Chlorogenic acid as an antiherbivore defence of willows against leaf beetles

In the leaves of 13 Finnish willow species, the content of a phenolic, chlo rogenic acid, was found to vary from 0 up to 18 mg g(-1) D.W. Effects of pu re chlorogenic acid on insect feeding behaviour were tested using four comm on leaf beetle species which are in the field mainly found on willows with low-chlorogenic acid leaves. One species, Lochmaea capreae L., was invariab ly deterred by pure chlorogenic acid applied in naturally occurring concent rations on the willow leaves. Accordingly, in 2-choice laboratory feeding t rials L. capreae was found to prefer low-chlorogenic acid leaves of four wi llow species over high-chlorogenic acid leaves of Salix pentandra L. and S. myrsinifolia Salisb. When presented on the leaves of S. phylicifolia L, pu re chlorogenic acid inhibited also the feeding by Phratora polaris Sp.-Schn . Instead, chlorogenic acid had no significant effect on Ph. polaris when i t was presented on the leaves of another willow S. cinerea L. In laboratory , Ph. polaris did not show general preference for willow species with low c hlorogenic acid content in their leaves. Thus, the response of Ph. polaris to chlorogenic acid seems to depend on the plant species. Apparently variat ion in other traits such as leaf hairyness may easily override the potentia l effect of chlorogenic acid content on Ph. polaris. To two other leaf beet le species, Galerucella lineola F. and Plagiodera versicolora Laich., chlor ogenic acid is an ineffective deterrent even at unnaturally high concentrat ions. In laboratory, G. lineola and P. versicolora did not prefer willows w ith low chlorogenic acid content in their leaves. Thus, among four studied leaf beetle species, only L. capreae seems to be clearly affected by this p henolic. Therefore, overall importance of chlorogenic acid as a defence aga inst willow-feeding leaf beetles appears to be very limited.