The citrosa plant (Pelargonium spp., Geraniaceae) is being marketed under the name "Pelargonium citrosum," and promotional literature is making claims that it has been genetically engineered to repel mosquitoes by emitting a citronella scent. A field test was conducted to test the effectiveness of this plant as a repellent against Aedes mosquitoes, in comparison to 75 percent deet and non-treatment (control). In a flat, wooded area at the University of Guelph Arboretum, Guelph, Ontario, twelve volunteers were either treated with 2 ml of deet or 2 ml of deionized water applied to their hands and forearms, or were instructed to stand directly next to a potted citrosa plant. Effectiveness was calculated by counting the number of mosquitoes that bit the participants within each daily twelve-hour test period for five days. Deet was found to provide better than 90 percent protection from mosquitoes after eight hours, and better than 60 percent protection after twelve hours. No signifi cant difference was found between the non-treated (control) subjects and the citrosa treatment subjects. Researchers noted that the citrosa plant did not add any detectable scent to the air unless it was disturbed in some way, e.g., by the wind or a hand brushing up against it. Not only was the plant ineffective at protecting humans against Aedes mosquito bites, the mosquitoes were seen landing and resting on the citrosa plant on a regular basis. This observation supports the test results which indicate lack of repellency.

Promotional literature claims that citrosa has been genetically engineered to produce citronella oil, but chemical analysis has shown that citrosa essential oil contains only trace amounts of citronella (11 percent citronellol, 0.09 percent citronellal), and, in fact, closely resembles the essential oils of the other geranium plants.(*) The citrosa plant is morphologically similar to the cultivar Pelargonium "Rosé" (P graveolens L'Hérit. x Ait., Geraniaceae or rose geranium), which is part of the hybrid complex P x asperum Ehrh. ex Willd. = P graveolens L'Hérit. x P radens H. E. Moore. In fact, the citrosa plant is essentially indistinguishable from the P xasperum hybrid complex. Despite the marketing, "Pelargonium citrosum" is not a valid taxonomic designation. Based on the results of this study, the authors conclude that citrosa should not be marketed as a mosquito repellent.

[Matsuda, B. M., G. A. Surgeoner, J. D. Heal, A. O. Tucker, and M. J. Maciarello. 1996. Essential Oil Analysis and Field Evaluation of the Citrosa Plant "Pelargonium citrosum" as a Repellent Against Populations of Aedes Mosquitoes. Journal of the American Mosquito Control Association, Vol. 12, No. 1, 69-74.]

((*) Oil of Ceylon citronella grass (Cymbopobon nardus) has an average of 14 percent citronellal and 12 percent citronellol. The oil of java citronella grass (C. winterianus) has, on average 2 percent citronellal and 16 percent citronellol. -- Lecture by Tucker, A. and M.J. Maciarello, Is This Plant a Hoax?)

Article copyright American Botanical Council.

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By Ginger Webb