Phenylpropanoids, a class of common phenolic compounds in plants, may potentially be useful as pest repellents. We investigated the relationship between the chemical structure of coniferyl benzoate and its repellency to birds by comparing coniferyl benzoate to two analogous natural esters, corresponding alcohols, and benzoic acid. The absolute and relative feeding repellency of these compounds were assessed in choice (two-cup) and no-choice (one-cup) tests using European Starlings (Sturnus vulgaris). In addition, benzoin Siam (= gum benzoin Siam) was compared to coniferyl benzoate to ascertain if phenolics that naturally occur with coniferyl benzoate in benzoin Siam enhance its repellency. Two-cup tests suggested that coniferyl alcohol was the most repellent compound followed by 3,4-dimethoxycinnamyl alcohol, 3,4-dimethoxycinnamyl benzoate, cinnamyl alcohol, cinnamyl benzoate, coniferyl benzoate, and benzoic acid. The repellency of most alcohols relative to their corresponding ester reversed in the one-cup tests. One-cup tests suggested that 3,4-dimethoxycinnamyl benzoate was the most repellent substance followed by cinnamyl benzoate, benzoin Siam, 3,4-dimethoxycinnamylalcohol, cinnamyl alcohol, coniferyl alcohol, coniferyl benzoate, and benzoic acid. Three conclusions on structure-activity relationships were inferred from these data. First, benzoate esters are more repellent than their corresponding alcohols.Second, repellency is increased by electron-donating groups. Third, acidic functions decrease repellency. We suggest that one function of naturally occurring coniferyl and cinnamyl derivatives may be chemical defense. Genetically engineering agricultural crops to produce analogs of coniferyl alcohol, as an inherent defense against pests and pathogens, may be possible.
© 1992 by the Ecological Society of America.