GC-MS and adult mosquito repellency bioassay analysis of volatile components of Monarda species in Oklahoma
Joshua D. McCann, Jeff B. Hill, Ricardo Lemus, Abaid Ur Bhatti, J. Tim Smith, and Nancy L. Paiva. Southeastern Oklahoma State University, Durant, OK
Native American tribes were reported to use Monarda punctata and Monarda citriodora plant preparations to repel insects and for medicinal purposes. Gas Chromatography/ Mass Spectroscopy (GC/MS) was used to identify and quantify volatile compounds in acetone extracts of separated plant parts (leaves, stems, floral structures) pooled from field-harvested specimens. Individual plants were also analyzed to examine plant-to-plant variation. Thymol was highly abundant in all acetone extracts and essentials oils prepared using steam distillation from both species. Generally, the second most abundant compound in the distillates was isothymol (carvacrol), although the thymol to isothymol ratio varied among individual plants. Lower levels of myrcene, alpha-pinene, alpha- and gamma-terpinene, d-limonene and p-cymene and carvone were also detected. Repellency of the extracts was estimated by introducing adult mosquitoes into a ventilated T-shaped apparatus and monitoring the migration of mosquitoes towards either a test extract or a solvent control. These bioassays indicated that even dilute solutions of the essential oils from both species were highly repellent to adult mosquitoes. Individual compounds identified by GC/MS were also tested for repellency properties, alone and in combination, to test for possible synergistic relationships. Thymol was an effective repellent at levels which matched the concentration in the essential oil dilutions. Isothymol appeared equally effective, and assays with mixtures suggested that thymol and isothymol work in an additive fashion, not a synergistic one. Funding was provided by NSF-EPSCoR Educational Outreach Program (EPS-0132534), NIGMS-SCORE (5S06-GM008003-33), NIH-INBRE (P20RR016478-04, CFDA#93.389), and National Science Foundation-Oklahoma Louis Stokes Alliance for Minority Participation.