14TH INTERNATIONAL SYMPOSIUM ON INSECT-PLANT INTERACTIONS Estimating direct resistance in willows against a major insect pest, Phratora vulgatissima, by comparing life history traits Anna Lehrman 1 *, Mikaela Torp 1 , Johan A. Stenberg 1 , Riitta Julkunen-Tiitto 2 & Christer Björkman 1 1 Department of Ecology, Swedish University of Agricultural Sciences, PO Box 7044, SE-750 07, Uppsala, Sweden, and 2 Natu- ral Product Research Laboratory, Department of Biology, University of Eastern Finland, PO Box 111, FIN-801 01, Joensuu, Finland Accepted: 16 January 2012 Key words: Salix, Chrysomelidae, salicylate, bioassay, leaf chemistry, phenolic compound, perfor- mance, Coleoptera, Salicaceae Abstract Many Salix species (Salicaceae) have a high energy quotum and efficient nitrogen use, resulting in a high and stable biomass production, making these plants suitable as bioenergy crops. However, Salix coppices are sensitive to herbivory by chrysomelid beetles, such as Phratora vulgatissima L. (Coleo- ptera: Chrysomelidae), and to maintain high biomass yields over several years, durable plant resis- tance is necessary. We have developed a reliable and efficient bioassay that can be used as a tool for detection of resistance against P. vulgatissima in Salix. Screening of six clones of one susceptible (Salix viminalis L.) and one resistant (Salix dasyclados Wimm.) willow species, by the response of several life history traits of larval and adult P. vulgatissima, identified egg production as the most reliable trait for resistance. We show that a 2-week bioassay of oviposition rate is sufficient for a trustworthy estima- tion of both total egg production and survival of adult leaf beetles. In addition, to elucidate the varia- tion in resistance among the studied clones, leaf concentrations of various phenolic compounds were compared with the key life history traits of the beetles. Phenolic compounds, especially salicylates, have previously been reported as the main factor controlling Salix resistance against P. vulgatissima. Indeed we found higher levels of salicylates in the resistant clones and larval performance was nega- tively correlated to the concentration of salicylates in the leaves. In addition, we found indications of negative effects of a luteolin derivative and quercetin-3-glucoside on leaf beetle performance, but fur- ther studies are needed to elucidate the specific roles that the different phenolic compounds play in plant resistance. Introduction The demand for renewable sources of energy is increasing. In Sweden and other parts of Europe, willows (Salix spp.; Salicaceae), are grown as a short rotation coppice crop to produce biomass as a source of bioenergy. Willows are well suited for this purpose due to their high energy quota and efficient nitrogen use: high biomass yields with a relatively low input of fertilization (Karp & Shield, 2008). Growing willows as an energy crop provides farmers with an alternative and profitable use of land that may be unsuit- able for production of feed and fodder. In addition, wil- lows can be used in soil remediation (Jensen et al., 2009; Mleczek et al., 2009) and cleaning of waste water (Arons- son et al., 2010). A major challenge in using perennial crops like willows is to maintain a high biomass production over many years. Tolerance toward abiotic factors, such as frost and drought, and tolerance and resistance against biotic fac- tors, such as pathogens and herbivores, is therefore impor- tant (Weih et al., 2008). Insect herbivores, particularly leaf-feeding chrysomelid beetles, often reach high densities in willow plantations (Bjo ¨rkman et al., 2004; Peacock et al., 2004) resulting in severe damage to the plants. The *Correspondence: Anna Lehrman, Department of Ecology, Swedish University of Agricultural Sciences, PO Box 7044, SE-750 07, Uppsala, Sweden. E-mail: anna.lehrman@slu.se Ó 2012 The Authors Entomologia Experimentalis et Applicata 144: 93–100, 2012 Entomologia Experimentalis et Applicata Ó 2012 The Netherlands Entomological Society 93 DOI: 10.1111/j.1570-7458.2012.01244.x