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) cu ltivar Autumn Bliss and the aphid-susceptible cultivar Malling Jewel were c ollected from the newly emerging crown leaves, and also from the group of f our more mature leaves immediately below the crown. Resistance and suscepti bility status of the leaves to infestation by the large raspberry aphid. Am phorophora idaei, were determined by bioassay with the insect just prior to collection of the wax. Analysis showed the waxes to consist of a complex m ixture of free fatty acids; free primary alcohols and their acetates; secon dary alcohols; ketones; terpenoids including squalene. phytosterols, tocoph erol and amyrins; alkanes and long chain alkyl and terpenyl esters. Composi tional differences which may relate to A. idaei-resistance status were noti ceably higher levels of sterols, particularly cycloartenol, together with t he presence of branched alkanes, and an absence of C-29 ketones and the: sy mmetrical C-29 secondary alcohol in wax from the resistant cultivar Bliss. There were also differences between the cultivars in the distribution of in dividual amyrins and tocopherols and in the chain length distribution for h omologues of fatty acids, primary alcohols and alkanes, and these may also be related to resistance to A. idaei. Emerging leaves had lower levels of p rimary alcohols and terpenes, but higher levels of long-chain alkyl esters, and in general, more compounds of shorter chain-length than the more matur e 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 leve ls of the compounds of shorter chain length found in the newly emerged youn ger leaves at the crown of the plant. (C) 1999 Elsevier Science Ltd. All ri ghts reserved.