Locust swarms threaten crops in Madagascar-FAO

* Swarms beginning to spread to east and north-FAO

* Madagascar govt sees potential risk for 460,000 households

* Action needed before rainy season, says FAO

ROME, Aug 12 (Reuters) - Madagascar is at risk of a significant plague of crop-eating locusts, the U.N. Food and Agriculture Organisation said on Thursday, calling for action before the rainy season.

An unknown number of immature swarms of Malagasy migratory locust have formed up and moved out of the country's south-western corner, where they are usually contained, and have begun to spread east and north, as far as Maintirano, FAO said.

It said the government estimated 460,000 rural families could be at risk.

"A very small part of an average swarm eats the same amount of food in one day as about 2,500 people," FAO said in a statement.

"A major, months-long control campaign will be necessary starting in advance of Madagascar's upcoming rainy season, which begins in mid-October, to stop locust numbers from growing and prevent them from reaching plague proportions."