Maryland Grows

Post-Labor Day, you may be packing up your beach gear and saying an unofficial goodbye to summer, but at least one of the season’s staples is sure to stick around—mosquitoes. With the hottest days of the year behind us, and pleasant autumn temperatures ahead, you may find yourself spending even more time outdoors with mosquitoes than before.

Adult female mosquitoes can generally continue their activities—flying, feeding, laying eggs—in temperatures above 50˚F. The average September temperature here in Maryland over the past five years (2020–2024) was 70.4˚F. You could even continue to be bitten into October, as the recent five-year average temperature was 59.6˚F. It may not be until November, which had an average temperature of 49.1˚F over the past five years, that you will be rid of mosquitoes for the season.

There are many popular methods to repel mosquitoes, often marketed by stores or trending online, that claim to be supported by science, but that don’t really work. Many of these involve plants, and may be especially tempting for gardeners. Before you succumb to the hype, let’s take a look at some common mosquito repellent myths.

Aromatic plants

There are thousands of articles for gardeners that advise planting basil, beebalm, catnip, fennel, garlic, geraniums, lavender, lemon balm, lemongrass, marigolds, mint, rosemary, sage, and more to keep your yard mosquito-free. The idea is that, because mosquitoes use olfactory cues to find you, a heavily perfumed garden will mask your scent and prevent mosquitoes from detecting your presence. Yet, I have been bitten while watering my lemon balm, which is between my catnip and mint.

This myth persists based on assertions that these plants have been proven to repel mosquitoes—but there is a huge caveat. Scientific studies of the repellency of these plants test essential oils applied directly to the skin, not human proximity to a whole plant. Essential oils, the compounds that give the plant its scent, are extracted through pressing, boiling, distilling, filtering, and/or using solvents on the leaves, and are therefore more potent than the scent of a whole, untouched leaf on a live plant. I have searched and have yet to find a single scientific study that tests the efficacy of plants at repelling mosquitoes simply by growing nearby. Personal experience tells me the scent is not strong enough to mask my presence, especially since mosquitoes use additional cues like carbon dioxide exhalation, heat, and movement.

Essential oils

Many plant essential oils do demonstrate repellency when applied directly to human skin, but not as effectively as EPA-approved repellents containing DEET, picaridin, or oil of lemon eucalyptus (which is actually not an essential oil, but an extract that has been refined and enriched for its active ingredient). This is because mosquitoes have multiple chemical receptors that they use to find you, and essential oils block fewer of those receptors and for a shorter amount of time than DEET, picaridin, or oil of lemon eucalyptus. For example, one study found that participants using DEET did not get a single mosquito bite for six hours, while the complete protection time (no bites) of citronella oil was only 10.5 minutes and that of fennel oil was 8.4 minutes. Though the essential oils continued to show some repellency for a couple hours (fewer bites than using no repellent at all), you have to ask yourself if your goal is to be bitten less, or to not be bitten at all. Be warned that repellents containing only essential oils are not regulated the same way by the EPA, and manufacturers may tout the incomplete protection time on the bottles. One study that tested some commercially available repellents containing cedar, citronella, lemongrass, geranium, and peppermint essential oils found that the complete protection times were all below 20 minutes.

Citronella candles

Speaking of citronella, candles promising to repel mosquitoes rely on the same premise as above: the citronella smell will mask your own. This means the same limitations apply: not all of the mosquitoes’ chemical receptors will be blocked, and cues such as carbon dioxide, heat, and movement will still be available. Perhaps that is why one study found that citronella candles had no effect on mosquito attraction to human test subjects.

So, how to enjoy your time outdoors as the weather cools down? Scientifically-backed methods include wearing loose clothing and/or thicker fabrics that cover your arms, legs, and feet, so that mosquitoes can’t reach your skin; applying EPA-approved repellents; and sitting in the path of a fan, as mosquitoes are not very strong fliers and will be blown off-course by the moving air. The American Mosquito Control Association also recommends mosquito coils and Therma-cell devices that release a synthetic pyrethroid insecticide into the air, which can offer a radius of protection so long as it’s not windy; don’t use these in conjunction with a fan, or you will blow your protection away. Ideally, you will pair these repelling tactics with source reduction to prevent large populations of mosquitoes from sharing your outdoor space in the first place.

By Sarah Rothman, Postdoctoral Fellow and Adjunct Faculty in the Department of Environmental Science and Technology at the University of Maryland. Read more by Sarah.