1Constituents of the essential oil of Tagetes minuta and their relative amounts (%) according to Gas chromatography-linked mass spectrometry analysis. http://ojvr.org/index.php/ojvr/article/downloadSuppFile/358/368
2Dose-dependent repellent response of Hyalomma rufipes (male and female ticks pooled) to Tagetes minuta essential oil. http://ojvr.org/index.php/ojvr/article/downloadSuppFile/358/369
3Effective concentration needed to repel 50% of ticks (EC50). http://ojvr.org/index.php/ojvr/article/downloadSuppFile/358/370
DiscussionThe essential oil of T. minuta used in this study was rich in terpenes based on GC and GC-MS analysis. Chemical analysis carried out on different species of Tagetes grown in Northern Italy indicated that dihydrotagetone, tagetones, ocimenones and piperitone occurred in Tagetes erecta, T. minuta, Tagetes patula and Tagetes tenuifolia (Marotti et al. 2004). These compounds were also present in the essential oil of T. minuta evaluated in this study.The results obtained in this study indicate that the essential oil of T. minuta has tick repellent and growth inhibitory properties. Tick repellency by the essential oil of T. minuta, corroborates studies by Lwande et al. (1999) who further showed that this was because of one of its constituents, beta-ocimene. Even though it is important to evaluate individual compounds in suitable bioassays for repellency (Lwande et al. 1999), whole oil, such as the one used in this study, may cause increased bioactivity compared to individual compounds because of synergistic effects. Ticks have highly efficient sensory organs. The tick’s sensory organ, the Haller’s organ, is situated on the dorsal surface of each foreleg and it has both olfactory and gustatory chemosensilla (Sonenshine 1991). Olfactory chemoreceptors or sensilla perceive volatiles, whilst gustatory chemoreceptors perceive stimuli on contact (McMahon, Kröber & Guerin 2003). Despite the necessity to explore plant based repellents as tick control agents, there is still a future need to improve on the longevity of effective, yet extremely volatile repellents in order to compete with registered compounds. Several studies deal with the improvement of formulations of plant oils to increase their longevity through the development of nano-emulsions, improved formulations and fixatives (Maia & Moore 2011). Kaaya and Saxena (1998) used petroleum jelly as a carrier for plant extract during an in vivo study. A further approach that could be relevant in the sustainable management of ticks is through the disruption of their life cycle by targeting engorged immature stages. This may result in the reduction of tick infestations to low and controllable levels, hence reducing the tick population during favourable climatic conditions. During this study, moulting of engorged nymphs of H. rufipes was significantly (P < 0.05) delayed by 60%. This could be attributed to tagetone, one of the identified constituents of T. minuta used in this study, possessing growth inhibitory properties (Jacobson 1983). These results are in agreement with findings of another study; the essential oil of a variety of T. minuta (genotype TM-1) deterred oviposition in Tribolium castaneum Herbst (Coleoptera: Tenebrionidae) by 81% and suppressed its egg hatchability by 91% when applied at a dosage of 70 000 ppm on filter paper (Alok et al. 2005). The bioactive compound(s) are very likely to be of a volatile nature as no direct contact was established between ticks and the extracts. The rate at which the volatiles diffuse from the glass vials could not be determined, but the delay in moulting indicates that the bioactive constituents of T. minuta should be very effective to produce such results with a single dose. Other herbal products that contain essential oils such as citronella oil or Chrysanthemum spp. (containing pyrethrum), are available as commercial arthropod repellents (Fradin & Day 2002). The use of botanicals for the control of ticks is compatible with traditional practices in Africa and Asia, where most resource-poor farmers use plant materials to treat endoparasites and ectoparasites of livestock (Lans & Brown 1998; Madge 1998). Traditional knowledge about the use of these plants is transferred through successive generations, especially in rural communities. Knowledge about the use of individual plant species, however, varies between localities in Africa, and scientific validation of their uses may increase the range of plants available for tick control. This may reduce the burden substantially on plant species that are at risk of extinction. Conclusion The results obtained in this study suggest that T. minuta is a potential source of tick control agents. Although T. minuta is a common weed in rural areas, it is unlikely that high enough concentrations of the volatile oils would be reached to affect ticks when animals are housed closely together with certain plants scattered on the ground. The extracted essential oil of T. minuta, however, may be of use in the integrated control of H. rufipes or other insects. Acknowledgements The National Research Foundation, South Africa, provided funding for the project. MEDUNSA (now merged with the University of Limpopo) and the University of Pretoria provided laboratory equipment. Colleagues in the Department of Biology, University of Limpopo, and Dr D. Katerere and Dr P. Soundy from the University of Pretoria made valuable contributions. Competing interests The authors declare that they have no financial or personal relationship(s) which may have inappropriately influenced them in writing this paper. Authors’ contributions S.R.M. (University of South Africa) and J.N.E. 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