Evaluation of 2 ornamental plants Citronella—Scented Geranium (Geraniales: Geraniaceae) and prostrate rosemary (Lamiales: Lamiaceae) for spatial repellency of host-seeking mosquitoes

Abstract

Mosquitoes are important vectors of pathogens, including West Nile virus, in California’s Central Valley. Public interest in plant-based mosquito repellents is widespread. Local residents frequently inquire whether ornamental plants provide mosquito protection however, the effectiveness of plants as spatial repellents is uncertain. To address this, we screened 6 candidate ornamental plants using laboratory Y-tube olfactometer bioassays, plants screened included: white yarrow (Achillea millefolium, Linnaeus), lemongrass (Cymbopogon citratus Stapf), lavender (Lavandula angustifolia, Miller), prostrate rosemary (Salvia rosmarinus ‘Prostratus’, Spenner), white sage (Salvia apiana, Jepson), and Citronella—Scented (CS) geranium (Pelargonium citrosum, L’Héritier). Citronella—Scented geranium and prostrate rosemary ranked highest in qualitative repellency scores and were selected for further examination based on retail availability. These 2 species were then evaluated in replicated field trials in both urban and rural locations over 16 wk. Treatments were rotated among sites using a Latin square design and compared against standard surveillance controls and a metofluthrin-based spatial repellent (Thermacell E90). Neither plant treatment reduced catch rates of host-seeking mosquitoes relative to controls under passive or mechanically damaged conditions at either location, while the metofluthrin treatment produced consistently lower mosquito captures at both locations. These results suggest that whole ornamental plants do not provide spatial repellency under realistic field deployment conditions.