5 Minutes
Why Iceland remains largely mosquito-free
Despite abundant freshwater habitats such as ponds and marshes near airfields and settlements, Iceland has stayed largely free of established mosquito populations. Biologists point to the island's climate as the primary barrier. Long winters combined with frequent freeze–thaw cycles in autumn and spring repeatedly freeze and refreeze shallow standing water, interrupting the aquatic stages of mosquito development and preventing stable populations from taking hold.
Mosquito biology and climatic constraints
A mosquito's development follows four stages: egg, larva, pupa and adult. Female mosquitoes deposit eggs on or near water. Those eggs hatch into aquatic larvae, which grow, then transform into pupae — a non-feeding, immobile stage from which the adult emerges. Crucially, larvae and pupae require liquid, unfrozen water to complete these stages.
Cold tolerance strategies in other regions
Some cold-adapted mosquito species survive extreme winters by pausing development. In parts of the Arctic, eggs can enter dormancy and endure months locked in ice before thawing in summer. In milder northern zones, mosquitoes may overwinter as eggs, larvae in protected unfrozen pools, or even as sheltered adults in burrows. But Iceland's climate sits between these categories: the repeated freeze–thaw cycles damage eggs and aquatic stages before maturation, making it difficult for mosquitoes to complete their life cycle.
Geothermal waters and chemical limits
Iceland's geothermal features remain unfrozen in winter, and at first glance they could appear to be refuges for mosquito development. However, two constraints make geothermal pools poor mosquito nurseries. First, many geothermal pools are too warm for cold-adapted larval stages, creating thermal conditions outside the tolerance range of species evolved for high latitudes. Second, geothermal waters often have unusual mineral and chemical compositions (e.g., high sulfur, low oxygen) that are hostile to insect larvae. Together, these factors reduce the likelihood that geothermal pools would sustain breeding populations.
Climate change: a growing concern
Rising temperatures and shifts in seasonal patterns could alter Iceland's mosquito-free status. Warmer springs and autumns would extend the windows when standing water remains unfrozen, increasing the chance that eggs, larvae and pupae complete development. Scientists are already documenting poleward range expansions of tropical and subtropical mosquito species elsewhere — for example, Aedes mosquitoes moving northward in the continental United States as winters warm.
Historical examples illustrate how quickly a mosquito-free region can change. Hawaii was free of mosquitoes until the 19th century, when ship-borne introductions established populations that spread rapidly in the archipelago's favorable climate. In Hawaii, warming has even pushed some vector species into higher-elevation forests that were once too cool.
Public-health implications and vector risk
Even if mosquitoes become more common in Iceland, the risk that disease-bearing species such as Aedes aegypti and Aedes albopictus will establish there remains relatively low under current climate projections. These species require tropical to subtropical thermal regimes to sustain year-round transmission of viruses like dengue and chikungunya. Modeling for northern Europe suggests that, despite warming, conditions suitable for dengue transmission will likely remain limited through mid-century. That said, increases in travel and trade raise the chance of episodic introductions, and surveillance is key to early detection.
Expert Insight
"Iceland’s combination of prolonged freezes and frequent thaw events is unusually hostile to the aquatic stages of mosquitoes," says Dr. Elena Vargas, a climate entomologist at the University of Copenhagen. "What will be decisive is not only mean temperature rise but changes in seasonality — longer frost-free periods in spring and autumn would matter far more for establishment than a small increase in summer temperature alone."
Dr. Vargas adds that monitoring freshwater habitats around transport hubs and conducting targeted larval surveys during warm seasons would provide early warnings if range expansions begin.
Conclusion
Iceland’s scarcity of mosquitoes is best explained by climatic constraints rather than a lack of breeding grounds. Repeated freeze–thaw cycles, unsuitable chemical and thermal conditions in geothermal pools, and short windows of unfrozen standing water interrupt the mosquito life cycle. However, climate change could extend periods favorable to larval development, increasing the risk of future establishment. Continued surveillance, coupled with climate modeling and public-health preparedness, will determine how this near-mosquito-free status evolves in the decades ahead.
Comments