Giant clouds of desert locusts have invaded a huge chunk of North and sub-Saharan Africa, threatening to devastate some of the world's poorest countries. Experts estimate this year's locust infestation is the worst in nearly two decades. And, without assistance from the international community, some fear they may even cause a famine. Lisa Bryant tracked the locusts in Mauritania, one of the countries hardest hit by the plague.

Summer rains have brought a green fuzz of vegetation across the arid, scrubby plains of Brakna, a region in southern Mauritania near Senegal. The first shoots of millet, sorghum and watermelon crops are sprouting in small, tilled fields. But farmers like Salek Ould Salek fear these promising signs of a good harvest might be quickly wiped out.

Speaking in Arabic dialect, Mr. Salek says hundreds of desert locusts swarmed into the region just the other day. They stayed a short while before flying off, but he fears they'll return, and decimate his young crop. If so, he says he and his family will lose their only source of livelihood.

This Mauritanian farmer isn't the only one worrying. Good rains and other favorable conditions this year have produced record numbers of locusts. They first swarmed into northern African countries like Algeria and Morocco. Now, they are traveling in search of food on currents of moisture and wind as far south as Senegal and Mali. Without effective eradication efforts, says Mohamed Lemine, a regional locust expert for the U.N. Food and Agricultural Organization, they could spread east to the horn of Africa, before returning to North Africa by the year's end.

Mr. Lemine says impoverished countries like Mauritania don't have the sprayers, insecticides, jeeps and airplanes to defeat the infestation. The U.N. has launched an appeal for $50 million in international assistance over the next few months. So far, it has received only a fraction of that sum. The worst-case scenario, Mr. Lemine says, could be a famine.

At the Mauritanian government's locust fighting center, in the country's capital, Nouakchott, deputy director Mohamed el Hassan Jaavar points to a map of the country, sprinkled with dots indicating where swarms of locusts have been spotted. So far, the voracious insects have destroyed date palm plantations in Mauritania's desert oases, forage for nomadic herds, millet, watermelon and cassava crops. Swarms of insects have even swooped into Nouakchott, bouncing off vehicles and crunching underfoot.

Mr. Jaavar says, because Mauritania is a transit corridor for locusts flying between northern and sub-Saharan Africa, it's more affected by the infestation than other countries.

Some 70 percent of Mauritanians depend on subsistence farming for their livelihood. He says during the last major locust infestation in 1988, many people lost everything. The government says it needs $20 million in aid to prevent that scenario from recurring.

Locust experts, like Yacoub Habad, are now scouring the Mauritanian desert and scrub lands of the south, looking for large swarms of locusts. They travel in teams and they're often out for weeks at a time. The government says this year it needs 30 teams to fight the locusts by land, and four to fight them with sprayers by air. So far they've got less than half the needed equipment and manpower.

Mr. Habad heads south from Nouakchott on a fiery hot afternoon in a four-wheel drive truck. The scenery changes from rose-colored sand dunes to clumps of thorn trees and spiny green vegetation. We pass the white tents of nomadic herders, and a few settlements of gray cement houses. Suddenly, a small cloud of locusts swirls in the blazing sky.

Mr. Habad says this is a fragment of a swarm, too small to be sprayed with insecticide, or even to be reported to the locust center in Nouakchott. The insects are finger-sized, yellow, and fat with eggs they will soon lay. They lay three sets of eggs and their larvae devour everything in sight. Young, pink-colored locusts can eat their body weight in food each day.

The locusts have already damaged the crops of this farm, near the southern village of Boghe. Mr. Habad asks a female farmer, Hawa Mint Touboul about what happened.

"Did a lot of locusts come or just a few?" Mr. Habad asks.

"A lot came," Mrs. Touboul replies.

They settled near her house for a while. Already, they've eaten part of her young watermelon seedlings. She's afraid they'll return.

The locusts also settled on pasture just a few miles away, where Yal Mohammed Suleiman was minding his herd of skinny cows.

Speaking in the French he learned in grade school, Mr. Suleiman says there were so many locusts, he couldn't even see the sky.

Local people try to kill locusts by burying larvae in holes, but Suleiman says that traditional method doesn't work very well. Still Mr. Suleiman says he remains hopeful this will be a good farming year. The rains have come early. That's good news for his cows and his crops. But that's also good news for millions of hungry desert locusts, which don't show signs of leaving Mauritania anytime soon.