The UN food agency has warned  that concentrations of locusts in southwestern Madagascar were at risk of turning into a plague and could threaten the livelihoods of 13 million people.

The Rome-based Food and Agriculture Organisation (FAO) called for a new campaign to contain the crop-devouring insects, saying an estimated 300,000 hectares of locust-infested territory needs to be treated by next May.

"We must break the locust population dynamics in order to prevent further developments that could affect the island for years," FAO locust officer Annie Monard, who is coordinating operations in Madagascar, said in a statement.

The UN agency said it has been helping anti-locust efforts on the island since last year, including by using for the first time on a large scale a biopesticide based on a fungus that is lethal to locusts and grasshoppers.

"While such efforts prevented the 2010 locust upsurge escalating into a plague... weather and ecological conditions in the first half of this year triggered a renewed build-up of locust populations," it added.

Locust plagues are massive concentrations of the insect -- sometimes in the billions -- that can destroy crops and pastures in their path.