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Mosquitoes are masters of speed and subtlety. Rarely do we catch one on our skin mid-bite. And when we do, the damage is often already done: the blood has been sucked, the itch-inducing saliva has been secreted, and, in some cases, a disease has already been transmitted.

This elusiveness means we have to rely on insect repellents for the best chance of evading bites. Not all mosquito-deterring products are created equal, though, says Dina Fonseca, a molecular ecologist and chair of the department of ecology at Rutgers University. Store shelves may be lined with a plethora of sprays and gadgets such as bracelets, candles, and bug zappers purported to ward off the insects, but Fonseca and other experts say only those with a few key active ingredients are effective. And as greenhouse gas emissions drive up global temperatures and make already mosquito-prone habitats even buggier, repellents may become increasingly essential.

Scientific American spoke with experts in mosquito behavior and avoidance about which products are worth the purchase and how they protect us from the pests.

What essential active ingredients should people look for in bug sprays?

Despite the vast number of brands and concoctions on the market in the U.S., only those containing Environmental Protection Agency–registered active ingredients such as diethyl toluamide (DEET), p-Menthane-3,8-diol (PMD) or icaridin (also known as picaridin) have been scientifically proved effective. Spray based on DEET, however, have long been considered supreme, Fonseca says. “It is the oldest tried-and-true gold standard among all of the repellents,” she says. “Back when I was a graduate student doing field work in a mosquito-filled bog, DEET was my best friend.”

The U.S. Department of Agriculture developed this synthetic chemical in 1946 for Army personnel deployed to mosquito-infested environments. At that time, it was used in a spray—nicknamed “bug juice”—that was 75 percent DEET and 25 percent ethanol. Bug juice irritated skin with scratches or cuts, and it held mosquitoes off for only a couple of hours. But later, the USDA and Army produced a new, longer-lasting, and gentler formula called extended duration topical insect/arthropod repellent (EDTIAR), which had no ethanol and contained just 33 percent DEET. It also included polymers that thickened the spray to help slow its evaporation—and it remains the go-to repellent for today’s military.

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