The bugs are among the world's most destructive agricultural pests
Liberia's president has declared a state of emergency in response to a plague of crop-destroying army worms.
Ellen Johnson-Sirleaf said all possible resources would be used to fight the insects, which have spread to next-door Guinea and are nearing Sierra Leone.
Some 400,000 residents in 80 villages had been affected, the UN Food and Agriculture Organisation (FAO) said.
The "worms" - which are actually caterpillars - are among the world's most destructive agricultural pests.
Guinea has started spraying, and Sierra Leone has announced it will mobilise chemicals and personnel to its border.
Worst in decades
Liberia has already appealed for international help to carry out aerial spraying against tens of millions of the invading insects.
It is the West African country's worst infestation of armyworm in three decades.
Creeks and rivers - which some villages rely on for drinking water - have been polluted by the massive amount of faeces from the swarm.
Mrs Johnson-Sirleaf said there are indications the insects had reached several villages in neighbouring Guinea.
Speaking during her annual message to lawmakers, she said: "I hereby declare a state of national emergency with particular emphasis on the existing and potentially affected counties.
"The technical team has identified and classified the species and has commenced spraying in the affected areas. Water and food are being supplied to the most vulnerable and displaced."
An agriculture ministry-led command post, manned by fewer than 100 pest control workers, has been set up to contain the infestation.
FAO experts are in Liberia helping efforts to control the swarm.
The agency's Winfred Hammond told the BBC: "It's quite an alarming situation for us, in a country where food-security is a big challenge. The areas affected have all been consumed by the army worms."
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.