Interactions of gall-formers and leaf-chewers on a tropical tree fern: evidence for non-repulsion and co-occurrence between insect guilds.

Abstract

Host plant selection by herbivores is driven by a complex array of cues, including leaf traits and previous leaf damage. Herbivore-associated cues to host selection at the plant and leaf scale aid understanding of mechanisms responsible for host preference that might translate into increased performance, as well as processes structuring herbivore populations mediated by interactions. We investigated how changes induced by a galling insect in the tropical fern Cyathea phalerata act as repellent or attractant cues for sawfly feeding and the effects of leaf size on herbivory levels. We recorded gall abundance, damage by chewers, leaf size, plant nutritional quality, phenolic concentration and leaf anatomical traits between galled and non-galled leaf samples. Galled samples contained less N, higher levels of phenolics and higher C/N ratio. However, leaf-chewing damage did not differ between galled and non-galled leaves. The gall structure was avoided by chewers, as it had high concentrations of phenolics, lignification and suberization. Larger leaves sustained higher gall abundance, but leaf size did not have a significant effect on chewer damage. A co-occurrence index calculated for both guilds indicated that galls and chewers exhibited a distribution that did not differ from random, reinforcing that the two guilds on C. phalerata do not show patterns of repulsion such as those maintained by interspecific competition. Sawflies dismissing chemical cues indicate that the increase in phenolics caused by galling insects does not generate increased protection of the galled pinnules. Our results highlight ferns as key resources for herbivores and as a potential plant group to study new research avenues on plant-insect interactions.