Phenolic secondary compounds as determinants of the host plant preferences of the leaf beetle Agelastica alni

Summary.

When the palatability of five plant species was studied in laboratory feeding trials, Alnus incana L. was the preferred host for an oligophagous leaf beetle, Agelastica alni L. In addition, Salix phylicifolia L. and S. caprea L. were consumed moderately, but S. myrsinifolia Salisb. and S. pentandra L. were rejected. The feeding preferences in the laboratory well predicted the distribution of A. alni adults on the alternative host species in nature. As a potential explanation for the observed host utilization pattern, willows were found to contain more condensed tannins in their leaves than the preferred grey alder, A. incana. However, (+)-catechin, a basic structural unit of condensed tanninmolecules, showed no deterrent activity against A. alni. In contrast, both chlorogenic acid and a salicylate-type phenolic glucoside, salicin, inhibited the feeding of A. alni in bioassays with pure phenolic compounds. Thus, high leaf content of chlorogenic acid and salicylates appear to largely explain the poor palatability of S. myrsinifolia and S. pentandra for A. alni. On the other hand, as A. alni prefers A. incana over even S. phylicifolia with little potent deterrents in its leaves, leaves of A. incana probably contain some specific stimulants for A. alni. Experiments with pure phenolic compounds suggest that these stimulants may overcome the effects of potential deterrents tested on A. incana leaves. This means that the effects of individual secondary compounds on A. alni may be modified by the background composition of other leaf chemicals.