FLORIDA'S LAW ENFORCEMENT agents destroy about 100 000 marijuana plants every year. And that's just 20 per cent of the estimated total grown there. Many of the crops lie deep in the Everglades, where there is dense vegetation to camouflage them and alligators to deter inquisitive state officials. But last April, Jim McDonough, director of Florida's drug control policy, decided to get tough. He suggested spraying the Everglades with a fungus that would kill off the marijuana but leave other plants untouched.

When the story appeared in newspapers in July, it enraged environmentalists and provoked a lawsuit from a pro-marijuana group. Some critics described it as a form of biological warfare. The St Petersburg Times urged that the "killer fungus" should not be released. And several months before the story broke, David Struhs, secretary of the Florida Department of Environmental Protection (DEP), had expressed fears that the fungus would mutate and attack other plant species.

McDonough's suggestion was based on research into biocontrol funded by the US Department of Agriculture (USDA), aimed at finding a cheap and environmentally friendly alternative to herbicides. The Florida office of drug control policy now says both the media and the DEP had misunderstood McDonough's proposal. He never suggested actually spraying the fungus over the swamps, the office maintains, but merely wanted to test it in a quarantine facility in Gainesville. Indeed, the DEP has since sanctioned this proposal, but according to Albert Wollermann, the office's lawyer, there are no immediate plans to go ahead with the tests.

Selective killers

The Florida eradication scheme may have been shelved, at least temporarily, but the USDA continues to spend $23 million a year on research into biocontrol agents that would selectively kill coca plants, from which cocaine is derived, and opium poppies. And some of those are poised to move out of the greenhouse and into the open.

Biocontrol of weeds is certainly not a new idea, but in the past it has usually involved insects.
The fungus at the centre of the Florida row is a variety of Fusarium oxysporum. Fusarium species infect the vascular system of a number of plants, from bananas to wheat, causing them to whither and die. The Florida scheme was based on work carried out by a researcher at Montana State University, Bozeman, called David Sands, who suggested that this particular variant would be lethal only to cannabis.

Sands did initially have a grant from the USDA to look at using Fusarium to control marijuana. But when he approached the Florida state government it was as head of his own company, Ag/Bio Con. The USDA says it stopped funding his research a few years ago, when lab tests showed the fungus was only marginally effective against cannabis. "The results were mediocre," says Eric Rosenquist, leader of the USDA's international programmes, who oversees the agency's funding for narcotics biocontrol, "If it's that mediocre in the greenhouse, it's unlikely to work in the field."

Ag/Bio Con continues its research in this area. And recent evidence suggests that he has improved the technology. The university began to release documents relating to the research. In some, Sands discusses patent applications he has made on a process for "virulence enhancement" of bioherbicides. It's not clear what this enhancement consists of, but in a letter he says that he developed it after USDA funding stopped.

In the meantime, the USDA is collaborating with the UN on a program. They hope to use the Pleospora papaveracea fungus to control opium poppies. But its main effort is concentrated on another variety of Fusarium oxysporum that attacks coca plants.

This fungus was discovered accidentally when it wiped out a test plot of coca being grown in Hawaii. Since then, USDA researchers have worked on manufacturing large amounts of the fungus in a form that is easy to store. More importantly, they have assured themselves that it will attack only coca plants. Since most pathogens evolve with their hosts, they can often survive only in that host. This selectivity can be confirmed in the lab by trying to persuade a fungus to infect first close relatives of the target plant, then progressively more distant relatives, until researchers are convinced no other plants will be affected. "We've done host specificity studies," says Rosenquist of the anti-coca fungus. "We're convinced of its safety.

If the anti-cannabis fungus is now more effective, it could spell disaster for farmers who grow industrial hemp. Masterson points out that Montana, where some of the work on the fungus has been done, borders the Canadian province of Alberta, where hemp has been grown industrially since 1998. If the fungus somehow spreads to fields in Alberta, it could damage the legal crop.

"For the hemp industry, it would be devastating for the fungus to get out," says Douglas Brown, a director of the WestHemp Cooperative in Vancouver, British Columbia. "There would be millions of dollars of losses. If this fungus is looking for Cannabis sativa, it's not going to differentiate between high-THC and low-THC varieties."

And even if the fungus stays put, it could destroy wild cannabis that has adapted to conditions in the areas where it is released. Losing the wild plants could make it harder to breed hemp with the traits future farmers will need, says Brown.

So the next step for the USDA will be to convince the government of one of the coca-producing nations--such as Peru, Bolivia or Colombia--to let field experiments take place there. Peruvian farmers think that this has already happened. They have accused the US of testing an anti-coca fungus that has since spread to bananas, yucca and tangerine crops.

Rosenquist denies this. And he stresses that work won't start without permission from the country concerned. But even with the country's consent, says Mooney, it's dangerous. "The solution to narcotics is not just to destroy the crops. It's a fundamental social problem, and it's not going to be solved by a silver bullet from an airplane."