Multiple functions of volatiles in flowers and leaves of Elsholtzia rugulosa (Lamiaceae) from southwestern China

Although the roles of volatile compounds have been examined separately in plant–herbivore or plant–pollinator interactions, few studies have focused on how plant scents can attract effective pollinators, repel ineffective pollinators, and defend against attacks by insect herbivores. We explored the functional significance of volatile compounds that impart a strong odor to Elsholtzia rugulosa, a shrub species in southwestern China. We monitored the pollinating honey bee Apis cerana, as well as two occasional visitors – Vespa velutina and a Bombus sp. – and an herbivorous beetle Oxycetonia jocunda. Behavior experiments using Y-tubes showed that honey bees were attracted primarily by floral scent while hornets and bumble bees were repelled by both the flowers and leaves. Analysis via gas chromatography-mass spectrometry revealed that these tissue types differed in their compositions and relative amounts of volatile compounds. When the plants were damaged, the average relative amounts of Elsholtzia ketone rapidly increased in the flowers and leaves. Furthermore, herbivorous beetles were strongly repelled by damaged tissues, suggesting a potential defense signaling function by these compounds. Our findings again demonstrate that scents have multiple functions in the interactions among plants and insects.