|Article Summary of Essential oils of Cupressus funebris, Juniperus communis, and J. chinensis (Cupressaceae) as repellants against ticks (Acari: Ixodidae) and mosquitoes (Diptera: Culicidae) and as toxicants against mosquitoes
Being able to control exposure to mosquito and tick-borne illnesses through repellants would have an impact in both the scientific community and the world as a whole. Mosquitoes and ticks are vectors for various illnesses such as dengue fever, Chikungunya, Rift Valley fever, yellow fever, malaria and Lyme disease. Many of the previous illnesses can reach epidemic levels, which is why repellants or toxicants are frequently used to try to reduce the amount of the biting arthropods.
While being able to control vector insect populations with repellants and toxicants is a good form of reduction for the various illnesses, repeated exposure to synthetic repellants shows a growing resistance. Ergo, finding alternative methods of controlling exposure to these vector arthropods is an issue of great concern. Plant and seed oils are already being used in natural based insecticides that are low risk environmentally due to the fact they are naturally occurring. With the need for an alternative method of protection and the growing resistance to synthetic repellants and toxicants, a natural repellant may prove to be more effective.
The researchers in this study were attempting to identify the compounds that make up each of the plant oils and then test the effectiveness of each as a repellant. After each test, they could then further isolate which compounds appeared to be the most effective as repellants or toxicants. This topic was of value for the fact that if the researchers could isolate a naturally occurring compound that worked as an effective and safe repellant, the occurrence of tick and mosquito borne illness around the world could be reduced. The hypothesis of the researchers stated, “One purpose of this study was to characterize the chemical composition of the essential oils of C. fineries, J. chinensis, and J. communis by analysis using gas chromatography (GC) and GC-mass spectrometry (GC-MS). The second purpose was to evaluate these oils as repellants against the ticks A. americanum and I. scapularis and as repellants and adult and larval toxicants against the yellow fever mosquito, Aedes aegypti (L)” (Carroll et al, 2011, p. 259).
Materials and Methods
The researchers conducted tests on living subjects, ticks and mosquitoes, using the repellants and controls of synthetic repellant and ethanol. When testing the ticks, “J. scapularis and A. americanum nymphs were tested three to five, and three to six months after molting, respectively” (Carroll et al, 2011, p. 259). The adult mosquitoes that were tested had been aged five to nine days.
The techniques used for the experiments varied based on how each type of arthropod would approach and bite the target. For the ticks they used 15 cm Petri dishes with 9 cm Petri dishes glued inside, creating a moat when water was added, and a filter paper suspended into the middle Petri dish. Many species of ticks climb up vertical surfaces when searching for a meal so the filter paper was suspended as the vertical surface for the tick to climb. Test solutions consisted of ethanol controls, deet, and essential oils from J. communis, C. funebris and J. chinensis.
The mosquito experiment design consisted of shielding the arm of a human test subject but leaving a small 4x8 cm opening for mosquitoes to land and bite. A small cloth could be treated with test solutions and placed near the opening. Test solutions consisted of ethanol controls, deet, and essential oils from J. communes, C. funebris and J. chinensis. “The protocol was approved by the University of Florida Human Use Institutional Review Board-01 (Study #636-2005)” (Carroll et al, 2011, p. 261).
The result of this study was that the researchers found that all three essential oils were repellant to the ticks, with J. communis being the most effective of the three. Two of the three oils were not consistently effective at repelling the female mosquitoes but, again, the oil of J. communis showed a minimum effective dosage (MED) that was similar to the deet. Toxicity in the experiment was negligible. There did not appear to be anything surprising or unexpected in the study.
The hypothesis of this study focused on two things: one was in characterizing the chemical composition of the essential oils, which was accomplished, and the other was to evaluate the oils as repellants and toxicants for ticks and mosquitoes. The repellant properties of the oils against ticks showed promising results while the repellant properties against mosquitoes could lead to further testing or a new delivery method.
One limitation of this study is the small sample of plants being tested. The researchers mention various other plants that have shown strong repellant and toxicant properties. In order to increase the chances of finding a more effective natural repellant for ticks and mosquitoes carrying these vector illnesses, increased experimentation will be needed in the future with more varied plant sources.
Modification of testing methodology may need to be considered as well. When testing for the effectiveness of repellants for the mosquitoes on human test subjects, it would be more effective to test the solution in an application format that would be used in real life situation. Testing a solution that is safe for application directly to the skin may be more successful with mosquitoes.
Carroll, J. F. &Tabanca, N. & Kramer, M. & Elejalde, N. & Wedge, D. E. & Bernier, U. R. & et al. (2011). Essential oils of Cupressus funebris, Juniperus communis, and J. chinensis (Cupressaceae) as repellants against ticks (Acari: Ixodidae) and mosquitoes (Diptera: Culicidae) and as toxicants against mosquitoes. Journal of Vector Ecology, Vol. 36, no. 2, 258-268.