Mosquitoes Can Be Trained to Treat DEET as an Invitation to Bite

Imagine studying hard for a test, acing it, and then finding out the rules of the game have quietly changed. That is a little bit what scientists are dealing with when it comes to mosquitoes and DEET bug spray. A new study published in the Journal of Experimental Biology has found that mosquitoes can actually be trained to associate the smell of DEET with food, potentially turning one of humanity's most reliable insect repellents into something that attracts them instead of driving them away. It is a discovery that has real consequences for the hundreds of millions of people around the world who rely on DEET to stay safe from mosquito-borne diseases.

DEET, which stands for diethyltoluamide, has been protecting people from mosquito bites since it was developed for the United States Army in 1946. By 1957, it had entered everyday civilian use, and for decades it was considered nearly foolproof. Mosquitoes simply despised the smell and avoided anything coated with it. However, a research team led by biochemist Clément Vinauger at Virginia Tech has been building a much more complicated picture of mosquito intelligence, and their newest findings suggest that DEET's reliability is not guaranteed under all conditions.

Mosquitoes are surprisingly sophisticated animals. With over 3,500 known species on Earth, they have evolved — meaning developed over many generations — remarkable abilities to sense and track their targets. Vinauger's team has previously shown that mosquitoes remember and avoid people who swat at them, use a combination of smell and vision to track humans with precision, and can even distinguish between different types of soap on a person's skin. Their brains are constantly processing information and updating their behavior based on experience, much like a video game character that learns from each level to become harder to beat.

In this new study, the team focused on the yellow fever mosquito, known scientifically as Aedes aegypti. This species is responsible for spreading serious illnesses to tens of millions of people annually, including dengue fever, Zika virus, yellow fever, and chikungunya. To test whether mosquitoes could learn new associations with DEET, the scientists used a method called Pavlovian conditioning — a training technique named after Ivan Pavlov, the researcher who famously taught dogs to expect food at the sound of a bell, eventually making them drool at the sound alone even when no food was present. The mosquitoes were restrained behind a fine mesh and repeatedly allowed to feed on a warm bag of blood while the scent of DEET was introduced in the final seconds of each feeding session. After just four repetitions, more than 60 percent of the trained mosquitoes tried to bite when exposed to the DEET scent alone.

The experiment became even more striking when the trained mosquitoes were offered a real choice. Study co-author Ayelén Nally from the University of Buenos Aires held out both hands — one treated with DEET at normal concentrations, the other bare. Untrained mosquitoes avoided the DEET-coated hand as expected. The trained mosquitoes, however, were actively drawn toward it. The scientists also found that sugar could be used instead of blood as the reward during training and the mosquitoes would still learn the same association, suggesting this is about how the mosquito brain processes any kind of reward, not just a blood meal specifically.

So what does this mean for your summer camping trip or your bug spray routine? Importantly, scientists are not telling anyone to stop using DEET. It remains one of the most effective repellents available, particularly in tropical regions where diseases spread by mosquitoes are a serious public health concern. The key takeaway from Vinauger's team is about how DEET is used. Applying it in smaller amounts and reapplying it regularly keeps the concentration high and consistent, making it harder for mosquitoes to ever get a successful blood meal while the scent is present. If the concentration fades and a mosquito still manages to feed, that is exactly the window in which this kind of learning can happen. Treated clothing faces a similar challenge, since DEET levels in fabric drop over time. As mosquito-borne illnesses continue to spread into new regions partly due to climate change, understanding the full picture of how mosquitoes think and learn is becoming one of the most important challenges in public health science.

Source: Popular Science