This is how mosquitoes survive the dry season

Malaria mosquitoes can survive the dry season in the African Sahel, which lasts around seven months, in a dry sleep. Scientists found this out by tagging mosquito larvae in two villages in Mali. About 18 percent of the Anopheles coluzzii mosquitoes most likely bridged the time between the rainy seasons by dry sleep.

The scientists led by Roy Faiman from the National Institute of Allergy and Infectious Diseases in Rockville (Maryland, USA) present their approach and their results in the journal "Nature Ecology

Normally, Anopheles mosquitoes only live for a few weeks. Due to the harsh environmental conditions, they are unlikely to survive the long dry season in the Sahel zone, during which no rain falls for months. However, when the rainy season usually begins in June, the mosquitoes are already present in large numbers after just a few days.

This time is far too short for them to hatch from eggs and get past the larval stage. This "dry-season malaria paradox" has puzzled experts for nearly a century, the study authors write. So far it was not clear whether the mosquitoes spend the dry season elsewhere and then fly back or whether they fall into a dry dormancy (also called summer dormancy or aestivation) on site.

To clarify the question, Faiman and colleagues conducted a large-scale experiment in the Malian villages of Thierola and M'Piabougou: they enriched the open ponds with heavy water - that is, water with deuterium in place of hydrogen.

In later laboratory tests of captured Anopheles coluzzii mosquitoes, the researchers were now able to determine whether the mosquitoes spent their larval stage in one of the ponds, which dry up during the dry season. The measurements showed that shortly before the start of the dry season, around 33 percent of the mosquitoes in both villages had an increased deuterium level.

Even during the dry season, at the end of March or beginning of April, many malaria mosquitoes appear for a short time. During this time and at the beginning of the rainy season, the researchers collected mosquitoes and determined the deuterium value. They calculated that around 18 percent of the mosquitoes buzzing around had not left the area around the two villages during the dry season. The remaining mosquitoes come from other regions, for example from the rice-growing region of Niono, 140 kilometers away.

"This survival strategy could impact mosquito control and malaria elimination campaigns," writes Faiman's team. Mosquito sprays could also be used during the dry season, even if the mosquitoes are not active or visible during this time. The possible use of genetically modified mosquitoes to reduce the population should also take into account the dry sleep of mosquitoes, the researchers emphasize.

The researchers' findings represent a significant advance in understanding the dry season malaria paradox, writes Peter Armbruster of Georgetown University in Washington (District of Columbia, USA) in a comment, also in Nature Ecology

They clearly demonstrated that dry sleep is a mechanism used by Anopheles coluzzii mosquitoes to survive the dry season. An elusive but important goal must now be to find the places where the mosquitoes keep their dry sleep in order to be able to kill them there.