An invasion of armyworms is stripping southern Africa of key food crops and could spread to other parts of the continent, experts warned on Tuesday at an emergency meeting of 16 African nations.

South Africa, Zimbabwe, Malawi and Zambia are among the countries where the fall armyworm has invaded fields of maize, a staple crop throughout the region.

The fall armyworm is a new threat in southern Africa. It is mostly associated with the Americas, where it has been since 1957, said David Phiri, the UN Food and Agriculture Organization coordinator for the region.

The pest is devastating to crops and so far impossible to eradicate.

"They have not managed to control it because it keeps evolving," Phiri said. "But we have to find a plan for managing it."

Food insecurity will worsen without a solution, he said. "What is of particular concern is that the pest has affected countries that are the main producers of maize."

South Africa's Agriculture Ministry said this month little was known about how the armyworms arrived or what their long-term effects will be. "It may become a migratory pest similarly to the African armyworm and may migrate in large numbers from one area to another, causing great damage."

Affected crops also include sorghum, soybeans, groundnuts and potatoes, the ministry said.

Worst drought

The region already has been struggling with what the UN has called its worst drought in more than three decades.

Zimbabwe, once prosperous and an exporter of maize, already has been battling food shortages for years. More than four million people, or a third of the population, are in dire need of assistance, according to the World Food Program.

Ringson Chitsiko, the permanent secretary in Zimbabwe's Ministry of Agriculture, said the government has begun distributing pesticides and providing information to farmers.