Mosquito Alert confirms five-year high in spring tiger mosquito numbers in 2020

The number of tiger mosquito sightings reported via the Mosquito Alert app in the first half of 2020 is 70% higher than in 2018, which was, until now, the year with the most early-season sightings of the insect. However, 2015 still holds the record for the greatest number of tiger mosquito sightings over a full season.

Analysis of all the tiger mosquito sightings reported between 2015 and 2020 reveals that there were three times as many in May and June 2020 than in the same months in any previous year, apart from 2018. It is difficult to predict how the rest of the 2020 tiger mosquito season will evolve though, as experience shows that spring trends cannot be extrapolated to the summer. Bearing in mind that the tiger mosquito has a biological cycle of around 15 days, its future population size largely depends on water being available over the summer to enable it to reproduce.

Warm temperatures and frequent rain in spring 2020 gave the tiger mosquito ideal conditions for reproduction. This is clearly reflected in the number of sightings reported to Mosquito Alert in the first half of the year. In June, AEMET, Spain’s meteorological agency, proclaimed the spring of 2020 the country’s fourth hottest since 1965 and fifth wettest this century. The lockdown owing to COVID-19 may also have contributed to the rise in tiger mosquito numbers, by making it impossible to take the necessary preventive measures in many second homes on the Mediterranean coast.

Predicting the tiger mosquito season

Mosquito Alert’s research team is currently studying how climate variables affect the explosion in tiger mosquito reproduction in spring. Determining the nature of the relationship in question will enable scientists to make predictions regarding the start of the insect’s season. To that end, it is necessary to factor in not only spring temperatures and rainfall, but also winter environmental conditions. In a mild winter, a greater number of eggs laid in late autumn will survive to hatch in spring.

It will hopefully soon be possible to make predictions about the initial stage of the tiger mosquito season, just as we are already able to forecast the weather. Anticipating how the season will evolve in summer is complicated, however, as it depends on summer storms and on human activity, such as artificial irrigation.

 Accumulated sightings

Mosquito Alert’s accumulated sighting curves are based on sightings reported via its app. To produce the curves, the sighting data is corrected for sampling effort, which is estimated on the basis of the number of people with the app activated in a particular area on a given day, and of the modelled likelihood of them making sightings on that day.

The correction in question is important where estimating tiger mosquito abundance is concerned. The significance of 10 reported sightings is not the same in an area where there are 20 users with the app activated as it is in an area where there are five such users. To perform the correction, Mosquito Alert needs to know how many people with the app activated are present in a specific area, and great care is taken to ensure user privacy when obtaining that information.

Data collection with guaranteed privacy

To fit abundance data to models, the Mosquito Alert app collects information on the approximate position of a user’s device at random intervals. The app therefore does not pinpoint the device’s exact position but indicates its presence in a 2×2 km grid (i.e. within an area of two million square metres). The Mosquito Alert server is merely informed that a device is present in a particular grid at a certain time.

To further protect user privacy, app locations are linked to a randomly generated identifier that prevents association with any other information that could compromise an individual’s anonymity. Users can turn off the function that records their approximate position whenever they wish, despite the data in question being vital for calculating tiger mosquito abundance.

An example of citizen science work

Mosquito Alert is a cooperative, non-profit, citizen science project coordinated by the CEAB-CSIC, CREAF, ICREA and Pompeu Fabra University (UPF). It is currently supported by "la Caixa" Foundation, the European Research Council, the European Commission and Barcelona City Council. It was initially also co-funded by Dipsalut (Girona Provincial Council’s public health body) and the Spanish Foundation for Science and Technology (FECYT). Its aim is to combat the spread of invasive species of mosquitoes that transmit infectious diseases that are emerging or re-emerging in Europe.

The main resource Mosquito Alert offers the public is its eponymous app, which can be used to send photographs of mosquitoes and/or their breeding sites. The app records the GPS location and other details of each sighting, and the data obtained is processed by a team of experts.