Barley exposed to aerial allelopathy from thistles (Cirsium spp.) becomes less acceptable to aphids

1. Recent studies have shown that plant-plant interaction via chemicals (allelopathy) can affect insects. Here the effects on aphid acceptance of barley after exposure to volatiles and root exudates from two common weeds, the thistles Cirsium arvense and Cirsium vulgare, were investigated. 2. Settling by bird cherry-oat aphid, Rhopalosiphum padi, was significantly reduced on barley plants that had been exposed to volatiles from Cirsium species for 5 days. Settling by Sitobion avenae was also reduced on Cirsium-exposed plants, whereas settling by Metopolophium dirhodum was not. 3. In olfactometer tests, Cirsium-exposed barley was significantly less attractive to R. padi than was unexposed barley, indicating that exposure causes a change in the volatile profile of barley. 4. Exposure of barley to root exudates from Cirsium species had no effect on R. padi settling. 5. The results lend weight to the theory that the effects of plant-plant allelopathy can extend to higher trophic levels.