Sciences, Uppsala, Sweden, 3 Rothamsted Research, Department of Biological Chemistry , Harpenden , United Kingdom There is extensive knowledge on the effects of insect herbivory on volatile emission from vegetative tissue, but little is known about its impact on floral volatiles. We show that herbivory by phloem-feeding aphids inhibits floral volatile emission in white mustard Sinapis alba measured by gas chromatographic analysis of headspace volatiles. The effect of the Brassica specialist aphid Lipaphis erysimi was stronger than the generalist aphid Myzus persicae and feeding by chewing larvae of the moth Plutella xylostella caused no reduction in floral volatile emission. Field observations showed no effect of L. erysimimediated floral volatile emission on the total number of flower visits by pollinators. Olfactory bioassays suggested that although two aphid natural enemies could detect aphid inhibition of floral volatiles, their olfactory orientation to infested plants was not disrupted. This is the first demonstration that phloem-feeding herbivory can affect floral volatile emission, and that the outcome of interaction between herbivory and floral chemistry may differ depending on the herbivore's feeding mode and degree of specialisation. The findings provide new insights into interactions between insect herbivores and plant chemistry. - Funding: This work was funded by a grant from Carl Tryggers Stiftelse to Martin Pareja and by Mistra through the PlantComMistra program. Funding for equipment from Stiftelsen Tornspiran is acknowledged. Rothamsted Research receives grant-aided support from the Biotechnology and Biological Sciences Research Council (BBSRC) of the United Kingdom. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Volatile organic compounds emitted from plants mediate an array of ecological interactions. They play important roles in plant defence as herbivore deterrents [1,2,3] and in the attraction of predators and parasitoids to herbivore damaged plants [for reviews see 4,5]. Furthermore, volatiles emitted from flowers play a vital role in mediating the mutualism between plants and their pollinators [6]. For many years pollination ecology and plant defence were studied separately [7], as adaptations to distinct and independent selective pressures. However, there have been advances in the integration of pollination and plant defence [8], in particular highlighting how flower traits are often evolutionarily constrained by selective pressures acting on plant defence [9]. It has even been argued that, in evolutionary terms, floral volatiles were originally involved in other aspects of plant behaviour such as defence [10]. Floral volatiles have the potential to mediate not only interactions of plants with pollinators, but a series of direct and indirect interactions with other organisms such as herbivores, predators and microorganisms [11]. There is growing awareness of the ecological importance of the connection between the chemistry of pollination and that of plant defence [12,13,14] but, despite the extensive evidence of the effects of different types of herbivore on the production of volatiles by vegetative tissues, only a handful of studies have explored the relationship between the emission of floral volatiles and damage to vegetative tissues. Effmert et al. [15] found that, although caterpillar-damaged vegetative tissues emitted herbivore-induced volatiles, there was no effect of chewing damage on floral volatiles. Theis et al. [16] found increased volatile terpenoid emission from male flowers after mechanical simulation of chewing. Kessler et al. [17] found that damage by Manduca spp. caterpillars caused reduced emission of the floral volatile benzyl acetone along with major changes in flower phenology. The damage in the cited studies was caused by leaf chewing, which is known to elicit strong jasmonate (JA) signalling and reduced levels of salicylate (SA)-mediated responses [18,19,20]. Phloem feeding herbivores, such as aphids and whiteflies, often elicit SA-mediated signalling [21,22,23]. Biosynthesis of SA is related to the shikimate pathway that leads to phenylalanine and phenolic compounds via cinnamic acid [24]. Benzenoid compounds, derived via cinnamic acid metabolism [25], are prominent in floral scents [26], so damage by phloem feeders is predicted to have different effects on floral volatile emissions to those caused by chewing insects. Even within the phloem-feeding guild, plant responses may vary depending on the degree of specialisation of the herbivore [27], and evidence is emerging that phloem-feeding aphids may be able to actively manipulate plant defences [28]. In a study of aphid-induced plant volatile emission, we tested the effect of feeding on vegetative tissue by the mustard aphid Lipaphis erysimi on the emission of floral volatiles in white mustard, Sinapis alba (Brassicaceae). Surprisingly, instead of inducing production of floral volatiles, the aphid caused a substantial reduction in emissions. We designed the current study to explore this effect and to address the following hypotheses: (i) insect herbivore feeding on vegetative tissue affects the identity and/or quantity of volatile emissions from flowers, (ii) the impact of herbivory depends on the herbivores feeding mode (aphids compared with larvae of the diamondback moth Plutella xylostella) and on the degree of specialisation (L. erysimi compared with the highly polyphagous peach-potato aphid Myzus persicae) (iii) herbivore-induced changes in floral volatile emission disrupt pollinator attraction and reduce chemical apparency to the herbivores natural enemies. The results demonstrate for the first time that a phloem-feeding herbivore can affect the volatile chemistry of flowers. A specialist aphid inhibits floral volatiles more strongly than a generalist aphid but a chewing herbivore has no effect Emission of floral volatiles from S. alba was investigated in response to feeding by a specialist aphid L. erysimi, a generalist aphid M. persicae and a chewing moth larva P. xylostella. Volatiles emitted by flowers were collected by air entrainment and analysed by gas chromatography. To analyse changes in the floral blends after herbivore attack, a discriminant analysis using compositional data was carried out. Aphid (...truncated)