ORIGINAL PAPER Host plant selection by a monophagous herbivore is not mediated by quantitative changes in unique plant chemistry: Agonopterix alstroemeriana and Conium maculatum Eva Castells Æ May R. Berenbaum Received: 29 December 2007 / Accepted: 14 February 2008 / Published online: 4 March 2008 Ó Springer Science+Business Media B.V. 2008 Abstract Host plant selection by ovipositing females is a key process determining the success of phytophagous insects. In oligophagous lepidopterans, host-specific plant secondary chemicals are expected to be dominant factors governing oviposition behavior; distinctive compounds can serve as high-contrast signals that clearly differentiate confamilial hosts from non-hosts increasing the accuracy of host quality evaluation. Agonopterix alstroemeriana (Clerk) (Lepidoptera: Oecophoridae) and Conium macul- atum L. (Apiaceae) form an extremely specialized plant- herbivore system, with A. alstroemeriana monophagous on C. maculatum, a plant with few other insect herbivores at least in part due to its virtually unique capacity among plants to produce piperidine alkaloids. Here we have studied the response of A. alstroemeriana oviposition to unique host plant secondary metabolites, piperidine alka- loids, and widespread compounds, mono- and sesquiterpenes, in a concentration-dependent fashion. Rates of oviposition were negatively correlated with Z-ocimene concentrations. To confirm the deterrent properties of this monoterpene for A. alstroemeriana oviposition, we con- ducted a choice experiment using artificially damaged C. maculatum plants, with higher emission of volatiles, and undamaged control plants. Damaged plants were less pre- ferred as oviposition sites compared to the controls. The lack of association between oviposition and piperidine alkaloids, defenses unique to Conium species, suggests that quantitative changes of these species-specific chemicals do not play a predominant role in host selection by the monophagous A. alstroemeriana. Keywords Agonopterix alstroemeriana Á Apiaceae Á Conium maculatum Á Host plant selection Á Lepidoptera Á Oecophoridae Á Oviposition Á Piperidine alkaloids Á Specialization Á Terpenes Introduction Host plant selection by ovipositing females is a key process critical for the survivorship, performance, and fitness of their offspring, especially in those species with low mobility of immature stages such as many Lepidoptera (Thompson and Pellmyr 1991). Because plants have evolved a wide array of toxic chemical defenses against herbivores, the mechanism underlying host choice is expected to be highly related to the plant chemical com- position (Honda 1995; Jaenike 1990). The ability to discriminate among plant chemical patterns varies depending on the degree of specialization (Janz and Nylin 1997; Bernays 2001; Egan and Funk 2006; Wee and Singer 2007). The Neural Limitation hypothesis predicts that, while generalists evaluate host suitability among different species with diverse chemistries, monophagous or oli- gophagous insects should have a greater capacity to differentiate host plant chemistry at the intraspecific level (Jaenike 1990; Janz and Nylin 1997). A narrow range of hosts, with reduced diversity of plant secondary chemicals, Handling editor: Robert Glinwood. E. Castells Á M. R. Berenbaum Department of Entomology, University of Illinois at Urbana- Champaign, 320 Morrill Hall, 505 S Goodwin Ave., Urbana, IL 61801, USA Present Address: E. Castells (&) Departament de Productes Naturals, Biologia Vegetal i Edafologia, Facultat de Farma `cia, Universitat de Barcelona, Av. Joan XXIII s/n, 08028 Barcelona, Catalonia, Spain e-mail: e.castells@ub.edu 123 Arthropod-Plant Interactions (2008) 2:43–51 DOI 10.1007/s11829-008-9032-9