The alfalfa was first, gnawed at the roots. The barley went next, followed by the beets and tomatoes. Then, one morning in August, Francisco Javier Ponferro went to his melon patch. "From the outside, they looked fine. But when you picked them up, you realized they all had big holes in them."

Like many farmers across central Spain, Mr. Ponferro has seen the face of evil, and it is small and furry. A plague of voles, some 750 million strong, is eating through the fields of Castilla-León, devouring crops, decimating local economies, and carrying disease. While no one seems to know what caused the invasion, creative tactics to combat it are flourishing as farmers, frustrated with weak local response, take matters into their own hands.

The voles first appeared in large numbers three months ago in the provinces of Palencia and Valladolid. With their short reproductive cycles (21 days) and hearty appetites, they soon spread across the plains. "They like tender greens, such as alfalfa, so they go for that first," says Francisco Ávila, an alfalfa farmer in Vilbañez. "But when that runs out, they move on to other things – like my beets."

Officials say some 260,000 hectares of land (1,000 square miles) have been affected, wreaking €30 million worth of damage. Francisco Salvador, spokesperson for the Palencia branch of COAG, an organization for agricultural unions, says, "People are frightened. If the voles are still there in a month, when we start planting, we don't know what we're going to eat."

Juan José Luque, a biologist at the University of Valladolid, admits that scientists don't know the source of the invasion. "Some of these species go through cycles where their population explodes," he says. "It's our hypothesis that the confluence of one of those natural cycles with last year's mild winter plays a role."

He adds that there is not enough evidence yet to connect the problem to global warming. "But if winters stay mild, like last year, it would be logical to expect more of these population explosions."

The recent shift from traditional dry farming to more profitable irrigated agriculture has also made Castilla-León's farmland more appealing to the critters.

Among locals, rumors of more insidious origins are spreading. "These aren't normal moles – they're not blind, for one thing," charges Rufino Reyerto, who lives in Reseno. "They're hybrids that have been produced in a lab." In neighboring Vilbañez, vegetable farmer Ponferro harbors deeper suspicions. "The government created this new mole so that birds of prey would have food. It's the environmentalists' fault." [Editor's note: Many Spanish farmers called the animals moles. Juan José Luque, a biologist at the University of Valladolid, confirmed that they are voles, a rodent not closely related to the mole.]

However murky the outbreak's causes, its cure remains more obscure. In an effort to drive the varmints above ground and toward tubes filled with poison or pits that will be set on fire, the government last week began sponsoring controlled field burning in some towns. But Professor Luque says there is no research to prove that will work. "It would be more effective to do what northern countries have done, and flood the fields," he says. Denmark, Sweden, and the US have had some success with that method.

Mr. Salvador says that his organization is contemplating roadblocks or other collective actions because they're so frustrated. "In most towns, we have seen nothing from the government."

Many are rigging traps or digging elaborate canals around fields to drown the voles when they stop to drink. Armed mostly with shovels, boys in towns like Fresno el Viejo have set up vole-hunting competitions. Ponferro sends out his three dogs each morning. "They caught 50 today," he says proudly. [Editor's note: The original version of this paragraph incorrectly referred to the animals as moles. They're voles.]

At lunchtime, he and other farmers in dusty Vilbañez escape the brutal August sun in the town cafe. The talk quickly turns to voles. Ávila notes that his alfalfa is only two years old – it takes six for the plant to really take root. When he goes out at night to check his crop, the voles are so plentiful that his fields ripple with movement. "If this keeps up," he says glumly, "I don't know what we're going to do."