Two galling insects (Hartigiola annulipes and Mikiola fagi), one host plant (Fagus sylvatica) – differences between leaf and gall chemical composition

Gall-inducing organisms change the chemical properties of galled organs and host plants due to the development of gall tissues that are supposed to shelter the galler and provide it with nutrients. Hartigiola annulipes and Mikiola fagi represent the gall midge family (Diptera; Cecidomyiidae). They share a host plant species, the common beech (Fagus sylvatica), whose leaves they use for galling. Their galls are single-chambered and occur on the upper side of the leaf blade. The morphologies of their galls are different, but there are a lack of studies comparing the impact of both species on the common host. Thus, this was the aim of this study. We analysed the total contents of carbon, nitrogen, soluble carbohydrates, starch, soluble phenolics and tannins in galls, galled leaves with removed galls and leaves without galls collected in two different forest stands in western Poland (a Scots pine forest with artificially-planted beech trees and a natural beech forest). The influence of the studied gallers on the leaves is species- and forest-dependent. Perhaps seasonal changes and the level of infestation are also responsible for the chemical changes. The content of soluble carbohydrates in the galls of H. annulipes reaches an optimized level and is independent of the forest type. A high infestation level by H. annulipes is manifested itself in an elevated content of total soluble phenolics and tannins in leaves, while gall tissues do not accumulate soluble phenolics. The low levels of nitrogen in the gall tissues of both gallers leads to the rejection of the nutrition hypothesis; however, M. fagi galls act as sinks for soluble carbohydrates.