The first desert locust swarms have left their breeding areas in North Africa and moved south to Mauritania, Senegal, and Mali, threatening the Sahel region with its worst plague in 15 years unless action is taken, the U.N. Food and Agricultural Organisation (FAO) has warned. Mauritania has been spraying swarms of locusts with insecticide since June and a senior agriculture ministry official in neighbouring Senegal told IRIN that the first swarms had already landed in the north of the country.