Plant-Derived Chemicals as Tick Repellents

Ixodes ricinus is the main vector of Lyme borreliosis and Tick-borne encephalitis in Europe. Repellents provide a practical means of protection against tick bites and can therefore reduce transmission of tick-borne diseases. In laboratory tests, pieces of cloth treated with MyggA Natural® (a commercial insect repellent) or with the essential oils of Corymbia citriodora (30%), Lavandula angustifolia (1 and 30%), Pelargonium graveolens (1 and 30%), Hyptis suaveolens (10%), Salvadora persica, Pistacia atlantica, Juniperus phoenicea (20%) and methyl jasmonate (MJ) (0.5%, 1.0%, 2.0% and 5.0%) showed strong repellent activity against I. ricinus nymphs. In a tick-infested woodland in east-central Sweden, we tested by randomized, standardised methodology the potential anti-tick repellent activity of MyggA Natural® (roll-on), two concentrations of MyggA Natural® spray, RB86 (a commercially available insect repellent for horses), the essential oil of C. citriodora and three concentrations of MJ. Each substance was dissolved in acetone and applied separately to 1 m2 cloths which were then pulled over vegetation. Nymphal tick numbers on the cloths were recorded at 10-m intervals and differed significantly between treated cloths and the untreated control and also between collectors. Volatile compounds from fresh and dried leaves of H. suaveolens and the essential oils of H. suaveolens (from Laos and Guinea Bissau) and S. persica, P. atlantica and J. phoenicea (from Libya) were collected by solid phase micro-extraction (SPME) and the constituents were identified by gas chromatography - mass spectrometry (GC-MS). Two main sesquiterpene hydrocarbons in the H. suaveolens oil were β-caryophyllene and humulene. These were modified by oxidation and sulphidation to obtain effective tick repellent compounds with lower volatility. In all three oils from Libyan plants the main monoterpene hydrocarbons were α-pinene, sabinene, β-pinene, myrcene, α-phellandrene, 4-carene, β-phellandrene and γ-terpinene. The selected plant species contained numerous volatiles known to have insecticidal, acaricidal, and/or insect repellent properties