The effect of Psephodiplosis rubi (Diptera: Cecidomyiidae) leaf galls on the secondary metabolite profiles of two congeneric host plants

Galling insects have the unique ability to control the secondary chemistry of their host plants. Galled plant tissue often has higher concentrations of these secondary compounds compared with surrounding plant material. In this study, we used nuclear magnetic resonance (NMR) spectroscopy metabolomics techniques to investigate the effect of galls induced by Psephodiplosis rubi Kolesik, on the profiles of secondary metabolites in its two Rubus host plants in subtropical rainforest. We isolated four triterpenoid compounds from both Rubus species. Concentrations of individual triterpenes were not significantly different in galled and non-galled leaves; however, the concentration of 2-oxopolomic acid was significantly negatively correlated with gall abundance. At the interspecific level, non-galled leaves from both Rubus species were chemically indistinguishable in principal component analysis analyses, while galled leaves showed slight chemical differences. Surprisingly, there was very little difference in metabolic profiles of galled and non-galled leaves at the intraspecific level. Using NMR, we were able to detect slightly different chemical responses in two congeneric host plant species induced by P. rubi galls. We demonstrate how NMR can be used to detect interspecific chemical differences of plants in ecological studies.