Epicuticular wax composition in relation to aphid infestation and resistance in red raspberry (Rubus idaeus L.)

Epicuticular waxes from the aphid-resistant red raspberry (Rubus idaeus) cultivar Autumn Bliss and the aphid-susceptible cultivar Malling Jewel were collected from the newly emerging crown leaves, and also from the group of four more mature leaves immediately below the crown. Resistance and susceptibility status of the leaves to infestation by the large raspberry aphid, Amphorophora idaei, were determined by bioassay with the insect just prior to collection of the wax. Analysis showed the waxes to consist of a complex mixture of free fatty acids; free primary alcohols and their acetates; secondary alcohols; ketones; terpenoids including squalene, phytosterols, tocopherol and amyrins; alkanes and long chain alkyl and terpenyl esters. Compositional differences which may relate to A. idaei-resistance status were noticeably higher levels of sterols, particularly cycloartenol, together with the presence of branched alkanes, and an absence of C₂₉ ketones and the symmetrical C₂₉ secondary alcohol in wax from the resistant cultivar Bliss. There were also differences between the cultivars in the distribution of individual amyrins and tocopherols and in the chain length distribution for homologues of fatty acids, primary alcohols and alkanes, and these may also be related to resistance to A. idaei. Emerging leaves had lower levels of primary alcohols and terpenes, but higher levels of long-chain alkyl esters, and in general, more compounds of shorter chain-length than the more mature leaves. During bioassay A. idaei displayed a preference to settle on the more mature leaves. This may be due to greater wax coverage and higher levels of the compounds of shorter chain length found in the newly emerged younger leaves at the crown of the plant.