Every summer, millions of people reach for a bottle of DEET-based bug spray before stepping outside — trusting that familiar chemical smell to keep mosquitoes at bay. It is a ritual built on decades of confidence in one simple idea: mosquitoes hate this stuff.
But a new study is complicating that picture. Under controlled lab conditions, mosquitoes did not just tolerate the smell of DEET — they chased it. The findings raise quiet but pointed questions about assumptions we have long taken for granted.
A repellent that became a lure
Researchers at the University of Tours worked with lab-bred Aedes aegypti mosquitoes to see whether their response to DEET could be reshaped through conditioning. The method was straightforward: a bag of warm sheep’s blood was introduced into the mosquitoes’ enclosure for 30 seconds, and during the final ten seconds of that feeding window, DEET was wafted in. The process was repeated four times.
After just four conditioning rounds, more than 60 percent of trained mosquitoes attempted to feed when exposed to DEET alone — with no blood present.
Researchers then moved to a live test. Study co-author Ayelén Nally, a chemical ecologist at the University of Buenos Aires, offered both hands to the trained mosquitoes — one coated in DEET, one bare. Roughly 60 percent of the conditioned insects went for the repellent-covered hand. The team also found that mosquitoes could learn to associate DEET with sugary food, not just blood, suggesting the conditioning effect was not tied to any single kind of reward.
What scientists thought they knew about DEET
For decades, the working assumption was simple: DEET worked because of its chemical properties. Either it was toxic or unpleasant to mosquitoes, or it interfered with their ability to detect human scent. Mosquitoes stayed away.
Study co-author Claudio Lazzari, a behavioral physiologist at the University of Tours, explains that the insects’ reactions to repellents are more malleable than previously assumed. The new findings build on a growing body of research suggesting mosquito behavior around repellents is not a fixed, hardwired response — and this study goes further than earlier work.
It is the first to demonstrate that DEET can actually draw mosquitoes in — not just fail to repel them, but actively attract them — under conditioned circumstances. That distinction matters. It shifts the conversation from “DEET sometimes loses effectiveness” to “DEET can become a signal mosquitoes seek out.”
Why Aedes aegypti makes this especially important
The species at the center of this study is not a minor player. Aedes aegypti transmits the viruses responsible for yellow fever, dengue, chikungunya, and Zika — and mosquitoes as a group kill more than one million people worldwide each year through the pathogens they carry.
Research published earlier this year found that mosquitoes have been biting humans for over one million years, thought to have evolved a preference for human blood around the time Homo erectus migrated into Southeast Asia. DEET has long been one of our most reliable tools for managing that ancient, dangerous relationship. The study does not challenge that reliability — but it does suggest our understanding of how the tool works, and how it can fail, needs updating.
When repellent starts to wear off, the risk goes up
Experts who reviewed the study point to a specific window of concern. The conditioning effect is most likely to form not when DEET is freshly applied and at full concentration, but when it begins to fade — that is when mosquitoes may first encounter the smell alongside a blood meal, and when the association can take hold.
Nina Stanczyk, a chemical ecologist at ETH Zurich who was not involved in the study, notes that the most important practical takeaway is to reapply repellent regularly, as directed on product labels. The risk is not in using DEET. It is in using it once and assuming it will last all day.
Study co-author Clément Vinauger of Virginia Tech is direct on this point. DEET remains “the gold standard in terms of protection,” he says, and should not be abandoned. The findings are a reason to use it correctly, not to stop using it.
What this means — and what we still don’t know
Scientists still do not fully understand the mechanism by which DEET repels mosquitoes in the first place. That gap makes the new findings harder to interpret — and more urgent to follow up on.
Medical entomologist Leon Hugo of the University of Queensland, who was not part of the study, writes that the research “raises the prospect mosquitoes may actually be attracted towards DEET in some cases” — not just in the lab, but potentially in real-world scenarios where repeated exposure occurs. Insect behavior, it turns out, is shaped by learning and experience, not chemistry alone. Whether wild mosquito populations exposed repeatedly to DEET in the field could develop similar conditioned responses remains unknown — but if they can, guidance around repellent application, product design, and public health recommendations may all need to shift alongside that understanding.