A groundbreaking new experimental study published Thursday in the *Journal of Experimental Biology* has upended long-held assumptions about how the world’s most widely used insect repellent, DEET, works against disease-carrying mosquitoes. The research, led by an international team of scientists, demonstrates that mosquitoes can be conditioned to associate the scent of DEET with the promise of a food source — even learning to prioritize biting human skin that has been treated with the repellent.
Lead researcher Claudio Lazzari, an emeritus professor at the University of Tours’ Insect Biology Research Institute in France, was quick to clarify that these surprising lab results, collected under highly controlled experimental conditions, do not undermine DEET’s proven track record as the global gold standard for mosquito bite prevention. Developed in the 1940s in the United States, DEET has been credited with saving millions of lives by reducing transmission of life-threatening mosquito-borne illnesses, and it remains the go-to repellent recommended by the World Health Organization for disease control.
Despite DEET’s success, Lazzari noted that the scientific community has long lacked a full understanding of why the compound deters mosquitoes in the first place. Researchers have debated for decades whether DEET is toxic to the blood-feeding insects, blocks their ability to track human scent, or simply produces an odor that mosquitoes find naturally unpleasant. To answer this question, the team turned to a classic conditioning experiment modeled after Ivan Pavlov’s famous dog studies, which first demonstrated how animals can learn to link unrelated stimuli to rewards.
For their tests, the researchers focused on *Aedes aegypti* mosquitoes — the species responsible for spreading dangerous pathogens including dengue fever, Zika, yellow fever, and chikungunya. In initial trials, mosquitoes were placed in mesh enclosures and presented with warm sheep’s blood, which the insects immediately fed on, as expected. When DEET scent was introduced alone, mosquitoes predictably moved away from the food source, consistent with DEET’s repellent properties.
The conditioning phase followed: over three repeated trials, scientists gave mosquitoes access to warm blood for 20 seconds, releasing DEET scent during the final 10 seconds of feeding. After this training, mosquitoes were re-exposed to DEET scent with no blood present. Astonishingly, more than 60% of the trained mosquitoes still attempted to bite the test fabric, conditioned to link the DEET scent with food. To confirm the result, a researcher offered the trained mosquitoes a choice between two bare hands: one untreated, and one covered in DEET. The insects overwhelmingly chose the DEET-coated hand. The team repeated the experiment using sugar instead of blood (to reflect mosquitoes’ natural diet of plant nectar in the wild) and observed the same outcome.
Co-author Clement Vinauger, a researcher at Virginia Tech in the United States, explained that the study challenges the long-standing paradigm that DEET repels mosquitoes purely through its inherent chemical properties. “What we are showing is that the mosquito’s brain can rewrite that response based on experience,” Vinauger said. “What the insect has learned matters just as much as what the chemical does. That, I think, is a paradigm shift.” Lazzari added that the repellent effect does not stem from DEET being toxic to mosquitoes; instead, mosquitoes’ innate avoidance of the compound is a learned response to how their brains interpret the chemical scent, which can be reshaped by experience.
Lazzari emphasized that this learned preference for DEET is extremely unlikely to occur in natural settings, as the specific controlled conditions of the lab experiment are not easily replicated in the wild. He reiterated that the public should continue to follow product instructions for DEET-based repellents, which remain a critical tool for preventing mosquito-borne disease. The findings, however, point to an urgent need for continued research into new repellents that are more effective, more eco-friendly, and less likely to trigger allergic reactions in human users, building on this new understanding of mosquito behavior.