Plants as Sources of Natural and Effective Acaricides : Against Ixodes ricinus  (Acari: Ixodidae)

Ticks and tick-borne diseases are major health hazards worldwide, with increasing numbers of cases of Lyme disease and tick-borne encephalitis reported yearly. Meanwhile, concerns about the environmental impact and safety of chemical acaricides are driving research into alternative control methods, such as plant-based acaricides. I evaluated eight plant species for their toxicity and repellency against nymphs of Ixodes ricinus (Acari: Ixodidae), the most important life cycle stage of tick-borne infection of humans. Paper I examines the toxicity of the principal active component of the essential oil (EO) of lemon eucalyptus (Corymbia citriodora), p-menthane-3,8-diol (PMD). At 4 h of exposure time (ET), lethal PMD concentrations for 50% mortality (LC50) were 0.035–0.037 mg/cm² and for 95% mortality (LC95) were 0.095-0.097 mg/cm². For 0.1 mg/cm², lethal times for 50% mortality (LT50) were 2.1-2.8 h and for 95% mortality (LT95) were 3.9-4.2 h. An open filter assay gave the most consistent results of five methods tried. Paper II investigated the toxicity of ylang-ylang oil (YYO) and star anise oil (SAO), two naturally occurring, commercially available and inexpensive EOs. Oils were tested at 0.05, 0.1, 0.2, and 0.4μl/cm², and dead nymphs counted at 30-min intervals up to 5h and then at 24, 48 and 72h. For YYO, an exposure of 4.4h resulted in LC95 for 0.4 μl/cm² and LC50 for 0.2μl/cm². The LT95 was 3h for 0.4 μl YYO/cm² and 4.3 h for 0.2 μl/cm². For SAO, the highest concentration (0.4 μl/cm²) only reached LC50 at 14 h and LT95 was 24h. Thus, YYO is a much stronger acaricide but SAO still showed significant toxicity. Paper III investigated two plants of traditional medicinal or economic importance in Libya -Salvadora persica, (Miswak) and Rosmarinus officinalis (Libyan Rosemary). EOs were extracted from wild-collected leaves by steam distillation. Oils were tested on I. ricinus nymphs and their chemical composition analysed by GC-MS. R. officinalis EO at 0.5 and 1µl/cm² exhibited 20% and 100% mortality, respectively, after about 5h of ET. The LC50 and LC95 for 1µl/cm² R. officinalis oil were 0.7 and 0.95 µl/cm², respectively. S. persica oil at 1µl/cm² gave 95% repellency up to 1.5h, reducing to 50% at around 5.45 h, but no significant mortality even after 24h ET. GC-MS analysis showed both oils to be rich in the monoterpenes 1,8 cineol, α-pinene and β-pinene with values of 20.8%, 5.9% and 16.8 %, respectively, for S. persica and 24.07%, 13.03% and 2.45%, respectively, for R. officinalis. Paper IV investigated EOs extracted from leaves of three additional native Libyan plants - Artemisia herba alba (white wormwood), Origanum majorana (oregano) and Juniperus phoenicea (Ar-aar). At 1µl/cm², the LT95 for both A. herba and J. phoenicea EO was 2h versus 72 h for O. majorana oil. GC-MS analyses gave plant specific combinations of the monoterenoids α-pinene, 1,8-cineol, camphor, linalool, terpinene-4-ol, α-terpinol, β-caryophyllene and β-thujanone. EO of A.herba alba contained most of the oxygenated monoterpenes, which all are all known to have insecticidal activity. Taken together, all the EOs used in this study show a broad spectrum of effects against I. ricinus nymphs, making them good candidates for controlling ticks and, thereby, the diseases they carry