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

Authors: FLORIO, JENNA - National Institutes Of Health (NIH); VERU, LAURA - National Institutes Of Health (NIH); DAO, ADAMA - University Of Bamako; YARO, SEYDOU - University Of Bamako; DIALLO, MOUSSA - University Of Bamako; SONOGO, L. - University Of Bamako; DJIBRIL, S. - University Of Bamako; HUESTIS, DIANA - National Institutes Of Health (NIH); OUSMAN, YOSSI - University Of Bamako; TALAMAS, E. - Florida State University; Chamorro, Maria; FRANK, H. - University Of Florida; BIONDI, M. - Non ARS Employee; BARTLETT, CHARLES - University Of Delaware; STROBACH, UDI - Non ARS Employee; LINTON, YVONNE-MARIE - Walter Reed Army Institute; CHAPMAN, ERIC - University Of Kentucky; REYNOLDS, DON - University Of Greenwich; FAIRMAN, ROY - National Institutes Of Health (NIH); KRAJACICH, BENJAMIN - National Institutes Of Health (NIH); WEETMAN, DAVID - Liverpool School Of Tropical Medicine; DONNELLY, MARTIN - Liverpool School Of Tropical Medicine; LEHMANN, TOVI - Nih, National Institutes Of Allergy And Infectious Diseases

Submitted to: Ecography
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/2020
Publication Date: 10/30/2020
Citation: Florio, J., Veru, L., Dao, A., Yaro, S.A., Diallo, M., Sonogo, L.Z., Djibril, S., Huestis, D., Ousman, Y., Talamas, E.T., Chamorro, M.L., Frank, H., Biondi, M., Bartlett, C., Strobach, U., Linton, Y., Chapman, E., Reynolds, D., Fairman, R., Krajacich, B., Weetman, D., Donnelly, M., Lehmann, T. 2020. Massive windborne migration of Sahelian insects: diversity, seasonality, altitude, and direction. Ecography. 10(20523):1-14. https://doi.org/10.1038/s41598-020-77196-7.
DOI: https://doi.org/10.1038/s41598-020-77196-7

Interpretive Summary: If insects, and particularly pest species, were able to travel long distances the implications in our ability to manage and protect crops, natural resources and human health would be significant. Measuring the number and kind of minute insects moving at high-altitude is challenging and therefore is rarely attempted. An innovative method to sample high-flying insects (40–290 meters) was used using sticky nets mounted on tethered helium-filled balloons. Insects were sampled over Mali during peak wet and dry season. While almost half a million insects were caught, the study focused on thirteen ecologically and phylogenetically diverse species. We found that flight activity of all species peaked during the wet season every year over multiple nights, suggesting regular migrations. For most species studied, frequent large-scale movement occurred from the more lush south to the drier north during the start of the wet season and, albeit less so, from the north to the south during the start of the dry season. Additionally, data suggests different species are capable of taking advantage of different wind currents that can transport them to various directions and directions in the African Sahel.

Technical Abstract: Long-distance migration of insects is 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 the 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 Malian villages, using sticky nets mounted on tethered helium-filled balloons. Nearly half a million insects were caught, and members of twelve insect orders were identified in a preliminary sorting of the collections. Thirteen ecologically and phylogenetically diverse species captured at altitude were identified in 222 netting samples, obtained during 125 collections in 96 nights. Use of control nets (raised momentarily to >40 m during system launch and retrieval) confirmed that the insects were captured at altitude, not near the ground. 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 intensity with respect to altitude, timing, aerial temperature, humidity, and wind speed, but these factors accounted for a modest fraction of the variance in aerial activity. 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. Extrapolating aerial density to estimate the seasonal number of migrants crossing Mali at latitude 14°N suggests numbers in the trillions, even for the modestly abundant taxa. Assuming 2–10 hours of flight, the distances traversed exceed tens and even hundreds of kilometers per night. Two migration strategies were proposed: “residential Sahelian migration” and “round trip migration”. The unprecedented magnitude and diversity of long-range windborne migrations underscore their role in Sahelian and neighboring ecosystems.