Ecological Entomology (2020), DOI: 10.1111/een.12922 Novel evidence for systemic induction of silicon defences in cucumber following attack by a global insect herbivore TARIKUL I S L A M, 1,2 BEN D. MOORE 1 and SCOTT N. JOHNSON 1 1 Hawkesbury Institute for the Environment, Western Sydney University, Penrith, New South Wales, Australia and 2 Department of Entomology, Bangladesh Agricultural University, Mymensingh, Bangladesh Abstract. 1. Silicon (Si) is a benefcial nutrient that has been reported to ameliorate many abiotic and biotic stresses in plants, including insect herbivory. Insect herbivory has been shown to induce Si defences in plants, although the magnitude and nature of induction remain largely ambiguous. In particular, it is unclear whether herbivore induction of Si defences is confned to attacked tissues (local) or occurs elsewhere in the plant (systemic). 2. We grew cucumber, Cucumis sativus L. plants (var. Burpless F1 and Beit Alpha), an intermediate Si accumulator, hydroponically under Si-supplemented or Si-free conditions and measured the level of Si induction caused by a polyphagous chewing insect, the cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). We also examined the impacts of Si on insect performance by conducting in vitro feeding assays on excised leaves (ex situ) and intact leaves on plants (in situ). 3. Herbivory signifcantly increased Si accumulation both locally in attacked leaves (21% increase in Beit Alpha and 17% in Burpless F1) and systemically in non-attacked leaves (19% increase in Beit Alpha and 10% in Burpless F1). Si supplementation signifcantly increased % foliar Si and C:N ratio, while signifcantly decreasing larval relative consumption (RC) and relative growth rate (RGR) in the in situ assays. In ex situ assays, however, Si only reduced larval RGR when fed on Beit Alpha plants. 4. Our results confrm that Si-based defences can also operate in moderate Si-accumulating plants and, for the frst time, that insect herbivory induces systemic Si accumulation equivalently between plant varieties. Key words. Feeding assays, herbivore performance, induced defence, silica, variety. Introduction Plants have developed a repertoire of physical and chemi- cal defences to combat insect herbivores (Gatehouse, 2002; Mithofer & Boland, 2012). Broadly, plant defences are clas- sifed as constitutive (i.e. defences that are continuously expressed) and induced (i.e. defences that are activated when challenged by herbivores) defences. Induction of plant defences is often more effcient than constitutive defence, particularly when defences are metabolically costly and herbivore attack is Correspondence: Tarikul Islam, Hawkesbury Institute for the Environ- ment, Western Sydney University, Hawkesbury Campus, Ground Floor, Building R2, Locked Bag 1797, Penrith 2751 NSW, Australia. E-mail: tarikul.islam@westernsydney.edu.au sporadic and can be anticipated due to previous herbivory (Kar- ban & Myers, 1989; Karban et al., 1999; Arimura et al., 2005). Herbivores can induce defences locally in attacked tissues only or systemically in un-attacked tissues beyond local responses (Wu & Baldwin, 2010). Both types of induction include trig- gering of plant signalling molecules by herbivores, including peptides, reactive oxygen species (ROS), and jasmonic acid (JA) (Karban & Myers, 1989; Kant et al., 2015). Particularly, JA and its derivatives (e.g. methyl jasmonate) have been linked to the elicitation of systemic plant defences under insect attack (Kant et al., 2015). Physical defences such as silicon (Si) deposition (silicifcation) in plant tissues are mostly regarded as qualitatively constitutive defences, that could be induced quan- titatively (McNaughton & Tarrants, 1983; Massey et al., 2007; Hartley et al., 2016). Specifcally, herbivore feeding and © 2020 The Royal Entomological Society 1