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Volatile compounds were isolated and identified from a crude extract of garry oak, Quercus garryana, foliage which was known to be attractive to the tachinid fly Cyzenis albicans. Candidate compounds were identified by the combined use of gas chromatography–mass spectrometry, and infra-red spectroscopy. Specific oak-leaf volatiles were field-tested in 2 different years in an apple orchard. Compounds were applied singly or in combination to individual apple trees; the number of C. albicans entering the canopy of each tree was observed and the number of eggs they oviposited on foliage was estimated. Borneol was the only compound that attracted flies when compared with the other treatments, but this did not result in more fly eggs being oviposited on borneol-treated trees. None of the compounds tested resulted in a greater number of Cyzenis eggs being oviposited. A greater abundance of borneol in oak foliage than in apple foliage, and its attractiveness to C. albicans, may explain the aggregation of flies in response to feeding-damage among oak trees and the absence of this pattern among apple trees.

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