Mosquito wars: what works to keep these little buggers away?

It's summer time, and with it comes outdoor dining, sports, and strolls through the neighborhood. But the loveliest evening can be spoiled by mosquitos, who torment us as they suck our blood and leave itchy welts behind.

How can we keep these pests away? Do citronella candles work? How about Victoria's Secret Bombshell perfume? (No, I'm not kidding. Read on.)

First, about mosquitos: they are more than just a nuisance. They also carry diseases, including West Nile virus, which now affects the entire U.S., and far more deadly diseases in other countries, including malaria, yellow fever, and Zika virus.

(Aside: West Nile virus first appeared in the U.S. in 1999, in New York, after a mosquito apparently hitched a ride on a plane from somewhere in the Middle East. A few years later it started spreading rapidly across the country, and now it's basically everywhere. West Nile fever usually manifests as a flu-like illness, but about 1 in 150 people get severe, life-threatening symptoms.)

One of the most popular ways to keep mosquitos off the backyard patio is easy to spot on a summer night in my neighborhood: burning citronella candles (such as this one from Cutter), which contain a natural oil made from grass whose scent is supposed to repel mosquitos. These are very popular and widely sold, but do they work?

Fortunately, the Journal of Insect Science published a study just last year, by Stacy Rodriguez and colleagues from the University of New Mexico, that gives us an answer. The scientists purchased a dozen different products, all from Amazon or local stores, and ran a very nice experiment to figure out how well each product worked.

Here's the setup: the scientists placed a cage full of mosquitos near a human volunteer, who sat 1-3 meters away, with a gentle breeze blowing from the humans towards the mosquitos. (This made it easier for the mosquitos to smell the humans, and also meant that they had to fly against the breeze if they wanted to bite the subjects.) The scientists applied each mosquito repellent (or device) to the volunteers, opened the cage, and counted how many mosquitos were attracted. They also ran controls where the subject had no protection.

So what worked? First off, with no protection, about 88% of the mosquitos were attracted to the human subjects. The three products that worked best at repelling mosquitos were:

1. OFF!® Clip On™, where just 27% of the mosquitos were attracted
2. Cutter® Lemon Eucalyptus, with 30%, and
3. Ben’s® Tick & Insect Repellent, with 34%. 

Nothing else worked nearly as well as these, although several products reduced the proportion of mosquitos from 88% down to 60-70%.

Notably, some of the products did not work at all, including citronella: Cutter Citro Guard had no effect on the mosquito's attraction to the human volunteers. Other failures were Invisaband™ and Mosquitavert, wrist bracelets containing geraniol oil, and the PIC® Personal Sonic Mosquito Repeller, a clip-on ultrasonic device that emits a sound that mosquitos presumably don't like. Mosquitos basically ignored these devices.

So what's the secret in the products that do work? The OFF! device contains metofluthrin, which appears to be the most effective repellent on the market. Lemon eucalyptus oil is a natural product that is nearly as effective, and Ben's Tick & Insect Repellent contains DEET, which has long been known as an effective defense against mosquitos.

The only one of these products that purports to work on a whole area (like your backyard patio) rather than just one person is the citronella candle, which unfortunately just doesn't work. So if you want your garden party guests to be protected, you may have to keep on hand a basket full of products with metofluthrin, lemon eukalyptus oil, or DEET.

And what about Victoria's Secret Bombshell perfume? Well, the same scientists looked at Bombshell in a 2015 study, where they included the perfume thinking that it would attract (rather than repel) mosquitos. Surprisingly, it had the opposite effect: even though DEET and metofluthrin are more effective, the scientist found that:

"Victoria Secret Bombshell repelled mosquitoes quite effectively 120 min post application."

And what is the active ingredient? According to the same study, that's unknown.