Fate of green ash, Fraxinus pennsylvanica phloem phenolics in larval emerald ash borer, Agrilus planipennis

Phenolics are a diverse group of compounds widely distributed in terrestrial plants. They are known to have deleterious effects on herbivores wherein they act as antifeedants/ feeding deterrents, pro-oxidants, antidigestants and/or toxins. Emerald ash borer, Agrilus planipennis Fairmaire is an invasive wood boring beetle in North America and a specialist on ash trees (Fraxinusspp.). The secondary chemistry of Fraxinus spp. is enriched with phenolics. Larvae of A. planipennis are well adapted to feed on phenolic-rich phloem tissues and cause extensive damage to the tree. In this study, targeted metabolomics of F. pennsylvanica (green ash) phloem, fourth instar larval tissues (midgut tissue, hindgut-Malpighian tubules, carcasses), midgut content, and frass revealed clear differences in phenolic profiles. Presence of new compounds in larval samples was observed and indicates potential transformation of ash phenolics by the emerald ash borer larvae. Few compounds were detected in phloem and larval samples indicating their excretion without modification. Overall this study demonstrates that A. planipennis larvae are metabolically capable to process ash phenolic compounds.