Mosquitos in town: how to prevent disease risk (and still green your city)
In cities, parks take the blame for mosquito problems. However, the real issue is usually a few streets away. Drains and standing rainwater create ideal breeding grounds for larvae. As a result, controlling adult mosquitoes in parks makes little sense, says environmental scientist Louie Krol in his PhD thesis.
For centuries, the Netherlands was a paradise for mosquitos. ‘Low-lying areas with brackish water were especially suitable for malaria-carrying species,’ explains Krol. It was not until around 1970 that the country was officially declared malaria-free.
Even so, the Netherlands remains vulnerable to mosquito-borne diseases such as the Usutu and West Nile viruses. Climate change plays a role in this. Milder winters and more extreme weather give mosquitos a better chance to survive and remain active for longer. After dry periods, heavy rainfall can leave water standing in many places. ‘Mosquitos are true pioneers: they quickly colonise these temporary pools, where, in the absence of natural predators and under the right conditions, they can develop rapidly and safely.’
‘Look at the bigger picture when dealing with mosquitoes’
Krol studied how landscape and land use affect the spread of the common mosquito, Culex pipiens, and where the risk of disease transmission is highest. ‘You can’t look at the mosquito in isolation,’ he says. ‘Disease transmission only occurs when several factors come together: the water where larvae develop, hosts and pathogens, environmental design, and climate. By studying how these elements interact, you can identify where interventions actually make a difference.’
‘We have created in cities unintentionally a paradise for mosquitos.’
The city’s infrastructure as a hidden mosquito breeding ground
During fieldwork in Oegstgeest and Leiden, Krol looked at how mosquitos use urban areas. According to him, cities unintentionally create ideal conditions for them. Water collects everywhere in the city’s infrastructure: street drains, gutters, rain barrels, garden pons, and other human-made containers. In these still, sheltered waters, leaves and other organic material accumulate: perfect food for mosquito larvae.
This creates a striking division in the city. Krol found that larvae are mainly concentrated in the grey infrastructure, while adult mosquitos move to sheltered, wind-free spots —like parks—to feed. Female mosquitos then return to the city to lay their eggs. ‘If you only look at adult mosquitos, it seems as though parks are the source of the problem,’ Krol says. ‘In reality, streets drains and other urban water sources are the main breeding sites.’
Tackling the source: simple measures that help
A few small interventions can already make a big difference here. Krol’s favourite solution is simple: ‘little ladders in the street drains!’ These allow amphibians, such as frogs, to move in and out safely so they can feed on mosquito larvae, reducing the number of adults.
‘Climate-adaptive measures, such as wadi systems, are effective.’
Climate-adaptive measures, such as wadi systems -shallow channels that temporarily collect rainwater and let it drain away slowly- can also be effective, provided they are properly designed and maintained. ‘If water remains for more than seven days, mosquitoes can lay eggs that develop into adults,’ Krol explains. Well-managed wadi systems combine water storage with a low risk of mosquito nuisance.
Green space is not the problem – design is key
Many people worry that adding more green space to cities will automatically lead to more mosquitoes. According to Krol, that does not have to be the case. With thoughtful design, we can influence where mosquitoes, people, and birds come into contact, reducing the risk of disease transmission.
‘We can create cities that are greener, more climate-resilient, and less prone to mosquito problems—a win-win for people and nature.’
‘If you create a small park surrounded by many breeding sites, you end up concentrating mosquito nuisance in one place,’ he says. Larger, well-connected green areas allow adult mosquitoes to spread out, lowering local concentrations.
Until now, researchers often focused only on the total amount of green space or water in an area. But that says little about how these elements are arranged. ‘Whether you have one large forest and a lake, or many small, fragmented green patches with bits of water, makes a big difference,’ Krol explains. By looking more closely at how green space and water are distributed, it becomes clearer where larvae develop and where adult mosquitoes gather. ‘This allows for targeted measures, helping us create cities that are greener, more climate-resilient, and less prone to mosquito problems—a win-win for people and nature.’
Thesis and promotion
Louie Krol defends his thesis ‘Interplay of landscape and climate on Culex pipiens mosquitoes in the Netherlands’ on 31 March in the Academic building. His promoters are Professor Peter van Bodegom, Dr Maarten Schrama and Dr Gertjan Geerling (Radboud Universiteit).