Massive windborne migration of Sahelian insects: Diversity, seasonality, altitude, and direction

Abstract

Knowledge on long-distance migration of insects is especially important for food security, public health, and conservation–issues that are especially significant in Africa. During the wet season, the Sahel nourishes diverse life forms which are soon purged by the long dry season. Windborne migration is a key strategy enabling exploitation of such ephemeral havens. However, our knowledge of these large-scale movements remains sparse due to the virtual invisibility of insects flying at altitude. In this first cross-season investigation (3 years) of the aerial insect fauna over Africa, we sampled crepuscular and nocturnal insects flying 40–290 m above ground in four Sahelian villages in Mali, using sticky nets mounted on tethered helium-filled balloons. Nearly half a million insects were caught, representing at least thirteen insect orders following preliminary sorting of the collections. At least 100 insect families were determined to have been captured at altitude in samples collected on 222 nets, obtained in 125 collections over 96 nights. Control nets (raised momentarily to >40 m during system launch and retrieval) confirmed that the insects were captured at altitude, not near the ground. Thirteen ecologically and phylogenetically diverse species were studied in detail. The flight activity of all species peaked during the wet season every year across localities up to ~100 km apart, and occurred over multiple nights, suggesting regular migrations. Species differed in flight altitude, seasonality, and correlations with aerial temperatures, humidity, and wind speed. All taxa exhibited frequent migrations on southerly winds, accounting for the recolonization of the Sahel from southern source populations. “Return” southward movement at the end of the wet season occurred in most taxa but no selectivity for such winds was detected. Extrapolation of aerial density to estimate the seasonal number of migrants crossing Mali at latitude 14°N suggested numbers in the trillions, even for the modestly abundant taxa. Assuming 2–10 hours of flight, the nightly distances traversed exceed tens and even hundreds of kilometers. Two migration strategies were proposed: “residential Sahelian migration” and “round trip migration”. The unprecedented magnitude and diversity of long-range windborne insect migrations highlight the importance of this life strategy in their impact on Sahelian and neighboring ecosystems.

Environmental drivers of annual population fluctuations in a trans-Saharan insect migrant

“…Despite the typically dry winter conditions prevalent in the Savanna/Sahel, these regions still support high populations of insectivorous birds through the winter (38,48). Furthermore, mass aerial movements of insects frequently occur above this part of Africa as evidenced by aerial captures of huge numbers of migrating insects above the Sahel of Mali (5,12) and lengthy stopovers by aerially foraging common swifts to feed up in these regions on their spring and autumn migrations (49). Painted ladies are thus just one of many insect species that breed in the moister regions further south before spring migration.…”

“…insect migration | population dynamics | painted lady butterfly | Lepidoptera I nsect migration occurs on an enormous scale (1), with billions of individuals undertaking multigenerational migrations between seasonally favorable climatic zones around the globe (2)(3)(4)(5)(6). These long-range migration cycles profoundly influence terrestrial ecosystems via the large-scale transfer of biomass, energy, and nutrients (4)(5)(6)(7)(8), the provision of ecosystem services (8)(9)(10), impacts on agricultural productivity (11), and spread of disease (5,12); thus, it is imperative that we better understand insect movement patterns. Recently, there has been a step change in our knowledge of the year-round spatial distribution and migratory routes of a few well-studied species (13,14), particularly the monarch butterfly (Danaus plexippus) (15,16) and (to a lesser extent) the painted lady butterfly (Vanessa cardui) (17,18).…”

The ‘migratory connectivity’ concept, and its applicability to insect migrants

“…One such region is the savannah / Sahel zone of West Africa, where the progressive advance, in summer, of the Inter Tropical Convergence Zone is followed by a belt of intense convective rainfall in an otherwise arid zone. A wide range of windborne migrant insects move north into the Sahel on moist south-westerly monsoon winds in early summer, to take advantage of renewed growth of vegetation and other resources produced by the monsoon rains [74,75]. Later (September-October) the Inter Tropical Convergence Zone retreats southwards again, and north-easterly 'Harmattan' winds are re-established which allow the progeny (or, in a few special cases, the original immigrants) to move south-westwards out of the increasingly dry Sahel.…”

“…Later (September-October) the Inter Tropical Convergence Zone retreats southwards again, and north-easterly 'Harmattan' winds are re-established which allow the progeny (or, in a few special cases, the original immigrants) to move south-westwards out of the increasingly dry Sahel. The utility of this atmospheric circulation is such that large numbers of species exploit it, most of them completely unstudied [75], but including tiny species such as mosquitos and other Diptera which are entirely windborne [75,76] to large species such as grasshoppers and locusts (Orthoptera) that actively fly downwind [77]. Nothing is known about the degree of migratory connectivity in such populations; however, the windborne nature of the transport and lack of self-directed movement suggest that is it likely to be non-existent or at least extremely low.…”

The effects of high-altitude windborne migration on survival, oviposition and blood-feeding of the African malaria mosquito,Anopheles gambiaes.l

“…arabiensis has not been found at altitude. Additionally, aerial density of mosquitoes was higher when ground-level wind was slower (Huestis et al , 2019; Florio et al , 2020: PREPRINT).…”