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NAIROBI - The United Nations is warning countries in northwest Africa to and begin early control measures as swarms of desert locusts move into the region.

According to the Food and Agriculture Organization (FAO), locust activity has intensified over the past three months, with large infestations of the crop-ravaging pests arriving in Algeria, Libya and Tunisia.

"Surveys and control operations are particularly urgent in locations where winter and early-spring rains have created suitable breeding conditions," said Cyril Piou, FAO locust monitoring and forecasting officer, in a last month.

"If left unchecked, these bands could develop into small swarms from May to June, increasing the risk to crops and pastureland," he added.

THE FAO is calling on affected nations to conduct surveys where locust breeding is likely to occur, an area that spreads from south of the Atlas Mountains in Morocco to the Sahara in Algeria, southern Tunisia and western Libya.

Here's what you need to know.

What causes locust swarms?

Desert locusts usually form , creating a mass of hungry insects that can cross continents and seas in search of food.

They are found in desert and semi-arid areas across 30 countries - from Mali in West Africa to India in South Asia.

Scientists have linked to some infestations, particularly those that hit eastern Africa in 2020.

In 2020, massive swarms invaded , ravaging crops, decimating pasture and deepening a food crisis from Kenya to Ethiopia, where millions already faced food insecurity.

Why are desert locusts such a threat?

Desert locusts are the world's , threatening livelihoods, food security, the environment and economic development, says the World Bank.

Swarm sizes vary from 1 square km - which can contain 40 to 80 million locust adults - to several hundred square km.

One swarm sighted in northern Kenya in 2020 was reportedly , more than twice the size of New York.

A swarm spanning 1 square km, containing about 40 million locusts, can eat the same amount of food in a day as about 35,000 people, 20 camels or six elephants.

A swarm the size of Paris eats as much in a day as half the population of France.

They usually fly with the wind and can travel more than 150 km in a day.

What can be done to stop locusts?

The best way to control desert locusts is by spraying them with pesticides or bio-pesticides from vehicles or .

Control by natural predators and parasites is limited since locusts can quickly escape most natural enemies. Eventually, locust swarms decrease by natural mortality or migration.

(Reporting by Nita Bhalla; Editing by Lyndsay Griffiths)