Old unproven ultrasonic repellant emerges in a new form
Wouldn't it be the coolest thing ever if you could download an app to your cell phone that would turn your phone into a safe, free mosquito-repelling machine? That's what some apps are claiming. The idea sounds reasonable. The app emits a sound that only mosquitoes can hear, but mosquitoes hate it, so they stay away.
The problem is that the idea isn't new or untested. In fact, ultrasonic repellants that claim to work on the same principal have been around for decades. Before smart phone apps, the sound was emitted from plug-in or battery-powered electronic devices. Those are still on the market too. Radio stations have even tried broadcasting the sound. So the devices have been tested numerous times, in various forms. And guess what? They fail the tests. Sometimes, they even attract more mosquitoes.
If the idea is such a failure, why didn't such devices disappear years ago? Well, some of the manufacturers offer detailed explanations that seem as if the idea ought to work, so people are willing to give it a try, especially if the device only costs a few dollars. Also, mosquito activity is so variable, depending on weather, location, and other factors like odors of perfume or sweat, that people can attribute fewer bites to turning on the sound, when the real reason may be something else and they'd have fewer bites anyway. There's also the possibility that some sound does actually repel some mosquitoes, but the right combination of sound, species, season, or whatever, never occurred during the actual tests. If that's the case, though, you may never hit upon the right combination either.
You can read reviews and decide for yourself whether the devices are actually working for those who say they do, or whether it's coincidence, or whether they've stumbled on some perfect combination that has eluded testers.Mosquitoes can hear, but...
First, let's get one objection out of the way. Mosquitoes don't have ears, but they can hear. If you're worried that your speaker isn't good enough, the sound might not be truly ultrasonic, so even an ordinary cell phone speaker can probably play it. Depending on the particular app, the sound is probably right at the edge of human hearing, so children or those with excellent ears might even be able to hear it, although adults might not. One reviewer said the "kids went crazy" when he played it in a classroom.
Ultra-sonic repellants usually make one of two claims: either it's the noise of dragonfly wings (or sometimes bats), which frighten off mosquitoes because dragonflies and bats are mosquito predators. Or it's the sound of a male mosquito calling for a mate, which will keep biting females away because "once a female mosquito has her eggs fertilized she will actively avoid any further contact with the male," as this app explains.
The problem is that there's no evidence female mosquitoes avoid males after they've mated. In fact, "research has shown that male mosquitoes are actually the ones attracted by the female flight sound and females normally have a very weak sensitivity for sound compared with the males." As those who've frequented a swampy area or pond in the daytime and been annoyed by mosquitoes while watching dragonflies darting about, "mosquitoes do not vacate an area hunted by dragonflies," as a Rutgers entemologist noted.Failed tests
But that doesn't matter if the anti-mosquito apps, or other ultrasonic repellants, actually work due to some other unknown reason. The problem is that in tests, they consistently don't.
One controlled trial tested three different frequencies from 3 to 11 kHz over eighteen nights in nine pairs of houses. No one in each pair houses knew whether the ultrasound was playing in their house, until the test was over. "A total of 7485 mosquitoes (10% Anopheles, 62% Culex, 27% Mansonia and 1% Aedes) were caught, 23 per house per night. There was no significant difference in landing rate between the houses with ultrasound device and the houses with placebo for any species of mosquito." The authors concluded, "The ultrasound device used was not effective against mosquitoes in this strictly controlled trial."
Another test, of frequencies between 20 and 70 kHz, put four species of mosquitoes in a flight chamber, to see if they would fly toward the source of the sound to get to a bait that imitated the smell of human breath and skin. The result? "For all species there was no significant difference between the numbers trapped when the devices were switched on or off, when all devices were tested simultaneously."
The most surprising result came from a test in which researchers counted the number of times Aedes aegypti mosquitoes tried to bite in a test chamber, during a three-minute period, either with the ultrasonic tone on or off. "The mosquito biting rates for five sound frequencies (ranging from 9.6 kHz to 18.2 kHz) initially demonstrated a significant increase." The biting rate decreased when the tone was turned off, and increased by about one-third again when an 11.8 kHz tone was turned back on.
Was that test just a unique reaction? Perhaps so, but the results of 15 completely different tests on the landing rates of mosquitoes done by different researchers, showed a landing rate equal or greater when the ultrasonic tone was turned on, in 12 of them. Some of the difference was only a few more landings out of a hundred, not significant. But geez, you'd expect the landing rates to be significantly lower if the ultrasonic repellants worked, not the same or higher.
Back in 2001, the FTC warned manufacturers that they needed to be careful making claims about ultrasonic insect and pest repellants without evidence. If such repellants worked, you'd expect manufacturers to be adding summaries of the scientific studies that showed how effective they were, but the claims still tend to be general ones, with cautions such as the devices won't work for all mosquitoes, or you need to experiment to find the right frequency.Why not try?
So is buying an ultrasonic repellant worth it? I'd say no. A free app? Well, if it doesn't place anything harmful on your phone, it won't cost anything to try, but so far, there's no evidence to indicate it will work. However, there are a couple of things to keep in mind. If the sound is at the range of human hearing, it might be audible--and very annoying--to babies or pets, who can't say "Turn that darn thing off!" the way older kids could, so make sure they don't seem upset. And also don't assume it's protecting babies or young children, who may not mention they're being bitten.
A cheap, completely non-toxic, perfectly safe, non-chemical mosquito repellant would be wonderful. Cell phone apps probably aren't the answer now, though. The closest thing that probably does exist and is proven to work, is mosquito netting, and it's available in various forms, everything from clothing to baby stroller covers. Netting is safe, effective, and non-toxic, but still not as cool and convenient as an anti-mosquito app would be, if only ultrasonic repellants worked.
 A. Enayati, J. Hemingway and P. Garner, "Electronic Mosquito Repellents for Preventing Mosquito Bites and Malaria Infection (Review)," The Cochrane Library (3:2010), 4.
 Ibid., 8.