Constitutive and herbivore-induced volatiles in pear, alder and hawthorn trees

Qualitative and quantitative differences among pear cultivars were found in constitutive and Cacopsylla-induced volatiles, depending on experimental treatment of the trees (i.e., uninfested and partly or completely infested by psyllids). Blend differences were also found between pear cultivars and wild-type pear, alder and hawthorn-the latter trees are frequently present in pear orchard hedgerows. Interesting differences were found in the presence of methyl salicylate and (E,E)-alpha-farnesene, two compounds previously found to mediate attraction of predatory bugs towards psyllid-infested pear trees. Methyl salicylate is expressed constitutively and is induced systemically by infestation in the whole plant of all four cultivars. (E,E)-alpha-farnesene on the other hand showed also systemic induction in Bartlett, NY10355 and Beurre Hardy, but in partially infested Conference trees it was induced locally, only in herbivore-damaged leaves. No methyl salicylate or (E,E)-alpha-farnesene were identified in honeydew. In field collected headspace samples of alder leaves infested by aphids and leaf beetles we found methyl salicylate but no (E,E)-alpha-farnesene, whereas in uninfested hawthorn neither were identified. Insight in the variability of damage-related pear volatiles will have important implications for integrated pest management in the field.