Can aphid-induced plant signals be transmitted aerially and through the rhizosphere?

Abstract

Aphids, through their close association with plants, cause systemic release of semiochemicals. These may have negative effects on subsequent aphid colonisation and can also have positive roles with insects that are antagonistic to aphid development, for example parasitoids. One of the semiochemicals involved in host selection by aphids is methyl salicylate, and since this compound was shown to have a role as a plant stress signal, the hypothesis that aphids might facilitate identification of new plant signals was examined. Confirmation was obtained during an investigation of avoidance of unsuitable hosts by the lettuce aphid, Nasonovia ribis-nigri. (Z)-Jasmone was identified as a plant-derived semiochemical acting negatively for a number of aphid species, and positively for insect antagonists such as parasitoids and predators. However, when the compound was employed at 0.1 ppm in air above intact plants, these plants then attracted aphid parasitoids long after the (Z)-jasmone itself was no longer detectable. A specific interaction was proposed, since the (Z)-jasmone appeared to be selectively taken up by the plants. Aerial interactions between intact barley plants from different cultivars, which may be differentially releasing stress associated signals, can also influence acceptability to aphids. Furthermore, it has been shown that exudates from the roots of aphid-infested plants, grown hydroponically or in soil, cause intact plants to become more attractive to parasitoids.