The arrival of U.S. Marines has disrupted the illegal drug trade in opium-rich Helmand province _ and that's not necessarily a good thing.

Farmers in Rig district say the troops have driven away many of the smugglers they relied on to transport their opium poppy across 75 miles (120 kilometers) of open desert to a market on the Pakistani border.

"The people from Khan Neshin will still be growing opium," said Fathi Mohammad, referring to the capital of Rig district. "But it will be more difficult for them to sell it."

As the U.S. prepares to send 30,000 more troops to Afghanistan, the experience in this rural district illustrates how the illegal drug trade complicates efforts to win over the population.

Many people in Helmand, which produces more than 50 percent of the world's poppy, rely on the heroin-producing crop to make a living. Disrupting the trade could undermine the goodwill generated by military and civilian development projects, such as dredging irrigation canals and opening schools.

The Obama administration tried to neutralize the drug issue earlier this year when it reversed a Bush-era policy of destroying poppy crops in Afghanistan, a tactic officials said did little or nothing to reduce the amount of drug money flowing to the Taliban and simply drove the population into the insurgents' camp.

The Marines patrolling the dusty fields and irrigation canals in Rig district have been careful to stress to residents that they are here to battle the Taliban, not cut into their livelihoods.

"Our guys have gotten pretty good at telling the farmers, 'Don't worry, we're not going to burn your opium crop,'" said 2nd Lt. Doug Toulotte, a member of the Marine battalion posted in Rig.

But the presence of U.S. forces can still have knock-on effects that harm farmers. Another 8,500 Marines are headed to Helmand by the end of 2010 as part of the increase in troops.

The remaining smugglers operating in Rig are paying farmers one-fifth of what they used to pay for poppy, said 1st Sgt. Mohammad Daoud, a member of the Afghan border police training with the Marines.

"The price has gone down because it is harder for the smugglers to transport the drugs," he said.

Further, the district governor views poppy as un-Islamic and has demanded that farmers not grow it, a position the Marines from the 4th Light Armored Reconnaissance Battalion say they must support _ even though it complicates their efforts to win hearts and minds.

"Anywhere that's under our control, we will destroy their opium crop," said Gov. Massoud Balouch, who comes from a wealthy family in Lashkar Gah, the capital of Helmand.