INTRODUCTION Euphydryas butterflies (Lepidoptera: Nymphalinae: Melitaeini) have been the subject of much ecological research (Ehrlich & Hanski, 2004). For instance, a great deal of effort has been devoted to the investigation of ovi- position behaviour and the understanding of the evolution of diet preferences (see revisions in Singer, 2003, 2004). One of the central themes underlying these issues is the fact that Euphydryas species lay eggs in clusters of sev- eral to many dozens, a strategy that has important impli- cations for their population biology and the searching behaviour of females. Thus, it has been suggested that the choice of egg-laying sites may be particularly important for batch-layers since females make just one or only a few decisions, which will have great repercussions on offspring survivorship. In line with this argument, the dis- crimination phase in searching Euphydryas and other Melitaeini females is much longer than in most butterflies that lay eggs singly (Singer, 2004; C. Stefanescu, pers. obs.). This complex searching behaviour often results in inter- esting patterns in the spatial distribution of eggs on host plants (Singer & Wee, 2005); some of these patterns are unrelated to plant quality per se and are commonly reported in other butterfly taxa. For example, for an indi- vidual plant the risk of attack is highly influenced by its spatial location. In particular, isolated plants tend to receive more eggs of Euphydryas editha than clumped plants, a phenomenon that can be explained in terms of the mobility patterns of searching females (e.g., Rausher et al., 1981; Singer & Wee, 2005; see also Mackay & Singer, 1982; Root & Kareiva, 1984). Several other physical factors also influence the acceptability of host plants. Thus, Murphy et al. (1984) showed that egg densi- ties in a population of Euphydryas chalcedona declined as distance from nectar sources increased, a pattern that is also reported in other non-related butterflies (e.g., Grossmueller & Lederhouse, 1987). In another study, Williams (1981) showed that females of Euphydryas gil- lettii inhabiting a thermally constrained montane habitat choose warmer microsites for egg-laying, resulting in egg clusters being clumped on host plant leaves orientated towards the sun. As well as these factors, variation in plant quality is also a major cause of non-random patterns in egg laying. Intrinsic properties of plant quality (i.e., its chemical pro- file) may vary both at the individual and population levels (Singer & Parmesan, 1993; Singer & Lee, 2000), ren- dering some hosts more acceptable than others to ovipo- siting females. As recently shown by Nieminen et al. (2003), complex tritrophic interactions may underlie these preferences. These authors found a positive correlation between the concentration of iridoid glycosides in plants of Plantago lanceolata and Veronica spicata and their Eur. J. Entomol. 103: 569–574, 2006 ISSN 1210-5759 Females of the specialist butterfly Euphydryas aurinia (Lepidoptera: Nymphalinae: Melitaeini) select the greenest leaves of Lonicera implexa (Caprifoliaceae) for oviposition CONSTANTÍ STEFANESCU 1 , JOSEP PEÑUELAS 2 , JORDI SARDANS 2 and IOLANDA FILELLA 2 1 Butterfly Monitoring Scheme, Museu de Granollers-Ciències Naturals, Francesc Macià 51, E-08400 Granollers, Spain; e-mail: canliro@teleline.es 2 Ecophysiology Unit CSIC-CEAB-CREAF, CREAF (Center for Ecological Research and Forestry Applications), Edifici C, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain; e-mails: josep.penuelas@uab.es, j.sardans@creaf.uab.es, i.filella@creaf.uab.es Key words. Lepidoptera, Nymphalinae, Euphydryas aurinia, Lonicera implexa, insect-plant interaction, oviposition cues, plant size, foliar chlorophyll concentration, spectral reflectance, Mediterranean area Abstract. In Mediterranean habitats, the specialist butterfly Euphydryas aurinia oviposits on Lonicera implexa. Previous work has shown that ovipositing females select and lay a higher number of egg clusters on certain plants. In this paper the results of a field study aimed at assessing whether females use plant size and/or plant or leaf greenness (i.e., chlorophyll concentrations) as cues for oviposition are described. Size of plants did not appear to be an important factor in determining host plant selection, probably because even small plants provide enough resources for the young larvae to reach the diapausing stage and because last instar larvae, the most likely to face resource depletion, can move great distances in search of food. Measurements of both spectral reflectance and chlorophyll concentration of plants failed to reveal differences between host and non-host plants. On the other hand, reflectance and chlorophyll concentration of leaves were found to be important in oviposition choice as egg clusters were generally located on the greenest leaves with the highest chlorophyll contents. This suggests that females use visual cues to select the leaves that will provide optimal growth opportunities for newly hatched larvae. Although there was some indication that plants receiving a greater number of egg clusters also had more leaves of high chlorophyll content, multiple egg batches on single plants could also be a consequence of females being attracted by the presence of conspecific egg clusters. 569