The insect is one of the world's most rapacious agricultural pests
Swarms of moth caterpillars that have devoured crops and plagued some 20 villages in central Liberia are now attacking neighbouring areas of Guinea.
Liberia's agriculture minister said it was the worst swarm of the insect, known as army worm, in three decades.
The bug, which can grow up to 5cm (2in) in length, is one of the world's most destructive agricultural pests.
The UN Food and Agriculture Organisation (FAO) has sounded the alarm over the infestation.
"There is a need for immediate action otherwise it may create problems for food production," said Tim Vaessen, of the FAO.
Liberia - which already imports 70% of its staple food, rice - declared a state of emergency in Bong county at the weekend.
Local residents reported the caterpillars were also infesting buildings and homes in their search for food.
"They are the most dangerous types - the most destructive types of caterpillars, and in fact each female lays between 500 and 1,000 eggs," Liberian Agriculture Minister Chris Toe told the BBC.
"It is also a health problem, because with them in the trees, and with their faeces going into the waters, all the water turns black and our people cannot drink that water."
The last time swarms of such caterpillars attacked on this scale was in the late 1970s, he added.
The moth caterpillars can lay waste to an entire crop in a matter of days once they reach maturity.