The Citronella Geranium in the Garden Bed Isn't Repelling a Single Mosquito

Nurseries sell them as mosquito plants. Vector control district websites have listed them as helpful. Homeowners plant lemongrass, citronella geranium and rosemary around patios every spring on the assumption that something in the leaves pushes mosquitoes away.

A study published in the Journal of Medical Entomology put seven of those plants in front of mosquitoes under laboratory and field conditions for sixteen weeks. Not one of them repelled anything. Lemongrass, the most heavily marketed of the group, actually pulled mosquitoes in.

The work came out of the Sacramento-Yolo Mosquito and Vector Control District in Elk Grove, California, where biologist Mario Novelo and colleagues kept running into the same question at outreach events. "People want to be proactive about mosquitoes, and they always ask about plants," he said. "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."

What the olfactometer showed

In the lab, mosquitoes were offered a choice in a Y-tube device: air that had passed over a plant, or plain air. White yarrow, lemongrass, lavender, prostrate rosemary, white sage, citronella-scented geranium and — as a known negative control — sunflower.

The numbers came back flat. Just over half of the mosquitoes preferred plain air to citronella geranium. Half preferred plain air to rosemary. White sage drew 47%. Lavender 38%. Lemongrass mustered only 31% choosing the plant side, which made it a statistically significant attractant rather than a repellent.

Then came the part that matters more than a lab rig. Over sixteen weeks, in both rural and urban habitat, the researchers set BG-Sentinel traps next to citronella geranium, next to prostrate rosemary, and next to a Thermacell device emitting metofluthrin, with a bare trap as the control. They ran the plants two ways — undisturbed, and mechanically damaged to force more volatiles into the air.

Nine species of Culex and Aedes were caught across the sites. Neither the geranium nor the rosemary reduced catches in any configuration, and the effect held for both genera. The metofluthrin device did suppress catches, which is how the researchers knew the setup could detect a real repellent if one were present.

Why the plant still isn't the problem

Novelo is careful not to tell anyone to rip out the garden. "That does not mean people shouldn't grow them. They look good, smell nice, and can attract beneficial insects," he said. "It also doesn't mean their concentrated essential oils are ineffective, because those can work."

That distinction is the whole finding. Volatile compounds from these species can repel mosquitoes when they are concentrated and formulated into a product. A living plant releases them at a fraction of that dose, across a footprint measured in inches, in whatever direction the wind happens to blow. A lemon eucalyptus or citronella product that carries an EPA registration has been tested at the concentration it is sold at. A shrub has not been tested at all, because until now nobody had.

What does hold up, per the study's authors: EPA-registered repellents — DEET, picaridin, oil of lemon eucalyptus and IR3535 among them — and the unglamorous work of dumping standing water.

Texas is still in the middle of the season

That advice is not academic in Texas this month. The state's week 40 arbovirus report, covering the week ending Oct. 10 and published Oct. 13, counted 1,389 West Nile-positive mosquito pools, 80 human West Nile cases — 16 of them fever, 64 neuroinvasive — and four deaths. The presumptive viremic blood donor count stood at 28.

The county table is dominated by the big metros: Harris with 413 positive pools, Dallas 188, Montgomery 162, Fort Bend 161, Tarrant 159. Trapping and spraying programs are still running in most of them, and the season historically stretches into November before a hard freeze shuts it down.

Which leaves a straightforward calculation for anyone who planted a citronella geranium this spring and skipped the repellent. The plant is fine. It just isn't doing the job it was sold to do.