NOUAKCHOTT, Oct 3, Locusts have invaded Mauritania's capital Nouakchott for the third time in as many months, forming massive new swarms munching their way through what remains of the city's greenery, residents said on Sunday. West Africa's worst locust infestation for more than a decade has wreaked havoc in some of the world's poorest nations, causing substantial damage to grain crops across thousands of hectares (acres) in a region where many are subsistence farmers.

"The swarms seen in recent days in Nouakchott have come from the reproduction of the first generation which came down from the north," said Mohamed El Hacen Ould Jaavar, head of operations at Mauritania's desert locust centre (CLAA).

"They're winged juveniles starting their migration and fattening up before they set off. They're very hungry." Nouakchott's main soccer pitch and President Maaouya Ould Sid'Ahmed Taya's palace gardens were eaten by the first locust invasion in early August.

The insects have destroyed 40-50 percent of Mauritania's crops and up to 60 percent of its pastures, according to Environment Minister Ahmedou Ould Ahmedou. Only an eighth of the affected area in Mauritania has been sprayed with pesticide.

--More 2215 Local Time 1915 GMT NNNN