Diversity, dynamics, direction, and magnitude of high-altitude migrating insects in the Sahel

Long-distance migration of insects impacts food security, public health, and conservation–issues that are especially signifcant in Africa. Windborne migration is a key strategy enabling exploitation of ephemeral havens such as the Sahel, however, its knowledge remains sparse. In this frst cross-season investigation (3 years) of the aerial fauna over Africa, we sampled insects fying 40–290 m above ground in Mali, using nets mounted on tethered helium-flled balloons. Nearly half a million insects were caught, representing at least 100 families from thirteen orders. Control nets confrmed that the insects were captured at altitude. Thirteen ecologically and phylogenetically diverse species were studied in detail. Migration of all species peaked during the wet season every year across localities, suggesting regular migrations. Species difered in fight altitude, seasonality, and associated weather conditions. All taxa exhibited frequent fights on southerly winds, accounting for the recolonization of the Sahel from southern source populations. “Return” southward movement occurred in most taxa. Estimates of the seasonal number of migrants per species crossing Mali at latitude 14°N were in the trillions, and the nightly distances traversed reached hundreds of kilometers. The magnitude and diversity of windborne insect migration highlight its importance and impacts on Sahelian and neighboring ecosystems.