Eucalyptus citriodora essential oil is a rich source of citronellal (80%) and is reported to be non toxic, non-irritant and non-sensitising so can be used on the skin.
Other less favoured sources of citronellal are Lemongrass (cymbopogon c.) and Lemon Tea Tree (Leptospermum petersonii). Lemongrass can be irritating and sensitising in some individuals. Tea tree is possibly sensitising in some individuals. Tea tree left in clear glass rapidly oxidises with para-cymene content increasing from 3% to 30% enough to cause chemical burns. Tea Tree left in amber glass hardly oxidises.
It works on mosquitos as well.