How to chew gum: the post-ingestion fate of foliar secondary compounds consumed by a eucalypt herbivore

Abstract

Herbivorous insects require mechanisms to deal with defence compounds produced by their host plants. Despite an array of secondary compounds associated with defence, eucalypts are hosts to many insect species that readily obtain nutrients also produced by these plants. Gonipterus weevils are foliage-feeding eucalypt specialists as larvae and adults, with a notable characteristic of protecting their eggs with a hardened frass-like substance. The aim of this study was to assess plant, weevil frass and egg capsule chemistry to determine how the weevil eliminates plant secondary metabolites. We hypothesised that noxious compounds would be metabolised prior to elimination and that egg capsules would be composed of frass and additional substances. Weevils were fed on Eucalyptus globulus plants for seven days, with their frass and egg capsules collected daily, and the damaged, first, fully-expanded leaves of the host collected at the end of the assay. Compounds present in each sample were extracted in hexane and analysed by gas chromatography-mass spectrometry. The most abundant compounds in each sample were waxes and terpenoids, and metabolism of 1,8-cineole was evident, with two metabolites that may have semiochemical activity. Comparative analysis revealed significant differences between all samples, with shared compounds varying in relative proportions and exclusive compounds in sample type. These findings contribute to the understanding of Gonipterus physiology and highlight the differences between frass and the cover of egg capsules.