SELDOM can such a laborious public policy have been devoted to such a futile end. For the past 15 years or so Colombia has used crop-dusting planes operated by American contractors to spray around 130,000 hectares (321,000 acres) a year of its land with glyphosate, a powerful weedkiller, in an attempt to wipe out the coca crop that provides the raw material for cocaine.

Put all that land together, and it amounts to an area almost as big as the state of New Jersey.

The defenders of spraying, who include the drug warriors in Washington, DC, claim that it has played a vital role in cutting coca cultivation in Colombia to a third of its peak of the late 1990s, and with it the output of cocaine (though by less). But studies have linked the spraying to increased rates of skin and respiratory diseases, and of miscarriages among farmers’ families.

In March a research arm of the World Health Organisation reclassified glyphosate as “probably carcinogenic”. The finding is rejected by Monsanto, its manufacturer, and some independent scientists. But it prompted Juan Manuel Santos, Colombia’s president, to ask his officials this week to halt aerial spraying by October and find an alternative.

That will not be easy. In Peru the government conducts manual eradication, sending workers to yank out coca plants one by one. But Colombia’s countryside is littered with landmines planted by guerrillas who rely on drug revenue, making manual eradication dangerous. To try to preserve the spraying programme American officials leaked a report that coca planting rose by 40% in Colombia last year. That happened in part because FARC guerrillas are seeking to raise cash ahead of a possible peace agreement.

Yet Mr Santos is right not to be swayed by such scaremongering. Few public policies in Latin America are as ineffective as coca eradication. Daniel Mejía of Bogotá’s University of the Andes has found that to be sure of wiping out just one hectare of coca 30 must be sprayed. Manual eradication does not work much better. Nowadays coca farmers replant straight away. Latin Americans may be generally poor at business innovation but coca is an exception: in Peru farmers have adopted high-density planting and drip irrigation to raise productivity. Some have doubled their harvests, to four a year. Rather than destroy the crop, eradication just pushes it into new areas. Total cocaine output in the Andes has remained abundant enough to supply the world without anything more than occasional price spikes.

So what is to be done? Many, including this newspaper, believe that legalising cocaine is the least bad option. Such is the fatigue with the drug war in the Americas that some presidents, including Mr Santos, have begun to muse about that. But legalisation remains decades away.

Meanwhile, Latin American democracies suffer the lethal, corrosive power of the drug gangs. That was on display on May 1st in Jalisco, in Mexico, where a newish mob answered a government crackdown by shooting down a helicopter with a rocket-launcher, killing six soldiers, and setting up roadblocks across the state. Peruvians are becoming used to gangland killings in Lima; in a recent poll 72% said Peru is “on the way” to becoming a “narco-state”. That is an exaggeration, but the drug trade has penetrated politics and the courts as well as the police.

No state can ignore the kind of challenge posed in Jalisco. Mexican officials stress that dismantling drug mobs is only part of a broader strategy that includes strengthening the police and crime prevention in vulnerable communities. But they are struggling to turn those intentions into reality.

The same applies farther south. Peruvian officials talk up their coca eradication. But the state barely disturbs organised crime. Peru is now the world’s biggest source of cocaine and supplier of counterfeit money, and a rising money-laundering centre. Yet the government seizes less than 10% of estimated cocaine output and few of the chemicals used in its manufacture, and almost nobody has been charged with money laundering.

Colombia does much better at interdicting the cocaine trade, frequently seizing up to half of estimated production. Bello’s suggestion to Mr Santos would be to forget about eradication altogether. After all, harvested coca accounts for less than 10% of the value of cocaine exports (and much less of retail value in foreign markets). Forget, too, about consumers and small-scale dealers.

Instead, redouble efforts to go after the cocaine laboratories, the chemical suppliers, the traffickers and the money launderers, with better intelligence and better policing. And continue to argue for legalisation.