Arrests of illegal migrants along the U.S.-Mexican border have dropped by more than a third since U.S. National Guard troops started helping with border security, suggesting that fewer people may be trying to cross.

U.S. Border Patrol agents arrested 149,238 fewer people from the start of July through November, down 34 percent from the same period last year, according to monthly figures provided Tuesday by U.S. Border Patrol spokesman Mario Martinez.

Arrests also had dropped by 9 percent for the same period from 2004 to 2005. If the downward trend continues, it would be the first sustained decrease in illegal immigrant arrests since shortly after the Sept. 11, 2001, terrorist attacks.

National Guard troops started arriving along the border June 15, and 6,000 were in place by August.

Victor Clark, a Mexican migration expert in Tijuana, says many migrants fear they will confront U.S. soldiers on the border.

"The presence of the National Guard has had a big impact on migrants," he told The Associated Press on Tuesday.

Border Patrol officials usually attribute a drop in arrests to fewer people crossing.

"We have seen some tangible results," Martinez said. "But we'll have to see over the next few months if it holds up. We are optimistic."

The National Guard troops are not allowed to detain migrants and have been limited to monitoring surveillance cameras and body heat detectors, but they have freed Border Patrol agents and "have helped us tremendously to detect illegal migration traffic," Martinez said.

The United States plans to expand the Border Patrol from just over 11,000 agents to about 18,000 by 2008. The U.S. also plans to build 700 miles of additional border fence.

Other measures may also be deterring crossers. In July, U.S. and Mexican officials started working together to prosecute human smugglers on both sides of the border.

U.S. immigration officials also have been raiding U.S. companies for illegal workers. Earlier this month, 1,300 people were detained in a sweep of meatpacking plants in six states.

Added to that, smugglers have increased their fees, charging as much as $3,000 to hide migrants in their cars and drive them across the border. Before the National Guard troops arrived, the price was about $2,000, migrant activists say.

Still, border experts say the downturn may be temporary while smugglers search for new routes and migrants come up with the money to pay the higher fees.

Edgar Velasquez acknowledges it's become tougher to cross. He spent three days walking in freezing temperatures through the remote mountain country west of Tucson, Ariz., and still was caught.

Agents found a body in those mountains Dec. 19. But that did not deter Velasquez, who said he planned to slip across the Arizona border during the holiday week when he hoped the U.S. patrols will be short-handed as agents take vacations.

"I imagine they also want to be with their families," said Velasquez, resting in the border city of Nogales before embarking on his illegal odyssey to reach a construction job in Florida.

Gustavo Soto, a spokesman with the U.S. Border Patrol Tucson sector, said smugglers often tell migrants there are less border agents out in the desert on holidays or when the weather is bad, "even though we have surveillance on the border 24/7" and 365 days a year.

Some migrants are simply giving up after a single try, something that was almost unheard of only a few years ago.

Esther Ardia walked for nearly three days as temperatures dropped to 14 degrees in the Arizona desert, trying to get back to her job at a North Carolina pine tree farm.

Ardia, 21, couldn't keep up with the group of about 30 illegal migrants and was abandoned by her smuggler after her legs cramped up. She was picked up by the Border Patrol and returned to Mexico.

"I knew it would be hard, but I thought I could make it," said Ardia. "It's very hard. I'm not going to try (to cross) again."