More than half of the population could be at risk of hunger as the plant-devouring insects scour the countryside, the U.N.'s Food and Agricultural Organization said.
An FAO locust control expert said the plague posed a major threat to the Indian Ocean island, a country struggling with poverty.
"Nearly 60 percent of the island's more than 22 million people could be threatened by a significant worsening of hunger," Annie Monard told the BBC.
"The last [locust plague] was in the 1950s and it had a duration of 17 years so if nothing is done it can last for five to 10 years, depending on the conditions," she said.
More than $22 million in emergency funding was needed so a full-scale spraying campaign could be initiated, the FAO said.
"FAO estimates that about two-thirds of the island country will be affected by the locust plague by September 2013 if no action is taken," the organization said in a statement.