Mosquito Repellant Plants? Caveat Emptor

mosquitoI sometimes wonder why God made mosquitoes. They’re so… annoying! No one enjoys getting bitten. It’s more than just the never-ending itch—they carry some very nasty diseases such as malaria, yellow fever, and chikungunya. Even here in the U.S. we’re at risk of West Nile, dengue, viral encephalitis, and now zika.

Lavendula - Lavender @DianeBrunjes-GlenEagle 22july05 LAH 003rMy Facebook feed is suddenly overflowing with lists of plants that supposedly repel mosquitoes. Just plant these flowers and herbs and your yard will be pest-free! Or, as one post on Pintrest claimed, “Plant a Mosquito Control container so you can sit and unwind in the evenings.” Unfortunately, anything that simple raises a red flag for me. Can controlling mosquitoes really be as easy as planting marigolds and lavender? It took some research to get past the hype, but I eventually found some scientific studies that look at this question.

The first question is, how do these blood-suckers find us? I’ve known for years that mosquitoes are attracted to the carbon dioxide we exhale, but it turns out that that’s only part of the story. They also can smell compounds such as cholesterol (on our skin, not in our blood), and uric and other acids. Plus they’re attracted to movement and heat.

Some plants—wild sage (Latana camara), thyme, lemon thyme, catnip, marigolds—contain oils with strong scents that might mask these body odors. In most cases, these scents don’t scare the buggers off, they just overwhelm the mosquitoes’ sensitive sense of smell. And of course they won’t disguise your body heat, or the fact that you’re alive and moving.

Artemisia versicolor 'Sea Foam'_XG_20090826_LAH_9736A few more plants, such as wormwood (Artemisia, left), santolina, tansy, and lavender, contain oils that actively repel mosquitoes. (Other plants may actually attract mosquitoes. And note that “citronella geranium” isn’t at all the same plant as the grass that citronella comes from, and it does nothing to deter mosquitoes. In fact, they happily land on it.)

Thymus - Thyme @CSUtilXeriscapeGarden 9Aug2006 LAH203rHowever—and this is a big however!—these plants don’t release their oils until their leaves are brushed or crushed, or even better, burned. In other words, you’d have to mangle your geraniums and thyme (right) in order for them to work as a scent screen. (The one exception I found to this rule is basil, which emits its scent even if untouched.) Even then, the scent doesn’t travel very far, and the slightest breeze will waft it away.

Moreover, different species of mosquito respond differently. Most research is aimed at thwarting the mosquito that carries malaria, Anopheles gambiae, and may not apply to your local pests. (Here in Colorado we contend with Aedes vexans and Culex tarsalis.) The studies were looking for a low-cost, effective way to protect people who don’t have access to DEET or window screens from mosquito-borne diseases.

It’s probably not a good idea to go burning your catnip patch just so you can enjoy an evening outside, but what about citronella candles? Do they work? A study done in Ontario, Canada, revealed that subjects sitting next to citronella candles received just over one bite per minute, while those without any candles received two bites per minute. So yes, they work. Sort of. I don’t know about you, but I’m not happy getting “only” one bite per minute—I don’t want any bites!

Colorado State Univ. Field Day

But what about all those lovely lists of herbs and flowers that supposedly keep your patio pest-free? Should you surround yourself with ageratum (aka floss flower, left), basil, bee balm, catmint, catnip, eucalyptus, garlic and onions, lavender, lemon balm, lemon grass, marigolds, mint, pineapple weed, pennyroyal, and rosemary? Sure, if you want to. Go ahead and enjoy the flowers, but don’t expect them to keep the mosquitoes away.

So what can we do? Find out next month!

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