The rapid return of mosquitoes to African semi-desert regions when the dry season ends was an unsolved mystery.
A surprising solution to the puzzle is the long-range migration of mosquitoes on high-altitude winds.
During the long dry season in the semi-desert region of Africa known as the Sahel, malaria transmission ceases because the mosquitoes that can transmit the disease (termed malaria mosquitoes or vectors) disappear, along with the surface water required for the development of the next generation of mosquitoes.
Yet with the first rains that end the dry season, adult numbers surge more quickly than can be explained by resumed breeding in newly rain-filled sites.
Evidence to explain this adult population boom has remained elusive for decades.
Insect flight typically occurs close to the ground, in a habitat patch that provides all of the insect’s essential resources such as food, shelter, mates and breeding sites.
Among malaria vectors, this type of foraging flight rarely exceeds a distance of five kilometres2.
By contrast, during long-distance migration, insects ascend to altitudes as high as 2–3 km, where fast air currents transport them downwind for hundreds of kilometres in a few hours3.
This behaviour is beneficial3 for insects moving in seasonally favourable directions.
The migration of monarch butterflies (Danaus plexippus) between North America and Mexico is one of the most widely known insect migrations, but the extent to which other insects engage in long-distance migration is under-appreciated, because these high-altitude flights are undetectable without technology such as radar.
The type of radar that can detect larger insects (those heavier than 10 milligrams) had been mainly used to track just a few agricultural pests, until a 2016 study of the southern United Kingdom4 used such radar to investigate insect migration in general.
This study revealed that an estimated 16.5 billion insects migrate annually at high altitude (defined in this case as a height of more than 150 metres) above the 70,000 km2 study area, indicating that wind-borne insect migration can occur on a strikingly large scale.
Current radar technology does not detect small insects (lighter than 10 mg) such as mosquitoes, which must instead be tracked by sampling using aerial nets.
In the UK study4, such insect capture provided evidence that three trillion small insects undertake high-altitude migrations, a number that substantially exceeds that of the larger radar-tracked insects in the same area.
Seasonal patterns in the direction of high-altitude winds can enable consistent routes for these bioflows (Fig. 1).
Huestis and colleagues studied four villages in the Sahel region of Mali.
The possibility that wet-season mosquito populations are re-established there by adults flying from the nearest year-round populations was excluded in a previous study5 by this team.
This is because the distance of more than 150 km to such sites is prohibitively long for self-powered mosquito flight.
A second possibility is that mosquitoes maintain a local presence and survive during the dry season, hidden away in a state of dormancy termed aestivation.
Important, albeit indirect, support for this hypothesis came from extensive population time-series analysis from that earlier study5, which showed beyond reasonable doubt that a mosquito vector species called Anopheles coluzzii persists locally in the dry season in as-yet-undiscovered places.
However, the data were not consistent with this outcome for other malaria vectors in the study area — the species Anopheles gambiae and Anopheles arabiensis — leaving wind-powered long-distance migration as the only remaining possibility to explain the data5.
Both modelling6 and genetic studies7 support the idea of long-distance migration to explain the seasonal dynamics of malaria mosquitoes in the Sahel, but many researchers have instead long discounted this phenomenon as being rare, accidental and inconsequential.
This entrenched attitude has been difficult to dispel given the challenge of obtaining compelling direct evidence.