When the locusts’ DNA spoke

A swarm of locusts flying over the Cape Verde Islands. This photo was taken by my colleague Maurice Balmat in 1988, during an invasion. The desert locust (Schistocerca gregaria) primarily lives in the Sahara. When solitary, it is inoffensive. Once gregarious, it is a serious threat to crops in a vast area from Mauritania to India, and North Africa to Kenya. The damage caused is substantial.

A fair amount is known about gregarious populations during invasions. We know that swarms are highly mobile and can travel hundreds of miles a day. In 1988, many swarms crossed the Atlantic Ocean, reaching many Caribbean islands and the northern coast of South America. In contrast, until recently, solitary populations, during remission periods, were largely a mystery. In the past, they were considered as sedentary and threatened by drought in a Saharan environment where rainfall is scarce. Then scientists became suspicious: and if the solitary individuals were also able to fly and migrate ? The main problem was that, in the solitary populations, this flight activity is nocturnal. How to study migrations of insects in very low density at night ? They will remain unnoticed in the vastness of the Sahara. We had a few indices. We had some radar studies showing the existence of these flights at night. In addition, data accumulated over the years by the locust control organizations showed a different distribution of these solitary populations according to the seasons. It could be assumed the existence of seasonal migrations... but it was not a real proof. Genetics would bring the solution.

With a colleague from CIRAD, M.P. Chapuis, and with the help of most locust control organizations, we organized an extensive sampling of solitary individuals throughout the habitat area of this species. This work lasted several years, from 2008 to 2012. Samples were received in Montpellier from Mauritania, Algeria, Niger, Sudan, Pakistan... and DNA ended up talking.

With microsatellite genetic markers we hoped differentiate populations, and trace migration routes between reproduction areas of these solitary populations. The results were both surprising and novel. In fact, the data showed a single large population without genetic structure, from the Atlantic to Pakistan! This means that these solitary populations are much larger during remission periods than previously thought. Above all, there is now evidence that these solitary populations move quickly, are regularly mixed across the whole of their remission area, and can quickly find suitable reproduction zones.

Our findings radically change our vision of the solitary populations. Desert locust is fully adapted to its Saharan environment. Will the locust control organizations make a good use of these results to improve the preventative strategy? This is another story.

Chapuis M.P., Plantamp C., Blondin L., Pagès C., Lecoq M., 2014. Demographic processes shaping genetic variation of the solitarious phase of the desert locust. Molecular Ecology, 23: 1749-1763.

Photos: Left - Swarm of Desert locust in Cape Verde (M. Balmat, 1988). Notice the yellow coloration of this mature swarm. Right - Desert locust in its solitary phase, in Niger (M. Lecoq, 1989).