Tracking mosquitoes over time: testing the role of aestivation in dry season persistence of a malaria vector

Tracking mosquitos over extended time or large distances remains a challenge. Mosquitoes developing in [²H]-enriched water are marked for life with an elevated proportion of the stable isotope, providing opportunity to test the hypothesis that Anopheles coluzzii persists in the Sahel through the dry season via aestivation. A large-scale experiment took place in 2017 in two Malian villages. We aimed to estimate the contribution of aestivation to persistence of mosquitoes through the 7-month dry season by marking at least 10% of the A. gambiae s.l. adults by the end of the wet season and assess the proportion of marked adults through the dry season, and especially after the first rain in June 2018. If aestivation is the only way A. coluzzii persists, the frequency of marked mosquitoes should be similar throughout. Finding no marked mosquitoes would be evidence against aestivation. The marked adult proportion by the end of the wet season was 38.8% (N=57/147). Five months after the last larval site dried up, 38.2% (N=42/110) remained marked. Eight months after the onset of the dry season, 3% (N=6/200) showed clear enrichment. When accounting for signal deterioration over time and inter-village dispersal, the resulting proportion of marked mosquitoes provides direct evidence of population-wide aestivation in coluzzii alone, supporting annual malaria resurgence shortly after the wet season onset.