Warmer European summers are widening the active window for invasive mosquitoes. Temperature thresholds, diapause and transmission risk explain why the season is stretching.
Why the mosquito season is changing
The European Centre for Disease Prevention and Control reports that Europe is warming, with longer, hotter summers and more frequent heat waves and flooding. Those conditions favour invasive mosquitoes such as Aedes albopictus and Aedes aegypti. Between 2013 and 2023, Aedes albopictus went from being established in 8 EU and EEA countries and 114 regions to 13 countries and 337 regions, spreading northwards and westwards. Aedes aegypti has been established in Cyprus since 2022. ECDC Director Andrea Ammon warned that continued spread could bring more cases, and possibly deaths, from dengue, chikungunya and Zika. The season is not simply longer everywhere; it is shifting in where it starts, how long it lasts and how much of the year transmission is possible.
The temperature thresholds that govern survival
Aedes albopictus needs specific temperatures to establish and stay active. The ECDC expert factsheet lists a mean winter temperature above 0 degrees C so eggs can overwinter, and a mean annual temperature above 11 degrees C for adult survival and activity. Optimum development occurs between 25 and 30 degrees C, and there is generally little adult activity below 9 degrees C. Populations have also established in areas with lower mean temperatures, between 5 and 28.5 degrees C, than scientists previously suspected. That gap between laboratory thresholds and field observations matters. It means the map of risk is not fixed by a single number, and monitoring must track local conditions rather than assume a species stops at a historical line.
Diapause and the winter survival trick
Temperate populations of Aedes albopictus survive winter through diapause, a dormant state triggered by shorter days. The ECDC factsheet states that diapausing eggs are generally produced below 13 to 14 hours of daylight, and that hatching suppression usually lasts long enough to outlast winter. This cold hardiness is measurable: diapausing eggs from European populations survived a cold spell of minus 10 degrees C, whereas tropical eggs survived only minus 2 degrees C. Diapause helps explain why a species associated with warm climates can hold on in cooler parts of Europe and then emerge when conditions improve. It also means the start of the season depends on day length and temperature together, not on warmth alone.
When adults are active across Europe
Activity windows differ sharply by latitude and local climate. In northern Italy, adult females were most abundant from May to September and peaked in late July. In Greece, the species is continuously active for more than eight months of the year and peaks in October. In Italy, some populations remain active throughout winter. Warming is expected to make most of Europe more suitable for establishment, with Western Europe, including Belgium, France, Luxembourg and the Netherlands, expected to provide favourable conditions within the next decades. These patterns show that a warmer climate does not just lengthen one uniform season. It reshapes timing, creating earlier starts in some places and extended activity in others. These regional differences mean a warming climate can extend mosquito activity in one area while shifting its peak timing elsewhere.
Temperature and chikungunya transmission
Temperature also shapes how well a mosquito can transmit a virus. Heitmann and colleagues, writing in Eurosurveillance in 2018, infected Aedes albopictus from Germany and Italy with a chikungunya strain from a French outbreak and kept them at 18, 21 or 24 degrees C for two weeks. Transmission rates exceeded 35 percent across conditions. The German population reached its highest rate, 50 percent, at 18 degrees C, while the Italian population reached 63 percent at 18 and 21 degrees C. The authors concluded that current chikungunya risk in Europe is limited mainly by where the vector lives, not by temperature, and that countries with established Aedes albopictus should run entomological surveillance and monitoring. Temperature governs how quickly the virus incubates inside the mosquito before it can be passed on during a later bite.
Projected shifts in transmission risk
Ryan, Carlson, Mordecai and Johnson modelled temperature-dependent transmission by Aedes aegypti and Aedes albopictus, publishing in PLOS Neglected Tropical Diseases in 2019. Their thermal bounds were 21.3 to 34.0 degrees C for Aedes aegypti and 19.9 to 29.4 degrees C for the more heat-limited Aedes albopictus. By 2050, the number of people exposed to at least one month of suitable transmission is projected to rise by roughly half a billion. By 2080, severe scenarios add nearly a billion more people for Aedes aegypti. For Aedes albopictus, the largest numbers appear in intermediate scenarios, because warming exceeds its upper thermal limit in many places. Europe shows some of the largest projected increases, including longer transmission seasons.
What this means for public health
The ECDC reported 1,133 human West Nile virus cases and 92 deaths in the EU and EEA in 2022, the highest count since 2018, with 1,112 cases locally acquired. In the same year, 71 locally acquired dengue cases were recorded in mainland EU and EEA, 65 in France and 6 in Spain, matching the total reported between 2010 and 2021. These figures show that mosquito-borne disease is already a European public health issue, not only a tropical one. Practical responses include surveillance of adult and larval mosquitoes, removing standing water, and using intact physical barriers such as correctly fitted screens. The aim is accurate risk information, not alarm. Surveillance data of this kind help health agencies target control measures where mosquito activity and local transmission risk overlap.
Sources
- Aedes albopictus - Factsheet for experts: European Centre for Disease Prevention and Control
- Increasing risk of mosquito-borne diseases in EU/EEA following spread of Aedes species: European Centre for Disease Prevention and Control
- Experimental risk assessment for chikungunya virus transmission based on vector competence, distribution and temperature suitability in Europe, 2018: Eurosurveillance
- Global expansion and redistribution of Aedes-borne virus transmission risk with climate change: PLOS Neglected Tropical Diseases
- West Nile virus - historical data: European Centre for Disease Prevention and Control