Plant odour and sex pheromone are integral elements of specific mate recognition in an insect herbivore
Specific mate recognition strongly relies on the chemical senses in many animals, and especially in nocturnal insects. Two signal types lend to premating olfactory communication in terrestrial habitats: sex signals blend into an atmosphere of habitat odorants, where plant volatiles prevail. We show for the first time that males of the African cotton leafworm Spodoptera littoralis perceive female sex pheromone and volatiles of its plant host cotton as a unit, rather than as independent messages. In clean air, S. littoralis males are attracted to flawed pheromone signals, such as single synthetic pheromone components or even the pheromone of a sibling species, Oriental leafworm S. litura. Presence of host plant volatiles, however, strongly reduces the male response to deficient or heterospecific pheromone signals. That plant cues enhance discrimination of sex pheromone quality confirms the idea that specific mate recognition in noctuid moths has evolved in concert with host plant adaptation. A participation of host plant odour in sexual communication suggests that mate recognition is under natural and sexual selection. Moreover, shifts in either female host preference or sex pheromone biosynthesis give rise to new communication channels that have the potential to initiate or contribute to reproductive isolation.