Volatile Interactions Between Undamaged Plants: Effects and Potential for Breeding Resistance to Aphids

Abstract

Various theories about why vegetational diversity may affect herbivore abundance have been put forward over the years. In this chapter, we discuss one possible mechanism that has not been widely studied, involving plant volatiles and using an aphid (Rhopalosiphum padi L.) and one of its hosts (Hordeum vulgare L., barley) as test organisms. Volatiles from neighbouring plants of a different species or even from plants of the same species have been found to alter aphid acceptance of the receiving plant. Similar effects have been found earlier with volatiles from damaged plants, but here the volatile-emitting plants are apparently undamaged. In the majority of plant combinations tested, host acceptance is decreased but only when certain emitters are combined with certain receivers. Exposure of barley to volatiles from the common weeds Chenopodium album L., Cirsium spp. and Solanum nigrum L. resulted in reduced host acceptance by the aphid, but exposing barley to volatiles from many other weed species had no effect. The same was true for intra-specific interactions; only when certain barley genotypes were exposed to volatiles from specific barley genotypes did the aphids respond differently. Such induced effects correlated with aphid growth rates in a set of barley genotypes representing a wide range of host suitability to the aphid. Pedigree information suggested that the ability to become induced is heritable in barley, something that might be exploited in breeding. More crop/pest combinations should be investigated for these effects, and favourable interactions should be exploited in new cropping systems as plant mixtures or in monocultures with chemical elicitors applied according to forecasts of pest attacks.