Hormaphis hamamelidis fundatrices benefit by manipulating phenolic metabolism of their host.

We investigated the pattern and potential adaptive value of phenolic concentrations in galls induced by the aphid Hormaphis hamamelidis on leaves of Hamamelis virginiana. By the time that founding females began reproduction, galls had higher concentrations of condensed tannins and lower concentrations of hydrolyzable tannins than leaves. Galled and ungalled leaf laminas never differed significantly in any phenolic measure. Condensed tannin concentrations also were positively related to the number of offspring per gall when gall dry weight, another important correlate of fecundity, was accounted for. This could indicate the prior sink strength of the gall. Polyphenols may act as a repository for excess carbon drawn to the gall by increased sink strength, or be an indication of the fundatrix' ability to manipulate host physiology. This study is the first to demonstrate a tangible, quantitative association between phenolic accumulation in galls and gall-former reproductive performance, and illustrates that condensed tannins may play roles other than plant defense.