Below-ground herbivory limits induction of extrafloral nectar by above-ground herbivores

Annals of Botany 115: 841–846, 2015 doi:10.1093/aob/mcv011, available online at www.aob.oxfordjournals.org Below-ground herbivory limits induction of extrafloral nectar by above-ground herbivores Wei Huang1, Evan Siemann2, Juli Carrillo3 and Jianqing Ding1,* 1 Received: 17 October 2014 Returned for revision: 21 November 2014 Accepted: 23 December 2014 Published electronically: 13 February 2015  Background and Aims Many plants produce extrafloral nectar (EFN), and increase production following aboveground herbivory, presumably to attract natural enemies of the herbivores. Below-ground herbivores, alone or in combination with those above ground, may also alter EFN production depending on the specificity of this defence response and the interactions among herbivores mediated through plant defences. To date, however, a lack of manipulative experiments investigating EFN production induced by above- and below-ground herbivory has limited our understanding of how below-ground herbivory mediates indirect plant defences to affect above-ground herbivores and their natural enemies.  Methods In a greenhouse experiment, seedlings of tallow tree (Triadica sebifera) were subjected to herbivory by a specialist flea beetle (Bikasha collaris) that naturally co-occurs as foliage-feeding adults and root-feeding larvae. Seedlings were subjected to above-ground adults and/or below-ground larvae herbivory, and EFN production was monitored.  Key Results Above- and/or below-ground herbivory significantly increased the percentage of leaves with active nectaries, the volume of EFN and the mass of soluble solids within the nectar. Simultaneous above- and belowground herbivory induced a higher volume of EFN and mass of soluble solids than below-ground herbivory alone, but highest EFN production was induced by above-ground herbivory when below-ground herbivores were absent.  Conclusions The induction of EFN production by below-ground damage suggests that systemic induction underlies some of the EFN response. The strong induction by above-ground herbivory in the absence of below-ground herbivory points to specific induction based on above- and below-ground signals that may be adaptive for this above-ground indirect defence. Key words: Extrafloral nectar, indirect defence, induced defence, above- and below-ground interactions, herbivory, specificity, tallow tree, Triadica sebifera, flea beetle, Bikasha collaris. INTRODUCTION Plants are frequently attacked by various shoot and root herbivores and have evolved a diverse array of defensive strategies (Agrawal, 2007; Mithöfer and Boland, 2012). Induced defence is thought to be a particularly effective and efficient strategy for plants through creating a specific and targeted defence response (Frost et al., 2008; Karban, 2011). In many cases, induced defence is systemic, crossing above- and below-ground boundaries, such that shoot herbivores can affect root defence level and root herbivore performance, and vice versa (Johnson et al., 2009; Rasmann et al., 2009; Kutyniok and Muller, 2012; Erwin et al., 2014; Huang et al., 2014). Thus, cross-talk of induced defence may determine herbivore population dynamics and ultimately affect community stability in both above- and below-ground compartments (Bezemer and van Dam, 2005; Erb et al., 2008; Soler et al., 2013). Induced defences function either directly by reducing the impact of herbivores, or indirectly by increasing attraction of herbivore natural enemies (Kessler and Heil, 2011; Mithöfer and Boland, 2012; Hanley et al., 2013). Interactions between induced above- and below-ground direct defences are fairly well studied (Kaplan et al., 2008a; Rasmann et al., 2009; Huang et al., 2013; Erwin et al., 2014). In general, intra-guild feeding increases induced direct defence responses of plants through shared signalling pathways, while inter-guild feeding, which often activates different signalling pathways, weakens induced direct defences of plants, probably through negative crosstalk (Soler et al., 2013). However, indirect defences are less well studied, and the effects of herbivore interactions on induced indirect defences are not well understood, particularly between shoot and root herbivores feeding simultaneously. Indirect defence can be classified as information-providing traits (e.g. volatile organic compounds) as well as resource-providing traits (e.g. extrafloral nectar) (Arimura et al., 2005). To date, limited studies have focused mainly on the former (Erb et al., 2008; van Dam and Heil, 2011). For example, maize plants attacked by root herbivores produced more (E)-b-caryophyllene than plants attacked by both shoot and root herbivores (Rasmann and Turlings, 2007). In cotton, plants increased (Z)-3-hexenyl acetate upon attacks by shoot herbivores, and the presence of root herbivory strengthened this increase (Olson et al., 2008). C The Author 2015. Published by Oxford University Press on behalf of the Annals of Botany Company. V All rights reserved. For Permissions, please email: Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan, Hubei 430074, China, 2Department of Ecology and Evolutionary Biology, Rice University, Houston, TX 77005, USA and 3 Department of Entomology, Purdue University, West Lafayette, IN 47907, USA * For correspondence. E-mail Huang et al. — Below-ground herbivory affects extrafloral nectar production 842 MATERIALS AND METHODS Study organisms Triadica sebifera is a rapidly growing, subtropical tree of southern China (Zhang and Lin, 1994). Triadica produces EFN both on glands at the bases of petioles and on the undersides of leaf margins in response to leaf damage (Carrillo et al., 2012a). Recently, we have shown that specialist herbivory induced greater EFN production than generalist herbivory (Wang et al., 2013), and that leaf-chewing herbivores induced EFN while phloem-feeders did not, indicating specificity in this defence response (Carrillo et al., 2012b). Bikasha collaris is one of the most abundant chewing insects on Triadica in China. Adults feed on leaves producing irregular scars while larvae feed on roots forming tunnels (Huang et al., 2011). Preliminary host range tests indicated that both adults and larvae are monophagous specialists that feed exclusively on Triadica (Huang et al., 2011). Bikasha pass through more than five generations per year in Wuhan, China, and adult and larval life stages of different generations can feed on the same plant simultaneously. Seeds and seedlings We collected seeds of Triadica from a natural population near Wuhan (31 330 N, 114 070 E). We removed the seeds’ waxy coats by soaking them in water with laundry detergent (10 g L1) for 2 d and then stored them in sand at a depth of 5–10 cm in a refrigerator (4 C) for 35 d. We sowed the seeds in growing medium (50 % topsoil and 50 % sphagnum peat moss) in an unheated greenhouse at Wuhan Botanical Garden, Ch (...truncated)