Spain detects insecticide-resistant tiger mosquitoes for first time

In the Basque town of Basauri, researchers have found something that until now existed only in Italy: tiger mosquitoes carrying a genetic mutation that allows them to survive the insecticides Europe is allowed to use against them. The discovery, made through an international study analyzing nearly 2,500 mosquito specimens from across the continent, marks the first time this particular adaptation has been documented in Spain. It also signals a widening problem. The same mutation has now turned up in seven other European countries—France, Malta, Switzerland, Bulgaria, Turkey, Romania, and Georgia—suggesting that what was once an isolated phenomenon is becoming something far more widespread.

The tiger mosquito, scientifically known as Aedes albopictus, arrived in Europe more than two decades ago from Southeast Asia. Spain first encountered it in 2004 in Sant Cugat del Vallés, near Barcelona. Since then, the insect has spread steadily along the Mediterranean coast, into parts of the Cantabrian region, and inland, its eggs capable of surviving moderate cold. What makes this mosquito particularly troubling is not merely its presence but what it carries. It transmits dengue, zika, and chikungunya—diseases endemic to tropical regions that, until recently, seemed safely distant from European shores. Yet in the summers of 2018 and 2019 alone, Spain recorded seven cases of locally transmitted dengue. Italy has seen repeated outbreaks of chikungunya, with 230 cases in 2007 and 436 in 2017. France has documented cases of both dengue and zika. The mosquito has made the impossible routine: a traveler returns home infected, a tiger mosquito bites them during the window when the virus is most present in the blood, and suddenly a tropical disease has a European foothold.

The mutation researchers found—designated 1016G—confers resistance to pyrethroids, the only class of insecticide authorized for use in the European Union. In Basauri, between one and eight percent of the mosquitoes tested carried it. The same genetic signature appeared in samples from Nice and Perpignan in France, Luqa in Malta, Basel in Switzerland, Rome and Bari in Italy, Burgas in Bulgaria, Istanbul and Igneada in Turkey, Bucharest in Romania, and Batumi in Georgia. Daniel Bravo, an applied entomology researcher at the University of Extremadura who led the Spanish portion of the study, emphasizes that the problem extends beyond what the data currently shows. Seven other Spanish locations—scattered across Catalonia, the Balearic Islands, Valencia, Andalusia, Extremadura, and Madrid—were tested but showed no sign of the mutation. Yet this absence may be misleading. The sample sizes from some areas were small enough that the mutation could easily have gone undetected. Bravo and his colleagues suspect the adaptation is far more prevalent than current findings suggest.

The emergence of insecticide-resistant insects mirrors the broader crisis of antibiotic-resistant bacteria, but with a crucial difference: this mosquito is a disease vector. The resistance appears linked to the widespread use of pyrethroids not just against mosquitoes but in agriculture and against household pests like bedbugs. Mikel Bengoa, a doctor of entomology and technical manager at the pest control company Anticimex, had already detected resistant tiger mosquitoes in Peñíscola, Castellón, back in 2017 through biochemical testing. The new study explains the mechanism behind what he and others had observed. The problem, Bengoa notes, is that rotating between different pyrethroid formulations—the standard recommendation—is nearly impossible when so few options exist. If resistance continues to spread unchecked, authorities will face an escalating trap: increase pyrethroid concentrations to maintain effectiveness, knowing this damages the environment and harms non-target insects, or watch as the insecticides gradually lose their power entirely.

The breeding grounds for tiger mosquitoes reveal why control is so difficult. Ana García Pérez, a researcher at Neiker, the Basque government's agricultural and environmental research institute, explains that the mosquito completes its entire life cycle in roughly two weeks—from egg to adult in as little as fourteen days under favorable conditions. More critically, it does not breed in the large water bodies that authorities can easily monitor and treat. Instead, it thrives in the small accumulations scattered across private property: saucers beneath flowerpots, discarded cans, half-empty swimming pools, toys left outside. Eighty percent of breeding sites exist in private gardens and patios, beyond the reach of municipal control campaigns. This is where the responsibility shifts from government to citizen. Bravo stresses the need for rational use of pyrethroids in homes and businesses, but also emphasizes that without public participation—without people emptying standing water from their yards—the battle cannot be won.

The stakes are not abstract. If resistant mosquitoes establish themselves across Europe and continue to transmit tropical diseases, and if the insecticides meant to control them become ineffective, the continent faces a scenario it has largely avoided for decades: endemic transmission of dengue, zika, and chikungunya. The window to prevent this remains open, but it is narrowing. Monitoring must intensify. Resistance must be tracked. And the small pools of water in backyards across Spain and Europe must be emptied, because it is there, in the ordinary spaces of private life, that the mosquito waits.