Coevolution of Lepidoptera and their host plants: The Salient Aroma Hypothesis

Abstract Host plant specialization by moths and butterflies has been a primary research focus in the field of coevolution and ecological specialization. However, factors underlying the evolution of host specificity remain largely unclear despite decades of research. Here, we demonstrate that host plant specialization is closely linked to diurnality in the Lepidoptera: diurnal butterflies and moths tend to specialize in their range of host choices, whereas nocturnal moths tend to be relative generalists. We further show that plants, on average, release larger amounts of volatile organic compounds (VOC) during the day than at night. Given that plant VOCs are important chemical cues for host searching in herbivores, we propose the ‘Salient Aroma Hypothesis (SAH)’. Under SAH, herbivores that are active during the period when plant VOCs are more available are better able to discriminate between potential host species and therefore have more specialized diet breadth. Conversely, those active at night, when VOCs are not released as much, are more likely to have wider diet breadth. Antennae are the primary organs used to perceive VOCs. Differences in their morphologies also support predictions from the SAH: Diurnal females have larger antennae relative to their body sizes than nocturnal females. Our study underscores the importance of plant VOCs in Lepidoptera evolution, suggesting a possible evolutionary pathway for host specialization in herbivorous insects.