"It is probably only a matter of time before most of the region will be affected," said David Phiri, sub-regional coordinator for Southern Africa of the UN's Food and Agriculture Organisation (FAO).
Phiri added that it is "even more concerning that the pest could be here to stay."
The warnings came at the close of an emergency regional summit in Harare to tackle an outbreak of crop-eating armyworms that has already caused damage to staple crops in Zambia, Zimbabwe, South Africa and Ghana.
Reports also suggest Malawi, Mozambique and Namibia are affected.
"Southern Africa is currently the epicentre of the fall worm problem," Phiri said. "There are other countries that have already been affected...and others that are at risk."
Experts and representatives from 13 southern and east African countries gathered for the three-day emergency meeting to thrash out solutions, agreeing "to increase surveillance, co-operation and share information" on the armyworms.
The first fall armyworms were spotted in Nigeria and Togo last year, with one theory saying they arrived in Africa on commercial flights from South America or in plants imported from the region.
The caterpillars eat maize, wheat, millet and rice - key food sources in southern and eastern Africa, where many areas are already struggling with shortages after years of severe drought.
They also attack cotton, soybean, potato and tobacco fields.
Chemical pesticides can be effective, but fall armyworms have developed resistance in their native Americas.
"There is still a lot of work that we need to do to begin to practically control the trans-boundary pest and disease problem," Phiri said.
Brazil spends about $600m to fight the pests every year, according to Phiri.
"We agree that prevention is more effective and less costly," Phiri said.
In December, Zambia deployed its national air force to transport pesticides across the country so that fields could be sprayed.