High food prices are encouraging many Afghan farmers to switch from opium poppies to wheat and other legal crops, a top U.N. official said Friday.

An oversupply of opium has pushed prices down 20 percent a year over the last four years, said Antonio Maria Costa, executive director of the Vienna-based U.N. Office on Drugs and Crime.

Anti-drug campaigns have made opium farming harder, and income from wheat has tripled since 2007, partly due to drought, the office said in a new report.

In 2007, opium brought 10 times as much money as wheat, the office. This year, it brought in three times as much.

"It has been very difficult to repress an activity while its profitability was so high," Costa told a news conference in Rome. "When profitability gets lower, people stop cultivating."

Last year, opium farmers cultivated 476,903 acres; this year, they cultivated 388,000 acres, the U.N. said.

Afghanistan remains the world's largest producer of opium — the main ingredient for the production of heroin — growing 90 percent of the global supply.

Poppy production remains high in the south of the country, where the Taliban-led insurgency is strongest, Costa said. U.S., U.N. and other Western officials say that the insurgency is funded by some of the proceeds from the multibillion dollar drug trade — perhaps as much as $100 million a year.

Surging food prices — stoked by high oil prices, growing demand, flawed trade policies and speculation — have triggered protests around the world in recent months. The Rome-based U.N.'s Food and Agriculture Organization recently said the food price surge in past months pushed the number of hungry people to 923 million worldwide.