After three years in operation, a U.S.-financed program to wean Bolivian farmers from growing coca leaf, the primary material for cocaine, is in trouble.

"I'm going to go back to growing coca if the government doesn't do something. There's no alternative development here - just words," said Zacarias Mamani, 62, who substituted pineapples for his coca plants two years ago.A crackdown in Colombia has hurt the once-dominant Medellin, Colombia, cartel, but cocaine trafficking is back up to record levels. A cartel based in Cali, Colombia, now controls a trade that has spread across South America.

In Bolivia, impoverished farmers are paid $2,000 for each hectare (about 2 1/2 acres) of coca plants they destroy. But the Bolivian government failed last year - and probably will again this year - to reach the U.S.-imposed target of destroying 7,000 hectares of coca.

"It's going to be almost impossible. The problem is that we haven't combined eradication with alternative development. The farmers don't trust us anymore," said Enzo Fernandez, field coordinator of the eradication program in the Chapare, a vast jungle area where an estimated 150,000 peasants grow coca leaf that produces 30 percent of the cocaine consumed in the United States.

South Americans, many of whom view the drug trade primarily as an economic problem, are expected to press Mr. Bush to provide more money to develop their fragile economies. They argue that police action without economic development would push more peasants into the drug trade and destabilize their fragile democracies by fueling discontent.

U.S. officials argue that South American countries, plagued by corruption, can't effectively spend any more money sent for alternative development. They say the only way to make coca too expensive to produce is heightened interdiction, including bringing local militaries into the anti-narcotics battle.

Carlos Saavedra, Bolivia's interior minister, says the U.S. economic assistance is insufficient.

"We need to substitute an economy that is legal . . . Interdiction is not the solution for combatting narco-trafficking," Mr. Saavedra said.

Richard Bowers, the U.S. Ambassador to Bolivia, defends the basic approach that has been taken.

"It's not valid to say we are not spending enough on economic development," Mr. Bowers said. There is the holy trinity of eradication, interdiction and economic development. We are balancing this correctly."

Bolivian President Jaime Paz Zamora last year negotiated the surrender of seven top drug traffickers, and U.S.-trained anti-drug police have forced cocaine-processing laboratories further into the jungle.

But Mr. Paz Zamora has resisted U.S. pressure to bring the Bolivian military into the fight, fearing it may turn a largely peaceful battle into a bloody war.

Last year, the United States trained and equipped two Bolivian army battalions for anti-narcotics operations. The first battalion carried out one operation before being decommissioned. The second battalion has yet to leave its barracks.

Bolivians involved in alternative development efforts say they have largely failed because of poor planning, inadequate funding and corruption. In 1989 and 1990, about a quarter of the country's 100,000 acres of coca were voluntarily eradicated - but almost as much acreage of new coca was planted, officials say.

Last year, the United States conditioned $66 million in aid on the eradication of 17,000 acres of coca. The goal was missed by about 15 percent.

"The Bolivians came here two months ago and asked me if I wanted to take out my coca," said Gabriel Agreda, sitting outside his wood shack in San Pedro. "I resisted. If I cut my plants, I will starve."

Economists say farmers are reluctant to give up the hearty coca plant

because it requires little care, can be harvested four times a year and always has a ready market.

Armando Ferrufino, regional director of the U.S.-financed alternative development program, which has a $4.5 million budget this year, says most of the 10,000 farmers who have received government credit to buy substitute crops - such as pineapples, bananas and oranges - cannot afford the 13 percent annual interest rate on it.

Many who substituted other crops for coca now say they made a mistake

because they have no income until the crops are harvestable. A new orange tree, for example, takes four years to produce fruit. And once the harvest begins, farmers complain, they can't transport their products to market.