A new study in the Journal of Insect Science concludes “many of the products … tested that were marketed as repellents do not reduce mosquito attraction to humans.” Sprays with DEET or PMD rated as most effective in repelling the Aedes aegypti mosquito.

A search for “mosquito repellent” on Amazon.com delivers more than 28,000 product results. For a regular consumer, it can be difficult to find the ones that truly work among a sea of products that make bold claims.

Researchers at the Molecular Vector Physiology Laboratory at New Mexico State University are working to make the search for the best mosquito repellents a little easier. And their latest study offers some clear guidance: Spray-on repellents containing DEET or PMD (oil of lemon eucalyptus) are the way to go. Several “wearable” devices such as bracelets and sonic repellers, as well as a candle, were all found to be quite ineffective compared to the sprays. Their study is published online this week in the Journal of Insect Science.

“These findings are extremely important for consumers because they need to be aware that there are mosquito repellent products available that are ineffective,” says Stacy Rodriguez, laboratory manager at the Molecular Vector Physiology Laboratory at NMSU. “While the labels of many products make strong claims, some products simply don’t work.”

You might remember the NMSU team from their 2015 study comparing “natural” repellents to DEET, which went viral for its finding that a popular brand of perfume worked fairly well as a repellent, too. This time around, Rodriguez and her colleagues devised a new experiment setup to control a multitude of variables including wind speed in testing each repellent’s effect on the Aedes aegypti mosquito’s attraction to a human.

The study used a wind tunnel in which a human subject with repellent was placed upwind of a three-part screened cage containing mosquitoes, whose movements toward or away from the test subject were measured after a 15-minute test period.

For each test, a human wearing (or, in the case of the candle, sitting next to) a repellent sat in a wind tunnel, upwind from a three-compartment, screened cage containing mosquitoes, at a distance of 1 meter or 3 meters. A steady 2-meters-per-second breeze in the tunnel carried the subject’s scent toward the mosquitoes, which began the test in the center compartment of the cage. After 15 minutes, during which the mosquitoes were free to roam into either the compartment closer to the human or further away, the doors between compartments were closed and the researchers counted the mosquitoes in each compartment.

“This is a pretty effective test to evaluate attraction,” Rodriguez says. “It is very accurate and can be done without having the subject bitten by the mosquitoes. The wind tunnel allowed us to standardize the wind speed that carried the scents from the human bait to the mosquito. Erratic wind patterns can lead to distorted and hard to interpret results in such experiments. Our wind tunnel data turned out to be highly replicable.”

They study tested five wearable devices (OFF!^® Clip-On, PIC^® Personal Sonic Mosquito Repeller, Mosquitavert^® Repellent Bracelet, Mosquito-No!^™ Repellent Bracelet, and InvisabandTM), one candle (Cutter^® Citro Guard), and five sprays (Cutter^® Lemon Eucalyptus, All Terrain^® Kids Herbal Armor^™, Avon^® Skin-So-Soft Bug Guard Plus Picaridin, Repel^® Sportsmen Max Formula^®, and Ben’s^® Tick & Insect Repellent).

The only wearable device that fared well in the study was OFF!^® Clip-On, which features a nebulizer to vaporize its repellent chemical, Metofluthrin. The sonic repeller and bracelets showed no significant reduction in mosquito attraction.

“Although the active ingredients in some bracelets may be mosquito repellents, we hypothesize that the concentrations that are emitted by all of the bracelets that we tested were too low to have an effect,” the researchers note in the study.

The five spray-on repellents tested showed significant, though varying, levels of reduction in mosquito attraction in the test. Cutter^® Lemon Eucalyptus (30 percent oil of lemon eucalyptus, known by its chemical acronym, PMD) and Ben’s^® Tick & Insect Repellent (98 percent DEET) were the most effective. “This finding confirms the findings of several other studies that found DEET and PMD the most effective and longest lasting mosquito repellents currently available,” the researchers write.

Rodriguez and her colleagues say consumers should seek out the most effective repellents to avoid mosquito bites. “At a time where vector-borne disease like Zika is a real threat, the most egregious danger to the consumer is the false comfort that some repellents give them protection against Ae. aegypti when they actually offer none,” they write.

What’s next for the researchers at the NMSU Molecular Vector Physiology Laboratory? Says Rodriguez: “We are always interested in testing products and we will probably test more in the future. We are also interested in creating sticky glue traps for mosquitoes similar to fly traps.”