Birch ( Betula spp.) leaves adsorb and re‐release volatiles specific to neighbouring plants – a mechanism for associational herbivore resistance?

Summary• Plant‐emitted semi‐volatile compounds have low vaporization rates at 20–25°C and may therefore persist on surfaces such as plant foliage. The passive adsorption of arthropod‐repellent semi‐volatiles to neighbouring foliage could convey associational resistance, whereby a plant’s neighbours reduce damage caused by herbivores. • We found that birch ( Betula spp.) leaves adsorb and re‐release the specific arthropod‐repelling C 15 semi‐volatiles ledene, ledol and palustrol produced by Rhododendron tomentosum when grown in mixed association in a field setup. In a natural habitat, a higher concentration of ledene was released from birches neighbouring R. tomentosum than from birches situated > 5 m from R. tomentosum . Emission of α‐humulene, a sesquiterpene synthesized by both Betula pendula and R. tomentosum , was also increased in R. tomentosum ‐neighbouring B. pendula . • In assessments for associational resistance, we found that the polyphagous green leaf weevils ( Polydrusus flavipes ) and autumnal moth ( Epirrita autumnata ) larvae both preferred B. pendula to R. tomentosum . P. flavipes also preferred birch leaves not exposed to R. tomentosum to leaves from mixed associations. In the field, a reduction in Euceraphis betulae aphid density occurred in mixed associations. • Our results suggest that plant/tree species may be protected by semi‐volatile compounds emitted by a more herbivore‐resistant heterospecific neighbour.