Weed–barley interactions affect plant acceptance by aphids in laboratory and field experiments

Increased botanical diversity can lead to suppression of insect pests. One route by which botanical diversity is increased in crops is through the occurrence of weeds, which increasingly interact with crop plants as organic production expands. However, the mechanisms by which this might affect insect herbivores are poorly understood. This study examined whether volatile chemical interactions between weeds and barley, Hordeum vulgare L. (Poaceae), can affect plant acceptance by the bird cherry oat aphid, Rhopalosiphum padi L. (Hemiptera: Aphididae). In laboratory experiments, exposure of barley to volatiles from Chenopodium album L. (Amaranthaceae) and Solanum nigrum L. (Solanaceae) resulted in significantly reduced aphid acceptance compared with unexposed plants. In a series of field experiments in which the occurrence of weeds was manipulated in plots of barley, significantly lower aphid acceptance was recorded on barley plants grown in plots with C. album compared with barley plants in weedless plots. The results indicate that interaction between weeds and barley can affect aphid–plant interactions in the field as well as in the laboratory and provide further evidence that the effects of chemical interactions between visibly undamaged plants can extend to higher trophic levels.