Madagascar is in a race against time to raise enough money to tackle its worst plague of locusts since the 1950s. Locusts have already infested over half of the island's cultivated land and pastures, causing the loss of 630,000 tonnes of rice, corresponding to 25% of food consumption.

At least 1.5m hectares (3.7m acres) could be infested by locusts in two-thirds of the country by September, warns the Food and Agriculture Organisation (FAO). Findings from a damage assessment indicate that rice and maize crop losses due to locusts in the mid- and south-western parts of Madagascar vary, on average, from 40% to 70%, reaching up to 100% in some plots.

Madagascar's agriculture ministry declared a national disaster in November. The food security and livelihoods of 13 million people are at stake, about 60% of the island's population. Around 9 million people depend directly on agriculture for food and income.

"We don't have enough funds for pesticide, helicopters and training," said Alexandre Huynh, the FAO's representative in Madagascar. "What is extremely costly is to run helicopters [needed to spray pesticides]. We have to start in September, and we have two to three months to prepare. We need $22.4m [£15.1m] but we are quite short of that. Discussions are going on with donors."

Adults locusts on a rock in Isalo national park, Madagascar. Photograph: Tiphaine Desjardin/FAO

The FAO has been issuing warnings since August last year, calling for financial support. José Graziano da Silva, its director general, said prevention and early action are key: "If we don't act now, the plague could last years and cost hundreds of millions of dollars. This could very well be a last window of opportunity to avert an extended crisis."

Control of the locust upsurge would have cost $14.5m (£9.8m) in 2011-12, but the FAO received only half the funding. The timing of this infestation could hardly be worse for Madagascar, which has been in the grip of a political and economic crisis since a coup in 2009, when Andry Rajoelina, the mayor of the capital Antananarivo, seized power.

Rajoelina's coup turned Madagascar into a pariah state. The EU, US and other countries suspended aid and the African Union suspended Madagascar's membership until a return to the state of law. The first elections since the coup were scheduled for 24 July, but, faced with sorting out the legitimacy of contested candidates, the government has postponed elections to 23 August.

Poverty has risen sharply in the past five years. According to the World Bank, the proportion of people living below the poverty line – already high before the crisis – may have risen by more than 10 percentage points. With 92% of the population living on $2 a day, Madagascar is one of the world's poorest countries. A subsequent drop in tourism has led people to plunder the forests that sustain the island's biodiversity.

The aid cut-off contributed to the locust crisis as the government allowed its locust surveillance programme to be run down. Those employed by the anti-locust national centre, part of the department of agriculture, to monitor the locust populations in the swamps in the south stopped receiving salaries, and the monitoring programme effectively ground to a halt. Normally, once the locust population starts to rise, people spray pesticide to bring the numbers down to safe levels.

A closeup of a locust. Photograph: Ministry of Agriculture in Madagascar

"When we were aware of the issue, it was too late," said Huynh, who has been in Madagascar since 2009. The FAO has focused on the most important swamps in the south, but managed only to restrain the spread of locusts. "There were not enough funds for a full programme. We avoided a major human crisis, but not enough to get rid of the problem, which has reached alarming proportions," he said. The FAO is now looking for funding for an intensive spraying campaign involving three helicopters.

Once locust outbreaks get out of hand, the cost can be extremely high. Madagascar's last outbreak in the 1990s took five years to contain and cost $50.6m. The FAO says a three-year programme to return the locust plague to safe levels requires more than $41.5m over the next three years.

Norway has agreed to contribute $513,000, and the FAO and Madagascar are in negotiations with the World Bank over a $10m grant.