The plague is Liberia's worst infestation of armyworm for three decades
The UN has warned Liberia could soon face a second wave of crop-destroying armyworms as the pests reproduce.
Liberia's President Ellen Johnson-Sirleaf declared a state of emergency this week and appealed for international aid to fight the insects.
Some 400,000 residents in 100 villages have been affected by tens of millions of the insects, the UN says.
The worms - which are actually caterpillars - are among the world's most destructive agricultural pests.
It is the West African country's worst infestation of armyworm for three decades.
Both Liberia and neighbouring Guinea, which has also been affected, have been carrying out aerial spraying against the insects.
Sierra Leone, which is also at risk, has mobilised chemicals and personnel to its border with Liberia.
A Sierra Leonean expert, Ibrahim Shamie, said that the worst was yet to come.
"The second emergence will be devastating. We had the experience in Sierra Leone, in 1979, about 30 years ago," he told the AFP news agency.
"When the second emergence occurs, that will be the biggest population."
UN Food and Agriculture Organisation (FAO) spokesman Winfred Hammond said Liberia's farming production was at serious risk if the pests - which are currently reaching the pupa stage - were not brought under control.
"Definitely we have a crisis on our hands," he was quoted as saying by the Associated Press news agency.
"At this stage they go into the soil, they are there for about seven to 12 days, before the adult moths will emerge."
These moths can reproduce quickly, laying up to 1,000 eggs within a week which grow into caterpillars within days, he said, launching an emergency appeal for $1.2m (£840,000).
Creeks and rivers - which some villages rely on for drinking water - have been polluted by the massive amount of faeces from the insects, which are eating vital crops including banana, plantain, coffee and cocoa.
FAO experts are in Liberia helping efforts to control the swarm, which analysts suggest may be the result of an unusually long rainy season last year.
The invasion began in Bong County before spreading into neighbouring Lofa and Gbarpolu counties and threatening villages over the border in Guinea.
With each female laying between 500 and 1,000 eggs, the caterpillars (of the genus Spodoptera) can devour an entire crop in a matter of days once they reach maturity. They grow up to 5cm (2in) in length.