They come in cloud-like swarms of up to 50 million. They swoop down on crops and within six hours can leave a field practically barren, moving on to destroy vast stretches of land.
Locusts, which have plagued Africa for thousands of years, once again threaten most of the continent - as well as an area including Saudi Arabia, Pakistan, and India.
Initial spraying of chemical pesticides from the air and ground in Sudan and Saudi Arabia against one species of locust appears to have ``greatly reduced'' the chances of a major plague in some regions, according to the United Nations Food and Agriculture Organization (FAO). But the situation remains ``volatile'' in many areas because of favorable breeding conditions for locusts, say reports by the FAO.
Last year, an international campaign of ground and aerial spraying halted a major infestation of locusts and grasshoppers - both members of the Acrididae family - and saved most crops. Experts hope to duplicate the effort this year.
``It's a start of a potential plague, [but] there is a good hope it can be stopped,'' says Rasik Skaf, a Syrian who is senior officer at FAO's Locust Control Center in Rome. The FAO is coordinating international efforts to halt the insects.
The threat may be greater this year. But the Africans are better equipped and trained - through their own resources and those of foreign donors left over from last year's campaign.
``Everybody is trained and ready to go,'' says Julia Taft, coordinator of US assistance to the campaign for the United States Agency for International Development.
This year the FAO will give greater attention to spotting locusts while they are still breeding on the ground, says Ms. Taft. Last year, some locusts in Sudan were not detected before they began to move. Many ground surveys of the infestation were not done in time. Once locusts swarm, they are harder to stop. Pesticides must then be sprayed ahead of the swarms so that the locusts die when they land and eat the poisoned vegetation. Much of this spraying is done from the air.
But some environmentalists and a researcher with the US Department of Agriculture point out that the potential long-term effects of the massive chemical sprayings are not known. One concern: Some of the natural predators of the locusts may have had their numbers significantly reduced by the spraying.
There have been some calls for greater use of natural predators and the National Audubon Society recommends more research on parasites and viruses to control locusts and grasshoppers. But Skaf says use of natural predators has failed. In an emergency such predators ``cannot cope with the large numbers'' of insects, he says. Critics charge, however, that the FAO is not investing enough in research on use of natural predators.
The total 1986 cost for the fight against locusts in Africa was $60 million, including about $10 million from African nations. The US share was about $9 million, making the US the single largest donor in the campaign. US officials expect to spend another $10 million on the antilocust effort this year. The value of the crops saved in 1986 was in the ``hundreds of millions'' of dollars, according to Skaf. Taft notes that the US contribution saved the US government much more in famine relief.
Many regions of Africa had just begun to emerge from the severe damage of the l983-84 drought. Despite the locust and grasshopper infestation that devastated some crops and stored surpluses last year, food production in Africa rose about 2 percent from the previous year.
In a telephone interview from his office in Rome, Skaf outlined the scope of this year's infestation - estimated to be one of the biggest in recent years.
Grasshoppers now threaten an area bordered roughly by Mauritania and Gambia on Africa's west coast, to Chad and Cameroon, including parts of northern Nigeria.
Desert locusts are endangering crops in nations from Morocco, in Africa's northwest corner, to as far away as India; as far north as southern Soviet Union and as far south as Tanzania. The red locust is a threat in parts of southern and eastern Africa. So far, species of the brown locust have been confined to South Africa, Namibia (South-West Africa), and Botswana, in southern Africa.
The African migratory locust is now ``under control'' in the Mali area, according to Skaf.
Some 50 airplanes will be used in this year's campaign, about the same as last year, along with other transport equipment and ground sprayers. Massive amounts of chemical pesticides will be used.
Environmentalists in and outside of the US government, as well as some African officials, have expressed concern over the use of some of these chemicals.