Normally, they're just harmless little green grasshoppers that live solitary lives.
But sometimes, when rains are plentiful and the desert foliage blooms, millions of locusts in West Africa merge into massive swarms, change to a dusty-yellow color, and swirl into a munching frenzy.
This year, otherwise welcome rains have paradoxically spawned the worst plague of locusts in the region since the 1980s. One swarm is reportedly more than 40 miles long. They eat their weight in food every day - and are denuding what was supposed to be a bumper crop.
Now they're moving northward. A few have even reached Italy. The plague may last several years and could, one UN official says, cause more destruction than any current African conflict, including Sudan's humanitarian tragedy.
But there is hope of stopping it. Pest-control efforts include locals burying flightless locusts in trenches and scouts with global positioning devices calling in cropdusters, including six from the US that arrived this week. Some $200 million has been spent so far, but experts say more is needed.
"We still have the possibility of making an impact," says Clive Elliot, chief locust officer at the UN's Food and Agriculture Organization (FAO), who's based in Rome. But, he says, "The movement of swarms mainly to the north and northwest is already beginning, and it's becoming increasingly difficult to stop the next stage."
If rains in countries like Morocco, Libya, and Algeria are plentiful, the current generation could give birth to a much larger group of offspring. Though locusts only live three to five months, females can lay 100 eggs at a time. The concern is that the swarms may multiply over the next two or three years - and continue stalking the region, sparking widespread hunger, as happened during the 1980s.
One hope is for a strong wind to blow the pests into the Atlantic, which is what ended the 1980s plague (although a few survived and landed in Jamaica).
The tragedy is that UN and local officials have known this plague was coming for about a year. The UN says it warned the world and blames donors for not responding more quickly. Donors say they weren't warned clearly. The FAO has asked for $100 million to fight the pests. It's received some $40 million in pledges and $14 million in actual cash. The shortfall, says Mr. Elliot, means the FAO won't be as ready if bigger swarms migrate back to West Africa next July.
One reason for the ineffective response is the infrequency of the plagues. In the years between episodes, organizations set up to fight them have atrophied. The last plague was 1987-89. Before that, plagues hit in 1967-1969, 1949-1963, 1940-1948, and 1926-1934, according to the FAO.
One positive outcome is new cooperation among African nations. Morocco and Libya, for instance, sent cropdusters south to Senegal, Mali, and Niger, hoping to stop the locusts' northward drift. The pesticides killed many, but more are still headed north.
Swarms move by day and rest at night. They can travel more than 60 miles per day. Juvenile locusts - called "hoppers" because they can't fly - have voracious appetites. Farmers bury them in trenches.
In a bitter irony, the swarms can be a curse in a time of plenty. Strong rains create robust crops and a flowering of the arid landscape - good conditions for locust eggs.
At first, solitary locusts feast on the plentiful food. But when the rains stop, the vegetation dries out, and the locusts are forced together. Swarms are formed, scientists say, when locusts bump against each other. This transition, from solitary, shy insect to so-called "gregarious" swarming pest, "is at the heart of the issue," says Oxford University entomologist Stephen Simpson.
The frequency of swarms appears to be affected by the growing number of farms in the region, he says: "That humans plant crops in otherwise barren environments does encourage locusts to come together and congregate." Furthermore, gregarious females tend to give birth to gregarious offspring - thus prolonging the swarms.
So far up to 40 percent of pastures and 10 percent of vegetables have been damaged in West Africa, but a full report won't be available until early November.
Meanwhile, the high-stakes fight continues. "If we lose the battle in the next five weeks, we will have a tenfold increase in the locust swarms, and they will go north," the UN's top humanitarian official, Jan England, said last week. He added, "There is a much greater danger to livelihoods than [from] any of the wars in the African region at the moment, Darfur included."
• Material from wire reports was used in this story.