Butterflies are believed to use mainly visual cues when searching for food and oviposition sites despite that their olfactory system is morphologically similar to their nocturnal relatives, the moths. The olfactory ability in butterflies has, however, not been thoroughly investigated. Therefore, we performed the first study of odour representation in the primary olfactory centre, the antennal lobes, of butterflies. Host plant range is highly variable within the butterfly family Nymphalidae, with extreme specialists and wide generalists found even among closely related species. Here we measured odour evoked Ca²⁺ activity in the antennal lobes of two nymphalid species with diverging host plant preferences, the specialist Aglais urticae and the generalist Polygonia c-album. The butterflies responded with stimulus-specific combinations of activated glomeruli to single plant-related compounds and to extracts of host and non-host plants. In general, responses were similar between the species. However, the specialist A. urticae responded more specifically to its preferred host plant, stinging nettle, than P. c-album. In addition, we found a species-specific difference both in correlation between responses to two common green leaf volatiles and the sensitivity to these compounds. Our results indicate that these butterflies have the ability to detect and to discriminate between different plant-related odorants.

Searching for resources is often a challenging task, especially for small organisms such as insects. Complex stimuli have to be extracted from the environment and translated into a relevant behavioral output. A first step in this process is to investigate the relative roles of the different senses during search for various resources. While the role of olfaction is well documented in nocturnal moths, the olfactory abilities of the closely related diurnal butterflies are poorly explored. Here we investigated how olfactory information is used in the search for host plants and asked if these abilities varied with levels of stimulus complexity. Thus, we tested two nymphalid butterfly species with divergent host plant range in a two-choice olfactometer testing different combinations of host and non-host plants. The experiments show both the monophagous Aglais urticae and the polyphagous Polygonia c-album could navigate towards an odor source, but this ability varied with context. While mated females exhibited a preference for their host plant, unmated females of both species did not show a preference for host plant cues. Furthermore, both species showed inabilities to make fine-tuned decisions between hosts. We conclude that olfactory cues are important for butterflies to navigate towards targets. We argue that there are limitations on how much information can be extracted from host volatiles. These results are discussed in the light of neural processing limitations and degree of host plant specialization, suggesting the necessity of other sensory modalities to sharpen the decision process and facilitate the final oviposition event.