Several ornamental plants commonly claimed to be mosquito-repellent don’t actually keep mosquitoes away under real-world conditions, a new study shows. Researchers say people should rely on proven contact and spatial repellents for protection and removing standing water to reduce local mosquito abundance. (Photo by Ary Faraji, SLCMAD, Bugwood.org)

By John P. Roche, Ph.D.

Mosquitoes in the U.S. spread dangerous diseases, including West Nile virus, dengue fever, and Zika virus. Some ornamental plants create volatile compounds with repellent qualities, and these plants are sometimes planted in the hope that they will repel mosquitoes and reduce nuisances and the incidence of disease. But do these plants actually repel mosquitoes in natural conditions?

Researchers at the Sacramento-Yolo Mosquito and Vector Control District in Elk Grove, California, tested several ornamental plant species to see if they were effective at repelling mosquitoes. They report their findings in a study published in July in the Journal of Medical Entomology.

Mosquitoes that spread disease in northern California include Culex tarsalis and Culex pipiens, which can spread West Nile virus in California, and the yellow fever mosquito (Aedes aegypti), which can spread dengue fever in California. Cases of locally acquired dengue fever are rare in California, but West Nile virus is more common, with over 8,000 cases and over 400 deaths from West Nile in California since 2003.

Repelling mosquitoes to reduce disease incidence includes the use of contact repellents, which repel mosquitoes that touch the compound, and spatial repellents, which repel mosquitoes with volatile compound aerosols in the air.

Mario Novelo, Ph.D., biologist at the Sacramento-Yolo Mosquito and Vector Control District, and colleagues tested the potential spatial repellency of a variety of ornamental plants. It is already known that some volatile plant compounds can repel mosquitoes in concentrated form. But no one has previously tested if volatile compounds emanating from ornamental plants in practical, natural conditions can repel mosquitoes.

“This study started with a real-life question that comes up at almost every event and outreach activity we attend,” Novelo says. “People want to be proactive about mosquitoes, and they always ask about plants. Surprisingly, there was no definitive study showing whether live plants actually work, yet many vector control districts recommend them or even list them on their websites. I believe knowing is better than assuming, so we set out to test this public concern in the most scientific way we could.”

The study involved lab choice tests on seven plants and then field assays using two of the plants. In the laboratory assays, Novelo and colleagues tested white yarrow (Achillea millefolium), lemongrass (Cymbopogon citratus), lavender (Lavandula angustifolia), prostrate rosemary (Salvia rosmarinus), white sage (Salvia apiana), and Citronella-scented geranium (Pelargonium citrosum). As a reference, they also tested sunflower (Helianthus annuus), which is already known to not contain repellent compounds.

Researchers at the Sacramento-Yolo Mosquito and Vector Control District in Elk Grove, California, used a Y-tube olfactometer device used in their study to evaluate whether several common ornamental plants repel mosquitoes. (Figure originally published in Novelo et al. 2026, Journal of Medical Entomology)

In a Y-shaped device in the laboratory called an olfactometer, the investigators presented test mosquitoes with a choice of orienting to a tube with air blowing past an ornamental plant or orienting to a tube blowing only pure air.

Their laboratory results found that 52.9% of mosquitoes chose pure air over Citronella-scented geranium, 50.0% chose pure air over prostrate rosemary, 47.2% chose pure air over white sage, 40% chose pure air over white yarrow, 40% chose pure air over sunflower, 38.3% chose pure air over lavender, and 30.6% chose pure air over lemongrass. In other words, none of these plant species caused statistically significant mosquito repellency. In fact, lemongrass was a significant attractant of mosquitoes.

Field assays were also conducted in rural habitats and urban habitats over a period of 16 weeks. Because Citronella-scented geranium and prostrate rosemary were chosen the least by the mosquitoes in the laboratory assays (though not showing statistically significant repellency), the investigators tested these two species in the field tests.

Each field location used a BG-Sentinel trap to catch mosquitoes. The different treatments at different locations featured a mosquito trap adjacent to Citrinolla-scented geranium plants, prostrate rosemary plants, or a Thermacell spatial repellent device (emitting metofluthrin, a known and effective mosquito repellent), as well as a lone trap as a control. Traps were maintained each day, and mosquitoes caught in traps were taken to the lab after each 24-hour period.

Researchers at the Sacramento-Yolo Mosquito and Vector Control District in Elk Grove, California, conducted a study to evaluate whether several common ornamental plants repel mosquitoes. They purchased plants at a nursery, where some plants are commonly marketed as natural mosquito deterrents. (Photo by Mario Novelo, Ph.D.)

In a study to evaluate whether several common ornamental plants repel mosquitoes, researchers at the Sacramento-Yolo Mosquito and Vector Control District in Elk Grove, California, conducted field trials in which mosquito traps were placed adjacent to species of plant in question, such as Citronella-scented geranium plants, Pelargonium citrosum, shown here. (Photo by Mario Novelo, Ph.D.)

In a study to evaluate whether several common ornamental plants repel mosquitoes, researchers at the Sacramento-Yolo Mosquito and Vector Control District in Elk Grove, California, conducted field trials in which mosquito traps were placed adjacent to species of plant in question, such as prostrate rosemary plants, Salvia rosmarinus, shown here. (Photo by Mario Novelo, Ph.D.)

In a study to evaluate whether several common ornamental plants repel mosquitoes, researchers at the Sacramento-Yolo Mosquito and Vector Control District in Elk Grove, California, conducted field trials in which mosquito traps were placed adjacent to species of plant in question, such as prostrate rosemary plants, Salvia rosmarinus, shown here being set up by SYMVCD laboratory technician James Brodigan. (Photo by Mario Novelo, Ph.D.)

For both rural and urban habitats, at some locations, the investigators collected mosquitoes amid unmanipulated plants (passive emissions), and in other locations, they collected mosquitoes amid plants that had been mechanically disturbed to try to release more volatile compounds (mechanical damage).

In the field assays, they caught nine species of Culex and Aedes mosquitoes. The results paralleled those of the laboratory assays. In rural habitats, in both passive emission and mechanical damage trials, no significant repellency was observed for either Citronella-scented geranium or prostrate rosemary. Similarly, in urban habitats, in both passive emission and mechanical damage trials, no significant repellency was observed for either Citronella-scented geranium or prostrate rosemary. Importantly, neither Culex species or Aedes species were repelled by either of the plant species. As in the laboratory assays, in all the field assays, metofluthrin showed significant repellency.

From a practical standpoint, this study found that the plant species tested did not repel mosquitoes. “That does not mean people shouldn’t grow them,” Novelo says. “They look good, smell nice, and can attract beneficial insects. It also doesn’t mean their concentrated essential oils are ineffective, because those can work. But when it comes to protection from mosquitoes, people should rely on EPA-registered repellents and on removing standing water around their homes.”

EPA-registered repellents include DEET, citronella, and lemon eucalyptus oil. Metofluthrin, the repellent used in this study as a control, is also highly effective and is EPA registered.

Future research on this question could examine potential repellent effects of other ornamental plants species, or the repellent effects of formulations from the species tested by this study when they are made in stronger concentrations. In the meantime, enjoy your geraniums—but don’t expect them to repel mosquitoes.

John P. Roche, Ph.D., is an author, biologist, and science writer with a Ph.D. and postdoctoral fellowship in the biological sciences and a dedication to making rigorous science clear and accessible. He authors books and articles, and writes for universities, scientific societies, and publishers. Professional experience includes serving as a scientist and scientific writer at Indiana University, Boston College, and the UMass Chan Medical School; and as editor of science periodicals at Indiana University and Boston College.