The spray often ends up near house keys, sunglasses and phone chargers. We take it before going out, we spray it on our legs, on our arms, maybe even a little in the air as if it were enough to create a chemical cloud between us and the summer. Then they arrive. Mosquitoes. Tiny, stubborn, with that vocation for disruption that no condominium has ever been able to match.
For decades the DEET it is one of the most used active ingredients in mosquito repellents. The acronym stands for N,N-diethyl-meta-toluamide, a compound developed in the 1940s and still used today in sprays, lotions and other products against biting insects. Health authorities continue to consider repellents registered and used according to the label as an effective tool for reducing bites; ingredients listed include DEET, picaridin, IR3535, PMD or formulated lemon eucalyptus oil, and 2-undecanone. The point, however, lies in the way these products enter real life: sweat, heat, water, exposed skin, forgotten reapplications, now low concentrations. The CDC reminds you that the duration of protection varies greatly and that, when you start getting stung again, the product should be reapplied always following the instructions on the label.
The spray does not erase the problem
The European summer is becoming a more mosquito-friendly terrain. Longer warm seasons, milder winters, erratic rainfall and cities full of small standing water widen the window in which these insects can breed and sting. The European Center for Disease Prevention and Control has strengthened surveillance on dengue, chikungunya, Zika and West Nile precisely because the European situation changes from year to year, with invasive species now monitored at a regional level.
Here comes the most uncomfortable part. A study on Aedes aegyptithe yellow fever mosquito and one of the vectors of dengue, Zika, chikungunya and yellow fever, suggests that these insects may change their behavior after exposure to DEET. In the laboratory, after repeated exposures, some of the mosquitoes continued to approach and attempt to bite even in the presence of the repellent. In one step of the experiment, after four exposures, more than 60% of the insects attempted to bite when they only perceived the odor of DEET, as if that signal had been linked to the possibility of feeding.
The mechanism recalls, with all the necessary cautions, associative learning: a smell, a reward, a memory. A bit like Pavlov’s dogs, only with less salivation and more itchy ankles. Research on mosquitoes has long shown that these insects respond to odors more flexibly than we would like to think. They can learn from positive and negative experiences, change preferences, avoid stimuli associated with risk, or persist when a cue has been linked to a source of nutrition. Previous studies on Aedes aegypti had already observed a lower repellency of DEET after a previous exposure: three hours after contact, some females were less sensitive to the compound, with attenuated olfactory responses.
Does this mean DEET stops working? No. It means something more precise and less like a shouty headline: protection also depends on the contextthe concentration left on the skin, the species of mosquito, the time elapsed since application and the way in which the insect interprets the chemical stimuli around us.
Mosquitoes are not little machines
For a long time we have talked about repellents as an almost magical barrier. Spray, get out, you’re safe. Biology is less polite. DEET can act in multiple ways: it interferes with the insects’ ability to locate us, it can confuse some olfactory signals, it can be perceived directly and, under certain conditions, it can push the mosquito to move away after contact. Work published in Scientific Reports showed, in “arm-in-cage” experiments, that some topical repellents reduce bites mainly because they induce mosquitoes to detach themselves from the skin after contact, rather than always keeping them away at a distance like an invisible force field.
This detail changes the way you read the scene. The mosquito can get on the skin, touch, try, leave again. The repellent works within this chemical micro-deal. If there is too little of it on the skin, if an area remains uncovered, if the product has been applied poorly or has now partially evaporated, the insect can find an opening. And when an odor is perceived more as a trace than an obstacle, behavior can become less predictable.
Aedes aegypti, by the way, bites mostly at dawn and dusk, while Aedes albopictus, our more familiar tiger mosquito, follows similar windows. The CDC reminds us that different species have different activity schedules, with overlaps and local variations that make constant protection in at-risk areas sensible.
How to use it without getting ripped off
The practical consequence is very simple: mosquito repellent should be used well, without treating it like a bad perfume. It should be applied to areas of exposed skin, avoiding cuts, children’s eyes, mouth and hands. Face sprays should first be sprayed on the hands and then distributed carefully. After returning, the treated skin should be washed. Permethrin, however, remains for clothes, mosquito nets and equipment: it cannot be applied to the skin.
Concentration matters too. Products with higher percentages tend to last longer, but above about 50% DEET appears to offer little additional benefit in terms of durability against mosquitoes. The problem, therefore, is rarely solved by choosing the most aggressive bottle and leaving it behind for the whole evening. We need uniform coverage, the right timing, reapplication when required, light but covering clothing in critical hours, mosquito nets where needed, elimination of stagnation in saucers, buckets, private drains, toys left on the balcony.
The most interesting data from the study, despite the almost disturbing curiosity of learning mosquitoes, concerns precisely this: the fight against bites cannot be entrusted to a single automatic gesture. DEET remains an important tool, especially where viruses transmitted by mosquitoes are circulating, but using it incorrectly means giving it a job that it cannot do alone.
