SPECIAL ISSUE: HOST PLANT SELECTION AND FEEDING ECOLOGY Herbivore-induced plants do not affect oviposition but do affect fitness of subsequent herbivores on canola Chaminda De Silva Weeraddana* & Maya L. Evenden Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E9, Canada Accepted: 16 October 2018 Key words: Mamestra configurata, Plutella xylostella, Brassica napus, induced defences, glucosinolates, herbivory, plant–insect interactions, Lepidoptera, Plutellidae, Noctuidae, Brassicaceae Abstract The diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae), is oligophagous on plants in the Brassicaceae, and is considered one of the most significant insect pests of canola (Brassica napus L.), a major oilseed crop grown in the Prairie Provinces of Canada. The bertha armyworm (BAW), Mamestra configurata Walker (Lepidoptera: Noctuidae), is a generalist herbivore that prefer- entially feeds on canola plants. In the canola growing season in the Prairie Provinces of Canada, DBM feeding occurs prior to BAW herbivory. In this study, we test the effect of DBM herbivory on subse- quent host use by BAW. Oviposition by female BAW was not influenced by previous DBM-herbivory or mechanical damage of canola plants. Bertha armyworm larvae were deterred from feeding on canola previously damaged by DBM and larvae developed into lighter pupae when reared on DBM- damaged plants. Bertha armyworm pupae that developed from larvae fed on mechanically damaged plants had similar pupal weights to those fed undamaged plants. Adult BAW reared on canola with previous DBM feeding damage had marginally smaller wings than moths reared on canola treated dif- ferently. The combination of these results suggests that herbivory by the brassicaceous specialist, DBM, negatively influences host use and larval performance by the generalist, BAW, on canola. Introduction Plants encounter multiple herbivore attacks during a growing season (Stam et al., 2014) and have evolved con- stitutive and induced defences to protect themselves from herbivores. Constitutive defences include a thick cuticle, a thick cell wall, trichomes, and toxins (War et al., 2012). Induced defences include the production of secondary compounds (F€ urstenberg-H€ agg et al., 2013), which can influence the activity of the attacking herbivore and other species that subsequently use the induced plant (Viswa- nathan et al., 2005). In general, the feeding activity of early-season herbivores influences late-season herbivore activity. The timing of changes induced by early-season herbivores will determine which herbivores are influenced by the induction. Plant responses following induction can be rapid or delayed (Mathur et al., 2013). Early-season herbivores can increase plant resistance or susceptibility to subsequent late-season herbivores, especially those in the same feeding guild (Hernandez-Cumplido et al., 2016). These changes in plant susceptibility can be mediated by plant responses to feeding by the first herbivore (Poelman et al., 2008), or reduced plant tissue availability to the late- season herbivore based on the feeding damage and pattern of the early herbivore (Walling, 2000; Hernandez- Cumplido et al., 2016). Initial herbivory can deplete host resources through plant biomass reduction and induction of toxic defences that can negatively influence subsequent herbivores (Agra- wal, 2000). Plants damaged early in the season by the potato flea beetle, Psylliodes affinis Payk, receive fewer eggs from the clavate tortoise beetle, Plagiometriona clavata (Fabricius), on Solanum dulcamara L. late in the season (Viswanathan et al., 2005). In contrast, early-season her- bivory by Pieris rapae (L.) increases susceptibility to flea beetles, Phylotreta spp., on wild radish, Raphanus rapha- nistrum L. (Agrawal & Sherriffs, 2001). The ecological cost of plant defences is differentially influenced by the insect feeding guild (Poelman et al., 2008). For instance, infestation of Brassica oleracea L. with the sap-feeding cabbage aphid, Brevicoryne brassicae L., *Correspondence: E-mail: weeradda@ualberta.ca © 2019 The Netherlands Entomological Society Entomologia Experimentalis et Applicata 167: 341–349, 2019 341 DOI: 10.1111/eea.12762