The castles of medieval and Renaissance Europe must have been rich in sights, sounds and smells. I imagine the inhabitants tossing bones and other scraps for the dogs on the floor, and the heaps that would accumulate. It was probably helpful in those times to own at least two castles so that when “the middens became stinking”, the royalty could move on and the floors could finally be swept out. Just imagine the populations of fleas, ticks and lice evicted along with the detritus! The practice of strewing herbs on the floor to repel vermin and freshen the air between cleanings dates back at least to this era. Leaves of sweet flag, flowers of lavender and leafy stems of pennyroyal were among the herbs commonly used for this purpose.
These days, few of us have spare castles to retire to when the vermin take over. We have not only higher standards of sanitation, but also more effective ways of controlling populations of insect pests. Herbs still can play a part, though, particularly as we search for “natural” solutions from the garden. Countless plants have been used throughout the ages or may have potential for use as insect repellents.
Insects and Scents
Insects as well as other arthropods have an extremely acute sensitivity to odors. For example, tiny amounts of chemicals called pheromones produced by an insect can elicit sexual and other behavioral responses from others of its species; a male gypsy moth will react to a single molecule of pheromone from a female, even at a distance of several miles. Insects are likewise capable of detecting chemical scents from plants and other animals.
Insects and plants evolved together, and complex interactions have developed between them. Just as many plants can use odors to attract pollinators, they can also produce scents to ward off insects that might eat them. Paradoxically, certain substances that act as insect attractants at low concentrations will repel the same insects at higher concentrations. Olfactory repellents will work only if in sufficient concentration, especially if they are competing with such strongly attractive odors as that of human sweat. In the final analysis, repellents sometimes do not repel at all but rather counteract attractive odors by either masking them, altering them chemically, absorbing them, or inhibiting their formation. Each herb thus has a constellation of mechanisms by which it may “repel” insects.
The principal component of most leading mosquito and tick repellents sold today is N,N-diethyl-meta-toluamide (DEET). An effective repellent of biting flies, mosquitoes, ticks and chiggers, DEET is recognized as the standard against which other repellents are compared, yet today it carries a warning: it should not be ingested, inhaled or used in contact with skin. DEET has been shown to be toxic to humans and domestic animals, and reactions can be severe, rapid and sometimes fatal. The compound directly affects the central nervous system, causing seizures and comas; children are particularly susceptible to its effects in concentrated formulations.
Turning away from the laboratory to the plant world for alternatives gives us few definitive answers; much is still uncharted territory. Volumes of anecdotal evidence and historical accounts exist on the repellent properties of many herbs, but few well-designed scientific experiments have been performed to test these claims. Research on this subject, much of it done in Third World countries, often has been only preliminary or has yielded inconclusive results, or the statistics don’t hold up to today’s standards. However, evidence suggests that herbal repellency is a subject offering promise for future research.
Please remember that herbs carry their own set of dangers and warnings. “Natural” does not mean “harmless”. I have several shelves of books about plants that are poisonous on ingestion or contact with skin. Herbal preparations should not be consumed or sprayed directly on skin unless they’re known to be safe. If applied to clothes instead of bodies, the repellency lasts longer anyway.
When asked what herbal insect repellents I can recommend, I must say that I’m a conservative: I stick to those plants and chemicals that are generally recognized as safe by the U.S. Food and Drug Administration. If they’re OK to eat, then I feel safe applying them to my skin and clothing. I avoid using, either internally or externally, any plants that are questionable. At the same time, I’m not willing to enhance my own body odor with some stinky formulation, bugs or no bugs.
For The Herb Companion, I have devised an herbal formula designed to repel mosquitoes, ticks, flies and fleas. It is a fragrant combination of the oils of basil, juniper, palmarosa, citronella, rose geranium, rosemary, myrrh, cedarwood, pine and lemon mixed into a base of grain alcohol. The formula is based on personal experience and a review of the scientific literature on insect repellency; its effectiveness has not been extensively tested. I’d be interested to know about readers’ experiences with this or any other natural substances they find useful as insect repellents.
Let’s look at these herbs, as well as other possibilities from the garden, and how we might put them to use against pests.
Mosquitoes are the most thoroughly studied insects in the search for repellents because these bloodsuckers transmit numerous diseases, including malaria and yellow fever. Citronella (Cymbopogon nardus), several species of basil and juniper have been shown to be effective against mosquitoes. Rose geranium, rosemary and several species of cedar all have shown promise in preliminary testing.
The oil of citronella, a fragrant grass of southern Asia, has long been used as an insect repellent and is often sold in candles that are burned on the patio in hopes of driving away mosquitoes in the evening. I use citronella candles myself, although I also question how effective they are: it seems to me that year after year and dollar after dollar I still scratch as much. A more effective way to use the repellency of citronella is to apply the oil to skin or clothing. The biggest drawback to the use of citronella oil on skin, however, is that the repellency fades fairly rapidly as the oil evaporates, and so it must be reapplied often. One experiment showed the maximum protection time of citronella oil against the yellow-fever mosquito to be only 1 hour and 18 minutes.
Basils (Ocimum spp.) can be used as mosquito repellents by either crushing the leaves or applying the oil. The species found to be most effective include common sweet basil (O. basilicum), the sacred basil of India (O. tenuiflorum, formerly O. sanctum), and tree basil (O. gratissimum, formerly O. suave). Oil of juniper (Juniperus communis) can also fight off two mosquito species.
Another alternative is mugwort (Artemisia vulgaris), one of the most ancient of insect repellents. Europeans of the Middle Ages believed that it possessed wondrous virtues for fending off all manner of wild beasts and evil spirits. Since the time of Dioscorides in the 1st century A.D., this herb has been regarded as useful against moths. Today we know that mugwort is a repellent against the yellow-fever mosquito. I’ve not used it myself, but scientific studies indicate that mugwort can be effective either crushed onto clothing, applied as an oil on clothing, or burned as a fumigant.
Tansy is another traditional insect-repellent herb; growing it near the doorway of a home to discourage flies from entering is an old custom. If you can find its essential oil, by all means add it to my herbal insect repellant recipe. I omitted it from the formula only because it does not seem to be as readily available as the other oils, but evidence suggests that tansy’s reputation for repellency is earned.
An extract of sweet flag (Acorus calamus), especially in combination with an extract of turmeric (Curcuma longa) or pine (Pinus sp.), has been effective against yellow-fever mosquitoes. Many other plants have shown promise against mosquitoes; among them are yarrow (Achillea millefolium), sweet Annie, hyssop, German chamomile, bog myrtle (Myrica gale), lantana, marsh tea (Ledum palustre), sassafras, and sandalwood.
The ingredients of my herbal formula that might shoo off the common housefly are palmarosa, citronella, juniper and rose geranium. Preliminary tests have demonstrated some effectiveness in repelling various flies.
Other reported fly repellents include the oil of roots of sweet flag, extracts of the twigs of Kenyan myrrh (Commiphora boiviniana), and extracts of the roots of Mexican yellow chapote (Sargentia greggii). Other possibilities include bog myrtle, German chamomile, sandalwood, tomato and vetiver.
Apple-of-Peru (Nicandra physalodes) was touted at the turn of the century as the shoo-fly plant. This species, which has escaped in the American tropics but is still sometimes grown as a garden annual, contains chemicals that inhibit feeding in flies, but its ability to repel them has not been established.
The oils of lemon and pine are included in the recipe in hopes that they’ll keep fleas from mistaking me for a dog. For use in flea pillows and bedding for your pets, I recommend the dried leaves of pennyroyal, fleabane and California laurel, mixed together with these two oils.
Lemon oil is rich in the lemon-scented limonene. Limonene is documented as lethal to the cat flea and is sometimes marketed as a “natural” dip. While lemon and other citrus oils are indeed natural, toxic reactions and even deaths have been reported in dipped cats. Most self-respecting cats do not wear clothing, so soak bedding and pillows with lemon oil as a safer alternative to dipping.
Pennyroyal, a creeping perennial mint, is the herb best known for flea repellency. The first-century Roman scholar Pliny noticed its effectiveness against fleas, as did the eighteenth-century Swedish botanist Linnaeus, who named the plant Mentha pulegium, from the Latin word for flea, pulex. Native American Indians rubbed leaves of American pennyroyal (Hedeoma pulegioides) onto their skin as protection from insects. I find no good scientific studies on the repellency of pennyroyal or its primary constituent, pulegone, yet the anecdotal evidence is so rich that I feel they should be explored.
A word of caution about the use of pennyroyal: at least one woman has died from ingesting an ounce of pennyroyal oil, and rats have displayed brain degeneration after being dosed with it. So don’t drink pennyroyal tea or oil, or even apply it to your skin or your pets. Pennyroyal pillows to tuck into your dog’s or cat’s bed have produced no reported problems, however, and may actually work.
Another plant widely recognized as a flea repellent is fleabane (Conyza canadensis). Mrs. M. Grieve, in A Modern Herbal, states that if fleabane is burned, its smoke will drive off fleas and other insects. Culpeper says, “The smell is supposed delightful to insects, and the juice destructive to them.” Gerard, in his Herball, has another idea on how this plant got its name: “I thinke it is rather because the seed doth resemble a flea so much, that it is hard to discern the one from the other.” Chemical studies of the essential oils of fleabane have been published, but I know of none that address its insect repellency.
The Indians of Mendocino County, California historically used California laurel (Umbellularia californica) to repel fleas. The leaves of Boenninghausenia albiflora, a perennial herb of temperate India, have a disagreeable odor and a long reputation as an effective repellent of fleas; Indian scientists who tested the oil against the dog flea report that it works well. Another plant that may be repellent to fleas is vitex (Vitex negundo).
Oh, how I wish that I had a safe repellent of the deer tick, which carries Lyme disease, or a good repellent for the wood tick and the American dog tick, both of which carry Rocky Mountain spotted fever. The fear of these diseases has severely curtailed my field trips in the woods and swamps.
Opopanax or bisabol myrrh (Commiphora erythraea) is the subject of the only good scientific tests on natural tick repellency that I can find. This myrrh of the ancient Egyptians has been recently documented as killing the larvae on contact and repelling the adults of African brown ear tick, deer tick, lone star tick and American dog tick. Does the oil of the more readily available common myrrh (C. myrrha) likewise repel ticks? I put it in my formula because I thought it was worth a try.
Some other herbs indicated in the scientific literature as having tick-repellent qualities include rose geranium, rosemary and California laurel. Further testing is warranted.
When I had an office in a cockroach-infested lab in New Jersey, my favorite nontoxic control was a large jar filled with banana skins, fried pork rinds and other food scraps. I greased the inside edge of the mouth with Vaseline and then placed a small ramp leading up to it. Cockroaches used to literally “walk the plank”, fall in and never get out. I did not know it then, but scientific studies have documented what housewives have always known: that beef broth and oils of banana, sweet orange, apple and pineapple are good attractants for cockroaches. In this case, attracting them accomplishes the same end as repelling them. This type of trap can be effective in the home, particularly in relatively moist places such as under the sink.
If your grandma put bay leaves in her flour canisters, it was because of this herb’s long reputation as an insect repellent. Today, the effectiveness of bay (Laurus nobilis) in repelling cockroaches is well documented. Simply place fresh or dried bay leaves in and around cupboards, especially where they will be brushed and crushed in normal kitchen activity. I use and recommend bay leaf as a cockroach deterrent. Other plants shown to be repellent to cockroaches include Osage orange (Maclura pomifera), Japanese peppermint (Mentha canadensis), Scotch spearmint (M. gracilis) and vetiver.
Anyone who has had a child come home from school with head lice knows the horror of these creepy-crawlies. You wash everyone’s hair, wash all clothing and disinfect the furniture, but that doesn’t stop you from scratching your own head for days on end. Many shampoos contain natural pyrethrins, which do indeed kill the adults, but the eggs, or nits, are usually not affected. Some plants that have shown effectiveness against lice include tomato, rosemary and marsh tea.
What Doesn’t Work
Lemongrass (Cymbopogon citratus) as an applied oil is a poor repellent against both mosquitoes and flies. Likewise, eucalyptus (Eucalyptus globulus) has been shown repeatedly to be ineffective against mosquitoes and flies despite its long use for that purpose in both homemade and commercial preparations. Pterigeron bubakii, a plant that Australian aborigines use to repel fleas, has also proven ineffective in the laboratory.
Ads in the popular press during the past few years have ballyhooed Citrosa or Van Leenii geranium as repelling mosquitoes just by sitting on the patio. I’m sorry to say that scientists have not been able to confirm this claim. If Citrosa leaves are crushed, they exhibit 30 to 40 percent of the repellency of DEET formulations, but crushed lemon thyme (Thymus ¥ citriodorus) has 62 percent repellency. Your money would be more wisely spent on lemon thyme, but in those tropical areas where lemon thyme fails, then crushed Citrosa geranium may offer some benefits.
Art Tucker is a botanist whose herbal specialties include essential oils and perfumery. He is a research professor in the Department of Agriculture and Natural Resources at Delaware State University in Dover.
I highly recommend Common-Sense Pest Control by William Olkowski, Sheila Daar, and Helga Olkowski (Newtown, Connecticut: The Taunton Press, 1991). Also helpful are Glossary of Plant-Derived Insect Deterrents by Martin Jacobson (Boca Raton, Florida: CRC Press, 1990) and Handbook of Plants with Pest-Control Properties by Michael Grainge and Saleem Ahmed (New York: John Wiley & Sons, 1988).
Sources for Essential Oils
• Aromatherapy Seminars, 1830 S. Robertson Blvd., #203, Los Angeles, CA 90035. Brochure free.
• Cameron Park Botanicals, PO Box 19747, Raleigh, NC 27619-9747. Catalog $1.50 (refundable).
• Essential Oil Company, PO Box 206, Lake Oswego, OR 97034. Catalog free.
• Great Lakes Herb Company, PO Box 6713-HC, Minneapolis, MN 55406. Catalog $2.
• Olfactorium, 401 Euclid Ave., Suite 155, Cleveland, OH 44114. Catalog free.
• Quality of Life Associates, 4 Field Rd., Arlington, MA 02174. Brochure free.