Why does it seem like there are fewer mosquitoes this summer?

BarcelonaIn recent weeks, there has been a detected decrease in incidents caused by mosquito bites. Although high temperatures favor these insects, the extreme heat that Catalonia has suffered in recent months does not seem to please them much. Unlike mammals, mosquitoes do not have an internal thermal regulation mechanism. This makes them very vulnerable to heat and the dehydration it causes, leading them to seek refuge in vegetation during the day to survive.

"If humidity is low, they can start to greatly reduce their activity, taking refuge in cooler, more humid places, thus reducing the health risk and nuisance," explains Roger Eritja, an entomologist at Mosquito Alert, a citizen alert system that investigates the spread of mosquitoes that carry exotic diseases. Added to this is a matter of perception: "People find that there are fewer mosquitoes because the heat prevents us from doing outdoor activities where we usually encounter them," he adds.

Furthermore, mosquitoes have also experienced a change in their operational hours: they usually remain hidden during the hottest hours of the day and are active before sunrise or at dusk. Now, however, as the daily peak heat hours have increased, the insects rest for longer periods. All this has led to a decrease in the presence of mosquitoes.

Despite this, Joan Basí, a technician for urban mosquitoes at the Mosquito Control Service, explains to ARA that we should not lower our guard. "Mosquitoes are still here, it's just that perhaps we don't see them." Tiger mosquitoes lay their eggs in artificial containers such as pots or buckets where the female has detected the presence of water in the past, and marsh mosquitoes lay their eggs on mud and damp soil. All mosquitoes, however, depend on a key factor: water. Due to their aquatic development, their eggs need it to complete the larval stage. If there is no water, the eggs do not develop, and if they do, the larvae have a higher risk of not surviving because they are quite susceptible to heat. "This is why there are fewer mosquitoes," explains Basí.

A matter of time

But the mosquito technician also makes it clear that we should not rush: everything indicates that the decrease in mosquitoes is a temporary issue. Mosquito eggs, especially those of the tiger mosquito, survive extreme climatic factors, including high temperatures and drought. "They have come to stay," he argues. In fact, according to Heatwaves constrain the future persistence of mosquito vectors in Europe, a study from the specialized journal Global Change Biology, the strong resilience of the eggs of this species suggests that their presence will not be affected by the scenario of a warmer Europe with repeated heatwaves.

Although the drought has interrupted the mosquito reproduction cycle, it has not completely broken it. "It's a matter of it raining again," explains Basí, who adds that their return is inevitable. As soon as humidity and rainfall increase, phenomena very common after a heatwave, the optimal conditions for mosquito proliferation will be met. With the summer downpours that usually arrive between late August and early September, the eggs waiting for humidity will turn into larvae and reactivate their life cycle.

In fact, from Mosquito Alert, Eritja warns that, looking to the future, the general increase in temperatures can favor the transmission of exotic imported diseases, such as dengue or yellow fever, as it accelerates both the parasite's cycle and the vector's cycle. To face this risk, Mosquito Alert calls for citizen participation through the use of the platform to report the expansion of invasive mosquitoes and thus be able to design theoretical models on the risk of bites and disease transmission. "Prevention and healthcare are more important in such cases than simple ambient temperatures," he explains.