Accompanying the recent interest in mosquitoes and mosquito control is a dose of healthy curiosity concerning the widely advertised mosquito repellent plants. The following article, reprinted from the July/August, 1992 issue of Organic Gardening magazine (page 18), provides one of the clearest summaries of the subject I have seen. Enjoy.--D. R. Lewis
Grow Your Own Skeeter Skeedaddler?
Unless you're just back from a trip to Mars, you've no doubt seen the "citrosa" plant advertised in newspapers and magazines in the United States and Canada as "guaranteed to repel mosquitoes." The supplier claims the plant is a "unique genetic combination " of a scented geranium and citronella grass (which does produce the citronella oil used in mosquito-repelling candles and the like). Many of you have written to us asking if this good news could possibly be true.
Well, We're sorry to report that all the experts we've checked with say "no." In fact, Arthur Tucker, Ph.D.., plant fragrance specialist at Delaware State College in Dover, tells us that chemical analysis of this "miracle plant" revealed that it contains only 0.09 percent citronellal (one of the main ingredients in citronella oil). Some forms of lemon balm beat that by a factor of around 400 (38 percent citronellal), he points out.
"If you want to grow a plant that might help repel mosquitoes, there are several that would probably be better choices," Dr. Tucker says. He adds that G. A. Surgeoner, Ph.D., of the University of Guelph in Ontario compared a potted citrosa plant (just sitting there) and its crushed leaves to DEEP WOODS OFF (which contains the powerful chemical DEET, the active ingredient in most rub-on insect repellents) and found that the citrosa in a pot (as it is shown in the ads touting its alleged repellent powers) has no significant effect against mosquitoes.
Crushed citrosa leaves, however, do offer some protection - they have 30 to 40 percent of the repellency of DEET. But crushed lemon thyme (Thymus X citriodorus) has 62 percent! So lemon thyme should work better than the much-ballyhooed citrosa plant. Which is nice because thyme costs much less and is winter-hardy while the citros is not. (Lemon thyme is available from many herb suppliers, including Companion Plants, 7247 No Coolville Ridge Road, Athens, OH 45701; and Richter's, Goodwood, ONT, Canada LOC 1A0.
The important fact to remember is that no plant - citrosa, lemon thyme or even citronella grass itself (a very hard to find 6- foot-tall tropical plant) will repel skeeters just sitting in a pot. Plants release significant amounts of their repellent oils only when their leaves are crushed, Dr. Tucker says. If you want to go that route, rubbing the crushed leaves on your skin is probably the best way to use these sweet-smelling natural insect repellents. (Be sure and "patch test" yourself for any allergy to these leaves by testing a small amount repeatedly on your inner forearm for a day or so; if there's no irritating skin reaction or redness, it's likely safe for you to rub away!)
This article originally appeared in the May 26, 1993 issue, p. 82.