Repellent activity against Aedes aegypti and metabolomic profiling of Myrica gale L. essential oils from Irish boglands

Bog myrtle (Myrica gale L.) is an ethnobotanically significant bogland plant with historic use as an insect repellent. Essential oils, hydro-distilled from bog myrtle leaves and fruits collected from four locations in Ireland, were analysed by GC-MS and tested for repellence against Aedes aegypti, the yellow fever mosquito. Contact (arm-in-cage) and spatial (Y-tube olfactometer) repellence assays were used. Commercial essential oils from M. gale, Myrtus communis and Syzygium aromaticum (clove oil) were also tested. The most effective bog myrtle essential oil in both assays was a Clevenger-hydro-distilled fruit oil, MG4C. Whilst exhibiting significant mean complete protection time in the arm-in-cage assay, it also exhibited strong spatial repellence in the Y-tube olfactometer. Repellence in M. gale samples was linked to a higher monoterpene content reflected by the monoterpene to sesquiterpene (M/S) ratio calculated using GC-MS data. Furthermore, metabolomic analysis linked spatial repellence to α-phellandrene and myrcene, whilst quantification of key terpenes alluded to delta-3-carene, β-pinene, γ-terpinene and camphene potentially contributing to the observed effects. This study establishes that specific bog myrtle essential oils are effective mosquito repellents.