Anti-narcotics police chemically fumigated the Sierra Macarena national park last week, clearing its entire 4,600 hectares of coca.
The spraying destroyed coca capable of producing 16 metric tonnes of cocaine and was probably a big blow to the Revolutionary Armed Forces of Colombia (Farc).
But environmentalists complain that the spraying of herbicides harms the environment and causes health problems for those living in the area.
Local groups have promised a court battle to prevent any spraying in 11 of Colombia's other 50 protected reserves known to have coca.
Still others say the entire spraying strategy, a cornerstone of the war on drugs, is flawed and ineffective - even in record use - at stopping a sharp rebound in coca production.
Alvaro Uribe, the Colombian president, announced that the park would be fumigated by air after a 100kg bomb planted by rebels exploded on August 2, killing six peasants hired by the government to uproot the coca by hand.
The "world will have to understand that we must fumigate", he said.
Uribe said he wants to double aerial spraying, and his top military advisers want to expand the practice to the 11 other parks known to have coca.
"Those who think fumigating La Macarena, and perhaps other parks, will wipe out coca production are wrong"
Colombian newspaper El Tiempo
"It's the most efficient way to do our job," General Jorge Baron, head of the anti-narcotic police, said.
In addition to those killed by the bomb, 26 workers, soldiers, and police guards have been killed at the Sierra Macarena park since December, when the government launched a manual eradication drive there involving 3,000 troops - its biggest ever. About 200 other workers quit, fearing for their lives.
Washington has long urged Uribe to extend spraying to parks and provided the glyphosate herbicide, as well as Black Hawk helicopters used for protection, during the missions.
The US said in a March indictment of Farc leaders that the rebel group is the world's largest drug cartel, responsible for more than half of the cocaine produced in the world.
"If the Farc thought the government would allow coca to grow untrammelled in its national parks, they've obviously miscalculated," said James O'Gara, deputy director of supply reduction for the White House's Office of National Drug Control Policy.
But the chemical fumigation of a park in one of the world's most biodiverse countries drew sharp criticism from environmentalists and others.
"Those who think fumigating La Macarena, and perhaps other parks, will wipe out coca production, are wrong," the normally pro-government newspaper El Tiempo said last week.
"Instead, there will be more coca, and less park, as rebels destroy more forests, deeper inside the park, to continue planting."
The editorial echoed the belief of a growing number of Colombians and key US congress members that aerial spraying is failing.
Despite a record fumigation of 140,000 hectares last year in Colombia, the latest US government survey found that 26% more land was dedicated to coca production - a rise due in part to a near doubling of the satellite survey area on which the estimate was based.