The great diversity of specialist plant-feeding insects suggests that host plant shifts may initiate speciation, even without geographic barriers. Pheromones and kairomones mediate sexual communication and host choice, and the response to these behaviour-modifying chemicals is under sexual and natural selection, respectively. The idea that the interaction of mate signals and habitat cues facilitates reproductive isolation and ecological speciation is well-established, while the traits and the underlying sensory mechanisms remain unknown. The larva of codling moth feeds in the apple. We show for the first time that the response of male moths to female sex pheromone relies upon presence of a kairomone released from apple. In a non-host tree, attraction to pheromone alone is very strongly reduced, but is fully rescued by blending pheromone with the apple kairomone. This affords a mechanism how host plant shifts shape new mate-finding signals that can give rise to assortative mating and reproductive isolation.

Plant headspace analysis: Volatiles released from green, fruiting branches of apple and birch trees at the trapping site have been collected for chemical analysis by gas chromatography-mass spectrometry.